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Recent results in relative conductometric elemental microanalysis
MICROCHEMICAL
JOURNAL
Recent
10,
286-300
Results
(1966)
in Relative
Elemental
Microanalysis1
E. PELL, L. MACHHERNDL, Znstitute for Analytical
Conductometric
AND
H. MALISSA
Chemistry and Microchemistry, Vienna, Austria
Technical
University,
Received July 19, 1965
Our research on the automation of the elemental microanalysis differs in two significant points from the conventional method of Fritz Pregl, designed in the twenties of our century: (1) the combustion is carried out in an empty tube of refractory porcelain with one open end, at temperatures of 1300°C and above; (2) the final determination of gaseous combustion products is carried out in the relative conductometric way. The basic principles of relative conductometry were developed in 1957 (a), when the working method, originally used in the fields of iron and steel industry, was converted for use in organic elemental microanalysis. In 1960 H. Malissa (9) published some papers that dealt with the separate determination of carbon and sulfur, and showed that it was possible to determine simultaneously the elements of carbon, hydrogen, and sulfur. At about the same time, W. S.tuck (l-5)) who used a similar method, made an elaborate report containing data on the determination of carbon only. It was also in 1960 that S. Greenfield (3) described another apparatus for the determination of carbon; his apparatus was constructed differently than that used by Malissa or Stuck. The theoretical background of the relative conductometric method was dealt with in a paper of W. Schmidts and W. Bartscher (14) published in 1961. The determination of oxygen was carried out by F. Salzer (12) in 1962, and in the same year S. Greenfield (4) adapted his apparatus to determine hydrogen. The determination of sulfur was perfected by E. Pell, L. Machherndl and H. Malissa (11) in 1963, and in 1964 F. Salzer (13) completed his work on the combined determination of carbon (relative conductometric method) and hydrogen (an ampero1 Paper presented at the International Symposium 1965, held at The Pennsylvania State University, U.S.A., August 22-27, 1965. 286
on Microchemical University Park,
TechniquesPennsylvania,
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
287
metric method). H. Malissa and his co-workers succeeded in developing the relative conductometric determination into a useful method. Papers that contain only an application of one of the mentioned methods are not reported in this review. THEORETICAL
CONSIDERATIONS
As mentioned in the introduction, the method of elemental microanalysis using relative conductometry may be treated in two main sections. The first section contains the pyrolysis of the organic sample, for which two basic ways are possible: the so-called empty tube combustion where the sample is treated with oxygen only, and methods using tubes packed with various catalysts or oxygen donators, etc. The question of which of the two ways is the better can neither be answered easily nor unambiguously, and it is not surprising that the microanalysts, concerned with their problems, think themselves to be divided into two opposing groups. Our team follows the “empty tube line,” using a method which was suggested as early as 1911 by I. Marek (10). This idea was soon forgotten until the group of R. Belcher and C. E. Spooner (1) took up work in this direction in 1943 and proved its usefulness. A review about the work accomplished in this field until 1959 was given by J. E. Fildes (2). H. Malissa reported in 1956 and 1957 (8) about a modification of the empty tube method that involved a combustion tube with one end open, which permitted a simple insertion of the sample boat. Combustion occurs in a dynamic system at high flow rates using suction pumps after the furnace. With regard to this, the apparatus designed by G. Ingram (5)) who uses a stationary combustion system consisting of a quartz tube with a combustion section of enlarged diameter, should be mentioned. The theoretical background for the correct judgement of the combustion process in the empty tube was supplied by the detailed research work of G. Kainz and co-workers (6, 7). The results of their investigations show how the yield of sample oxidation products depends on the proportions between sample and oxygen, the temperature, and the duration of the combustion components in the heated zone. The best proportions of sample versus oxygen are controlled by three different factors: (1) The large diameter of the combustion tube, influencing (a) the performance of the combustion chain reaction, as the period of time available for diffusion of oxygen into the gaseous combustion mixture is proportional to the tube diameter; (b) the proportion of the mixture, as the amount of
288
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
oxygen available per time unit will also increase with tube diameter; and (c) the duration, which will also increase with increased diameter. (2) The oxygen flow rate will be great in respect to the speed of sample volatilization. The amount of oxygen supplied per time unit must be at least equivalent to the amount of volatilized sample. (3) By reduction of the volatilization rate, which also permits smaller oxygen flow rates. The time of duration in the heated zone depends, as maintained above in points 1 and 2, on tube diameter, oxygen flow rate, and the length of the zone. The calculation of the time of duration t of a given volume of gas in a section of a gas tube is given by the formula t=
V v (Tz/TI~’
where V is the volume between the area of gasification and the end of the heated zone, expressed by V = r%l; v is the oxygen flow rate in milliliters per minute; and Tz and T1 are the absolute temperatures in the hot zone and in the environment, respectively. An application of these formulas to the conditions as they have already been used for several years in our work shows that the requirements for optimal conditions are met in general. We use tubes of 20-mm diameter and apply flow rates of 54-230 ml per minute. The rate of sample volatilization can be varied to some extent by varying the speed of sample insertion. Our working temperature is at least 12OO”C, a fact that will influence the yield of oxidation under any circumstances in the desired direction. All these facts back our assumption that our way of combustion in the empty tube is scientifically well founded. In addition to this, it completely avoids all the disadvantages connected with the use of catalysts, simplifies work, and reduces costs considerably since porcelain tubes are used. A few remarks about the procedure of combustion used in our laboratory may be added, as this factor also influences the yield of products of combustion considerably. Three different types of sample insertion rods for microplatinum crucibles and boats have been tested and compared (11) (Fig. 1). The first model of this insertion rod was only suitable for crucibles. The oxygen flow passing over the crucible mouth will carry sample vapors and combustion products, which are blown out of the crucible against the wall
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
289
of the combustion tube. They will be mainly transported in the flow direction. The second model is suitable for platinum boats. Oxygen and sample vapors are mixed by a countercurrent movement, resulting in a double diversion of the oxygen flow. The third model, also designed for boats, delivers the sample vapors in the same direction as the oxygen flow. Introduction of sample vapor into the hot oxygen gas is therefore slow, permanent excess of oxygen is guar-
02
/
/ Ebv
Cokidqe
of Ouartz
Thermocouple
FIG.
1.
Sample insertion
I Boot of Platinum
rod for combustion
in an empty tube.
anteed, and complete mixing is effected by diffusion and turbulence, so that the reaction is smooth and quantitative. Test series were run with several substances, and all three types of insertion rods were used in turn. Results showed superior results with model III, as it is expressed in the over-all standard deviations: Model I, Model II, Model III,
crucible t_ 0.86 ‘i6 c cartridge I * 0.44 % C cartridge II t 0.19 % C
These results are in agreement with those obtained by previous tests in connection with the determination of sulfur in organic compounds by the relative conductometric method ( 11) .
290
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
On the basis of the results of these tests it was decided to use only insertion rod model III for the determination of hydrogen. The second section of using relative conductometry is concerned with the final determination of combustion products, which is carried out in our work only by the relative conductometric method. Carbon is converted to carbon dioxide and absorbed in dilute sodium hydroxide; and hydrogen is burned to give water, which reacts with calcium carbide to give acetylene, which is again converted to carbon dioxide and absorbed as above. Sulfur is converted to sulfur dioxide and absorbed in a dilute sulfuric acidhydrogen peroxide solution. Oxygen is converted to carbon monoxide, which is likewise oxidized to carbon dioxide and absorbed as above. ~ Hz0 + CaCz + CaO + &Ha -+ 2C02 2H + SO2 -+ Ha0 L
H20+C~H2+CO-+COz+Hz0
S + 02 -+ SO2 -+ SO3 --f H,SO,
The determination of carbon was the first to be subjected to the relative conductometric method; since this method has been discussed in a considerable number of papers, it is only mentioned here for the completeness of this report (8, 9, 15). For the determination of hydrogen, there were three different possibilities: (a) Direct absorption of water in concentrated sulfuric acid, resulting in a linear change of conductivity in the narrow limit of 99.83-99.78s. This method was successfully applied by S. Greenfield and R. A. D. Smith (4). (b) Reaction with calcium carbide and combustion of the acetylene formed to sodium hydroxide. (c) Conversion of water to carbon monoxide by reaction with carbon, oxidation to carbon dioxide, and absorption in sodium hydroxide. For several reasons we decided to use the second method: the reaction of calcium carbide with water vapor is rapid and, if a suitable branded reagent is selected, free of side reactions. The reaction itself is very sensitive, as will be demonstrated later on. Furthermore, we appreciated the fact that a mixture of acetylene and carbon dioxide is easily separated by means
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
291
of soda asbestos, which absorbs the carbon dioxide formed by the sample combustion. Water and calcium carbide may react according to the following reactions: CaCs + HZ0 + CZH2 + CaO CaCs + 2H20 * CzHs + Ca(OH),!
AG,, = - 25.32 Kcal/mole AG,, = - 42.36 Kcal/mole
(1) (11)
If the reaction products are included in the system, we find also: CaO + HZ0 + Ca(OH)2 CaCs + Ca(OH)s + C2H2 + 2CaO
AG,, = AGZ, = -
17.04 Kcal/mole 8.23 Kcal/mole
(III) (IV)
Based on the calculated AG-values cited above, it should be expected that reaction II is preferred thermodynamically. This is not the case because reaction II is a combination of reactions I and III. Reaction I has to occur at first, forming C2HZ and CaO, and reaction III and therefore also II start only when the carbide surface approaches exhaustion by increasing the load. If a sufficient excess of carbide is provided in practical work it seems granted, at least for a considerable time, that reaction I occurs exclusively. The determination of sulfur by measuring the conductivity of solution containing dilute sulfuric acid and hydrogen peroxide in comparison to a reference resistor could be extended into the ultramicro range. Sensitivity was increased from 0.5 to 0.05 pg S per millimeter recorder deflection by reducing the concentration of the absorption solution and adjusting the reference resistor. This successful adaption may be applied also for carbon and hydrogen determinations. EXPERIMENTAL
The apparatus consists of the following parts (Fig. 2) : (1) The gas purification unit for delivery of absolutely pure and dry oxygen, consisting of a quartz precombustion tube, kept at 1300°C; soda asbestos tubes for the absorption of carbon dioxide; two freezing traps for the removal of water cooled with an acetone-dry-cold cooling mixture; and a Sicapent tube for final drying. (2) The sample combustion train, with two tubes of refractory porcelain of I8 mm inner diameter, in a furnace for a maximum temperature of 1400°C. Immediately after the oxygen stream has left the furnace a system of
292
E. PELL, L. MACHHERNDL, AND H. MALISSA
synchronized constant volume pumps simultaneously divides the combustion products into three parts. Each part is passed into its measuring unit, which contains the reference and the measuring conductivity cell. (3a) The first part of the combustion products is used for the determination of carbon converted to carbon dioxide. After being dried over calcium chloride, the gas is bubbled through 0.02 N sodium hydroxide. The
s
FIG. 2. Schematic representation of apparatus for carbon-hydrogen-sulfur determination by relative conductometry. 1, Gas purification; 2, combustion tube; 3, combustion furnace; 4, division of combustion gases; 5, CaC, tube; 6, soda asbestos tube; 7, furnace for combustion of C,H,; 8, external absorber; 9 and 13, pumping system; 10 and 14, measuring cell; 11 and 15, reference cell; 12 and 16, recorder.
formation of sodium carbonate results in a reduction of conductivity. The conductivity signals of fresh sodium hydroxide and loaded hydroxide carbonate solution are amplified and fed into a modified Wheatstone circuit, which finally controls the phase-sensitive recorder motor. The response of the recorder is linear to the content of carbon dioxide. As it can be assumed that the determination of carbon by relative conductometry is generally known already, it will not be discussed further.
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
293
(3b) The second part of the combustion gas is used for determination of hydrogen. The gas will contain in any case water and carbon dioxide; by passing it over high purity calcium carbide, water is converted into acetylene. Carbon dioxide is removed completely by sodium asbestos, and acetylene is oxidized over copper oxide at 550°C to carbon dioxide, which is now passed into the measuring cell as used in part 1. The problem and main difficulty in the determination of hydrogen using the carbide way almost exclusively consisted in the selection and pretreatment of the calcium carbide. The product used for the first experiments was of technical grade with an assay of only 68%. It consisted of the usual big lumps which were crushed into smaller ones, and only the central parts of each one were meshed and sieved, using a set of sieves of 2, 1, 0.5, and 0.2 mm grain size. The different mesh fractions were stored and tried in the reaction tubes. The results of the first trial series were not encouraging at all. The activity of the calcium carbide decreased rapidly when it was used for a longer period, and the response of recorded carbon was not linear to the imput of the sample. Similar unsatisfactory results were obtained with a number of different grades of calcium carbide of different provenance until we happened to obtain a sample produced by Cyanamide of Canada with an assay of 92.8r/c, which yielded immediately far better and promising results so that it was decided to carry on with the experiments. The use of high purity calcium carbide finally led to a positive result of our work. It seems that carbide of low grade does not react in accordance with reaction I, but reactions I and III occur side by side in nonconstant rates. Furthermore, it was found that the preparation of the calcium carbide reaction tube needs special care. The necessary operations like crushing, sieving, and filling were carried out in a glove box, which was rinsed several times with pure and dry nitrogen and kept under slight pressure during work to prevent contamination by atmospheric humidity, After the first promising results with the new brand of carbide, we started detailed research work to establish the optimal conditions. The purification and combustion train, as well as combustion temperature, were left unchanged, but the test series was performed under variations of oxygen flow rate, which is effected by replacement of the pump pistons; and under variation of the particle size and filling height in the calcium carbide reaction tubes and the shape of reaction tubes and concentration of
294
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
the absorption solution. The acetylene combustion train again was left unchanged. After trying about 300 different variations, the optimal combinations were elected (see Table 1). Optimum performance is reached by a combustion which results in the rather remarkable sensitivity of 0.2 ug H for a l-mm recorder deflection with a coefficient of variation of about 1%. Sensitivity is increased with decreasing flow rate of combustion products and decreasing concentration of absorption solution; however, total time for one determination will increase in this case. TABLE 1 SENSITIVITY OF DETERMINATION OF HYD~ROCENIN CONNECTION WITH PUMPING SPEEDAND CONCENTRATIONOF ABSORPTIONSOLUTION Concentration
of absorption
solution
O.OZN
0.005N
Pumping speed (ccm/min)
Average wz H/mm
Average M H/mm
39 74 273
0.836 0.801 4.012
0.223 0.225 1.021
The conditions finally adopted for further work were fixed as follows: (a) Pumping rate from sample combustion train, 100-150 ml per minute; flow rate for passing the carbide reaction vessels, SO-55 ml per minute. (b) Particle size of calcium carbide, 0.5-1.0 mm; filling height, 30 mm. (c) Concentration of absorption solution, 0.02 N sodium hydroxide. A large number of organic compounds were analyzed under these conditions for carbon and hydrogen. As it can be seen from Tables 2 and 3, for this purpose we selected such compounds as recommended by IUPAC, and furthermore we tried to include all the common hetero elements. The interference of hetero elements or their volatile combustion products was eliminated by adding a scavenging tube with a silver wool filling working at 550°C followed by an absorption tube charged with Perhydrit (hydrogen peroxide-urea compound). It is necessary to calibrate the method by treating a series of test samples. Some typical results are shown in Tables 2 and 3 together with the respective statistical treatment of the results. The over-all standard deviation for results obtained by the empty tube
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
295
method and the relative conductometric finish of combustion products was found to be % 0.19% for carbon and * 0.16% for hydrogen, with a working time of 10 minutes for one determination. Hydrogen
2466
Carbon
FIG. 3. tion.
Typical
recordings
of relative
conductometric
2go2
carbon-hydrogen
determina-
The simplicity of operation and maintenance is demonstrated by a description of the method: (1) Weigh sample in platinum boat. (2) Rinse measuring cells once with absorption solution or doubledistilled water. (3) Fill cells with a certain semiautomatically adjusted volume or fresh solution. (4) Switch on connection to recorders. (5) Insert sample into furnace. (6) Read recorder deflections. (7) Compute results from recorder deflections. (8) Break connection to recorders.
Anthracene Naphthalene Benzoic acid Glucose Cholesterol Melamine Acetanilide m-Dinitrobenzene Hippuric acid -
Organic compounds: 94.34 93.80 68.85 40.00 83.86 28.57 71.09 42.87 60.30
-
0 0 0 N ON ON ON
% Calcd.
Hetero elements:
RESULTS
2
94.20 93.40 68.83 39.98 83.69 28.47 70.99 42.59 60.07
% Found
C
0.19
0.05
0.22 0.22 0.15 0.19 0.22 0.10 0.19
&S (%) 5.66 6.20 4.96 6.71 11.99 4.80 6.71 2.40 5.06
% Calcd.
OF CARBON-HYDROGEN DETERMINATION RELATIVE CONDUCTIMETRIC FINISH
TABLE
H
5.60 6.46 4.93 6.71 11.88 4.74 6.53 2.19 4.82
% Found
USING
0.22 0.09 0.24 0.19 0.06
0.05
0.12
0.11
0.25
5s (%I
0.23 0.24 0.22 0.48 0.26 0.35 0.27 0.12 0.32
C
1.85 1.90 3.68 8.68 1.24
0.75
2.43
1.07 1.70
H
Coefficient of Variation (“/o )
K 5 E 9
i 9
8 g 3 -v
b
E
3
3
M
p-Fluorobenzoic acid Hexachlorobenzene 5,7-Dibrom-8oxyquinoline o-Iodobenzoic acid Triphenylphosphine Sulfonal Sulfanilic acid Thiourea Phenylthiourea
Organic compounds:
35.68 33.90 82.45 36.78 41.60 15.75 55.23
NBrO
P SO SON SN SN
36.23 33.72 82.62 36.49 41.50 14.98 55.31
60.25 25.59
% Found
%
C Calcd. 60.01 25.31
JO
TABLE
3
0.13 0.26 0.11 0.14 0.22 0.07 0.18
0.18 0.09
%
1.66 2.03 5.72 7.01 4.08 5.26 5.30
3.60
Calcd.
OF CARBON-HYDROGEN DETERMINATION RELATIVE CONDUCTIMETRIC FINISEI
FO Cl
Hetero elements:
RESULTS
%
H
1.83 1.99 5.40 6.77 4.07 4.77 5.20
3.57
Found
USING
0.10 0.07 0.12 0.18 0.15 0.11 0.17
0.13
&s (%)
0.36 0.77 0.13 0.38 0.53 0.47 0.33
0.30 0.35
C
5.46 3.52 2.22 2.66 3.68 2.31 3.27
3.64 -
H
Coefficient of variation (%)
298
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
(9) Remove platinum boat from furnace. (10) Drain solution from cells. For maintenance it is necessary to: (1) Replace the carbide tube after lo-20 determinations. (2) Refill the absorption solution after 100-200 determinations. (3) Replace fillings of various absorption tubes after about 1000 determinations. (3~) The third part of the combustion products will serve for the determination of sulfur. The sulfur dioxide formed by the combustion is passed into a measuring cell containing 0.002 N sulfuric acid together with some hydrogen peroxide. In this case absorption results in an increase of conductivity that is recorded by comparison of voltage drops in the cell and on a reference resistor. The signal is again amplified and recorded as usual. In addition to our paper on the determination of sulfur, published in 1963 (II), I want to discuss the extension of this method into the ultramicro range. The necessary adjustment of the apparatus was simply effected by reducing the concentration of the absorption solution for one order of ten and adjusting the reference resistor according to the new requirements. The main trouble now arising consisted of a very noticeable thermal sensitivity and the increased influence of adsorption phenomena on the wall of the glass between tubing furnace and measuring cell. These two facts required a long period of detailed trouble shooting. Thermal influences were eliminated by coating the whole train with thick layers of foam rubber pads, and the adsorption was reduced by avoiding any unnecessary sharp bend or reduction of diameter in the connecting tubing, and making this connecting tube as short as possible. As already mentioned, the ultramicro apparatus brought an increase of sensitivity from 0.5 to 0.05 pg sulfur per millimeter recorder deflection. The weighing of the samples was effected on two different types of ultramicro balance: Mettler UM 7 and Cahn electro-balance No. 1.510. The determination of carbon and of hydrogen may be adjusted for ultramicro work. Investigations in this direction are underway, and we hope to report some results in due time. SUMMARY The relative conductometricmethod was further developedwith special attention to the determination of hydrogen in an attempt to contribute to the future automa-
RELATIVE
CONDUCTOMETRIC
tion of elemental microanalysis. sample are eliminated.
ELEMENTAL
All weighing
MICROANALYSIS
processes except the weighing
299 of the
Sample combustion was simplified by the introduction of the empty tube with one open end, which offers the following advantages: (a) application of high temperatures is limited only by the properties of the furnace and the combustion tube; (b) the expensive quartz tubes are replaced by the cheaper porcelain tubes; (c) the increase of oxygen flow rate in connection with appropriate diameter and length of tube results of a reduction of combustion time to a few seconds; (d) all possible types of final determinations could be used; and (e) the introduction of the proportional division of the combustion products enables the simultaneous determination of several elements. The application of the relative conductometric methods provides basic facts for preliminary research in the automation of elemental microanalysis, as it is possible to observe the start, the performance, and the finish of a chemical reaction without delay, which makes it possible to study the combustion process itself. The automatic recording of results is guaranteed by the high standard of electronic data handling. The presence of a number of hetero elements and their influence on the carbon and hydrogen determination was taken into account during this work. It could be shown that nitrogen, fluorine, chlorine, bromine, iodine, phosphorus, and sulfur have no influence, as they are eliminated quantitatively by an external absorber. The introductive work for the automation of the elemental microanalysis has come to a temporary end by carrying out test series on a number of selected test compounds and the calculation of standard deviations of the results obtained, which proved that the proposed method gives results of equal quality compared with conventional methods. REFERENCES 1.
R., AND SOONER, C. E., A new technique for the ultimate microanalysis of organic compounds. J. Chenz. Sot. 313-316 (1943). 2. FILDES, J. E., Empty tube combustion methods for the microelementary analysis of organic compounds. Rev. Pure Appl. Chem. 9, 117-137 (1959). 3. GREENFIELD, S., A conductimetric micro method for determining carbon in organic compounds. Analyst 85, 486-492 (1960). 3. GREENFIELD, S., AND SMITH, R. A. D., A conductimetric micro method for determining hydrogen in organic compounds. Analyst 87, 875-879 (1962). 5. INGRAM, G., The combustion of organic compounds by ignition in oxygen: The determination of carbon and hydrogen. Analyst 86, 411-414 (1961). 6. KAINZ, G, AND SCHEIDL, F., fiber die quantitative Oxydation organischer Verbindungen im “leeren Rohr.” II. Mitt. Der Einfluss der Versuchsvariablen. Mikrochim. Acta 1963, 902-910. 7. KAINZ, G., AND HORWATITSCH, H., Zur Kenntnis der Vorglnge bei der C-HAnalyse. tiber die Verbrennung in leeren Rohr. Z. Anal. Chem. 184, 363-370 (1961). 8. MALISSA, H., Beitrag zur raschen Bestimmung kleiner Kohlenstoffmengen in anorganischen und organischen Substanzen mit Hilfe der Leitfihigkeitsmessung. Mikrochim. Acta 1957. 553-562. BELCHER,
300 9.
E. PELL,
L. MACHHERNDL,
AND
H.
MALISSA
MALISSA, H., BeitrLge gur Mikroelementaranalyse. Mikrochim. Acta 1960, 127144. 10. MAREX, I., Organische Verbrennungsanalyse ohne Verwendung eines Sauerstoffiibertragers. J. Prakt. Chem. 84, 713-731 (1911). 11. PELL, E., MACHHERNDL, L., AND MALISSA, H., Die relativkonduktometrische Mikrobestimmung von Schwefel in organischen Substanzen. Mikrochim. Acta 1963, 615-627. 1.2. SALZER, F., Zur mikroanalytischen Sauerstoffbestimmung in organischen Substanzen mit relativkonduktometrischer Endpunktsanzeige. Mikrochim. Acta 1982, 835-867. 13. SALZER, F., Ein Schnellverbrennungsapparat mit elektrischer Endpunktsanzeige zur Bestimmung von Kohlenstoff und Wasserstoff in organ&hen Substanzen. Z. And. Chem. %5, 66-80 (1964). 14. SCHMIDTS, W., AND BARTSCHER, W., Grundlagen der konduktometrischen Kohlenstoffbestimmung. Z. Anal. Chem. 181, 54-59 (1961). 1.5’. STUCK, W., Eine Mikromethode zur schnellen Bestimmung von Kohlenstoff in organischen Substanzen. Mikrochim. Acta 1980, 421-428.
JOURNAL
Recent
10,
286-300
Results
(1966)
in Relative
Elemental
Microanalysis1
E. PELL, L. MACHHERNDL, Znstitute for Analytical
Conductometric
AND
H. MALISSA
Chemistry and Microchemistry, Vienna, Austria
Technical
University,
Received July 19, 1965
Our research on the automation of the elemental microanalysis differs in two significant points from the conventional method of Fritz Pregl, designed in the twenties of our century: (1) the combustion is carried out in an empty tube of refractory porcelain with one open end, at temperatures of 1300°C and above; (2) the final determination of gaseous combustion products is carried out in the relative conductometric way. The basic principles of relative conductometry were developed in 1957 (a), when the working method, originally used in the fields of iron and steel industry, was converted for use in organic elemental microanalysis. In 1960 H. Malissa (9) published some papers that dealt with the separate determination of carbon and sulfur, and showed that it was possible to determine simultaneously the elements of carbon, hydrogen, and sulfur. At about the same time, W. S.tuck (l-5)) who used a similar method, made an elaborate report containing data on the determination of carbon only. It was also in 1960 that S. Greenfield (3) described another apparatus for the determination of carbon; his apparatus was constructed differently than that used by Malissa or Stuck. The theoretical background of the relative conductometric method was dealt with in a paper of W. Schmidts and W. Bartscher (14) published in 1961. The determination of oxygen was carried out by F. Salzer (12) in 1962, and in the same year S. Greenfield (4) adapted his apparatus to determine hydrogen. The determination of sulfur was perfected by E. Pell, L. Machherndl and H. Malissa (11) in 1963, and in 1964 F. Salzer (13) completed his work on the combined determination of carbon (relative conductometric method) and hydrogen (an ampero1 Paper presented at the International Symposium 1965, held at The Pennsylvania State University, U.S.A., August 22-27, 1965. 286
on Microchemical University Park,
TechniquesPennsylvania,
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
287
metric method). H. Malissa and his co-workers succeeded in developing the relative conductometric determination into a useful method. Papers that contain only an application of one of the mentioned methods are not reported in this review. THEORETICAL
CONSIDERATIONS
As mentioned in the introduction, the method of elemental microanalysis using relative conductometry may be treated in two main sections. The first section contains the pyrolysis of the organic sample, for which two basic ways are possible: the so-called empty tube combustion where the sample is treated with oxygen only, and methods using tubes packed with various catalysts or oxygen donators, etc. The question of which of the two ways is the better can neither be answered easily nor unambiguously, and it is not surprising that the microanalysts, concerned with their problems, think themselves to be divided into two opposing groups. Our team follows the “empty tube line,” using a method which was suggested as early as 1911 by I. Marek (10). This idea was soon forgotten until the group of R. Belcher and C. E. Spooner (1) took up work in this direction in 1943 and proved its usefulness. A review about the work accomplished in this field until 1959 was given by J. E. Fildes (2). H. Malissa reported in 1956 and 1957 (8) about a modification of the empty tube method that involved a combustion tube with one end open, which permitted a simple insertion of the sample boat. Combustion occurs in a dynamic system at high flow rates using suction pumps after the furnace. With regard to this, the apparatus designed by G. Ingram (5)) who uses a stationary combustion system consisting of a quartz tube with a combustion section of enlarged diameter, should be mentioned. The theoretical background for the correct judgement of the combustion process in the empty tube was supplied by the detailed research work of G. Kainz and co-workers (6, 7). The results of their investigations show how the yield of sample oxidation products depends on the proportions between sample and oxygen, the temperature, and the duration of the combustion components in the heated zone. The best proportions of sample versus oxygen are controlled by three different factors: (1) The large diameter of the combustion tube, influencing (a) the performance of the combustion chain reaction, as the period of time available for diffusion of oxygen into the gaseous combustion mixture is proportional to the tube diameter; (b) the proportion of the mixture, as the amount of
288
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
oxygen available per time unit will also increase with tube diameter; and (c) the duration, which will also increase with increased diameter. (2) The oxygen flow rate will be great in respect to the speed of sample volatilization. The amount of oxygen supplied per time unit must be at least equivalent to the amount of volatilized sample. (3) By reduction of the volatilization rate, which also permits smaller oxygen flow rates. The time of duration in the heated zone depends, as maintained above in points 1 and 2, on tube diameter, oxygen flow rate, and the length of the zone. The calculation of the time of duration t of a given volume of gas in a section of a gas tube is given by the formula t=
V v (Tz/TI~’
where V is the volume between the area of gasification and the end of the heated zone, expressed by V = r%l; v is the oxygen flow rate in milliliters per minute; and Tz and T1 are the absolute temperatures in the hot zone and in the environment, respectively. An application of these formulas to the conditions as they have already been used for several years in our work shows that the requirements for optimal conditions are met in general. We use tubes of 20-mm diameter and apply flow rates of 54-230 ml per minute. The rate of sample volatilization can be varied to some extent by varying the speed of sample insertion. Our working temperature is at least 12OO”C, a fact that will influence the yield of oxidation under any circumstances in the desired direction. All these facts back our assumption that our way of combustion in the empty tube is scientifically well founded. In addition to this, it completely avoids all the disadvantages connected with the use of catalysts, simplifies work, and reduces costs considerably since porcelain tubes are used. A few remarks about the procedure of combustion used in our laboratory may be added, as this factor also influences the yield of products of combustion considerably. Three different types of sample insertion rods for microplatinum crucibles and boats have been tested and compared (11) (Fig. 1). The first model of this insertion rod was only suitable for crucibles. The oxygen flow passing over the crucible mouth will carry sample vapors and combustion products, which are blown out of the crucible against the wall
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
289
of the combustion tube. They will be mainly transported in the flow direction. The second model is suitable for platinum boats. Oxygen and sample vapors are mixed by a countercurrent movement, resulting in a double diversion of the oxygen flow. The third model, also designed for boats, delivers the sample vapors in the same direction as the oxygen flow. Introduction of sample vapor into the hot oxygen gas is therefore slow, permanent excess of oxygen is guar-
02
/
/ Ebv
Cokidqe
of Ouartz
Thermocouple
FIG.
1.
Sample insertion
I Boot of Platinum
rod for combustion
in an empty tube.
anteed, and complete mixing is effected by diffusion and turbulence, so that the reaction is smooth and quantitative. Test series were run with several substances, and all three types of insertion rods were used in turn. Results showed superior results with model III, as it is expressed in the over-all standard deviations: Model I, Model II, Model III,
crucible t_ 0.86 ‘i6 c cartridge I * 0.44 % C cartridge II t 0.19 % C
These results are in agreement with those obtained by previous tests in connection with the determination of sulfur in organic compounds by the relative conductometric method ( 11) .
290
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
On the basis of the results of these tests it was decided to use only insertion rod model III for the determination of hydrogen. The second section of using relative conductometry is concerned with the final determination of combustion products, which is carried out in our work only by the relative conductometric method. Carbon is converted to carbon dioxide and absorbed in dilute sodium hydroxide; and hydrogen is burned to give water, which reacts with calcium carbide to give acetylene, which is again converted to carbon dioxide and absorbed as above. Sulfur is converted to sulfur dioxide and absorbed in a dilute sulfuric acidhydrogen peroxide solution. Oxygen is converted to carbon monoxide, which is likewise oxidized to carbon dioxide and absorbed as above. ~ Hz0 + CaCz + CaO + &Ha -+ 2C02 2H + SO2 -+ Ha0 L
H20+C~H2+CO-+COz+Hz0
S + 02 -+ SO2 -+ SO3 --f H,SO,
The determination of carbon was the first to be subjected to the relative conductometric method; since this method has been discussed in a considerable number of papers, it is only mentioned here for the completeness of this report (8, 9, 15). For the determination of hydrogen, there were three different possibilities: (a) Direct absorption of water in concentrated sulfuric acid, resulting in a linear change of conductivity in the narrow limit of 99.83-99.78s. This method was successfully applied by S. Greenfield and R. A. D. Smith (4). (b) Reaction with calcium carbide and combustion of the acetylene formed to sodium hydroxide. (c) Conversion of water to carbon monoxide by reaction with carbon, oxidation to carbon dioxide, and absorption in sodium hydroxide. For several reasons we decided to use the second method: the reaction of calcium carbide with water vapor is rapid and, if a suitable branded reagent is selected, free of side reactions. The reaction itself is very sensitive, as will be demonstrated later on. Furthermore, we appreciated the fact that a mixture of acetylene and carbon dioxide is easily separated by means
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
291
of soda asbestos, which absorbs the carbon dioxide formed by the sample combustion. Water and calcium carbide may react according to the following reactions: CaCs + HZ0 + CZH2 + CaO CaCs + 2H20 * CzHs + Ca(OH),!
AG,, = - 25.32 Kcal/mole AG,, = - 42.36 Kcal/mole
(1) (11)
If the reaction products are included in the system, we find also: CaO + HZ0 + Ca(OH)2 CaCs + Ca(OH)s + C2H2 + 2CaO
AG,, = AGZ, = -
17.04 Kcal/mole 8.23 Kcal/mole
(III) (IV)
Based on the calculated AG-values cited above, it should be expected that reaction II is preferred thermodynamically. This is not the case because reaction II is a combination of reactions I and III. Reaction I has to occur at first, forming C2HZ and CaO, and reaction III and therefore also II start only when the carbide surface approaches exhaustion by increasing the load. If a sufficient excess of carbide is provided in practical work it seems granted, at least for a considerable time, that reaction I occurs exclusively. The determination of sulfur by measuring the conductivity of solution containing dilute sulfuric acid and hydrogen peroxide in comparison to a reference resistor could be extended into the ultramicro range. Sensitivity was increased from 0.5 to 0.05 pg S per millimeter recorder deflection by reducing the concentration of the absorption solution and adjusting the reference resistor. This successful adaption may be applied also for carbon and hydrogen determinations. EXPERIMENTAL
The apparatus consists of the following parts (Fig. 2) : (1) The gas purification unit for delivery of absolutely pure and dry oxygen, consisting of a quartz precombustion tube, kept at 1300°C; soda asbestos tubes for the absorption of carbon dioxide; two freezing traps for the removal of water cooled with an acetone-dry-cold cooling mixture; and a Sicapent tube for final drying. (2) The sample combustion train, with two tubes of refractory porcelain of I8 mm inner diameter, in a furnace for a maximum temperature of 1400°C. Immediately after the oxygen stream has left the furnace a system of
292
E. PELL, L. MACHHERNDL, AND H. MALISSA
synchronized constant volume pumps simultaneously divides the combustion products into three parts. Each part is passed into its measuring unit, which contains the reference and the measuring conductivity cell. (3a) The first part of the combustion products is used for the determination of carbon converted to carbon dioxide. After being dried over calcium chloride, the gas is bubbled through 0.02 N sodium hydroxide. The
s
FIG. 2. Schematic representation of apparatus for carbon-hydrogen-sulfur determination by relative conductometry. 1, Gas purification; 2, combustion tube; 3, combustion furnace; 4, division of combustion gases; 5, CaC, tube; 6, soda asbestos tube; 7, furnace for combustion of C,H,; 8, external absorber; 9 and 13, pumping system; 10 and 14, measuring cell; 11 and 15, reference cell; 12 and 16, recorder.
formation of sodium carbonate results in a reduction of conductivity. The conductivity signals of fresh sodium hydroxide and loaded hydroxide carbonate solution are amplified and fed into a modified Wheatstone circuit, which finally controls the phase-sensitive recorder motor. The response of the recorder is linear to the content of carbon dioxide. As it can be assumed that the determination of carbon by relative conductometry is generally known already, it will not be discussed further.
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
293
(3b) The second part of the combustion gas is used for determination of hydrogen. The gas will contain in any case water and carbon dioxide; by passing it over high purity calcium carbide, water is converted into acetylene. Carbon dioxide is removed completely by sodium asbestos, and acetylene is oxidized over copper oxide at 550°C to carbon dioxide, which is now passed into the measuring cell as used in part 1. The problem and main difficulty in the determination of hydrogen using the carbide way almost exclusively consisted in the selection and pretreatment of the calcium carbide. The product used for the first experiments was of technical grade with an assay of only 68%. It consisted of the usual big lumps which were crushed into smaller ones, and only the central parts of each one were meshed and sieved, using a set of sieves of 2, 1, 0.5, and 0.2 mm grain size. The different mesh fractions were stored and tried in the reaction tubes. The results of the first trial series were not encouraging at all. The activity of the calcium carbide decreased rapidly when it was used for a longer period, and the response of recorded carbon was not linear to the imput of the sample. Similar unsatisfactory results were obtained with a number of different grades of calcium carbide of different provenance until we happened to obtain a sample produced by Cyanamide of Canada with an assay of 92.8r/c, which yielded immediately far better and promising results so that it was decided to carry on with the experiments. The use of high purity calcium carbide finally led to a positive result of our work. It seems that carbide of low grade does not react in accordance with reaction I, but reactions I and III occur side by side in nonconstant rates. Furthermore, it was found that the preparation of the calcium carbide reaction tube needs special care. The necessary operations like crushing, sieving, and filling were carried out in a glove box, which was rinsed several times with pure and dry nitrogen and kept under slight pressure during work to prevent contamination by atmospheric humidity, After the first promising results with the new brand of carbide, we started detailed research work to establish the optimal conditions. The purification and combustion train, as well as combustion temperature, were left unchanged, but the test series was performed under variations of oxygen flow rate, which is effected by replacement of the pump pistons; and under variation of the particle size and filling height in the calcium carbide reaction tubes and the shape of reaction tubes and concentration of
294
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
the absorption solution. The acetylene combustion train again was left unchanged. After trying about 300 different variations, the optimal combinations were elected (see Table 1). Optimum performance is reached by a combustion which results in the rather remarkable sensitivity of 0.2 ug H for a l-mm recorder deflection with a coefficient of variation of about 1%. Sensitivity is increased with decreasing flow rate of combustion products and decreasing concentration of absorption solution; however, total time for one determination will increase in this case. TABLE 1 SENSITIVITY OF DETERMINATION OF HYD~ROCENIN CONNECTION WITH PUMPING SPEEDAND CONCENTRATIONOF ABSORPTIONSOLUTION Concentration
of absorption
solution
O.OZN
0.005N
Pumping speed (ccm/min)
Average wz H/mm
Average M H/mm
39 74 273
0.836 0.801 4.012
0.223 0.225 1.021
The conditions finally adopted for further work were fixed as follows: (a) Pumping rate from sample combustion train, 100-150 ml per minute; flow rate for passing the carbide reaction vessels, SO-55 ml per minute. (b) Particle size of calcium carbide, 0.5-1.0 mm; filling height, 30 mm. (c) Concentration of absorption solution, 0.02 N sodium hydroxide. A large number of organic compounds were analyzed under these conditions for carbon and hydrogen. As it can be seen from Tables 2 and 3, for this purpose we selected such compounds as recommended by IUPAC, and furthermore we tried to include all the common hetero elements. The interference of hetero elements or their volatile combustion products was eliminated by adding a scavenging tube with a silver wool filling working at 550°C followed by an absorption tube charged with Perhydrit (hydrogen peroxide-urea compound). It is necessary to calibrate the method by treating a series of test samples. Some typical results are shown in Tables 2 and 3 together with the respective statistical treatment of the results. The over-all standard deviation for results obtained by the empty tube
RELATIVE
CONDUCTOMETRIC
ELEMENTAL
MICROANALYSIS
295
method and the relative conductometric finish of combustion products was found to be % 0.19% for carbon and * 0.16% for hydrogen, with a working time of 10 minutes for one determination. Hydrogen
2466
Carbon
FIG. 3. tion.
Typical
recordings
of relative
conductometric
2go2
carbon-hydrogen
determina-
The simplicity of operation and maintenance is demonstrated by a description of the method: (1) Weigh sample in platinum boat. (2) Rinse measuring cells once with absorption solution or doubledistilled water. (3) Fill cells with a certain semiautomatically adjusted volume or fresh solution. (4) Switch on connection to recorders. (5) Insert sample into furnace. (6) Read recorder deflections. (7) Compute results from recorder deflections. (8) Break connection to recorders.
Anthracene Naphthalene Benzoic acid Glucose Cholesterol Melamine Acetanilide m-Dinitrobenzene Hippuric acid -
Organic compounds: 94.34 93.80 68.85 40.00 83.86 28.57 71.09 42.87 60.30
-
0 0 0 N ON ON ON
% Calcd.
Hetero elements:
RESULTS
2
94.20 93.40 68.83 39.98 83.69 28.47 70.99 42.59 60.07
% Found
C
0.19
0.05
0.22 0.22 0.15 0.19 0.22 0.10 0.19
&S (%) 5.66 6.20 4.96 6.71 11.99 4.80 6.71 2.40 5.06
% Calcd.
OF CARBON-HYDROGEN DETERMINATION RELATIVE CONDUCTIMETRIC FINISH
TABLE
H
5.60 6.46 4.93 6.71 11.88 4.74 6.53 2.19 4.82
% Found
USING
0.22 0.09 0.24 0.19 0.06
0.05
0.12
0.11
0.25
5s (%I
0.23 0.24 0.22 0.48 0.26 0.35 0.27 0.12 0.32
C
1.85 1.90 3.68 8.68 1.24
0.75
2.43
1.07 1.70
H
Coefficient of Variation (“/o )
K 5 E 9
i 9
8 g 3 -v
b
E
3
3
M
p-Fluorobenzoic acid Hexachlorobenzene 5,7-Dibrom-8oxyquinoline o-Iodobenzoic acid Triphenylphosphine Sulfonal Sulfanilic acid Thiourea Phenylthiourea
Organic compounds:
35.68 33.90 82.45 36.78 41.60 15.75 55.23
NBrO
P SO SON SN SN
36.23 33.72 82.62 36.49 41.50 14.98 55.31
60.25 25.59
% Found
%
C Calcd. 60.01 25.31
JO
TABLE
3
0.13 0.26 0.11 0.14 0.22 0.07 0.18
0.18 0.09
%
1.66 2.03 5.72 7.01 4.08 5.26 5.30
3.60
Calcd.
OF CARBON-HYDROGEN DETERMINATION RELATIVE CONDUCTIMETRIC FINISEI
FO Cl
Hetero elements:
RESULTS
%
H
1.83 1.99 5.40 6.77 4.07 4.77 5.20
3.57
Found
USING
0.10 0.07 0.12 0.18 0.15 0.11 0.17
0.13
&s (%)
0.36 0.77 0.13 0.38 0.53 0.47 0.33
0.30 0.35
C
5.46 3.52 2.22 2.66 3.68 2.31 3.27
3.64 -
H
Coefficient of variation (%)
298
E. PELL,
L. MACHHERNDL,
AND
H. MALISSA
(9) Remove platinum boat from furnace. (10) Drain solution from cells. For maintenance it is necessary to: (1) Replace the carbide tube after lo-20 determinations. (2) Refill the absorption solution after 100-200 determinations. (3) Replace fillings of various absorption tubes after about 1000 determinations. (3~) The third part of the combustion products will serve for the determination of sulfur. The sulfur dioxide formed by the combustion is passed into a measuring cell containing 0.002 N sulfuric acid together with some hydrogen peroxide. In this case absorption results in an increase of conductivity that is recorded by comparison of voltage drops in the cell and on a reference resistor. The signal is again amplified and recorded as usual. In addition to our paper on the determination of sulfur, published in 1963 (II), I want to discuss the extension of this method into the ultramicro range. The necessary adjustment of the apparatus was simply effected by reducing the concentration of the absorption solution for one order of ten and adjusting the reference resistor according to the new requirements. The main trouble now arising consisted of a very noticeable thermal sensitivity and the increased influence of adsorption phenomena on the wall of the glass between tubing furnace and measuring cell. These two facts required a long period of detailed trouble shooting. Thermal influences were eliminated by coating the whole train with thick layers of foam rubber pads, and the adsorption was reduced by avoiding any unnecessary sharp bend or reduction of diameter in the connecting tubing, and making this connecting tube as short as possible. As already mentioned, the ultramicro apparatus brought an increase of sensitivity from 0.5 to 0.05 pg sulfur per millimeter recorder deflection. The weighing of the samples was effected on two different types of ultramicro balance: Mettler UM 7 and Cahn electro-balance No. 1.510. The determination of carbon and of hydrogen may be adjusted for ultramicro work. Investigations in this direction are underway, and we hope to report some results in due time. SUMMARY The relative conductometricmethod was further developedwith special attention to the determination of hydrogen in an attempt to contribute to the future automa-
RELATIVE
CONDUCTOMETRIC
tion of elemental microanalysis. sample are eliminated.
ELEMENTAL
All weighing
MICROANALYSIS
processes except the weighing
299 of the
Sample combustion was simplified by the introduction of the empty tube with one open end, which offers the following advantages: (a) application of high temperatures is limited only by the properties of the furnace and the combustion tube; (b) the expensive quartz tubes are replaced by the cheaper porcelain tubes; (c) the increase of oxygen flow rate in connection with appropriate diameter and length of tube results of a reduction of combustion time to a few seconds; (d) all possible types of final determinations could be used; and (e) the introduction of the proportional division of the combustion products enables the simultaneous determination of several elements. The application of the relative conductometric methods provides basic facts for preliminary research in the automation of elemental microanalysis, as it is possible to observe the start, the performance, and the finish of a chemical reaction without delay, which makes it possible to study the combustion process itself. The automatic recording of results is guaranteed by the high standard of electronic data handling. The presence of a number of hetero elements and their influence on the carbon and hydrogen determination was taken into account during this work. It could be shown that nitrogen, fluorine, chlorine, bromine, iodine, phosphorus, and sulfur have no influence, as they are eliminated quantitatively by an external absorber. The introductive work for the automation of the elemental microanalysis has come to a temporary end by carrying out test series on a number of selected test compounds and the calculation of standard deviations of the results obtained, which proved that the proposed method gives results of equal quality compared with conventional methods. REFERENCES 1.
R., AND SOONER, C. E., A new technique for the ultimate microanalysis of organic compounds. J. Chenz. Sot. 313-316 (1943). 2. FILDES, J. E., Empty tube combustion methods for the microelementary analysis of organic compounds. Rev. Pure Appl. Chem. 9, 117-137 (1959). 3. GREENFIELD, S., A conductimetric micro method for determining carbon in organic compounds. Analyst 85, 486-492 (1960). 3. GREENFIELD, S., AND SMITH, R. A. D., A conductimetric micro method for determining hydrogen in organic compounds. Analyst 87, 875-879 (1962). 5. INGRAM, G., The combustion of organic compounds by ignition in oxygen: The determination of carbon and hydrogen. Analyst 86, 411-414 (1961). 6. KAINZ, G, AND SCHEIDL, F., fiber die quantitative Oxydation organischer Verbindungen im “leeren Rohr.” II. Mitt. Der Einfluss der Versuchsvariablen. Mikrochim. Acta 1963, 902-910. 7. KAINZ, G., AND HORWATITSCH, H., Zur Kenntnis der Vorglnge bei der C-HAnalyse. tiber die Verbrennung in leeren Rohr. Z. Anal. Chem. 184, 363-370 (1961). 8. MALISSA, H., Beitrag zur raschen Bestimmung kleiner Kohlenstoffmengen in anorganischen und organischen Substanzen mit Hilfe der Leitfihigkeitsmessung. Mikrochim. Acta 1957. 553-562. BELCHER,
300 9.
E. PELL,
L. MACHHERNDL,
AND
H.
MALISSA
MALISSA, H., BeitrLge gur Mikroelementaranalyse. Mikrochim. Acta 1960, 127144. 10. MAREX, I., Organische Verbrennungsanalyse ohne Verwendung eines Sauerstoffiibertragers. J. Prakt. Chem. 84, 713-731 (1911). 11. PELL, E., MACHHERNDL, L., AND MALISSA, H., Die relativkonduktometrische Mikrobestimmung von Schwefel in organischen Substanzen. Mikrochim. Acta 1963, 615-627. 1.2. SALZER, F., Zur mikroanalytischen Sauerstoffbestimmung in organischen Substanzen mit relativkonduktometrischer Endpunktsanzeige. Mikrochim. Acta 1982, 835-867. 13. SALZER, F., Ein Schnellverbrennungsapparat mit elektrischer Endpunktsanzeige zur Bestimmung von Kohlenstoff und Wasserstoff in organ&hen Substanzen. Z. And. Chem. %5, 66-80 (1964). 14. SCHMIDTS, W., AND BARTSCHER, W., Grundlagen der konduktometrischen Kohlenstoffbestimmung. Z. Anal. Chem. 181, 54-59 (1961). 1.5’. STUCK, W., Eine Mikromethode zur schnellen Bestimmung von Kohlenstoff in organischen Substanzen. Mikrochim. Acta 1980, 421-428.