Determination of physical constants

Chapter 4

Determination

of physical

constants

Most of the physical properties of organic compounds (melting point, boiling point, refractivity, solubility in various solvents, etc.) can be expressed numerically, and these data are called the physical constants. Other physical properties, such as the absorption or scattering of radiant energy of different wavelengths, yield a spectrum that may not only be characteristic of the given molecule, but may also allow the recognition of the functional groups present. F r o m these data—although they can be expressed numerically as wavelength values or wavelength ranges—no physical constants are derived in the classical sense, and therefore they will not be treated in this chapter. In this chapter, the treatment of the subject is based on practical requirements, that is, physical constants are considered only with respect to the determination of characteristic values for organic compounds. In a narrower, classical sense, physical constants are numerical data measured on pure substances and, when compared with literature data, are suitable for more or less reliable identification of the sample. As was mentioned above, the individual organic substances can be characterized better by their physical than their chemical properties. However, one physical constant is rarely sufficient for identification, and the determination of some chemical properties is usually also essential. When several, properly chosen physical constants are determined, relatively few measurements can yield sufficient information for identification. With a knowledge of some properties of the sample (qualitative analytical data, preliminary tests, etc.), when the task is the reliable identification with the compound suspected, accurate determination of one single physical constant may be sufficient.

64

Certain physical constants are also related to the structure of the molecule (e.g., refractivity, density and boiling point are related to each other and also to the structure) [ 1 ] . Physical constants are suitable for the identification of unknown organic substances, and also for the determination of the purity of the sample. They are often more sensitive than chemical reactions to the presence of contaminants. Thus, an unaltered value of a physical constant (which can be determined accurately) during a purification procedure proves the purity of the substance. Certain physical constants (e.g., melting point, boiling point, density, refractivity, solubility) can be found in the literature for many compounds, whereas others (e.g., specific rotation) are characteristic of different groups of compounds and are known for almost all of them. Other physical constants are not available in the literature, even if they were suitable for the characterization of various compounds. Published IR spectra can be regarded as excellent physical constants and are becoming available in increasing numbers. In identification work, the choice of the most characteristic physical constant (s) is of importance. In order to achieve simple and rapid approach to identification, usually a general informatory measurement is first carried out, followed by an accurate determination of the same physical constant or another one that is found to be more suitable for identification. Owing to the increasing number of known organic compounds, it often happens that 5-10 c o m p o u n d s have identical or nearly identical physical constants (e.g., melting and boiling points). Although the accuracy of the methods used in the determination of physical constants has greatly improved, facilitating distinction, this is offset by the fact that the melting and boiling points of many organic compounds, for structural and physical reasons, are not always sharp, but may cover a range. Thus, whereas about 50 years ago the determination of the melting point or boiling point was often sufficient for identification, today these measurements are hardly more than a preliminary test. The following requirements should be met by the physical constants utilized in identification work: 1. Yield accurate, reproducible and reliable results; 2. Be suitable for testing a large number of c o m p o u n d s ; 3. A sufficient difference should appear between numerical data of various compounds; 4. Literature data should be widely available; 5. Simple apparatus and rapid procedures should be required. 65

All these requirements are met by the determinations of melting and boiling points for solids and liquids, respectively. Literature data are available for almost all c o m p o u n d s known, but it must be pointed out that several old and unreliable data can be found in some books. This can be attributed partly to the insufficient purity of the samples and partly the fact that often these d a t a can be given as ranges only.

1. Determination of melting point In practice, the melting point of solid organic c o m p o u n d s is the temperature at which the crystals of the solid turn into a liquid (melt). In theory, the melting point is identical with the freezing point, but the latter is often affected by the p h e n o m e n o n of supercooling. Thus, the freezing point may be several degrees lower than the melting point, and further, it cannot be observed and measured as the characteristic physical constant. According to a more exact definition, the melting point is the temperature at which the solid and liquid phases are in equilibrium with each other (Fig. 7). As the equilibrium temperature can be measured only for large samples and by a complicated procedure, in practice, methods that measure temperatures near to the equilibrium state are applied (usually not deviating from it by more than 0.5°C). The accuracy of melting point measurements is affected by the rate of heating. An exact method would be to insert a thermometer (mercury thermometer, thermocouple or thermistor) in the sample, in close contact

Fig. 7. Solid-liquid phase equilibrium / — C r y s t a l ; 2—melt

66

with it. However, this would require large samples (several grams) and heating should be effected very slowly, but rapidly enough to transfer the heat of melting instantaneously to the matter. These two requirements, that is, slow heating and transfer of large a m o u n t s of heat in unit time, can hardly be satisfied. Further, in practice several grams of sample are rarely available for the determination of the melting point. Moreover, under the above conditions for the determination of the solid-liquid equilibrium temperature, the heatinsulating action of air entrapped between the crystals would slow the process of heat transfer when the vessel is heated on its outside. Melting proceeds from the walls towards the bulk of matter, and melting takes place over a period of time. The situation is the same, in the opposite sense, when heating is applied in the sample. The significant influence of the heat of melting on the determination of melting points has been eliminated by applying small sample sizes (about 1 mg), whose heat of melting is negligible in comparison with the heat capacity of the whole system. The thermometer is placed not in but near the substance, using a medium with appropriate heat-conducting ability in between. This can be a liquid bath or a metal block with adjacent bores. Earlier, the liquid used in the bath was concentrated sulphuric acid, but now paraffin oil or silicone oil are favoured. An important point is to have a sufficiently large volume of the bath (200 c m ) which can be heated to at least 200-300°C without boiling or extensive evaporation. The liquid should be colourless, transparent and not be discoloured on heating. The volume of the metal block should be at least 1 d m and made from a good heat-conducting metal (copper or aluminium). When a bath is used, the liquid is stirred while heating, either with a stirrer or by bubbling air through it (e.g., Thiele apparatus). The Maquenne block is designed for rapid but only approximate determinations of melting points. This is a flat metal rod with polished surface. The sample is placed on it in a thin strip, the rod is then heated at one end, so that its temperature decreases towards the other end. The strip of sample will melt with a sharp boundary, and the temperature of the rod at the boundary can be determined with a pyrometer with an accuracy of 2-3°C. This is identical with the melting point of the sample. When the apparatus is continuously heated, melting points can be determined very rapidly. 3

3

(A) D E T E R M I N A T I O N O F M E L T I N G P O I N T IN A CAPILLARY

For preparative organic chemical purposes, the accuracy of melting point determinations by the old "thermometer-capillary" method is usually satisfactory (Fig. 8). 7

67

h

(a)

1 1

(b)

1

I

(c)

V_y

(d)

Fig. 8. Apparatus for the determination of melting point ( a ) T h e r m o m e t e r a n d capillary t u b e in a Kjeldahl flask; / — t h e r m o m e t e r ; 2—glass capillary t u b e ; (b) t h e r m o m e t e r a n d capillaries in a s i d e - t u b e Kjeldahl flask; / — c a p i l l a r i e s ; 2 — t h e r m o m e t e r ; (c) T h i e l e m e l t i n g p o i n t a p p a r a t u s , using air s t i r r i n g ; 3—air;

(d)

metal block (Cu or AI) with t h e r m o m e t e r a n d capillaries

This is carried out as follows: 1-2 mg of the powdered sample is packed in a thin glass capillary of length 30-50 mm and i.d. 1-2 mm, to form a compact layer of thickness 2-3 m m at the bottom. A compact layer is obtained on dropping the capillary in a wider glass tube of length 0.5-1 m on to a wooden or hard rubber surface. The glass used for the capillary should be resistant to chemicals (e.g., Rasotherm or Pyrex), as common glass, containing large amounts of sodium, may release contaminants during heating and a depressed melting point will be obtained. A high purity of the inner walls of the capillary is also essential. In preliminary measurements, a thermometer with a wide measuring range (20-250°C) is used. In accurate measurements, with a knowledge of the approximate value, the use of a thermometer with a narrower measuring range is recommended, with divisions permitting readings to 0.5°C and estimations t o 0.1 °C. Calibration of the thermometer with four or five standard substances with known melting points may be advisable. Some standard substances are listed in Table 6. The packed capillary is fixed to the thermometer with a thin platinum wire, so that the sample is immediately adjacent to the bulb of the thermometer (Fig. 8a). 68

TABLE 6. Melting points of some pure substances Substance

Melting point (°C)

Substance

Melting point (°C)

1-Menthol Benzophenone 4-Nitrotoluene Naphthalene Vanillin Acetanilide Benzoic acid Urea Phenacetin

41.6 48.1 51.8 80.04^80.25 81.1-81.6 113.4-114.2 121.8-122.4 133.1-133.4 133.9-134.4

Salicylic acid Succinic acid Anthracene Phthalimide Dimethylglyoxime 4-Nitrobenzoic acid Phenolphthalein Anthraquinone N,N-Diacetylbenzidine

157-158 182.7 214.8-215.0 233.5 235.8 240-241 260.7-263 285-286 317

Both are immersed in the bath as deeply as possible, but keeping the open end of the capillary above the liquid level. A suitable vessel may be a Kjeldahl flask of capacity 250-500 c m , filled with liquid up to two-thirds of its volume. The bulb of the thermometer and the sample should be in the geometric centre of the flask. The thermometer is fixed in the mouth of the flask by means of a cork, with a second bore which serves for the stirrer and allows the release of air during heating. The flask is mounted and heated with a micro burner at a rate of 4°C/min. When the melting point of the sample is k n o w n approximately, the heating rate may, of course, be higher at the beginning, but at about 10°C below melting point it should be reduced to 4°C/min or, more preferably, to l°C/min. The liquid in the bath is stirred and the behaviour of the sample in the capillary is observed. The temperature is read when the crystals of the sample collapse and become transparent. This is usually 0 . 5 1.0°C higher than the melting point. However, if the thermometer is calibrated with standard substances under identical conditions, accurate results can be obtained. When the sample has solidified on cooling after the test it cannot be used in a repeated measurement, as a different (usually lower) value would be obtained. Different samples must be used for replicate measurements. 3

In the Thiele apparatus for melting point determinations, the liquid is stirred as a result of convection phenomena caused by heating or by passage of small air bubbles through it (Fig. 8c). The modified Thiele apparatus is heated electrically [ 2 ] , and its temperature can be maintained with an accuracy of ±0.1 °C. In practice, the flask shown in Fig. 8b is also used, where the capillaries containing the sample are placed in two side-tubes. As only the ends of the capillaries are in contact with the bulb of the thermometer, the bath should be stirred more vigorously to ensure a uniform temperature in the system. The advantage of this arrangement is that the observation of melting 7*

69

o-

6

o

Ni

u

° —

9 9

Fig. 9. Schematic diagram of the Haereus "FUS-O-MAT" apparatus 1—thermocouples;

2—electric o v e n ; 3 - s a m p l e in m i c r o t e s t - t u b e ; 4—regulator

for h e a t i n g ; 5—reference

thermocouple;

6—recorder

in two capillaries is possible. A similar situation is represented in Fig. 8d, where the two capillaries are in contact with the bulb of the thermometer and are observed through a hole bored in a metal block. Light is passed through the bore on to a detector, and the melting point is indicated by an increase in the intensity of the light due to the transparency of the melt. A c o m m o n source of error in the methods discussed above is that the capillary and the thermometer are observed separately. In recent apparatus, an optical arrangement is applied to project the images of the thermometer scale and the capillary beside each other. Thermocouples and thermistors can also be used instead of thermometers and the melting point is read on the calibrated scale of a millivolt meter. However, these are not better than conventional thermometers filled with mercury. Walish and Eberle [3] succeeded in measuring the temperature in the capillary proper and thus, applying the principles of differential thermal analysis, the melting and boiling points of organic compounds could be determined more correctly. The apparatus, the " F U S - O - M A T " is marketed by the Haereus Co. [4, 5]. In this apparatus, there is a thermocouple made from nickel and chromium -nickel wires (0.1 mm thick) placed in a quartz or platinum protective tube of i.d. 0.8 mm. This is immersed in a glass capillary (i.d. 1.2 mm, o.d. 1.5-1.8 m m ) containing the sample as a layer of thickness 10-15 mm. This requires 10-30 mg of sample. The measuring cell is placed in a well regulated thermostated electric oven containing another similar thermocouple in the walls. The two thermocouples are connected in a compensating mode. The third, kept at 0°C or 40°C, is the reference. A schematic diagram of the apparatus is shown in Fig. 9. 70

If the temperature of the oven is increased at an even rate (e.g., 1040°C/min), the recorder coupled to the thermocouples will draw a straight line as long as the temperatures of thermocouples are identical and compensating each other. When the temperature changes in the vicinity of thermocouple 1 owing to processes involving changes in the internal heat content of the sample (e.g., melting, which is an endothermic process, or recrystallization, which may be an exothermic process) the balance of the thermocouples in the differential circuit alters and current flows through the meter, and also the recorder, and a step or wave (similar to that in polarograms) is obtained. When the process is finished, the temperature equilibrium is re-established and a straight line is recorded again. The first break in the wave indicates the start of the process and the second represents the end of it. Steep steps indicate a "sharp" melting point, whereas a flat step is obtained when melting takes place over a temperature range. If several processes involving changes in internal energy contents take place before complete melting, several steps will appear in the curve. The melting curves of 2,4,6-tribromophenol and trichloroethylene are shown in Figs 10a and 10b, respectively. The literature melting point for the first compound is 94-96°C, and the new value is 93.2-93.3°C. According to the melting point curve, the old literature data value is wrong, and the recent value indicates the final part (93.1 °C) of the second of two slightly different melting point values. Melting starts at 90.0°C, but visually, or by means of the change in the intensity of a light beam, only continuous melting between 90.0°C and 93.3°C can be observed in the capillary. The investigations of Walish and Eberle [3] indicate that there are very few organic compounds that have a single, sharp melting point that could be given with an accuracy of 0.1 °C. F o r most organic compounds the melting point can be determined only to within + 1°C, partly because several slightly different melting phenomena occur subsequently and partly because they overlap completely. This situation cannot be improved by changing the experimental conditions, as it is due to the crystal structure properties of the individual organic compounds. Even in this situation, the method of Walish and Eberle is advantageous in comparison with the conventional capillary technique. There is a drawback, however, that the accuracy of measurement is limited because, in spite of the fact that the thermoelectric power of the N i — N i C r thermocouple is high, it does not vary perfectly linearly with temperature, and this causes errors in different temperature ranges. It is possible to construct a microvolt temperature scale corrected for this deviation, but it is more advisable to calibrate it with standard substances, especially as the heat capacity of the 71

^ ^ 9 1 . 9 °C

(a) 93.1 °C 90.9 °cjfgi 92.4 °C 90.9 °C

Fig. 10. Melting point, freezing point and boiling point curves recorded by the "FUS-O-MAT" apparatus (a) T r i b r o m p h e n o l ; h e a t i n g r a t e : 2 0 ° C / m i n ; (b) t r i c h l o r o e t h y l e n e ; heating r a t e : 2 0 ° C / m i n ; (c) m a r g a r i n e ; h e a t i n g r a t e : 2 0 C / m i n ; (d) light k e r o s e n e ; h e a t i n g r a t e : 2 0 C / m i n ; c h a r t speed 30 m m / m i n c

72

system consisting of the capillary, the thermocouple and the sample is higher than that in the capillary method. A great advantage of the technique is the short heating period, which allows rapid measurement, and here the heating rate is 10-20°C/min instead of the 4°C/min commonly used in the capillary technique. A similar apparatus was designed by Aleksandrov et al. [6] for purity control. The equilibrium temperature is measured with a platinum resistance thermometer at a heating rate of 0.02°C/min, with an error of 0.0001 °C. Contaminants present in amounts higher than 0.001% could be detected in this way. The apparatus was applied to the examination of very pure benzoic acid. Such an apparatus can also be utilized in recording the cooling (freezing) curve, (Fig. 10c) in the same test. (B) D E T E R M I N A T I O N O F M E L T I N G P O I N T O N THE H O T - S T A G E M I C R O S C O P E

A common deficiency of the above methods is the relatively large sample size which may have a non-negligible heat of melting. Moreover, the phenomenon of melting is observed through a liquid in the bath in a capillary, in a mass of crystals, and cannot always be seen clearly. Observation of the melting of separate crystals thus seemed desirable. This provides the possibility of observing the solid-liquid phase equilibrium and also changes in crystal shape, which are often very characteristic during heating to the melting point. The crystal shape can easily be observed under a microscope with 50-100-fold magnification, using a hot stage that can be heated slowly to the desired temperature. As difficulties were encountered in fitting the thermometer in the hot stage so as to indicate the temperature of the crystals accurately, the problem was not solved immediately. The above requirements were satisfied by the hot-stage microscope constructed by Kofler et al. [7], using a metal block with continuous electric heating through a resistance wire. The sample is placed in a glass chamber with good heat-insulating characteristics on the hot stage (a metal block) of the microscope, which carries a bore through which light is reflected from a mirror. The thermometer is inserted in a bore on the other side of the metal block so as to reach the point in the metal block where the temperature is the same as that of the crystals on the microscope slide. The apparatus is widely applied and well known, so a detailed description is not given here [ 7 ] . Today there is an advanced apparatus available in which a picture of the mercury thread is projected, by means of an optical system, next to a picture of crystals, and the temperature and melting phenomena can be observed simultaneously. 73

The apparatus can be used in the temperature range 50-260 C. Some apparatus provide an increased accuracy in the 80-180 C range. Others can be used at temperatures higher than 260 C, or can be cooled with liquid carbon dioxide in order to measure melting points of substances down to about — 55 C. In this test, only a few crystals (1-2 jig) of sample are required, preferably including both small and large ones. The crystals are observed under 50-80fold magnification, the temperature can be raised very slowly by means of a resistance wire and can be set to 1-2 C around a constant value, so that heating and heat loss of the apparatus are balanced. The usual heating rate near the melting point is 4 C/min. The melting points of several organic substances are indicated by melting or sublimation of crystals invisible to the naked eye a few degrees before the actual melting point. The field of the microscope becomes dim owing to the appearance of hardly visible spots. Subsequently, small crystals of the sample are slightly displaced, usually about 1 - 2 C below the melting point, then the actual phenomenon of melting is observed. The melting point is the temperature at which the small crystals melt entirely into drops and the edges and corners of medium-sized crystals start to melt while the large crystals are still unmelted. In this way, the melting point of p-aminoethylbenzoic acid ( 9 0 . 5 C ) can be determined within 0.5 C (Fig. 11). At a temperature 0 . 5 C higher than the melting point, all of the crystals melt into drops. The equilibrium state between the crystals and the melt is 90.5 C, but when the temperature is raised further very slowly, solid residues can still be observed in large melt drops (see Fig. 7). As in the capillary method, the melting point cannot be measured repeatedly on the same sample. The melting point of volatile or sublimating substances must be measured in closed systems. If the melting point is not too high ( > 100°C), the edges of the microscope slide are sealed. Preferably 0.5- 1.0 mg of sample is placed in a flat capillary (Fig. 8a and b) which is sealed by soldering and placed in the bore at the side of the hot-stage microscope. Even the hot-stage microscope is not suitable for the determination of "sharp" (that is, accurate to within ± 1 C ) melting points of all organic compounds, but it is superior to the capillary technique. With substances that show a wide melting range the subsequent melting phenomena can sometimes be observed at a slow heating rate near the melting point on the hot-stage microscope. Thus, not only can the melting point be determined accurately, but the behaviour of the crystals of the organic substance can be monitored during heating, and in the melt also. Further, changes in crystals shape (polymorphism) with increasing or C

C

74

8 9 °C

9 0 °C

90.5 °C

91 °C

Fig. 11. Melting phases of anaesthesin (ethyl ester of p-aminobenzoic acid) on the hot stage

decreasing temperature can also be noted, as well as sublimation phenomena before melting, etc. F o r example, the plate-rhombic crystals of diethylbarbituric acid start to be converted into long needles at about 175°C. This process continues u p to the melting point (184°C) and is so characteristic of the substance that it provides a reliable means for identification in a single crystal (Fig. 1). Other substances, e.g., crystal hydrates, have a double melting point. First the crystals melt or, more accurately, are dissolved in the water of crystallization, then the water of crystallization evaporates at elevated temperatures, the crystal d r o p solidifies, undergoes recrystallization, and finally the melting point of the anhydrous substance is reached. There are substances that solidify after reaching a first melting point and, as the crystal structure is altered or a structural rearrangement takes place, the melting point is also altered. Crystals that contain water or alcohol of crystallization can be recognized from the fact that the crystals grow dim already before reaching the first melting point owing to the fact that the equilibrium with the 75

water or alcohol of crystallization is destroyed under the influence of heat. The hot-stage microscope technique, like the capillary method, is also suitable for purity control of organic compounds. The melting point of contaminated substances is lower than that of the pure compound and it occurs in a definite temperature range. The melting processes can also be photographed through the microscope. When unknown substances are examined, first a preliminary test is carried out using a high heating rate and a thermometer with a broad measuring range, then the actual determination is effected more accurately, on a separate sample. In a modified version of the apparatus, the change in the intensity of a light beam passing through the sample is measured by means of a photocell, and the melting point is indicated by a sharp break on the intensity versus temperature curve. Visual observation is also possible with such apparatus [8, 9 ] . However, sharp changes are obtained only when relatively large amounts of crystals are present in the light path, and this may affect the sharpness of the melting point. The fact that the melting point of a mixture of organic compounds is lower than those of the components can also be utilized in identification work, by determining the so-called eutectic melting point. The sample and the pure substance suspected to be identical with it are mixed in a ratio of about 1:1, and n o melting point depression should appear when the two substances are identical. This is particularly useful when compounds with very similar melting points are to be distinguished. The test can also be effected by the capillary technique. However, the melting point of a mixture of organic compounds depends not only on the melting points of the components, but also on their proportions in the mixture. Such a melting point (or solubility) diagram is shown in Fig. 12. The simple diagram shown holds for the case when melts of c o m p o u n d s A and B are completely miscible and do not react with each other, no mixed crystals are formed and the molecular compound corresponding to the first eutectic does not form other molecular compounds.The above-described phenomenon is due to the appearance of molecular compounds that have melting points lower than those of the original compounds. When several kinds of molecular compounds can be formed with increasing temperature, several breaks will appear in the diagram ( £ £ ) , as shown in Fig. 13. Here, E is the eutectic temperature of A + B and £ relates to the molecular c o m p o u n d formed from the eutectic mixture and substance B, while M represents the highest temperature of the melt. When two (or three or more) l 5

x

76

2

2

1

^ O -.

Temperature

B + eutectic

50 % A + C

100 % B

50 % B

Composition

Fig. 12. Diagram of mixed melting for a binary mixture A— melting point of c o m p o u n d A; B—melting

p o i n t of c o m p o u n d B ; E— melting p o i n t of the eutectic m i x t u r e ; C - per cent

1 !

Temperature -

c o m p o s i t i o n of t h e eutectic m i x t u r e

Composition Fig. 13. Diagram of mixed melting with two intermediate eutectic mixtures

components form a molecule compound of definite composition, this formulation will have the lowest melting point and any other will melt at a higher temperature. The composition with the lowest melting point is called the eutectic mixture. The eutectic melting point is characteristic of the components and can be utilized in identification work (Table 7). However, the 77

TABLE 7. Melting points of some standard substances for the Kofler mixed melting point determination Substance

Melting point ( C)

Benzene 0-Naphthol ethyl ether Azobenzene Benzil Acetanilide Phenacetin Benzanilide Phenyl-p-acetylaminosalicylate Dicyanodiamide Saccharine Phenolphthalein

5.49 37 68 95 115 135 163 190 209 228 262

eutectic melting point of most organic compounds is not known. Therefore when applying the capillary technique, mixtures with various compositions (1:1,1:2,1:4) should be prepared and the lowest melting point is taken as the eutectic melting point. Melting should take place rapidly in this mixture. In other instances, only partial melting occurs and this is hardly observable in the capillary; the procedure is then troublesome and time consuming. When the hot-stage microscope is used, one mixture (e.g., 1:1) is sufficient. This is spread on the microscope slide in a thin layer (about 0.1 mm thick) (Fig. 14). When the two substances are finely powdered but not perfectly mixed, there are points in the layer where their proportions differ from 1:1, and the mixture is nearly or exactly identical with the eutectic composition. This layer will be seen to be semi-transparent under the microscope with about 50-fold

Fig. 14. Phenomenon of mixed melting on the hot stage

78

magnification. At the temperature corresponding to the eutectic melting point, the layer will melt at some points, where the composition is identical with that of the eutectic mixture. A further increase in temperature results in melting of larger sections and finally of the whole mixture. A slow occurrence of the first molten sections indicates that the eutectic composition is far from the 1:1 ratio, and the test should be repeated with another mixture (e.g., 1:2 or 1:4). The eutectic melting points of about 1200 organic compounds were determined by Kofler et al. in mixtures with some suitable standard substances; these data are given in the Appendix of Ref. [7]. Since then several similar sets of data have been published [10-14]. Such data are very useful when compounds with similar melting points are to be distinguished. In Table 8, eutectic melting point data for some substances with similar melting points are given for mixtures with two test substances (Table 8). TABLE 8. Eutectic melting point data of some organic compounds Compound

Melting point ( C)

Eutectic melting point with phenacetin with acetanilide ( C) (°C)

Urea Cinnamic acid Phenacetin Cinnamic anhydride Malonic acid Phenylpropionic acid Resorcyl aldehyde

135 135 135 135.5 136 136 136

102 84 90 79 62 53 70

122 98 135 96 84 71 84

TABLE 9. Chemicals used for mixed melting point determination Chemicals

Melting point ( C)

Temperature range of use (°C)

Azobenzene Benzil(dibenzoyl) Benzanilide (N-benzoylaniline) Salophen (p-aminophenylacetyl salicylate) Dicyandiamide (1-cyanoguanidine) Phenolphthalein

68 95 163

2 0 100 20 120 14(M70

190

17a 190

21(^212 263

19CK340 20a250

79

It can be seen from Table 8 that the two test substances allow a distinction to be made between substances that could not be achieved on the basis of melting point data alone. Here, acetanilide and phenacetin were used, and some other test substances suggested for use in other melting point ranges are shown in Table 9. In general, substances that have melting points near that of the sample are used, and other limitations in the measurement of mixed melting points should also be borne in mind (see p. 77).

2. Determination of freezing point In theory, the freezing and melting points of a substance are identical but not in practice. Theoretically, the freezing point is the temperature at which the liquid substance is transformed into the solid (crystalline) state on cooling. With some substances, identical or only slightly different melting and freezing points can be measured, but most organic compounds exhibit a freezing point that is lower than the melting point, sometimes by as much as 5 -8°C. This is caused by the phenomenon of undercooling. The undercooled liquid (melt) will solidify very rapidly, which makes possible a clearer determination of the freezing point than of the melting point. This does not mean that the freezing point of the undercooled melt represents a characteristic value, as the extent of undercooling is affected by the external conditions (particularly by the rate of cooling, crystal shape, etc.). The freezing point is determined in a simple apparatus shown in Fig. 15, provided that a sufficient amount of sample (4-5 g) is available. Water, icewater or, below 0°C, salted ice or solid carbon dioxide can be used for cooling. Vigorous stirring during cooling is necessary. The temperature of the freezing point is read when the liquid, which initially was clear and transparent, becomes opaque owing to the separation of small crystals. The capillary technique can also be applied to the determination of the freezing point, but as stirring of the sample cannot be effected in this instance, only the temperature at which the undercooled melt solidifies can be established. When a thermocouple is immersed in the capillary [3, 4 ] as described in connection with the determination of the melting point, the cooling curve of the sample can be recorded (Fig. 10c). The heat of crystallization will produce a break in the curve. The freezing point can also be determined with a hot-stage microscope, the liquid being heated to a few degrees above the melting point, then allowed to cool slowly. The freezing point is indicated by the appearance of the first 80

Fig. 15. Apparatus for the determination of freezing point

crystal in the liquid. It often happens that, rather than the original crystal shape, a crystal aggregate (sphaerulite) appears, that is, the needles seem to grow outwards from a central point. A dendrite structure is also frequently encountered. Several c o m p o u n d s do not yield crystals on solidification, particularly when the liquid is cooled rapidly, because internal friction increases very rapidly under such conditions and prevents crystallization, so that the liquid solidifies as a "glass". Undercooling frequently occurs with substances that have needle-shaped crystals, and is relatively rare with those which form plates. Owing to the phenomenon of supercooling, the determination of freezing point is applied mainly to fats, oils, waxes, etc., the melting ranges of which are very wide and often impossible to determine accurately. Their freezing points, however, can clearly be observed as the liquid becomes opaque. The accuracy of this measurement is sufficient for technical characterization. The freezing point of this type of substance is measured either in the apparatus shown in Fig. 15, or by recording the cooling curve by means of a thermocouple immersed in the capillary. Figure 10c shows, for example, the cooling curve of margarine recorded with the Haereus F U S - O - M A T apparatus. Kroeger et al. constructed a sensitive apparatus for recording the freezing point curve [15]. This is suitable, for example, for the determination of 0.001 mole % of contaminants in 4-aminopyridine. It is essential to maintain a temperature difference between the container and the walls of the oven of less than 0.005°C. 81

3. Determination of boiling point Boiling point is an important and characteristic physical constant, primarily for organic substances that are liquid at room temperature, but can be of importance for solids too. Substances that undergo sublimation are characterized by the temperature of sublimation at atmospheric pressure. Liquids evaporate at the surface at all temperatures. The vapour formed exerts a pressure in a closed space. This is called vapour pressure and can be very different for different liquids at the same temperature (2322.75, 9997.5 and 39 723.4 Pa for water, benzene and carbon tetrachloride, respectively, at 20°C). When the temperature is increased, the vapour pressure rises exponentially. The logarithm of the pressure of the saturated vapour (p) varies linearly with the reciprocal of absolute temperature (T): A logp=-- +£ where A and B are specific for a given substance and are independent of temperature in a certain range. O n heating, the vapour pressure increases until the vapour pressure in the bulk of the liquid is equal to the external pressure and evaporation starts in the bulk of the liquid, with the formation of vapour bubbles. At this point, the liquid starts to boil. Thus, the boiling point is the temperature at which the vapour pressure of the liquid is equal to the external pressure. As the vapour pressure depends on temperature, the boiling point also depends on the external pressure. The higher the external pressure, the higher the boiling point, and vice versa. The boiling point at 1 0 P a is called the "normal" boiling point and data relate to this pressure when not specified otherwise. The boiling point can be determined simply and rapidly for organic liquids and can be characteristic of molten solids also. When the boiling point is determined at a pressure other than atmospheric, this is indicated in parentheses after the boiling point value. Boiling points are measured at reduced pressures mainly for substances that decompose on heating before the boiling point is reached. In addition to pressure, boiling points may be affected by overheating. Overheating or retarded boiling can be explained as follows. The bubbles that form first in the bulk of the liquid when the temperature of boiling is reached are extremely small, and their surface are therefore very concave. At concave surfaces the vapour pressure is lower than at planar or less concave surfaces if 5

82

the liquid wets the walls of the vessel. As the vapour pressure at the surface of the bubbles is significantly lower than the external pressure, the bubbles are unable to grow in size and thus boiling is retarded, even if the boiling point has been reached. With a further increase in temperature, the vapour pressure at the surface of the bubbles becomes equal to the external pressure and boiling suddenly starts. At the same time, the temperature of the liquid falls to the normal boiling point or, more accurately, to a value lower that the boiling point, owing to the consumption of heat by the rapid evaporation. The process is repeated again and again and "bumping" boiling takes place. This phenomenon will not only falsify the boiling point results, but there is also the possibility that the liquid could run out of the vessel or into the condenser during distillation. Overheating is prevented by the presence of gases dissolved in the liquid or of small solid particles. The solubility of gases decreases with increasing temperature, thus facilitating the formation of larger bubbles. Solids with large amounts of gases adsorbed on their surface act similarly. Examples of such solids are pumice and porcelain pieces. Earlier, small tetrahedrons formed from platinum sheets were used to facilitate the formation of bubbles at the edges and apices. In order to prevent overheating during distillation, particularly when working at reduced pressures, air or an inert gas can be led into the liquid through a capillary. (A) D E T E R M I N A T I O N O F B O I L I N G P O I N T O N M A C R O - S C A L E WITH D I S T I L L A T I O N

In the determination of boiling points, temperature is not measured in the liquid proper, because of the adverse effects of overheating discussed above, which result in a varying temperature of the liquid. O h the other hand, the temperature in the vapour space also is not always equal to the boiling point, as the space high above the liquid level may be overheated by the heat source and additionally the temperature can be much higher at the walls of the vessel. The boiling point of a uniformly boiling liquid can be measured accurately by placing the bulb of the thermometer 1-2 cm above the liquid level. N o drops should reach the bulb directly from the liquid, and quiet boiling is therefore essential (see Fig. 16a). The K a h l b a u m flask (used in the determination of molecular wight, see p. 84) prevents the release of drops (Fig. 16b), but the upper device must be protected from overheating. The use of an electric heater with a large surface area and a relatively low temperature is recommended. The thermometer, when placed properly, meets the so-called wet vapour that has a temperature identical with the normal boiling point. 8

83

(a)

(b)

Fig. 16. Apparatus for the determination of boiling point suggested by Kahlbaum (a) Simple a p p a r a t u s ; (b) differential a p p a r a t u s ; 7 — c o n d e n s e r ; 2—thermometer;

3 — v a p o u r lift;

4—heater

In the fractional distillation procedure, the thermometer is installed in the outlet tube for vapours (e.g., in the side-tube of a Claisen flask), or in the upper part of the condenser attached to the vapour outlet tube of the fractionating flask (outside the cooled space, of course). The temperature measured there is not necessarily the real boiling point, but changes in temperature on the appearance of fractions with different boiling points can be more clearly observed. In the determination of the boiling point of a pure liquid, about 50 c m of the sample are distilled in a long-necked flask of capacity 200-250 c m . Both the heat source and the flask should be protected from air draughts. One d r o p should leave the condenser each minute. First, 3-4 c m of liquid are distilled off, and the thermometer is read at the beginning and at the end of the condensation of the next 5-40 c m fraction. If the difference between the two values is less than 1-2°C, the mean value is taken for the boiling point. With greater differences, the boiling point is taken as a range with the two limiting 3

3

3

3

84

temperatures (e.g., 8 6 - 9 0 C ) . D a t a with an accuracy of 0.1 °C are obtained only with very careful determinations. The above statements hold, of course, only for homogeneous and pure liquids. With liquid mixtures or contaminated homogeneous liquids the temperatures of the "first and last d r o p " can be of importance. These indicate the boiling points of the components with the highest and lowest values, and thus the presence of small amounts of contaminants with extreme boiling points can be recognized. The boiling point of an azeotrope is also a characteristic value. As has been mentioned above, atmospheric pressure also affects the temperature of the boiling point. In measurements for informatory purposes, the variation in atmospheric pressure can usually be neglected, but in accurate measurements corrections must be applied or the measured boiling points should be given together with the actual pressure during the measurement. The correct arrangement of the thermometer is also of importance and, if necessary, a second thermometer should be used to permit a correction to be made for the temperature difference between the vapour space and the external environment. Calibration with standard substances under identical c

TABLE 10. Substances suitable for calibration of thermometers for boiling point determination Substance

Boiling point (°C)

Substance

Boiling point (°C)

Bromoethane Acetone Chloroform Carbon tetrachloride Benzene Toluene Chlorobenzene Bromobenzene Cyclohexanol

38.4 56.11 61.27 76.75 80.10 110.62 131.84 156.15 161.10

Aniline Methyl benzoate Nitrobenzene Methyl salicylate p-Nitrotoluene Diphenylmethane a-Bromonaphthalene Benzophenone

184.40 199.5 210.85 222.95 238.3 264.4 281.2 306.1

conditions is advisable. Some standard substances suitable for this purpose are listed in Table 10. Thermocouples or thermistors can also be used instead of thermometers; these always require calibration, and their accuracy does not exceed that of thermometers in macro or semimicro scale determinations.

8*

85

(B) D E T E R M I N A T I O N O F B O I L I N G P O I N T O N THE SEMIMICRO-SCALE

When the volume of the sample available is only 2-3 c m , the boiling point is determined in the apparatus shown in Fig. 17. The test is carried out in a test-tube (about 15 mm i.d. and 150 mm long) (l \ and a small vessel with a double cup shape (2) is placed in it (the larger diameter of the cups is about 8 mm, the smaller diameter is 2 mm and the height of the double cup is about 15 mm). The test-tube is mounted in a metal holder about 20 mm high lined with asbestos, and the holder is fixed on an asbestos plate with a bore about 5 mm in diameter in the middle. A microburner is placed at this point. Thermometer 7J is mounted in the bore of a cork closing the end of the test-tube so as to reach the upper cup without touching the walls or the bottom. Thermometer T measures the ambient temperature to allow correct calculation. Thermometer T is used mainly in highly accurate measurements and can usually be omitted in practice. The test-tube is filled with the liquid up to the middle of the double cup (this requires about 2 c m of sample). The bottom of the test-tube is heated with a micro burner to quiet boiling, so that the thermometer senses the temperature 3

2

2

3

Fig. 17. Semimicro apparatus for the determination of boiling point l—Test-tube;

86

2—vessel

Fig. 18. The Smith Menzies apparatus for the determination of boiling point

of "wet vapour". If the boiling point of the sample is low (less than about 100°C), the walls of the test-tube protruding from the holder are covered with wet filter-paper for cooling. The thermometer is read 1-2 min after the start of boiling, when the condensed matter flows back on the walls of the test-tube and mixes with the boiling sample. A few grains of pumice are placed in the bottom of the test-tube to ensure quiet boiling. The Smith and Menzies bulb method is suitable for determination of the boiling points of smaller samples (0.1-0.2 c m ) . A bulb (about 10 mm in diameter) is blown to the end of a capillary of length 30-40 mm, and the capillary tube is bent as shown in Fig. 18. About 0.1-0.5 c m of sample is drawn into the bulb. The small vessel is fixed to the thermometer stem and immersed into a suitable bath. The height h between the open end of the capillary and the liquid level in the bath is measured. Heating is started slowly while stirring the liquid in the bath. Air will leave the bulb in the form of large bubbles. At the boiling point, small bubbles of the vapour of the sample appear instead of the large air bubbles, rising upwards in a line in the liquid, provided that the sample is insoluble in it. Heating is then stopped or reduced, and the temperature at which the evolution of bubbles ceases is read. This gives the boiling point at pressure p in the bulb, which is higher than the normal pressure, as pressure p differs from the pressure reduced to b by the pressure produced by the bath liquid layer of height h (mm) in the bath. Pressure p is calculated from the equation 3

3

0

where s is the density of the liquid in the bath. (C) D E T E R M I N A T I O N O F BOILING P O I N T O N THE MICRO-SCALE

When the amount of sample available is less than 0.1 c m , the determination must be carried out on the micro-scale, and several different methods based on various principles have been developed for this purpose. The accuracy of these measurements is not lower than that of those carried out on the macro- or semimicro-scale, but the boiling point is established by means of phenomena brought about by boiling of the sample. This requires skill of the operator. Instrumental methods that indicate changes in internal heat contents at boiling are very accurate. Of the simple techniques, the Emich and Siwoloboff methods have been widely used. The Emich method utilizes the fact that the vapour pressure of the liquid rises suddenly at the boiling point, and will displace the d r o p of sample still in 3

87

(a)

(b)

(c)

(d)

Fig. 19. Emich capillaries for the determination of boiling point on the microscale

Fig. 20. Siwoloboff apparatus for the determination of boiling point on the microscale

(a) E m p t y capillary; (b) filled a n d sealed capillary; (c) capillary with t h e air b u b b l e ; (d) t h e p o s i t i o n of t h e d r o p at the boiling point

liquid state. A capillary of length 70-80 mm and i.d. 0.5-1 mm is drawn to a point at one end (Fig. 19a). Some liquid (10-20 \x\) is drawn into the tube to a height of 5-6 mm (Fig. 19b), then the capillary is kept in horizontal position and the pointed end is soldered rapidly. In this way, air is trapped between the drop and the sealed end (Fig. 19c). The capillary is fixed to a thermometer and immersed in a bath so as to leave about 2 cm of the tube above the liquid level. Slow heating is started and the boiling point of the sample is reached when the liquid d r o p suddenly rises (Fig. 19d) and floats at the level of the liquid in the bath. The temperature is read at this moment. When the bath is cooled, the drop falls, and the test can be repeated. In the Siwoloboff method, a few microlitres of sample are inserted in a tube of length 70-80 mm and i.d. 2-3 mm in a layer about 10 mm thick. Another capillary (length 100 mm, o.d. 1 mm, i.d. 0.5 mm) with a restricted section at a distance 5-6 mm from the end (Fig. 20) is placed in the first tube. These tubes are fixed to a thermometer, immersed in a bath so as to leave the open end of the capillary above the liquid level, and heating is started. First air bubbles leave the inner capillary then, when the boiling point is reached, a series of vapour bubbles start to rise from the end of the inner capillary. Now the temperature is lowered, the end of the capillary and the thermometer are 88

observed, and the temperature is read at the moment when evolution of bubbles ceases and the liquid sample starts to enter the inner capillary where the condensation of vapour produced a reduced pressure. Here essentially the condensation temperature of the vapours is measured, but this coincides with the boiling point. According to experience, the cessation of evolution of bubbles and the penetration of liquid into the capillary takes place within a 1°C change of temperature. Slow heating at the boiling point (3-4°C/min) is necessary in order to prevent rapid boiling. The Siwoloboff method is also suitable for the determination of boiling points at pressures lower than atmospheric. In this instance, a ground-glass joint is applied at the open end connecting the tube to a manometer and a pump. Usually, pressures of not less than 1999.5 Pa are applied, as at lower pressures the correlation between boiling point and pressure varies widely and is uncertain. The Siwoloboff method was modified by Karr and Childers [16] by applying electric heating and measuring the temperature in the tube with an iron-constantan thermocouple. This has been used mainly in the determination of the boiling points of aromatic hydrocarbons. Cervenansky [17] applied the Siwoloboff method to the measurement of the boiling points of impure liquids and to the determination of the characteristic boiling points and boiling ranges of certain liquid mixtures. In the examination of liquid mixtures, however, the Emich method proved to be more advantageous. As the use of thermometers always involves subjective errors and boiling points cannot be read to within 0.1 °C in most instances, for a long time workers have attempted to determine boiling points on the basis of the relatively large change in internal energy content. Differential thermal analysis (DTA) seemed to be very suitable for this purpose; temperature is measured with a thermocouple or thermistor in an objective manner very precisely and reliably. Vazallo and Harden [18] first measured the boiling point at atmospheric pressure with a thermocouple, then Barrall et al. [19] carried out boiling point determinations in the range 3.999 x 10 —101.3 x 10 Pa. The sample (0.02 c m ) was absorbed in powdered carborundum (0.15 g) and examined in a D T A apparatus using powdered c a r b o r u n d u m as a reference. The temperature was increased at a rate of 8°C/min in nitrogen atmosphere. The boiling point curves of decane and some derivatives were recorded with an accuracy of a few hundredths of a degree. Similar measurements were reported by Kerr and Landis [20] using a D u P o n t Model 900 D T A apparatus. One determination required 10-20 min time. G a r n and Anthony [21] examined 20-100 jil samples in detailed studies on phase conversions. 3

3

3

89

The Haereus F U S - O - M A T apparatus mentioned in connection with melting point determinations is also suitable for the determination of boiling points, and the heating rate (20°C/min) applicable here is higher than in other apparatuses. The sample is placed in a test-tube of length 40 mm and diameter 5 mm in a layer about 5 m m thick. The NiCr—Ni thermocouple in a platinum envelope and a capillary (50 mm long, 1 mm diameter, sealed at one end) are immersed in the sample (the air leaving the capillary during heating will ensure quiet boiling). The boiling point curve of trichloroethylene recorded with this apparatus is shown in Fig. 10c. There is a rapid increase on the slowly rising curve at the boiling point (84.5°C) continuing until complete evaporation of the sample, (literature d a t a : 86.9, 87-87.2°C). The instrument can also be used in the analysis of liquids with a wide boiling range. Figure lOd shows the boiling point curve of light gasoline (taken from the manual describing the apparatus). In the instrument, the millivolt scale is calibrated to the actual thermocouple and indicates the temperature directly. In this way, the problems caused by non-linear changes in the thermocurrent with temperature are eliminated. Checking of the instrument with some standard samples is recommended. The boiling point data are important in identification work, and these values are given in handbooks for most organic compounds even for those which are solid at room temperature. It should be emphasized, however, that a large number of old and inaccurate data should be replaced in the future by values measured on chromatographically pure substances.

4. Determination of density The density of liquids can be determined easily, simply and rapidly, and these data are given in h a n d b o o k s for most liquids and some solids. As density measurements could be carried out relatively accurately in the last century, and the values are hardly influenced by the presence of contaminants, the old data are reliable and only the reference temperatures may differ from recently published values. Absolute density is the mass of substance per unit volume, while specific weight is the weight of matter per unit volume, where the gravitational acceleration constant, g, is also involved: m

sp.w.= 90

°# v

I0 kgm

1 A 3 l3

-

3

Relative density is the ratio of the absolute densities of two bodies:

and is a dimensionless number. As the masses of two bodies of equal volume (m , m ) give the same ratio as their absolute densities, the relative density can be expressed by means of masses of equal volume: 0

c

_

m

vt

°

The correlation between absolute and relative densities is

where S is the absolute density of the reference substance at t C. In organic chemistry, the reference substance is almost exclusively water at 4°C. Relative density at 4°C is nearly equal to absolute density. In handbooks, the reference temperature is usually stated, or else the data are mainly referred to 4°C. It may be given in the form of two values separated by an oblique line (e.g., 20/4, 20/20, 4/4 or 36/0), with the measurement temperature first and the reference temperature second. When the substance is solid at room temperature and can be melted without decomposition, the measuring temperature may be 60-90°C or, with gases, a negative temperature may be indicated (e.g., the density of ethane is 0.561 k g / m at - 1 0 0 ° C ) . Gases can also be characterized by the weight per m (e.g., ethane, 1.357 k g / m at 0°C). Older data without an indication of the measuring temperature are unreliable. c

vt

3

3

3

(A) D E T E R M I N A T I O N O F DENSITIES O F L I Q U I D S

The methods used are based either on the hydrostatic principle or the pycnometer technique. When a sufficient amount (0.2-0.5 d m ) of liquid is available, areometers provide a rapid means of determination, first using an areometer with a wide measuring range then another with a 0.1-0.2 density value range which allows the determination of density to within 0.001. A thermometer is combined with 3

91

areometers, and the temperature of the sample must be adjusted to the calibration temperature of the areometer. The density of organic liquids does not change uniformly with temperature, and therefore conversion of the values measured to another temperature requires the use of tables constructed for the actual liquid. The M o h r - W e s t p h a l balance method is also based on the hydrostatic principle. A 30-50 c m volume of sample is required and the density can be determined to within 0.001. The balance should preferably be calibrated against water or another liquid of known density prior to measurement, at the temperature to be used. The pycnometer technique makes possible a more accurate measurement of density for liquids and solids also. First the mass of the dry, empty pycnometer is measured, then it is filled with water up to the mark ( 2 0 C ) and weighed again. This gives the "water value" of the pycnometer. The pycnometer is emptied, dried and filled with sample at the same temperature as the water (drying of the pycnometer should be carried out without warming; preferably, it is rinsed with ethanol then with diethyl ether and the vapours are removed by aspiration). The pycnometer and the sample are weighed. The mass of the empty pycnometer is subtracted from this value and the result is divided by the water value, giving the relative density of the sample referred to water. In order to calculate the absolute density, the masses must be reduced to vacuum by applying the respective factors and their differences. In practice, this procedure can be omitted, as the maximum error involved is 0.11%, and density is calculated with the following equation: 3

where m and m are the weight of the sample and water, respectively, and S is the density of water at the actual temperature. These values are given in handbooks. Pycnometers with various shapes and designs are available. The simplest one is flask-shaped, with a capacity of 10-100 c m , and a capillary tube protrudes from the ground-glass stopper for adjusting the level of the liquid. A thermometer is also included in some pycnometers. The Ostwald-Spengler pycnometer is smaller and can be weighed on an analytical balance. For measurements on volatile substances, pycnometers which can be completely closed are used. The capillary pycnometer developed by Clemo and McQuillen [22] has a capacity of less than 1 c m (Fig. 21). Its length is a few centimeters, the diameter of the inner tube is 0.4 mm and the diameter of the v

vt

3

3

92

0.4 m m

Fig. 21. Micro pycnometer

capillary is only 4 jim at both ends; in such narrow capillaries there are no losses due to evaporation. The minimum amount measurable in the device is 2 m g , and densities can be measured to within 0.001. (B) D E T E R M I N A T I O N O F DENSITIES O F SOLIDS

The pycnometer technique is also suitable for the determination of the densities of solids. With organic substances, however, reliable results are obtained only when large crystals or pellets free from entrapped air can be prepared. Fine powders or crystals cannot be mixed with water without air bubbles attached to them. However, water is used when no organic liquid inert to the substance is available. The reference liquid is used first for calibrating the pycnometer, the sample is weighed in the pycnometer, then the pycnometer is filled with the liquid and weighed again. If G is the mass of the liquid of density d, G is the weight of the solid sample and G is the weight of the liquid added to the sample in the pycnometer, G — G will give the mass of liquid excluded by the sample. The density of the solid sample, d is calculated with the following equation: 1

3

2

l

3

x

In view of the difficulties mentioned, a more accurate and reliable method is the so-called floating method, but this is more time consuming. The basic principle is that solid particles float in a liquid that has the same density. Liquids recommended for use with organic substances insoluble in water are solutions of potassium mercury iodide or lead perchlorate [23]. A 78% saturated solution of the latter substance has a density of 2.6 at 15 C and thus can be used for almost all organic compounds. The solution is diluted with water until the sample starts to float, and the density of the liquid is determined by e.g., the pycnometer method. Thus, in principle, the results are as accurate as in the pycnometric determination of the density of liquids. Care should be taken to avoid adherence of air bubbles to the solid particles. C

93

5. Determination of solubility The determination of the solubility of organic compounds, from the qualitative point of view, is a preliminary test. It also allows conclusions to be drawn regarding some important physical and physical-chemical characteristics of the compound (e.g., apolar or polar nature). These data may also be necessary and useful for organic chemists when the task involved is recrystallization, extraction, etc., of the substance. Earlier, qualifications such as "readily soluble", "less soluble", "poorly soluble" or "insoluble" were regarded as sufficient. In tables of handbooks, usually water, ethanol, diethyl ether and the apolar solvents chloroform and carbon tetrachloride are mentioned. However, the extent of solubility or miscibility may be characteristic of a compound. Solubility in solvent mixtures is also of importance. By definition, solubility is the concentration of the sample in a solution that is in equilibrium with the solid substance. In organic chemical practice, the amount of substance (grams) soluble in 100 c m of solvent is determined. Solubility also depends on temperature, and with organic substances it increases with temperature, hardly any exceptions being known. The solubility of a given compound also depends slightly on the actual crystal modification and particle size, as very small crystals ( < 1-2 jam) are more soluble than larger crystals. Solubility is independent of the amount of excess solid. N o chemical reaction should take place between the solvent and the solute. In the determination of the solubility of solids, excess of solid phase should be present, and accurate determination and maintenance of temperature must be ensured. As the solubility equilibrium is attained very slowly, the process can be accelerated by starting from a higher temperature than that of the measurement and allowing slow cooling of the system to the desired temperature. Slow cooling, and also the presence of the solid phase, prevents supersaturation phenomena. When the solubility of the substance decreases with increasing temperature, the saturated state is approached from a lower temperature. After the saturated state has been reached, an aliquot of known volume is taken, the solvent is evaporated and the residue is weighed. The amount of the solute is preferably determined by chemical methods. On the macro and semimicro scale, the solubility of solid organic substances can be determined with the Buchbock apparatus (Fig. 22). The glass vessel is closed at the bottom with a fine silk sieve (3), which will hold the sample. Solvent is added to both the outer and the inner vessels. Pipette 1 is immersed in the inner vessel, which is rotated by an electric motor, and the 3

94

Fig. 22. Saturation apparatus design Buchbock (modified) / — P i p e t t e ; 2—glass s t o p c o c k ; 3

silk sieve

by

Fig. 23. Apparatus for the determination of solubility of gases in liquids / - G a s b u r e t t e filled with m e r c u r y ; 2—vessel with solvent

liquid mixed in this way will keep the solid in a suspended state. After completion of dissolution, stirring is stopped, the solid is allowed to settle, then the pipette is filled with the clear liquid up to the stopcock (2), removed from the system, emptied, and the amount of solute is determined by weighing the residue after evaporation of the solvent. The whole apparatus must be thermostated during the procedure. When the solubility of a substance is determined at different temperatures, the differential molar heat of solubility of the substance can be calculated by the Clausius-Clapeyron equation. On the ultramicro scale, the solubilities of crystalline substances have been determined by Armstrong and Copenhaver [24] on one or two crystals by leading vapours of solvents above the crystals placed on a microscope slide. Dissolution of the crystals was observed under a microscope. The time required was taken as a measure of solubility. Some anomalies explained by solvate formation were observed [25, 26]. Mutual dissolution, mixing of liquids. Water and organic liquids, and different organic liquids may be completely or only partially miscible with 95

each other. When no decrease in volume or formation of emulsions accompanies the phenomenon, the simplest way of operation is to shake 50 c m of each substance in a separating funnel with volume graduations until no volume change takes place, in the individual phases. This simple procedure can be used only at room temperature. A more accurate procedure that can be employed at various temperatures is to determine the composition of the mixture on the basis of the physical or chemical properties of one (or both) of the components. Refractometric or volumetric methods are very suitable for this purpose. Low water contents of organic liquids can be determined, for example, by the Karl Fischer titration. When the compositions of both phases are determined, two points on the solubility curve belonging to the same temperature are obtained. In industrial applications, the so-called aniline point being the mixing temperature of 1:1 mixture of gasoline and aniline, is of importance. A high aniline point indicates a large aromatic content of the gasoline, and vice versa. It is also possible to determine the solubility conditions for multicomponent liquid mixtures. The methods are similar to those developed for binary mixtures. The composition corresponding to the critical mixing point of the system at a given temperature can be established. Solubility of gases in liquids. Inorganic gases (elemental gases, carbon dioxide, sulphur oxides, etc.) are normally more soluble in water than in organic solvents. However, gaseous organic compounds are more soluble in organic solvents than in water. There are several exceptions. It is well known that the solubility of gases in liquids decreases with increasing temperature, and this is valid for both water and organic solvents. As dissolution of gases is accompanied by a decrease in volume, their solubilities depend strongly on the partial pressure, and therefore pressure and temperature must be carefully controlled during the measurement. In organic chemical handbooks the solubility of gases is given as cubic centimetres of gas in the normal state dissolved in 100 c m of solvent. In this measurement again both physical and chemical methods can be employed. Of the physical methods, the Ostwald apparatus (Fig. 23) measures the volume of gas absorbed by the solvent of known volume. The solvent must be entirely free from dissolved gases. The ratio of the volumes of burette 1 and vessel 2 in Fig. 23 can be adjusted to the measurement of the solubility of readily or poorly soluble gases, within certain limits. Very soluble gases, particularly in aqueous solutions, are preferably measured by chemical analyticaKjnethods. The Ostwald apparatus cannot be thermostated satisfactorily. Therefore, when the deviation from room temperature is significant, or solubility is to be determined at various temperatures, a more advantageous procedure is to 3

3

96

bubble the gas through the liquid at a given temperature until saturation. The process can be accelerated by applying a slight overpressure above the liquid. When the solubility of a gas is to be determined at pressures higher than 1.013 x 10 Pa (1 atm), a closed saturation vessel equipped with a manometer is used. 5

6. Determination of refractivity Although refractivity is not a very characteristic property of organic compounds, it can be determined very accurately and rapidly, but the presence of contaminants has an adverse effect. As refractivity data for various organic substances are known, the method can be employed in identification work and purity control. In theoretical examinations, the specific refractivity, correlated with density, or the molar refractivity, correlated with density and molecular weight, can be utilized. When a light beam falls from a transparent medium (vacuum, air or another gas, liquid or solid) on to the boundary with another transparent medium, its direction usually becomes altered. The extent of the deviation from the direction of the incident beam is called the refractivity (refractivity index). When the first medium is a vacuum, the absolute refractivity is measured. When the angle of incidence is denoted by a and the reflection angle by fc, we have sin a sin b The relative refractivity is the ratio of two absolute refractivities: "rel =

—

\

n

where n is the absolute refractivity of the medium with lower refractivity (e.g., 1.00029 for air) and n is the absolute refractivity of the second medium with higher refractivity (e.g., an organic substance). With a knowledge of n when n is measured n can be calculated, or, when the refractivity of air is neglected, n = n . In practice, the medium with n is a glass of known refractivity from which the light beam enters the sample. The light path can also be reversed when necessary. The refractivity depends, in addition to the nature of the sample, on the temperature and the wavelength of the light, and these values must be x

2

u

rel

2

rd

2

x

97

Fig. 24. Schematic diagram of the Abbe refractometer / — Prism

specified when quoting refractivities. Of the refractivity values given in handbooks, earlier data are often given at 15°C and the yellow light from a sodium lamp (subscript D). Temperature is given as a superscript to the refractivity value. For example, the refractivity of carbon tetrachloride is given as n = 1.4631. Instead of the symbol D, the actual wavelength may be indicated {D = 589.0 nm). In the recent literature, data may be referred to lines of the hydrogen gas spectrum [ C = 656.3nm (a), F = 486.1 nm (/?), G' = 434.1 nm (y)]. The symbol D or He refers to the He line of wavelength 587.5 nm. Earlier, values were quoted to four decimal places, but today values are measured to five decimal places. For this purpose, refractometers are used, designed for the determination of refractivity in liquids, fats and gases, but the refractivity of solids could be determined only indirectly for a long period. In practice, the refractivity of liquids and fats liquid below 1 0 0 X , and of organic substances with melting points lower than 100°C, have to be determined most frequently. The most widely applied apparatus is the Abbe refractometer and the Pulfrich refractometer, both working on the principle of total reflection, measuring the limiting angle. The Abbe refractometer (Fig. 24) is less accurate than the Pulfrich refractometer, but only a few microlitres of sample are required. This is placed in the aperture of the double prism (1) (about 0.15 mm apart from each other) and the prisms are thermostated. Usually white light is used for illumination, scattering being eliminated by the compensator in the apparatus (two, Amici prisms). The values read on the scale are related to the sodium D line. Refractivities between 1.3 and 1.7 can be measured with an accuracy of ±0.0002. l 5 D

3

98

In the Pulfrich refractometer, the light beam falls from the sample liquid at 90°C on to a prism of higher and known refractivity. The angle of reflection corresponding to this is measured. This angle will be identical with the limiting angle of total reflection at the boundary of the medium. The sample is inserted in the vessel attached to the thermostated prism, the side wall is illuminated with a homogeneous light beam and the angle of total reflection is read. This type of refractometer is more accurate than the Abbe refractometer, the error of measurement being less then ±0.0001. The principle of immersion refractometers is the same as that of the Pulfrich refractometer, but the prism is mounted at the end of a telescope tube in such a position that the light beam corresponding to the limiting angle of total reflection will pass along the axis of the telescope tube. The prism at the end of the telescope is immersed in the sample liquid, and is illuminated with a mirror through the sample so as to reach the plate of the prism at a very flat angle. There is an Amici prism system in the instrument, so that white light can be used. The prisms are exchangeable, and therefore refractivities can be determined between 1.3 and 1.6, with the accuracy provided by the Abbe refractometer. The immersion refractometers can be used for the determination of concentration by the use of a calibration series, provided that the refractivity of the solution is sufficiently dependent on concentration. Refractometers can easily be automated and are widely applied in process control systems. Standard substances suitable for checking the refractometers are, for example: 2,2,4-trimethylpentane, n ° 1.39145, n 1.38898; methylcyclohexane, n ° 1.42312, n 1.42058; and toluene, n ° 1.49693, n 1.49413. The refractivity of water is n ° 1. 33299, n 1.33250. Saylor [27] reported the microscopic determination of refractivity with an error of ±0.00001, using a series of glass prisms. In mineralogical studies, the refractivities of solid inorganic substances are determined by either the prism method, in which the critical angle is measured at the polished surface of a crystal, or by the use of liquids with identical refractivity. In the latter instance, liquids or solutions of known refractivity are mixed with Thoulet liquid (a solution of potassium tetraiodomercurate), yielding a medium with such a refractivity in which the crystals become invisible, as there is no reflection from the crystal plates. These two methods are not suitable for studies on organic crystals because such crystals are often too small and it is sometimes difficult to find a medium that is inert to the sample substance. The determination of the average refractivity of a crystal mass does not give accurate results. 2

2 5

D

2

D

2

D

2 5

D

2

D

2 5

D

9

D

2 5

D

99

(a)

(b)

Fig. 25. The Becke line at the border of crystal and melt (a) n
2

(b) n

{

>n

2

This fact is responsible for the situation that in earlier h a n d b o o k s refractivities are given only of solid organic substances that have low melting points, which could be examined with an Abbe refractometer. Kofler developed a method involving reverse application of the method of liquids with identical refractivity, which proved to be suitable for the determination of the refractivity of all solid organic substances that can be melted without decomposition and that will withstand temperatures 10-20 C higher than their melting points. The method is based on the comparison of the refractivity of glass powders of known refractivities with that of the melt of the organic sample. When small crystals are immersed in a liquid (or melt) that has a refractivity different from that of the crystals and the crystals are observed under a microscope with 50-70-fold magnification, the rays of light entering and subsequently leaving the crystals will be dispersed (bent), and thus the crystal will be surrounded by a bright line (a halo), which seems to move when the microscope tube is lifted or lowered. This bright line is called the Becke line (Fig. 25). When the refractivities of the liquid and the glass are different, a sharp black line can be seen around the crystals when focusing on the crystals. On lifting or lowering the microscope tube, a bright line (the Becke line) will immediately appear at the crystal boundaries, and this seems to be displaced towards the medium of higher refractivity. When the tube is lowered, the bright Becke line moves toward the medium of lower refractivity. The Becke C

100

Fig .26. Relationship between the refractivities of the crystal and the medium, based on the position of the Becke line

line becomes stronger (broader and brighter), as the difference between the refractivities of the two media increases (Fig. 26). The test is carried out with a series of glass powder samples with different refractivities. Such a series was produced first by the Schott Co. (Jena), and consisted of samples with the following refractivities: 1.3400, 1.4339; 1.4584, 1.4683, 1.4842, 1.4936, 1.5000, 1.5101, 1.5204, 1.5299, 1.5403, 1.5502, 1.5611, 1.5700, 1.5795, 1.5897, 1.6011, 1.6128, 1.6231, 1.6353, 1.6484, 1.6598, 1.6715, 1.6877. Recently, the Franz Kustner Nachf. Kg (Dresden A 21) has produced a series of glass powders as an accessory to the Boetius hot-stage microscope, with the following refractivities: 1.3400,1.4655,1.4953,1.5043,1.5151,1.5217, 1.5309, 1.5427, 1.5577, 1.5675, 1.5744, 1.5828, 1.5912, 1.6064, 1.6126, 1.6245, 1.6354, 1.6441, 1.6546, 1.6641 and 1.6741. When using a red filter in the hot-stage microscope, the specified values are related to the sodium D line. Thus, the behaviour of the Becke line will indicate whether the refractivity of the liquid or of the glass powder is the greater, and some indication regarding the extent of the difference can also be obtained by a skilled operator. O n this basis, another sample is prepared with another glass powder until the closest equality of refractivities is reached. However, this does not allow the accurate determination of the refractivity of the sample, and only a range can be given. In order to establish the exact value, the refractivities (those of the liquid or melt and of the glass) must be made identical. In this work, the fact that the refractivities of liquids and melts decrease with increasing temperature can be utilized, that of glass powder remaining almost unaltered (maximum 0.000001/°C). The sample with the glass powder with a refractivity slightly lower than that of the liquid (or the melt at 2 - 3 C above the melting point), is now heated slowly until the glass crystals appear to fade in the melt, but the shape can still be seen and the movement of the Becke line on moving the tube can still be observed. At the temperature of identical refractivities, the behaviour of the c

9*

101

Becke line will suddenly change and the direction of its movement on moving the microscope tube will be reversed. This phenomenon takes place over a range of 2-3°C, and can be explained simply by the fact that first the refractivity of the melt was higher than that of the crystals, then the state of identical refractivities was reached and, as the temperature increased further, the refractivity of the melt became lower than that of the crystals. It is advisable to carry out the determination with two glass powders, the second being the next lower in the series. The temperature of the melt must, of course, be increased to a higher value to reach identical refractivities. The temperature coefficient of the refractivity of organic compounds varies between 0.0002 and 0.0009. In practice, the mean value (0.0005) can be applied. In exact determinations, the temperature coefficient of the refractivity of a given organic compound is determined in the following way. Measurement is carried out with two glass powders. For example, the refractivity of phenacetin is 1.5101 at 134-135 C with one glass, and 1.5000 at 156-157°C with another glass. The difference between the two refractivities is 0.0101 unit and the difference between the two mean temperatures is 22°; thus the coefficient will be 0.0101/22 = 0.000459. Refractivities can be quoted in two ways. In simple instances, the observed limiting temperature of identical refractivities are given as superscripts to the refractivity of the glass powder used (e.g., the melting point of 2chlorobenzoic acid is 1 4 0 C and the refractivity is quoted as 1.5204 or 1.5101 ). The refractivities referred to the melting point are calculated as folloSvs, using the above example. When using the glass powder of refractivity 1.5204, the limiting temperatures were 146-148°C, mean value was 1 4 7 C , which is 7 C higher than the melting point. This is multiplied by the mean temperature coefficient of refractivity (0.0005) and the result is added to the refractivity index of the glass powder used, i.e., 1.5204 + 0.0035= 1.5239. Using the glass powder of refractivity 1.5101, the limiting temperatures were 166-169°C, the mean value 167.5C, which is by 27.5°C higher than the melting point. After multiplying with 0.0005, we obtain 0.013 75 + 1.5101 = 1.523 85. In practice, the test is carried out on a Kofler hot-stage microscope. As the behaviour of the Becke line can be observed most clearly when using parallel light rays, a plane mirror is applied and the diaphragm is restricted. The condenser of the microscope is removed if possible, and a red filter is placed in the light path. The sample (2-3 mg) is placed on a microscope slide and some glass powder is added. Unknown substances are usually tested first with a glass powder of refractivity 1.5217. The hot stage is heated to 2-3°C above the 146

166

C

102

169

1 4 8

melting point of the sample. The microscope field is focused first on the glass crystals, then the microscope tube is moved upward and downward to establish the behaviour of the Becke line, and thus the refractivity of the glass powder to be used next. A new sample is prepared and the procedure is continued until one finds the two glass powders that represent refractivities higher and lower than that of the sample. In a repeated test with the glass powder that has the nearest refractivity value, the temperature is increased slowly until the above-described behaviour of the Becke line is observed and the two limiting temperatures are determined. It is particularly advisable to repeat the measurement with another glass powder that has another refractivity (not so close to that of the sample) when the difference between the melting point and the temperature equality is small. Kofler et al. [28] gave refractivity data for nearly 1000 compounds (mainly drugs) using two different glass powders at two different temperatures. Several papers [9-13] have been published with data for additional compounds. Lacourt and Delande [29] identified 39 amino acids by the Kofler refractivity measurement technique. Kartnig [30] applied the method to the rapid detection and determination of H C H and D D T in the presence of each other. Refractivities of gases are determined with interferometers. The sample and the reference gases are examined in cells of length about 1 m. The coherent light beam is split before passing through the cells, then they are combined again, and the interference picture obtained is shifted when the refractivities of the two gases are different. The interference fringes are brought together optically, and the displacement required for this is measured and used for the calculation of the sample refractivity, the refractivity of the reference gas being known. The Rayleigh gas interferometer is the most widely used and is suitable, for example, for the determination of 0.1% of methane in the air of mines.

7. Determination of optical rotation Only molecules that contain an asymmetric carbon atom are capable of rotating the vibration plane of polarized light. These are optically active substances. Thus, optical rotation is not a generally applicable method of examination, but this is the only one by which enantiomorphic molecules, for example, can be distinguished. In general, optical rotation is measured in solutions, and the physical constant determined is the specific rotation:

103

CD 1

A

® Y--A

2

s

Fig. 27. Schematic diagram of the apparatus design for the measurement of optical rotation / — L i g h t s o u r c e ; 2 — p o l a r i z e r ; J — s o l u t i o n ; 4—analyzer;

5 —detector

where a is the angular rotation, / is the length of the column of liquid in decimetres and c is the concentration of the solution (g/100 c m ) . A subscript and superscript against the square brackets for [ a ] indicate the wavelength of light used (or the letter denoting it, e.g. D refers to the sodium D line with a wavelength of 589.0 nm) and the temperature of measurement (°C), respectively. The numerical value also carries a sign, and is followed by the concentration data and the specification of the solvent in parentheses, e.g., + 33.2°C (r = 2, methanol). Molecular rotation allows a comparison of the rotation of compounds with different molecular weights: 3

[M] With a knowledge of specific rotation, the concentration of solutions can be determined, e.g., sugar contents can be measured very accurately in this way. Optical rotation is measured in polarimeters, as illustrated in Fig. 27. Monochromatic light is polarized, then proceeds through the cell containing the sample solution (usually 1 dm long). The amount by which the analyzer must be rotated will give the angular rotation and the direction of rotation will reveal the levorotatory or dextrorotatory nature of the sample. In visual observations, the analyzer is rotated until both halves of the field of view are equally dark. In the half-shadow instruments, the field is divided into three parts and these are compared. The accuracy of measurement depends primarily on matching the intensities of the simultaneously viewed fields properly. Apparatus equipped with photoelectric detectors is more accurate. In some instruments, both the analyzer and the polarizer are fixed, and the rotation of the sample is compensated by means of the Faraday effect, that is, optical activity is brought about by a strong magnetic field in normally inactive substances. In spectropolarimeters the wavelength of light can be varied, usually between 800 and 250 nm, and an optical rotation versus wavelength curve suitable for structure elucidation can be recorded. 104

In the ultraviolet range the rotation of optically active substances is greatly enhanced. Measurements at different wavelengths may provide possibilities for the analysis of mixtures in which the optical activities of the components depend on the wavelength of light, that is, the rotational dispersion curves of the components are different. Optical rotation and its extent depend on the structure and configuration of the compound. In simple molecules the correlation is also simple, but in complex molecules the correlation between optical rotation and the absorption bands is complicated. Optical rotation at any wavelength can be regarded as the sum of several partial rotations, each belonging to an absorption band. When the structure or configuration of the molecule is altered, it may result in an alteration of all partial rotations, which makes the correlation between optical rotation and structure even more complex. This can be avoided if optical rotation can be measured at a wavelength within the range of an optically active absorption band. In this instance, the changes in rotation coupled with the Cotton effect for the band can clearly be observed and the general "background rotation" of other absorption bands does not interfere. The Cotton effect curves recorded in this way are particularly suitable for studies on the stereochemistry of oxo groups. In plane polarized light, the electric vector is limited to one plane, its magnitude varies periodically at a given point in the light beam and it corresponds to a normal planar wave. In circularly polarized light, the magnitude of the vector is constant, but its direction rotates clockwise or counter-clockwise, describing a spiral path through space. When the medium has refractivities that are different for the two circularly polarized rays, circular dichroism occurs. This is the difference between the absorption intensities of the two circularly polarized rays, being essentially the difference between the molar extinction coefficients of clockwise and counter-clockwise circularly polarized light. Measurement of circular dichroism can yield information on structure, configuration and conformation, the results can be interpreted relatively simply, and detailed quantitative evaluation can provide very important information particularly in studies on e.g., ketosteroids.

8. Determination of molecular weight Molecular weight is one of the most important physical characteristics of organic compounds. It is utilized in identification work, but its determination is essential in the elucidation of the structure of unknown compounds when the molecular formulae must be established, with a knowledge of quantitative elemental analytical data.

105

The molecular formula of a substance is regarded as correct only when the calculated and found molecular weight are identical. U p to about 100 molecular weight units usually no problems occur, particularly when the number of hydrogen atoms is small. The error of most of the molecular weight determination methods is 1-2%, that is, the molecular formula can be determined to + 1 hydrogen atom, when reliable elemental analytical data are available. Total elemental analysis (including the determination of oxygen) is desirable in order to avoid unnoticed errors in the measurement of the other components. There are several physical and physical-chemical methods for the determination of molecular weight, but none can be generally applied to the establishment of molecular weight in all types of compounds. In choosing the method to be applied, the order of magnitude of the molecular weight is of primary importance. This may be between 100 and 1,000,000 units, and the physical state, volatility, decomposition temperature and solubility of the sample in various organic and inorganic solvents must also be considered. As the error of determination is at least 2% at molecular weights between 100 and 500 units (except in mass spectrometry), agreement between the measured molecular weight and the molecular formula can be achieved only by determining other physical and chemical characteristics of the sample, particularly when the number of hydrogen atoms is to be established. For molecular weights higher than 500 (between 500 and 2000), only mass spectrometry yields accurate results. When molecular weight determination methods are applied to the elucidation of structural properties (activity, dissociation, association, formation of compounds, concentration, etc.), very accurate results are usually not required. Special methods are employed in the determination of even higher molecular weights values (polymers, biological substances), and in most instances only the average value is obtained. These special methods suitable for measurements in the 10,000-100,000 range with an error of 100-1000 molecular weight units, will be discussed at the end of the chapter. As in all fields of organic chemical analysis, an important requirement is to use small samples (a few milligrams). Therefore, small apparatus and small volumes must be used and a relatively large change to be measured must be achieved. In the following discussion only those techniques are discussed which meet the above requirements and are suitable for use in organic microanalytical laboratories without difficulty.

106

(A) D E T E R M I N A T I O N O F V A P O U R D E N S I T Y

The classical methods developed by Dumas, Gay-Lussac and Van Hoffmann are not used today; the Victor Mayer method is mainly employed on the micro- and semimicro-scale. The principle of the method is rapid evaporation of a known amount of volatile sample in a pre-heated space of known temperature and the volume of air or the weight of mercury driven out by the vapour is determined. The test is essential in examination of very volatile substances. O n the micro-scale, a variation of the procedure is to evaporate a known amount of sample in a known volume at a given temperature and then to measure the increase in pressure. Apparatus working on this principle are widely applied in the petroleum industry to the examination of very volatile fractions. There is an apparatus suitable for testing 1 c m of liquid [31], and the sample size can be even smaller when the apparatus is coupled with a preparative gas chromatograph [32]. The determination of density and thus the molecular weight of gaseous organic compounds can be carried out by the use of gas density balances. Parsons [33] used the G o w - M a c gas density balance in the determination of the molecular weights of gaseous pyrolysis products of organic compounds. 3

(B) O S M O T I C M E T H O D S

In the determination of the molecular weight of less volatile but soluble organic compounds, methods are based on the comparison of certain properties of solutions and the pure solvent. According to the Raoult-Van't Hoff law, the osmotic pressure, vapour pressure, boiling point and freezing point of solutions are correlated with the molecular weight of the solute. Thus, when the amounts of the solute and the solvent, that is, the concentration of the solution and the respective physical constant of the solvent, are known and that of the solution is measured, the molecular weight can be calculated. The method is applicable only when the solute molecules do not affect each other (dilute solutions, maximum 0.1 m o l e / d m , preferably 0.001 m o l e / d m ) , the solute particles are molecularly distributed, neither dissociation nor association phenomena alter the number of solute particles and no chemical reactions take place between the solute and the solvent. As in dilute solutions the changes caused by the solute are small, sensitive methods must be applied in these measurements. Determination of molecular weight by measuring the osmotic pressure. According to the Raoult-Van't Hoff rule, the osmotic pressure of ideal dilute 3

3

107

Fig. 28. Simple semimicro osmometer / - - C a p i l l a r i e s ; u l ; 2 - s o l u t i o n ; .?

solvent; 4

membrane

solutions is linearly proportional to the molar concentration. In theory, osmotic pressure can be measured simply, but in practice difficulties arise because of the lack of ideal semipermeable membranes. Furthermore, the measurement is time consuming and not very accurate. A simple apparatus designed for the measurement of osmotic pressure is shown in Fig. 28. In dilute solutions of polymers with high molecular weights, the osmotic pressure is equal to the pressure of only a few centimetres thick layers of the solvent, but the height of this solvent column can be measured very precisely with a microscope. Both aqueous and non-aqueous solutions are used. In aqueous solution, the semipermeable membrane is usually collodion, and in non-aqueous solutions denitrated collodion or cellophane is employed. The sample size is about 0.5 g, but this can be recovered when necessary. The method is used mainly in the examination of polymers with molecular weights between 20,000 and 500,000, in order to determine the average molecular weight based on the number of molecules. (C) D E T E R M I N A T I O N O F M O L E C U L A R W E I G H T BY M E A S U R I N G THE D E P R E S S I O N O F THE V A P O U R PRESSURE O F S O L U T I O N S

As mentioned above, the vapour pressure of solutions is lower than that of the pure solvent and proportional to the molar fraction: x =

(Po-P)

, Po where x is the molar fraction and p and p are the vapour pressures of the solvent and the solution, respectively. The lowering of vapour pressure (Po — P) be measured with a differential manometer. Samples of a several milligrams are required and the accuracy is about 2%, but the procedure is rather difficult, with a complicated apparatus. The fact that solutions of different molar concentrations in the same solvent have different vapour pressures and the phenomenon of isothermal distillation have been utilized in the determination of molecular weight on the 0

can

108

micro- and ultramicro- scales in simple apparatuses. When the solutions are enclosed in a small c o m m o n space, the solvent will evaporate from the solution of lower molar concentration and condense in the more concentrated solution until the molar concentrations in the solutions become identical. The changes in the volume or weight of the solutions are determined at the equilibrated state and compared with the initial state, and the molecular weight is calculated. Isothermal distillation takes place at a rate decreasing with time, and total equalization of concentrations is a function of the solvent vapour pressure and it requires at least 12 h, sometimes several days. A more rapid procedure is the dynamic version, in which the molecular weight is calculated from the rate of isothermal distillation measured. Usually graphical extrapolation is employed. The first, very simple variation of the "iso-osmotic" method was developed by Barger [34]. Solutions of a standard substance at different concentrations and of the sample at known but constant concentration were inserted alternately in a capillary as drops separated by air bubbles. The positions of the small liquid columns formed were measured under a microscope with about 100-fold magnification equipped with an eye-piece micrometer, then the capillary was thermostated for a certain period. The positions of the drops changed and the length of the liquid columns also changed depending on the relationship between concentration of adjacent drops, as they lost or received solvent from each other. If two adjacent liquid columns remained unaltered, then they had identical osmotic pressures, that is, identical molar concentrations. The molar concentration could be obtained by interpolation (error about 5%) or comparing the magnitude of the change of 2-3 pairs of liquid drops. However, problems arise from the fact that as the drops are inserted in the capillary one after another, adjacent solutions are slightly contaminated, and the initial concentration is also altered owing to diffusion through the thin liquid film wetting the walls of the capillary. The procedure was modified by Rast [35] and only one pair of liquid drops separated with an air bubble was inserted in each capillary (diameter 0.5-1.2 mm). The concentration of the standard solution was varied and that of the sample was constant in each capillary. The capillaries were closed at both ends, fixed on a glass plate in order of increasing concentration and the position of the drops were measured under a microscope. The set-up was thermostated for several days and the changes in the positions of the meniscuses and in the lengths of liquid columns were determined. Another dilution series was prepared from the standard solution that caused the smallest change in the capillary, the procedure was repeated and the result was obtained by interpolation from the changes in three capillaries exhibiting the least variation in column length. 109

Fig. 29. Molecular weight determination by isothermal distillation / and

2—solutions

(b)

(c)

Fig. 30. Signer-Clark apparatus for the determination of molecular weight (a) T o p view; (b) side view; (c) front view

Mutual contamination of the solutions can be avoided by inserting the standard and sample solutions in separate capillaries and placing them in a common larger closed tube with the open ends facing each other (Fig. 29). Signer [36] and Clark [37] inserted the two solutions in capillaries with fine divisions and placed them in a common space (Fig. 30). The changes in volume were read periodically and the measurement was completed when constant volumes were attained. The error of modified Barger methods is 2-3%. A disadvantage of these methods is their slowness, but it is very advantageous that any solvent or solvent mixture can be used. The main cause of error is the insufficient accuracy of the measurement of the length or volume of the liquid columns. Therefore, attempts have'been made for a long time to base the determination on weighing. Trutnowsky [38] measured the rate of weight gain of the solution in an atmosphere saturated with the vapour of the solvent and applied an empirical formula in the calculation. The procedure is rather troublesome. Szilagyi and Szilagyi-Pandur [39] developed and patented an apparatus (Mikromol Type OX-103) suitable for the determination of molecular weight 110

Fig. 31. Szilagyi-Szilagyi apparatus for the determination of molecular weight / — M e t a l c u p ; 2—lid of valve; 4—screw

with valve a n d suction t u b e ; 3 — h o u s e for securing the lid; 5—glass

vessels;

6 — s u c t i o n t u b e ; 7—screw of the valve

of organic compounds up to about 500 with an accuracy far better than that of earlier techniques. Isothermal distillation is accelerated by applying a vacuum. The apparatus (Fig. 31) consists of a metal block with a small dead volume that can be evacuated, and contains three glass vessels of volume about 5 ml with stoppers. The vessels together with the stoppers are weighed on a micro-balance. The sample is added to one of the vessels and different amounts of the standard to the other two vessels, which are weighed again on the micro-balance. A suitable solvent (about 2 ml) is added to each vessel, which are closed and weighed again. The vessels are opened and placed in the metal holder, which is evacuated and thermostated at 3 0 - 4 0 C for 2-4 days, depending on the vapour pressure of the solvent. The vessels are removed from the apparatus, closed and weighed again. The molecular weight is calculated with the equation c

M=

M

x

and

M =

111

where A is the weight of the sample, A and A' are the weights of the standard, 0 is the weight of the solvent in the sample vessel, 0 and 0' are the weights of the solvent in the standard vessels and M is the molecular weight of the standard substance. When the weight change of the solvents is 0 > O > 0 ' , the molecular weight can be calculated directly, otherwise a correction must be applied. The molecular weight of the standard should be in the region of the suspected molecular weight of the sample. x

2

2

X

2

2

x

2

x

2

(D) D E T E R M I N A T I O N O F M O L E C U L A R W E I G H T BY M E A S U R I N G THE ELEVATION O F BOILING P O I N T O F S O L U T I O N S (EBULLIOSCOPY)

Ebullioscopic measurements are also based on the Raoult law: the boiling point of a solution is always higher than that of the pure solvent, owing to the decrease in vapour pressure of the solution. When the criteria mentioned above for the conditions of the validity of the Raoult law are satisfied, the molecular weight can also be determined from vapour pressure measurements. Vapour pressure can be measured directly with a vapour pressure balance or a micromanometer. A correction factor obtained from the calibration of the apparatus with a standard substance is used in the calculation. The sample should have no measurable vapour pressure at the boiling point of the solution, in practice, the boiling point of the sample should be at least 150 C higher than that of the solvent. The elevation of the boiling point that occurs when 1 mole of solute is dissolved in 1000 g of solvent has been determined for almost all common solvents and the data are listed in Table 11. Solvents with a high elevation of the boiling point are preferred, the boiling point should be lower than the decomposition temperature of the sample, and no dissociation or association phenomena and no chemical reaction should take place in the solution. Thus, when the boiling point of the solvent, the boiling point elevation and the weights of the sample and the solvent (that is, the concentration of the solution) are known and the boiling point of the solution is measured, the molecular weight can be calculated with the following simplified equation: C

where c is the weight of the sample dissolved in 1000 g of solvent, A is the difference observed between the boiling points of the solvent and the solution and A is the molecular boiling point elevation of the solvent. As has been mentioned above, the Raoult law holds only for dilute solutions, thus the temperature difference A is small (e.g., in 0.1 m o l e / d m g

M

3

g

112

TABLE 11. Data for some solvents for determination of molecular weight based on elevation of boiling point Solvent

Boiling point (°C)

Molal boiling point elevation (°C)

Acetone Aniline Benzene 1,2-Dibromoethane Diethyl ether 1,4-Dioxane Diphenyl Ethanol Acetic acid Acetic anhydride Ethyl acetate Phenol Chloroform Naphthalene Nitrobenzene Pyridine Carbon tetrachloride Carbon disulphide Water

56.1 184.4 80.12 131.6 34.6 101.4 256.1 78.4 118.1 139.4 77.1 181.4 61.2 217.9 210.9 115.5 76.7 46.35 100.00

1.48 3.69 2.64 6.43 1.83 3.13 7.06 1.04 3.07 3.53 2.83 3.6 3.8 5.8 5.27 2.69 4.88 2.29 0.516

solutions in 1,2-dibromoethane it is only 0.64°C; the A value of the solvent is 6.43°C). Therefore the boiling points must be measured very accurately. O n the macro- and semimicro-scale, the well-known Beckmann thermometer can be used, there is a more sensitive version of the thermometer suitable for use on the micro-scale with which 0.002°C changes in temperature can be measured with satisfactory accuracy. The method was used first by Victor Mayer in the determination of molecular weight, the modern apparatus is shown in Fig. 16. In the differential ebullioscope, the boiling points of the solvent and the solution can be measured simultaneously with two Beckmann thermometers. O n the microscale, the Menzies-Wright ebulliometer is widely used. This is equipped with an inner heater and a special Menzies differential thermometer. The apparatus must be calibrated with a standard substance that has a molecular weight similar to that of the sample. In measurements on the semimicro-scale, 5-15 mg of sample are dissolved in about 5 c m of solvent and the temperature is measured with a thermistor suitable for the detection of temperature differences as small as 0.0003°C [40, 41]. Molecular weight was determined in even smaller samples (1-3 mg) by Perold and Schoning [41]. A M

3

113

special variation of the ebullioscopic technique was described by Weisz and Pantel [42], in which the sample and reference solutions are boiled in two vessels of identical size and the solvent is added to that with higher boiling point until equalization of boiling points. This could be accomplished very precisely with the use of two thermistors connected in a differential mode. Today, thermistors are widely used [43], but their specific resistance decreases very sensitively with increasing temperature and the relationship is not linear and therefore calibration is necessary. When they are connected in the differential (compensating) mode, a high precision can be achieved by establishing the compensated state. Several workers [ 4 4 - 4 6 ] have dealt with the application of thermistors in ebullioscopy. Thermistors have made possible the measurement of molecular weight from the temperature difference between solvent and solution drops hanging in a vapour space. Thorough thermostating and connection of the thermistors in the compensating mode are necessary. De Ros et al. [47] developed a semimicro method based on the depression of the vapour pressure of solutions. Solvents with boiling points between 40 and 120°C were used and molecular weights up to 500 could be determined. The dew-point was observed very accurately when the solvent evaporated and condensation started. (E) D E T E R M I N A T I O N O F M O L E C U L A R W E I G H T BY M E A S U R I N G THE D E P R E S S I O N O F THE F R E E Z I N G P O I N T ( A N D M E L T I N G P O I N T ) O F S O L U T I O N S ( A N D MELTS); ( C R Y O S C O P Y )

This is another application of the Raoult law. The freezing point of dilute solutions satisfying other requirements for the validity of the Raoult law is lower than that of the pure solvent and is proportional to the molecular weight of the solute. The molal depression of the freezing point is produced by 1 g-mole of solute in 1000 g of solvent. This value, in almost all instances, is higher than the numerical value of the elevation of the boiling point (water 1.85°C, benzene 4.9°C, nitrobenzene 6.8 C), and there are also solid substances suitable for use as solvents. C a m p h o r and its derivatives can be mentioned here, as their molal freezing point depressions are so high (35-40°C) that they can be measured with the usual thermometers in dilute solutions with satisfactory accuracy. When a solid solvent is used, the melting points of the solid solution and the solvent are measured. With a knowledge of the molal freezing point depression value for the solvent, and the weights of the sample and the solvent, the freezing points of C

114

the solvent and the solution are measured, and the molecular weight is calculated with the equation M=

^

where c is the weight of sample (in grams) in 1000 g of solvent, A is the molal freezing point depression of the solvent and A is the difference between the freezing points of the solvent and the solution. O n the macro-scale, a solution is prepared from about 20 c m of solvent and a few tenths of a gram of the sample, this is cooled below the freezing point and crystallization is started by adding a crystal of the solvent to the supercooled solution. The temperature rapidly rises to the freezing point. The freezing point of the solvent must also be determined in the same apparatus. Under the above conditions, that is, in about 0.05 m o l e / d m solutions, the temperature difference is a few tenths of a degree, and this can be measured with a Beckmann thermometer with an accuracy of 2 - 3 % . Thus the error of the method is less than 5%, and this is mainly due to the fact that the amount of solvent frozen out is neglected. When applying a correction for this (this is particularly advisable with water), the error of measurement can be reduced to 2 - 3 % . Smit et al. [48] introduced a cryoscopic technique in which the temperature versus heat content curve is recorded on about 2 mg of sample; either the solvent or the solute is the crystallizing substance, several solvents can be used and the method is also reliable for polymers. The accuracy is about 1%. O n the micro-scale, in 1-2 c m of solutions (0.005 g / c m %0.05 m o l e / d m ) , resistance thermometers (e.g., thermistors) provide a means for accurate measurement [ 4 9 - 5 2 ] . Hesse [53] described a simple apparatus in which 3 20 mg of sample in about 1 c m of solvent (benzene, dioxane, dimethyl sulphoxide) could be examined for molecular weight up to 1000 units with an error of 1-1.5%. A modified Beckmann cryoscope was developed by Perkins and Twentyman [54] for air-sensitive substances (1-2% error). The cryoscopic procedure is rather troublesome, particularly when working on the micro-scale. Rast [35] developed a very simple method using a micro-balance and a melting point determination apparatus only. A 1-2 mg amount of sample is weighed in a melting point capillary (i.d. about 2 m m ) then 20-30 mg of camphor are added and the capillary is weighed. The capillary is soldered without loss of camphor, then the contents of the capillary are melted in a flame and homogenized. The capillary is fixed to a thermometer in the same manner as in the melting point determination. M

q

3

3

3

3

3

3

10

115

The thermometer and the capillary are immersed in a heating bath, the temperature is increased and the contents of the capillary are observed through a magnifying glass until the disappearance of the last crystal. The melting point of the camphor sample used is also determined in the same way (this may be between 178 and 180°C) and the molal melting point depression is also established with a substance of known molecular weight. This may vary between 37 and 40 C (theoretical value: 3 7 . 7 C ) . The molecular weight of the sample is calculated with the equation f

where G and G are the weights (in grams) of the unknown substance and camphor, respectively. A is the difference between the melting points of the camphor and the mixture and A is the molal melting point depression for camphor. If the temperature is measured with an error of 0.5°C, the error of the determination is about 3%. If observation of the capillary and the melting process on the hot stage of a Kofler microscope with 50-fold magnification and a thermometer with 0.2°C divisions are used, the error of measurement can be reduced to less than 2%. An unfavourable property of camphor is its relatively high melting point, (176-180°C), below which temperature several organic substances undergo decomposition. Certain camphor derivatives have much lower melting points and relatively high A values, e.g., the melting point of dihydro-acyclopentadien is 50°C and the A value is 45.3°C. Solvents containing hydroxyl groups are usually avoided because of the possibility of association resulting in erroneous molecular weight results. Data for substances suitable for use in the Rast method are listed in Table 12. Staudinger [55] established a correlation between viscosity and molecular weight for certain substances, such as linear chain molecules. Thus, absolute viscosity is a

c

a

c

c

c

M

=

KM m

where M is the molecular weight of the polymer and K is an independent constant determined by the nature of the monomer and the solvent. A more correct form of this equation is m

M = Km

' M

a

where a varies between 0.5 and 2.

116

TABLE 12. Data for pure substances for determination of molecular weight based on depression of melting point Compound Aniline Anthraquinone Benzene Borneol Bornyl bromide Bornyl chloride Bornylamine Bromocamphor 2-Bromonaphthalene Cyclopentadecanone (Exaltone) Cyclohexanol 1,2-Dibromoethane 1,4-Dioxane Diphenyl 2,6-Dichlorocamphane (pinene dichloride) 2,6-Dibromocamphane (pinene di bromide) Dihydro-a-cyclopentadiene 2,5-endo-Ethylcyclohexanone 1,4-endo-Azocyclohexanon Acetic acid Phenol Hexachloroethane Isocamphane Camphor Camphoquinone Camphane Camphene Naphthalene 2-Naphthol Nitrobenzene Carbon tetrabromide 2,4,6-Trinitrotoluene

Melting point <°C)

Molal melting point depression (°C)

-6.2 266 5.49 204 90 131 164 76-77 59 65.6 23.9 10 11.3 70.5 174

5.87 14.8 5.07 35.8 67.4 46.5 40.6 11.8 12.4 21.3 38.3 12.5 4.7 8.0 56.2

170 50 178 141 16.6 41 187 65 176-180 199 49 39 80.4 123 5.7 94 81

80.9 45.3 32.9 32.2 3.9 7.27 47.7 44.5 40 45.7 31.1 64 6.9 11.25 6.89 86.7 11.5

(F) D E T E R M I N A T I O N O F M O L E C U L A R W E I G H T BY M E A N S O F MASS SPECTROMETRY

Mass spectrometry is the most advanced method for molecular weight determination, and very accurate results can be obtained. Single-focused instruments provide an accuracy of 0.2%, and with double-focused instruments the accuracy is better than 0.01%. 10*

117

The principle of operation of mass spectrometers is as follows. The sample is vaporized and introduced into the ionization chamber of the instrument. (The molecule must be stable under these conditions when the molecular weight is to be determined.) In the ionization chamber the molecules are exposed to an electron beam of varying energy (50-70 eV), and positively charged molecular ions are produced: AT+e-»AT +2e +

These molecular ions are formed in the vicinity of a positively charged ionaccelerator plate that has a variable potential of about 2 k V. The positive ions are reflected from this plate by electrostatic repulsion and form an ion beam under the effect of charged auxiliary plates. This ion beam is focused on the exit slit of the ionization chamber, then passes through a magnetic field produced by an electromagnet; this changes the direction of the ion beam, usually by 90°, and the particles in the beam move along different orbits defined by their m/e values, where m is the mass of the ion and e is the charge: m _

Hr

~e

IV

2

2

'

where H is the magnetic field strength, r is the radius of the orbit and Kis the potential of the accelerator plate. The ionization potential and the strength of the magnetic field are adjusted so as to make a given ion beam (containing particles with nearly identical m/e values) pass through the exit slit and fall on to the collector. The magnitude of the ion current is about 1 0 " A. This current is amplified electronically, and the current measured is directly correlated with the amount of the ion species examined. In double-focused instruments, the ion beam is focused in an electrostatic analyzer after the electrostatic repulsion step, then it reaches the magnetic analyzer. In this way, a deviation of 180° is achieved and the ion beams are better resolved. The mass spectrum is obtained by establishing the m/e values of the ion beams passing through the slit. The peak height indicates the relative frequency of the given species (as a percentage). The mass number of the ion species with the highest value (relatively few in the ion beam) will be the molecular weight of the compound examined, the smaller mass numbers are produced by isotopes, contaminants and molecular fragments. Thus, a distinction between 1,2-benzanthracene and 1,5-diphenylbenzene can be achieved with the simplest instrument (see the very simplified spectrum in Fig. 32). The double-focused instrument is much more efficient, for example, in the examination of an alkaloid with a high molecular weight, the elemental analytical data yield the possible molecular formulae C 5 H 5 4 N 0 (mol. wt. 1 2

4

118

4

8

100h o o

100h

1,2-benzanthracene, C H 1 8

o

If

1,5-diphenylbenzene, C

1 2

18 U H

M 230.31

M 228.30

i

228

i i

il

j

226 228 230

i i i

m/e Fig. 32. Diagrams of molecular weight recorded by a mass spectrometer

778) or C 4 2 H N 0 (mol. wt. 722). In a mass spectrometer of medium resolution, the mass of the molecule ion was found to be 718. The molecular formula modified on this basis allowed the two possibilities: C 3 H 5 o N 0 (mol. wt. 718.373) or C H N 0 (mol. wt. 718.336). The accurate formula was obtained from the spectrum recorded with a high-resolution mass spectrometer, the mass of the molecule ion was 718.3743, confirming the first formula. 4 8

4

6

4

4 2

4 6

4

4

6

7

(G) D E T E R M I N A T I O N O F M O L E C U L A R W E I G H T BY X-RAY D I F F R A C T I O N S T U D I E S

The volume of the unit cell of a crystalline substance can be determined by diffraction measurements and the molecular weight can be calculated from these data. When the density of the substance is known, the mass of the unit cell is also known. The weight of the unit cell can be calculated (related to 0 = 16) by dividing its mass by 1.66 x 1 0 ~ . The dimensions of the unit cell are given inAngstroms, as 1 A = 1 0 ~ c m \ the molecular weight is then obtained from the equation 2 4

3

M

2 4

=

where V is the volume of the unit cell, d is density and n is the number of molecules in the unit cell. The value of n can often be determined from the number of main positions of steric groups but, when the approximate size of the molecule is known, this can be inserted in the above equation, and the equation is solved for n. The value obtained is near to an integer if the molecular weight applied was reasonable. The integer obtained in this way can then be used in the calculation of the accurate molecular weight.

119

(H) D E T E R M I N A T I O N O F M O L E C U L A R W E I G H T O F P O L Y M E R S BY U L T R A C E N T R I F U G A T I O N

The method can be based on two different concepts: either the velocity of displacement of the solute in a strong field is determined (that is, the rate of sedimentation), or the distribution of the solute in the sedimentation equilibrium is established, that is, the state when the centrifugal force is just high enough that the movement of the solute towards the bottom of the vessel is compensated for by the diffusion. Recently, molecular weights have been determined from measurements in a state approaching equilibrium. The ultracentrifuges operate at a speed of about 60,000 rpm. The changes that occur in the cell during ultracentrifugation can be monitored by absorption techniques or Schlieren techniques (here the gradient of refractivity, being identical with the concentration gradient, is recorded photographically a n d interference phenomena are utilized; the distortion of interference fringes indicates concentration very sensitively). (I) O T H E R M E T H O D S FOR T H E D E T E R M I N A T I O N OF MOLECULAR WEIGHT

In colloidal solutions of polymers, the phenomenon of light scattering can be utilized for the determination of molecular weight. Electron microscopic methods can also be employed, as an electron microscope with good resolution is effective down to about 0.5 nm, that is, the polymer molecules can be "seen". In this way, when no other difficulties arise, molecular weights in the 10 range (e.g., viruses) can be determined. 6

9. Determination of viscosity [56-57] Viscosity (internal friction) is a qualitative characteristic of liquid organic compounds and its measurement is of importance primarily in technical tests. The viscosity of gases can also be measured. According to the Poiseuille formula, the absolute viscosity is

where r is the radius of the capillary, / is its length, p is pressure and v is the volume of the fluid flowing through the capillary in time t. Absolute viscosity depends on temperature; that of water is 0.01005 at 20°C and 0.00282 at 100°C. 120

Several methods and apparatus are available for the determination of viscosity. For small samples, the modified Ostwald capillary viscometer can be used with advantage. In the apparatus shown in Fig. 33 the time during which the meniscus of the liquid decreases from mark 1 to mark 2 is measured. The apparatus must be thermostated and the constants for the apparatus must be known but, when relative viscosity is to be determined, only the density (S) of the sample and the outflow time (t ) of a liquid with known viscosity and density (S ) need to be measured. The reference liquid is usually water. T h u s : w

w

n _

S-t

As the absolute viscosity of water is known, that of the sample can also be calculated. The Hoppler ball viscometer consists of a glass tube of precisely known dimensions and uniform diameter mounted in an oblique position (80°). This is filled with the sample fluid, and a ball falls downwards in the tube (the size of the ball used depends on the approximate density and viscosity of the sample), and the time elapsed as the ball covers the distance between two marks on the tube is measured. Viscosities between 1 0 " a n d 4 x 10 Pa s c a n be measured with this apparatus, that is, gases can also examined in this way. 5

Fig. 33. Ostwald capillary viscometer (modified)

2

Fig. 34. Traube stalagmometer

121

Vamos and Mozes published a detailed discussion of capillary viscometers [59]. Rotation viscometers are preferred for the examination of high-viscosity fluids, and Engler viscometers are used mainly in the oil industry. This apparatus consists of a metal vessel and 240 c m of fluid are discharged from it through a tube of 2.9 m m i.d. The time required for outflow is measured and related to that for the same amount of water at the same temperature. The results are expressed in Engler degrees, which can be converted into relative kinematic viscosity only by an approximate empirical equation 3

10. Determination of surface tension [59-62] Surface tension is a property correlated with molecular forces. According to one of the definitions, it is the excess free energy of unit surface area of a liquid as compared to the bulk of the liquid, that is, the work required for isothermal reversible production of 1 c m of additional surface. The dimension is work/area = force/length, and its unit is 1 0 N / c m . The surface tension of organic fluid ( l m J • m ~ ) are of the same order of magnitude (e.g., 22.75 for ethanol, 58.2 for formamide and 63.4 for glycerol at 20°C), but it can be greatly altered by surfactants (particularly that of water). Such agents reduce the surface tension and in this way, for example, the wetting ability of water is greatly enhanced. Of the several methods developed for the measurement of surface tension, the stalagmometric method is the most frequently used. The Traube stalagmometer (Fig. 34) yields relative results. Fluid of density 5 flowing out slowly from the apparatus forms a d r o p at the end of the outlet tube and, when it has reached a certain weight, equal to 2rny mJ • m ~ , it falls. The volume of the pipette (V) is known, the number of drops falling until complete draining (n) is established, and thus y can be calculated from the equation 2

- 5

2

2

2my The apparatus is calibrated with water, the d r o p number for water is determined (n ) and y is obtained. T h e surface tension of the sample can be calculated with the equation w

122

w

The stalagmometer must be absolutely clean; a mixture of sulphuric and chromic acids is recommended for cleaning, followed by washing with water and ethanol and rinsing with the sample fluid. Measurements can be carried out at room temperature, and small changes in temperature do not affect the result significantly (e.g., the surface tension of ethanol is 23.61 x 1 0 " N m " at 10°C, 22.75 x l ( T N m " at 20°C, 21.89 x l ( T N m " at 30°C and 18.43 x 1 0 N m " at 60°C). 3

3

- 3

1

3

1

1

1

11. Determination of thermal properties of organic compounds [58] The thermal properties of organic compounds are not as important as other physical properties in identification work, but this type of data may be very useful in technical applications. Several thermal methods have been developed, some of which have practical importance in organic chemistry and others mainly in theoretical research work. (A) D E T E R M I N A T I O N O F T H E R M A L C O N D U C T I V I T Y

Coefficients of thermal conductivity are listed in handbooks for many solid, liquid and gaseous substances, at various temperatures. This coefficient, denoted by / , has the units Wm~ K~ . The thermal conductivities of organic compounds are relatively low, gases have the lowest values, and solids the largest values. The thermal conductivity of solids and liquids can be determined by the stationary technique, when a constant energy stream is led through the sample and the temperature is measured at different points in the specimen. In the non-stationary technique, the rate of temperature change is measured in 1

l

Fig. 35. Determination of thermal conduction number by measuring the rate of cooling /—Specimen; 2—thermocouples; J - galvanometer

123

the specimen with changing temperature (heated or cooled). In this instance, thermal conductivity is characterized by the temperature conduction number that can be derived from Fick's second law: a =

10~ m /s, 6

C S

2

1

where a is the coefficient of thermal conductivity, C is the specific heat of the substance and S is its density. The measurement is essentially a recording of temperature changes due to heat transfer. Of organic compounds, the thermal conductivity of synthetic materials has the greatest importance, this is usually characterized by the temperature conduction number. The latter is determined by the non-stationary technique and a schematic diagram of the measuring apparatus is shown in Fig. 35. (B) O T H E R C A L O R I M E T R I C M E A S U R E M E N T S

[62,

63]

Heat of combustion is an important thermal characteristic of certain organic compounds (e.g., benzoic acid is used in the calibration of BerthelotMahler-Kroeker calorimeters). Another thermal measurement technique is the thermometric titration, in which concentration can be established from the heats of reaction of certain processes. Other thermal properties of organic compounds (e.g., specific heat) and the detection and determination of exothermic or endothermic processes in solutions or melts (e.g., heat of dissolution, heat of melting, heat of neutralization) on the micro-scale are studied by differential calorimetry or differential scanning calorimetry (DSC), which permits the measurement of changes in heat content in very small samples and very slow processes (0.001 °C/min). The essential part of the apparatus is a carefully insulated vessel containing two microcalorimeters, one holding the sample and the other being empty or containing the solvent, while the sample is dissolved in the other microcalorimeter. The temperature of the apparatus is changed slowly, a difference is produced by the sample or sample solution between the two calorimeters and this can be followed precisely with a thermocouple connected in a compensating mode (the accuracy is 1 0 ~ - 1 0 " C ) . A curve with a maximum is obtained and the integrated value is used in the calculation. The heat effect may be compensated with electric heating, and an instrument indicating the balanced state may be used. Several modifications of the differential calorimeters are known, such as adiabatic, flowing and dynamic-difference calorimeters. 6

124

7 o

12. Determination of relative permittivity (dipole moment) [64] Molecules of a substance, when placed in an electric field, will suffer reversible changes. The extent of this change depends on the electrical properties of nonconductive matter (dielectrics) and the electron structure of the molecules, and can be expressed numerically as the permittivity of the substance. The permittivity can be used for calculating the dipole moment of the molecules. The absolute permittivity (s) is a proportionality factor for the electrical polarization and the vector of the electric field strength: D = e,£ The relative permittivity is the ratio of the absolute permittivities of the sample (e ) and of a vacuum (e ): abs

0

_ abs rel — e

e

£o

Molecular polarization taking place under the influence of an electrical field, consists of three independent processes. In electron polarization, the electrons and nuclei of the molecules are displaced in opposite directions by the electrical field, and an induced dipole moment is thus produced. This is proportional to the field strength and is independent of temperature. In atom polarization, the nuclei with different effective charges in a molecule with polar bonds are displaced with respect to each other. T h e process is dependent on field strength and independent of temperature. Orientation polarization is characteristic of molecules that have a constant (permanent) dipole moment where the centres of positive and negative changes d o not coincide in the absence of an electrical field. Atomic and electron polarization takes place in this kind of molecule too, but the tendency of orientation of these molecules in a force field is the predominating factor in this instance. This process is not independent of temperature, as thermal motion counteracts the orientation polarization. The correlation between the permittivity of compounds and the dipole moment of the individual molecules is expressed by the Debye equation. In apolar molecules, the electrical field gives rise to electron or atom polarization a n d small induced dipoles are formed in the course of charge displacement. This is the case with symmetric organic molecules, e.g., hydrocarbons, and diatomic gas molecules. Polar molecules represent dipoles already in the normal state, the centres of positive and negative charges d o not coincide. They behave like small 125

permanent magnets in an electrical field and are orientated along the direction of the force field. Thus, it is a significant difference between apolar and polar molecules that in the former only the molecular structure is deformed, whereas in the latter the molecular structure is also altered. As has been mentioned, this orientation will alter the electrical properties of a dielectric (a solution of a polar molecule in an apolar solvent), and this can be measured. Electrostatic techniques, a.c. capacity measurements and techniques based on electromagnetic wave phenomena have been utilized for this purpose. Of these methods, in practice those based on a.c. capacity measurement (bridge method, resonance method and drifting methods) are the most widely applied. The dipole moment of simple organic molecules such as saturated hydrocarbons is zero, that of olefins, some aromatic hydrocarbons and heterocyclic compounds (toluene, thiophene, furane) is 0 - 1 , while ethers and primary amines have values between 1 and 1.6, alcohols and phenols between 2.4 and 3, nitriles and nitro compounds between 3 and 4 and sulphones and sulphoxides between 4 and 5.5. Several compounds containing polar bonds have dipole moments of zero. The electric dipole moments of chemical bonds are known, and the whole molecule shows the vectorial sum of the individual bond moments. In this way, a rough estimation of the dipole moments of molecules is possible. In structural investigations, dipole moments provide valuable information on the distribution of electrons. Inductive and mesomeric effects were confirmed first by dipole moment measurements.

References to Chapter 4 1. 2. 3. 4. 5. 6.

Gilmore, E. H., Menault, M., Schneider, V.: Anal Chem., 2 2 , 892 (1950). Powell, H. B., Mellon, E. K., Burow, D . F.: J. Chem. Educ., 41, 345 (1964). Walish, W., Eberle, H. G.: Mikrochimica Ada, 1031 (1967). Naumann, R.: Pharm. Ztg. 114, 36, 1283 (1968). Kreutzer, H.: Haereus Analysengerate, 2 9 - 3 0 (1970). Aleksandrov, Y. I., Varganov, V. P. Egerov, 1.1., Ivanov, K. A., Psavko, B. R., Frenkel, I. M.: Zh. anal. Khim., 21, 574 (1972); Ref., Anal, Ahstr., 887 (1973). 7 Kofler, L., Kofler, A., Brandstatter, M.: Thermo-Mikro-Methoden zur Kennzeichnung organischer Stoffe und Stoffgemische. Universitatsverlag Wagner GmbH. Innsbruck, 1954. 8. Kolb, A. K., Lee, C. L., Trail, R. M.: Anal. Chem., 39, 1206 (1967). 9. Mettler Instr. Corp.: Instrument for Automatic Determination of Melting and Boiling Points. FB.-l. 10. Kofler, A., Kolsek, J.: Mikrochimica Acta, 408 (1969). 11. Kofler, A., Kolsek, J.: Mikrochimica Acta, 1038 (1969). 12. Kofler, A., Kolsek, J.: Mikrochimica Acta, 367 (1970).

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13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

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