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Preliminary Operations in Quantitative Analysis
Chapter Five Preliminary Operations in Quantitative Analysis
INTRODUCTION Several basic laboratory operations are common to all quantitative analysis procedures. These are sampling, drying, weighing, and dissolving. Dissolving is the one operation not always necessary, since there are several instrumental methods in which the measurements are made directly on the sample. For the experienced analyst these operations are rather obvious and often are performed so routinely that little thought is given to them. In certain respects this is unfortunate since proper preparation for the measurement is as important as the measurement itself. In other words, the final measurement is no better than the preparation for the measurement.
SAMPLING The objective in obtaining a suitable sample is to take a portion that is representative of all the components and their amounts that are contained in the bulk sample. If the bulk sample is homogeneous, no problem is encountered in obtaining a laboratory size sample, regardless of whether it is solid, liquid, or gas. It is when the bulk sample is heterogeneous that special precautions must be taken in order to obtain a representative sample. For example, the iron content of a rod of iron alloy may vary widely through the length of the rod, on the surface, and in the center. If the rod was homogeneous, it wouldn't matter what section or part of the rod was selected as the sample, since the composition would be uniform throughout the rod. Unfortunately, blunders in sampling are made and much time and effort is wasted in trying to get good determinations using improper samples. 63
64
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
A bulk sample may be reduced to a laboratory size by a random choice or it may be reduced according to a statistically based plan, which theoretically gives every particle or portion of the substance an equal chance of appearing in the sample. In general, statistical sampling requires removal of portions from every section of the sample. These are subsequently combined, mixed, and resampled until a suitable laboratory size sample is obtained. The details of this general technique will differ according to the physical state of the substance being sampled. Sampling by random choice is difficult, as personal prejudices often exert an influence. Some deliberate thoughts about how to carry out the random choice are necessary before actually doing it. Nothing should be done to the sample which might inadvertently change its composition. Thus, crushing or grinding, labeling, and storage of the sample must be employed cautiously. Different techniques are employed for sampling gases, liquids, and solids. Heterogeneous mixtures such as emulsions, powders, suspensions, or aerosols require statistical handling. Only in homogeneous samples are the techniques simple and generally straightforward. Gases. There are three basic methods of collecting gases. These are expansion into an evacuated container, flushing, and displacement of a liquid. In all cases the collection vessels are of known volume, and the temperature and/or pressure within the vessel must be known. Usually, the collection vessels are made from glass (other inert material can be used), and they must be fitted with an inlet and an outlet both of which can be conveniently opened and closed. Contamination from previous samples can be a serious problem, and its elimination is affected by extensive flushing of the container with the gas to be sampled. The design of the sample device should easily permit this procedure. Air Sampling. Air is a complex mixture containing gases as well as particulate matter. Its actual composition is very dependent on the location and environment from which it is taken. Presently, because of pollution studies, much effort is directed toward monitoring the quality of air. In collecting an air sample and assuming that the determination of dissolved or suspended pollutants is what is being sought, two goals must be satisfied. 1. An accurate flow device must be used to measure the volume of air sample. Such a device must be calibrated as one would calibrate a piece of volumetric glassware, since these devices represent a measurement by volume. 2. A sample collector, such as a filter or an absorbing solution, must be used to trap the contaminants in the air. The actual efficiency of the collector
SAMPLING
65
must be determined experimentally with standards since few collectors of this type operate with 100% collection efficiency. Atmospheric sampling is much more difficult. Factors such as wind, temperature, or rain are variables which are difficult to overcome or control. The type of sampling method that is selected depends on the chemical and physical properties of the substances in the atmosphere that are being determined. In general, the atmospheric sample is passed through a series of fine filters or through a trapping solution. In the first case, which is excellent for isolating particulate matter, the filtering action is controlled by the porosity of the filtering device. In the latter, as the atmospheric sample is percolated through a column of solution, a chemical reaction traps the soughtfor components. Figure 5-1 illustrates several designs for collecting air samples. Liquids. Sampling pure or homogeneous liquids is straightforward and usually any sampling device which doesn't destroy the purity or homogeneity is an appropriate one. It is, however, a good idea to statistically sample even "pure" liquids. Heterogeneous liquid mixtures present a more difficult problem and the technique employed depends on whether the mixture is a suspension, an
Ί
.Sample air Sample m air
Calcium chloride-^;iVr<; drying tube
(a)
I
(c)
Sample air Direction of "airflow
\ ^ Sample collector
ιβΜΜΜΜΜΜΜΜΜΜΜβ γ
^ Airflow measuring device
Pump
(d)
F i g . 5 - 1 . Collection of air samples (a) by a trapping solution using a fritted absorber, (b) and (c) by a cold trap, and (d) by a general arrangement of absorber, flow-measuring device, and pump.
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PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
emulsion, a mixture of immiscible liquid phases, or a liquid containing solid residues. Additional complications arise if the liquid mixture is unstable (for example, an emulsion), contains volatile components, or contains dissolved gases. In general, aliquots are randomly withdrawn from various depths and from all locations in the liquid sample. These can be analyzed separately or combined to provide a composite sample statistically representative of the original sample. Solids. If a solid is homogeneous, any portion of it can be selected as being representatitive. For a heterogeneous solid a careful plan must be prepared which will statistically allow selection of all sections of the bulk solid. The sampling can be done by hand or by a mechanical sampling machine. The latter technique is useful if the bulk sample is a large mass. It is not always possible to statistically obtain a representatitive sample. Consider the difficult task of determining the composition of the moon's crust. Obviously, a statistical selection of the moon's surface was not possible. From the limited quantity of moon rocks and dust, the sampling was based partly on size and partly on physical state. The effect of particle size is introduced if a solid substance is being sampled, since composition of particles of different sizes can vary. In general, the conversion of a large sample to one suitable for analysis requires, first, reducing the sample to a uniform particle size and, second, reducing the mass of the sample. A uniform particle size is obtained by putting the sample through either crushers, pulverizers, mills, or mortars. Sieving can also be used. Whatever the procedure, it is necessary to ensure that contamination is not introduced into the sample by these operations. A very important part of the analysis of the moon rocks or of any material from beyond the earth is to establish whether or not there is organic matter present. A recent report on the analysis of a meteorite demonstrates the need for very careful handling of the samples to prevent organic contamination on earth. This is illustrated in the following example. Figure 5-2 shows a flow sheet describing the sampling technique employed on the meteorite (February 8, 1969, date of falling; February 15, 1969, date of sample collection; and March 1 and 10, 1969 date of duplicate analyses). The sample was egg shaped and weighed about 2.5 kg. A 250-g fragment was taken and portions of its surface and its interior used for analysis. The surface chips represented a depth of about 0.25 inch and included all of a fusion crust and a fresh surface break. This entire operation and all succeeding operations were carried out in a clean cabinet through which a filtered air stream was continuously passed. The flow sheet describes some of the details of the procedure. The actual data can be found
SAMPLING
67 250-g Fragment
40-g Surface chips Identical procedure carried out as with interior chips
200-g Interior chips 1/4 to 1 inch chips washed with MeOH rock crusher 120 Mesh powder CH3OH: benzene (1:3) (with I ultrasonic vibration)
Residue (For bound hydrocarbonfatty acid analysis)
"Organic solvent remove solvent by rotatory evaporator
|HF:HC1(1:1)
column chromatography
40% -40% Residue extract with benzene-MeOH
HF-HC1 solution extract with benzene
Combine these two extracts
n- Heptane fraction
GC-MS* analysis
Column chromatography as with initial organic extract
*GC-MS = Gas chromatography and mass spectrometry. tTLC = Thin layer chromatography.
Benzene fraction
GC-MS* analysis
MeOH fraction for fatty acid 1 analysis solvent \ removed BF3-MeOH Methyl esters TLCt T GC-MS analysis
Fig. 5-2. Flow sheet for analysis of meteorite sample. [From J. Han, B. R. Simoneit, A. L. Burlingame, and M. Calvin, Nature 222, 364 (1969).]
in the reference. From these data it was concluded that organic material found in the surface layer of this meteorite was of biological origin and could not be other than terrestrial contamination acquired during the short period of time before collection (February 8 to 15). Since the sample was contaminated it is doubtful that any conclusions can be drawn with respect to organic matter being originally present in the sample. Only correct sampling procedures will yield useful information. Regardless of how precise or accurate the chemical operations or measurements are, the collected data will not be better than the type of sample taken. Solid sampling can involve large masses such as a train-car load of coal or large amounts of small items such as pharmaceutical tablets. In quality control in the manufacture of pharmaceutical tablets a large number of tablets are randomly selected, weighed, and then pulverized into a powder. A weighed amount of the powder is taken for analysis and the results reported on a per tablet basis.
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PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
Clinical Samples. Clinical samples are most often whole blood, serum or plasma, cerebrospinal fluid, gastric juice, ascitic fluid in the abdominal cavity, pleural fluid in the chest cavity, synovial fluid in joints, draining wounds, urine, and tissue. Each of these samples presents its own unique problems with regard to the collection and handling of the sample. It is beyond the scope of this book to document these details. A factor that the analytical chemist or analyst encounters in clinical analysis more so than in industrial or enviromental analysis is that someone else is responsible for collecting the sample. The physician, nurse, or medical technician obtains the sample and the analyst must rely on their procedural ability to properly collect and handle the sample. For example, in collecting blood or other physiological samples the individual responsible for the collection must be aware of the following precautions. 1. The immobilization of the fluid flow in the individual should be minimized, since prolonged immobilization may alter the chemical values. 2. Samples of the fluid, in most cases, should not be taken while intravenous solutions are being administered. 3. Syringes must be scrupulously clean. 4. The container used for transporting the sample must be scrupulously clean. If anticoagulants are added the sample should be thoroughly mixed and this information provided to the analyst. 5. The amount of sample collected must be sufficient to allow the analyst to perform the determination. Different tests will require different amounts of sample, and consequently, the technician collecting the sample must be aware of this. 6. The analyst has no control over the diet, environment, etc. of the patient. The levels of the components of most clinical samples are an average, and the aforementioned factors have a significant effect on these levels. It is the responsibility of the individual collecting the sample to control these factors. 7. The analyst should be made aware of any special steps taken to preserve the sample, particularly if materials are added to the sample. Summary. Some analytical methods allow measurements to be made without destroying or changing the sample, while others consume the entire sample or require only trace amounts of sample material. It is also possible to analyze a surface by instrumental techniques. In this way the randomness of the entire surface of the sample can be determined. In summary, it is not possible to describe a set of general methods for sampling all substances under all conditions. However, the aim in all cases is statistical sampling, the use of chemical common sense, and cooperation between the analyst and the individual providing the sample. Many sources
DRYING
69
are available to aid the analyst in selecting the sampling procedure. These include general procedures,* clinical procedures,** and environmental procedures for airf and water. J DRYING Once a suitable sample is obtained, a decision must be reached on whether the analysis is to be made on the sample as received or after it has been dried. Most samples contain varying amounts of moisture, either because the sample is hygroscopic or because water is absorbed in the surface. Analysis on an "as-received basis'' treats water as part of the sample's composition and in handling the sample it is necessary that no gain or loss of water occur prior to analysis. Analysis on a dried basis means that the water has been removed. This is usually done by heating in an oven, a muffle furnace, or by Bunsen or Meeker burners. Techniques for this are shown in the chapter on laboratory techniques. Since heat is used for drying, it is possible to decompose the sample or to remove volatile components from the sample. In some cases a dried basis implies that this is true, while in others, perhaps the majority, only water removal is implied. Frequently, drying procedures are such that only a fixed amount of moisture is removed. An example of this is the removal of water to form a stable stoichiometric hydrate by controlled heating. Continued heating at an elevated temperature would be needed to remove the remaining water of hydration. Drying to a known hydrated state is an example of producing reproducible dryness, while the complete removal of water is the production of a state of absolute dryness. Regardless of which state is used, the handling of the dried sample must be rapid and its storage must be in the absence of water. In addition, the heated sample must be cooled, usually in a desiccator, before weighing. * P. D. La Fleur, Ed., "Accuracy in Trace Analysis: Sampling, Sample Handling, Analysis," Vols. 1-2, NBS Publication 422, U.S. Government Printing Office, Washington, D.C. (1976). I. M. Kolthoff, P. J. Elving, E. B. Sandell, "Treatise on Analytical Chemistry," Parts I to III, Wiley-Interscience, New York, 1963. ** J. S. Annino, "Clinical Chemistry: Principles and Procedures," third ed., Little Brown and Co., Boston, 1964. M. M. Wintrobe, "Clinical Hematology," Lea and Febiger, Philadelphia, 1961. t P. O. Warner, "Analysis of Air Pollutants," J. Wiley & Sons, New York, 1976. A. C. Stern, Ed., "Air Pollution," Academic Press, New York, 1968. t "Standard Methods for the Examination of Water and Waste Water," 13th ed., American Public Health Association, New York, 1970. W. Leithe, "The Analysis of Organic Pollutants in Water and Waste Water," Ann Arbor Science Publishers, Inc., Michigan, 1973.
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PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
WEIGHING Weighing of the sample is described in detail in the chapter on laboratory techniques. In ordinary analysis this measurement is one of the most accurate ones that can be made in the laboratory. Samples are usually weighed in triplicate and each of these is independently carried through the experimental procedure. An alternate technique is to weigh one sample and dissolve it in a known volume in a volumetric flask. Aliquots of this solution can then be taken for the quantitative determination. The weight of sample that is actually taken will depend on the concentration level of the substances in the sample that is being determined.
DISSOLVING Usually, after weighing the sample the next step is dissolution. If the sample is soluble in water the problem is solved, although occasionally the sample slowly hydrolyzes in water to produce insoluble hydrous oxides. Organic materials are usually dissolved in organic solvents or mixtures of organic solvents and water, if water alone is not a suitable solvent. Although the most direct and easily used solvent is water, there are a variety of chemical and instrumental procedures which require the use of a certain solvent composition. In other cases, the dissolving process is not even required. For example, in atomic emission, where the sample is excited by an arc or spark and the resulting radiant energy emitted is instrumentally analyzed, a solid or liquid sample can be used directly. There are general guidelines concerning the solubility of salts in water. These are briefly summarized in Table 5-1. Modern technology has resulted in a vast number of alloys and inorganic and organic mixtures. With such samples, the dissolution step is not a simple case of following the solubilities of salts. If the organic part of a mixture is to be analyzed, the solvents and techniques of organic chemistry must be used. For inorganic analysis, if water is not suitable, the sample must be dissolved in an acid or by fusion with a flux. Acid Treatment. In using acids several questions must be asked. Is a nonoxidizing or oxidizing acid needed? What kind of anion is desirable or most appropriate in solution? What are the chemical properties of the sample? Is it necessary to get rid of the excess acid? Knowledge of inorganic reactions and chemical common sense are needed to answer these questions. For example, H 2 S0 4 would not be used to dissolve a sample containing barium metal or ion, or HC1 would not be used to dissolve a silver metal or silver salt sample. In other situations acid treatment should
DISSOLVING Table 5-1.
71
General Guidelines for t h e Solubility of Inorganic Salts
Acetates are soluble, except a few sparingly soluble ones like AgC2H 3 02 and Hg 2 (C2H 3 02)2. Arsenates, borates, and carbonates are insoluble, except those of N H 4 + , Na + , and K + . The bicarbonates of these three and of Ba 2+ , Ca 2+ , Fe 2 + , Mg 2+ , Mn 2 + , and Sr 2+ are soluble. Many borates are soluble in NH 4 C1 solution, and all are soluble in hot dilute acids. Any salts of these ions with tervalent or quadrivalent metals are so completely hydrolyzed that they may be considered as not existing. Fluorides are insoluble, except those of Na + , K + , N H 4 + , Ag+, and Sn 2+ . F e F 3 is slightly soluble, but like many other metals, iron forms a very soluble complex ion with excess F~. Oxalates are insoluble, except those of Na + , K + , N H 4 + , and Fe 3 + . A number of oxalate complexes of other metals are soluble. Chromates are insoluble, except those of Na+, K+, NH 4 +, Ca 2+ , Mg 2 + , Zn 2+ , and Fe 3 + . Sulfides are insoluble, except Na 2 S, K 2 S, and (NH 4 ) 2 S. Slightly soluble sulfides are BaS, SrS, CaS, and MgS. Phosphates are insoluble, except those of N H 4 + , Na + , and K + . Phosphates of alkaline earth metals are soluble in acid but insoluble in neutral or alkaline solution. Chlorides and bromides of Ag + and Hg 2 2 + and iodides of Ag + , Hg 2 2 + , Hg 2 + , and P b 2 + are insoluble. PbCl 2 , PbBr 2 , and HgBr 2 are slightly soluble. Some basic halides are insoluble; for example, SbOCl. Thiocyanates are soluble, except Ag + , Pb 2 + , Hg 2 2 + , and Hg 2 + . Sulfates are soluble, except BaS0 4 , Hg 2 S0 4 , PbS0 4 , and SrS0 4 . Ag 2 S0 4 and C a S 0 4 are slightly soluble. Some basic sulfates are insoluble; for example, (BiO) 2 S0 4 . Sulfites are generally similar to carbonates. M g S 0 3 is slightly soluble. Nitrates are soluble, except a few basic salts like BiON0 3 . Nitrates do not exist for arsenic or tin (IV). Nitrites are soluble, except AgN0 2 , which is slightly soluble. Nitrites are very unstable to oxidation or reduction.
not be used because the sample may be volatilized. For example, acid treatment of carbonate or sulfide samples can result in the loss of C 0 2 or H 2 S, unless arrangements are made to trap the gases. But if the analysis is not concerned with the sulfide or carbonate content of the sample, then there is no difficulty. In some cases acid cannot be used because it causes parts of the sample to become passive and not take part in the reaction, usually because an oxide coating forms. It is difficult to describe acid conditions for dissolving inorganic samples since not all samples are simple pure metals, metal oxides, or alloys. For example, a high-temperature alloy might contain 13 different metals, including such elements as Nb, Ta, W, Zr, and rare earths. Sometimes, one technique is used to dissolve one part of the sample and another to dissolve the residue. As a general guide it is useful to classify the more common acid conditions according to whether they are oxidizing or nonoxidizing. The nonoxidizing acids are HC1, dilute H 2 S0 4 , and dilute HC104, while the oxidizing acids are HN0 3 , hot concentrated H 2 S0 4 , and hot concentrated HC10 4 . Dissolution of metals by the nonoxidizing acids is a process of hydrogen
72
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
replacement. Therefore, any scheme of relating the ability of metals to displace hydrogen can also be used to qualitatively predict solubility. The Table of Standard Reduction Potentials (see Appendix IV and Chapter 10) is such a scheme, and metals below hydrogen in the series will dissolve in a nonoxidizing acid. There are exceptions and these are usually due to a presence of a passive condition, oxide film formation, or insoluble salt formation. Briefly, HCl will dissolve the metals above hydrogen, salts of weak acids, and many oxides. Dilute H 2 S0 4 and HC10 4 are useful for metals above hydrogen, the difference being the solubility of the salts that are formed. Heat and concentrated H 2 S0 4 will often dissolve metals below hydrogen. However, the problem of solubility of sulfate salts should be considered. Nitric acid will dissolve metals below and above hydrogen since its oxidizing power will vary according to whether the acid is dilute or concentrated. In general, the metal is oxidized to its highest oxidation state. The principle limitations are that some metals (Al and Cr) become passive while others (Sn, Sb, and W) form insoluble acids. Sulfide salts and salts of oxidizable anions are also dissolved by HN0 3 . Perhaps the most powerful oxidizing conditions are obtained by using hot, concentrated HC10 4 which will dissolve all the common metals. It is often an advantage to use acids in combination. The most familiar mixture is aqua regia (1:3 HNO3-HCI). In essence, the features of both acids are included since the H N 0 3 furnishes oxidizing power, while HCl furnishes complexing properties and strong acidity. Solubility of many metal ions is maintained only in the presence of complexing agents. In fact, this is a useful technique for solubilizing an otherwise insoluble ionic salt. Sometimes the addition of bromine or hydrogen peroxide to mineral acids increases their solvent action. An added advantage of such combinations is the hastened oxidation (and destruction) of organic materials in the sample. Hydrofluoric acid is special in that it is a weak acid as well as a nonoxidizing acid, but yet it is still useful for solubilizing certain samples. Silicate samples, where silica is not to be analyzed are readily decomposed by HF, and the silica is volatilized as S1F4. Hydrofluoric acid is better than HCl in that it furnishes a good complexing anion, F~~. In certain cases, the complex is very difficult to decompose and it can, therefore, affect future chemical steps. Since HF can cause serious injury upon contact with the skin, a convenient way of creating HF conditions is to add NaF to an HCl solution of the sample. Perchloric acid when hot and concentrated is a potent oxidizer. If a dilute solution is boiled and the water is slowly evaporated, the oxidizing power increases, gradually reaching a maximum at 72% acid; at this point an azeotropic mixture is distilled. Other advantages of perchloric acid are that very soluble perchlorate salts are formed, it acts as a dehydrating agent, and the hot concentrated acid readily oxidizes organic materials. The latter technique, which is generally referred to as wet ashing and employs a HNO3-
DISSOLVING
73
HCIO4 mixture, requires careful handling since an improper procedure can result in a violent explosion. Nitric acid is added first, heated, cooled, and then the HNO3-HCIO4 mixture is added. Thus, the H N 0 3 acts as a moderating force by oxidizing the more reactive compounds at lower temperatures. Minerals and rocks, such as silicates, sulfides, phosphates, carbonates, sulfates, and very refractory minerals and oxides often require special treatment. The difficulties are enhanced if analysis of the anion is also desired as some of the more easily used acids can result in volatilization losses of this portion of the sample. Flux Treatment. The second general dissolving technique, fusion with a flux, is more potent than acid treatment for two reasons. First, since a flux is a fused salt media, the temperatures required for creating the condition (300° to 1000°C) are much higher than possible by the acid treatment. Second, there is a greater concentration of reagent in contact with the sample. The advantage of oxidizing or nonoxidizing conditions is not lost since there are fluxes which yield both of these conditions. The difficulties in using the fluxes are several. Special containers that will withstand the temperature as well as the reaction conditions are needed. Pt, Ag, Ni, Au, and Fe are some of the more common crucible materials. However, before choosing the appropriate crucible the reactivity of the flux must be known. Upon completion of the fusion and after cooling, the solid material is usually dissolved in water or dilute acid. The solution that is obtained has a very high salt content which often must be considered in the following chemical or instrumental steps. Other possible difficulties are impurities introduced by the flux, volatilization due to the high temperature, and spattering losses as a result of the reaction. Table 5-2 lists the common fluxes according to their oxidizing or nonoxidizing properties and also some of their applications. The fluxes can also be divided according to whether they yield acidic or basic conditions. Carbonates, hydroxides, and peroxides provide basic conditions, while boric oxide and pyrosulfates provide acidic conditions. Decomposition of Organic Matter. Decomposition of organic material can be done in several ways. The method that is finally chosen will depend on whether analysis of the organic matter is desired or whether only organic removal is desired. The wet ashing procedure, already mentioned, in which HCIO4-HNO3 is used, completely destroys organic matter. Some other chemical agents that can be used are metallic sodium in liquid ammonia, fuming nitric acid, and dry-ashing. Dry-ashing is the simplest to carry out. Usually, the sample is heated in an open crucible or dish in the presence of air or oxygen to red heat until all the carbonaceous material has been oxidized. The nature of
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
74
Table 5-2. Common Fluxes Flux
Crucible
Application Nonoxidizing
Na 2 C0 3 NaOH KOH B203 CaC03-NH4Cl
Pt Au, Ni, Ag Au, Ni, Ag Pt Ni
Silicates, phosphates, sulfates Silicates, silicon carbides Silicates, silicon, carbides Silicates, oxides Silicates (known as J. Lawrence Smith method for analysis of alkali metals) Oxidizing
Na 2 C0 3 mixed with KN0 3 ,, KC103 or Na 2 0 2 Na 2 0 2 Ιν2ο2θ7
Pt, Ni Fe, Ni (not Pt) Pt porcelain
Common oxidizing conditions for readily oxidized samples such as Sb, S, Cr, Fe Sulfides, acid-insoluble alloys such as ferrochromium, ferrotungsten, Ni, Mo, W, and Pt alloys Insoluble oxides and oxide-containing samples, iron ore, and Zr, Hf, and Th phosphates
t h e method requires some control to prevent loss of the desired portion of sample b y spattering or volatilization.
Questions Explain why heterogeneous samples require a statistical sampling procedure while homogeneous samples do not. Suggest a procedure that is suitable for obtaining a representative sample of milk of magnesia (a suspension of MgO in water). What type of problems are encountered when obtaining representative samples of gases? of liquids? of aerosols? of suspensions? Lead occurs in the environment as particulate matter. What kind of procedure should be used to obtain a representative atmospheric sample for a lead determination? In sampling air with the devices shown in Fig. 5-1 why would the pump not be placed in front of the sample collector? Why is it necessary to carefully monitor the flow rate in sampling air? If air contains 1 ppm Pb per liter of air, how many liters of air would have to be passed through a lead trapping solution in order to obtain 1 g of lead? Explain why it is preferred to reduce a solid heterogeneous sample to a uniform particle size.
QUESTIONS
75
9. What is the significance of an analysis reported on an ''as-received" basis? 10. Criticize the statement, "Reproducible dryness is the same as absolute dryness." 11. Suggest several reasons why desiccants are useful to the analytical chemist. 12. Suggest a procedure for dissolving each of the following: a. b. c. d.
Fe(N0 3 ) 3 Na 2 C0 3 CaC0 3 BaS0 4
e. f. g. h.
Sodium silicate Iron ore Zn Brass (Cu-Zn-Pb-Sn)
i. Dried blood j . Wool
INTRODUCTION Several basic laboratory operations are common to all quantitative analysis procedures. These are sampling, drying, weighing, and dissolving. Dissolving is the one operation not always necessary, since there are several instrumental methods in which the measurements are made directly on the sample. For the experienced analyst these operations are rather obvious and often are performed so routinely that little thought is given to them. In certain respects this is unfortunate since proper preparation for the measurement is as important as the measurement itself. In other words, the final measurement is no better than the preparation for the measurement.
SAMPLING The objective in obtaining a suitable sample is to take a portion that is representative of all the components and their amounts that are contained in the bulk sample. If the bulk sample is homogeneous, no problem is encountered in obtaining a laboratory size sample, regardless of whether it is solid, liquid, or gas. It is when the bulk sample is heterogeneous that special precautions must be taken in order to obtain a representative sample. For example, the iron content of a rod of iron alloy may vary widely through the length of the rod, on the surface, and in the center. If the rod was homogeneous, it wouldn't matter what section or part of the rod was selected as the sample, since the composition would be uniform throughout the rod. Unfortunately, blunders in sampling are made and much time and effort is wasted in trying to get good determinations using improper samples. 63
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PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
A bulk sample may be reduced to a laboratory size by a random choice or it may be reduced according to a statistically based plan, which theoretically gives every particle or portion of the substance an equal chance of appearing in the sample. In general, statistical sampling requires removal of portions from every section of the sample. These are subsequently combined, mixed, and resampled until a suitable laboratory size sample is obtained. The details of this general technique will differ according to the physical state of the substance being sampled. Sampling by random choice is difficult, as personal prejudices often exert an influence. Some deliberate thoughts about how to carry out the random choice are necessary before actually doing it. Nothing should be done to the sample which might inadvertently change its composition. Thus, crushing or grinding, labeling, and storage of the sample must be employed cautiously. Different techniques are employed for sampling gases, liquids, and solids. Heterogeneous mixtures such as emulsions, powders, suspensions, or aerosols require statistical handling. Only in homogeneous samples are the techniques simple and generally straightforward. Gases. There are three basic methods of collecting gases. These are expansion into an evacuated container, flushing, and displacement of a liquid. In all cases the collection vessels are of known volume, and the temperature and/or pressure within the vessel must be known. Usually, the collection vessels are made from glass (other inert material can be used), and they must be fitted with an inlet and an outlet both of which can be conveniently opened and closed. Contamination from previous samples can be a serious problem, and its elimination is affected by extensive flushing of the container with the gas to be sampled. The design of the sample device should easily permit this procedure. Air Sampling. Air is a complex mixture containing gases as well as particulate matter. Its actual composition is very dependent on the location and environment from which it is taken. Presently, because of pollution studies, much effort is directed toward monitoring the quality of air. In collecting an air sample and assuming that the determination of dissolved or suspended pollutants is what is being sought, two goals must be satisfied. 1. An accurate flow device must be used to measure the volume of air sample. Such a device must be calibrated as one would calibrate a piece of volumetric glassware, since these devices represent a measurement by volume. 2. A sample collector, such as a filter or an absorbing solution, must be used to trap the contaminants in the air. The actual efficiency of the collector
SAMPLING
65
must be determined experimentally with standards since few collectors of this type operate with 100% collection efficiency. Atmospheric sampling is much more difficult. Factors such as wind, temperature, or rain are variables which are difficult to overcome or control. The type of sampling method that is selected depends on the chemical and physical properties of the substances in the atmosphere that are being determined. In general, the atmospheric sample is passed through a series of fine filters or through a trapping solution. In the first case, which is excellent for isolating particulate matter, the filtering action is controlled by the porosity of the filtering device. In the latter, as the atmospheric sample is percolated through a column of solution, a chemical reaction traps the soughtfor components. Figure 5-1 illustrates several designs for collecting air samples. Liquids. Sampling pure or homogeneous liquids is straightforward and usually any sampling device which doesn't destroy the purity or homogeneity is an appropriate one. It is, however, a good idea to statistically sample even "pure" liquids. Heterogeneous liquid mixtures present a more difficult problem and the technique employed depends on whether the mixture is a suspension, an
Ί
.Sample air Sample m air
Calcium chloride-^;iVr<; drying tube
(a)
I
(c)
Sample air Direction of "airflow
\ ^ Sample collector
ιβΜΜΜΜΜΜΜΜΜΜΜβ γ
^ Airflow measuring device
Pump
(d)
F i g . 5 - 1 . Collection of air samples (a) by a trapping solution using a fritted absorber, (b) and (c) by a cold trap, and (d) by a general arrangement of absorber, flow-measuring device, and pump.
66
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
emulsion, a mixture of immiscible liquid phases, or a liquid containing solid residues. Additional complications arise if the liquid mixture is unstable (for example, an emulsion), contains volatile components, or contains dissolved gases. In general, aliquots are randomly withdrawn from various depths and from all locations in the liquid sample. These can be analyzed separately or combined to provide a composite sample statistically representative of the original sample. Solids. If a solid is homogeneous, any portion of it can be selected as being representatitive. For a heterogeneous solid a careful plan must be prepared which will statistically allow selection of all sections of the bulk solid. The sampling can be done by hand or by a mechanical sampling machine. The latter technique is useful if the bulk sample is a large mass. It is not always possible to statistically obtain a representatitive sample. Consider the difficult task of determining the composition of the moon's crust. Obviously, a statistical selection of the moon's surface was not possible. From the limited quantity of moon rocks and dust, the sampling was based partly on size and partly on physical state. The effect of particle size is introduced if a solid substance is being sampled, since composition of particles of different sizes can vary. In general, the conversion of a large sample to one suitable for analysis requires, first, reducing the sample to a uniform particle size and, second, reducing the mass of the sample. A uniform particle size is obtained by putting the sample through either crushers, pulverizers, mills, or mortars. Sieving can also be used. Whatever the procedure, it is necessary to ensure that contamination is not introduced into the sample by these operations. A very important part of the analysis of the moon rocks or of any material from beyond the earth is to establish whether or not there is organic matter present. A recent report on the analysis of a meteorite demonstrates the need for very careful handling of the samples to prevent organic contamination on earth. This is illustrated in the following example. Figure 5-2 shows a flow sheet describing the sampling technique employed on the meteorite (February 8, 1969, date of falling; February 15, 1969, date of sample collection; and March 1 and 10, 1969 date of duplicate analyses). The sample was egg shaped and weighed about 2.5 kg. A 250-g fragment was taken and portions of its surface and its interior used for analysis. The surface chips represented a depth of about 0.25 inch and included all of a fusion crust and a fresh surface break. This entire operation and all succeeding operations were carried out in a clean cabinet through which a filtered air stream was continuously passed. The flow sheet describes some of the details of the procedure. The actual data can be found
SAMPLING
67 250-g Fragment
40-g Surface chips Identical procedure carried out as with interior chips
200-g Interior chips 1/4 to 1 inch chips washed with MeOH rock crusher 120 Mesh powder CH3OH: benzene (1:3) (with I ultrasonic vibration)
Residue (For bound hydrocarbonfatty acid analysis)
"Organic solvent remove solvent by rotatory evaporator
|HF:HC1(1:1)
column chromatography
40% -40% Residue extract with benzene-MeOH
HF-HC1 solution extract with benzene
Combine these two extracts
n- Heptane fraction
GC-MS* analysis
Column chromatography as with initial organic extract
*GC-MS = Gas chromatography and mass spectrometry. tTLC = Thin layer chromatography.
Benzene fraction
GC-MS* analysis
MeOH fraction for fatty acid 1 analysis solvent \ removed BF3-MeOH Methyl esters TLCt T GC-MS analysis
Fig. 5-2. Flow sheet for analysis of meteorite sample. [From J. Han, B. R. Simoneit, A. L. Burlingame, and M. Calvin, Nature 222, 364 (1969).]
in the reference. From these data it was concluded that organic material found in the surface layer of this meteorite was of biological origin and could not be other than terrestrial contamination acquired during the short period of time before collection (February 8 to 15). Since the sample was contaminated it is doubtful that any conclusions can be drawn with respect to organic matter being originally present in the sample. Only correct sampling procedures will yield useful information. Regardless of how precise or accurate the chemical operations or measurements are, the collected data will not be better than the type of sample taken. Solid sampling can involve large masses such as a train-car load of coal or large amounts of small items such as pharmaceutical tablets. In quality control in the manufacture of pharmaceutical tablets a large number of tablets are randomly selected, weighed, and then pulverized into a powder. A weighed amount of the powder is taken for analysis and the results reported on a per tablet basis.
68
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
Clinical Samples. Clinical samples are most often whole blood, serum or plasma, cerebrospinal fluid, gastric juice, ascitic fluid in the abdominal cavity, pleural fluid in the chest cavity, synovial fluid in joints, draining wounds, urine, and tissue. Each of these samples presents its own unique problems with regard to the collection and handling of the sample. It is beyond the scope of this book to document these details. A factor that the analytical chemist or analyst encounters in clinical analysis more so than in industrial or enviromental analysis is that someone else is responsible for collecting the sample. The physician, nurse, or medical technician obtains the sample and the analyst must rely on their procedural ability to properly collect and handle the sample. For example, in collecting blood or other physiological samples the individual responsible for the collection must be aware of the following precautions. 1. The immobilization of the fluid flow in the individual should be minimized, since prolonged immobilization may alter the chemical values. 2. Samples of the fluid, in most cases, should not be taken while intravenous solutions are being administered. 3. Syringes must be scrupulously clean. 4. The container used for transporting the sample must be scrupulously clean. If anticoagulants are added the sample should be thoroughly mixed and this information provided to the analyst. 5. The amount of sample collected must be sufficient to allow the analyst to perform the determination. Different tests will require different amounts of sample, and consequently, the technician collecting the sample must be aware of this. 6. The analyst has no control over the diet, environment, etc. of the patient. The levels of the components of most clinical samples are an average, and the aforementioned factors have a significant effect on these levels. It is the responsibility of the individual collecting the sample to control these factors. 7. The analyst should be made aware of any special steps taken to preserve the sample, particularly if materials are added to the sample. Summary. Some analytical methods allow measurements to be made without destroying or changing the sample, while others consume the entire sample or require only trace amounts of sample material. It is also possible to analyze a surface by instrumental techniques. In this way the randomness of the entire surface of the sample can be determined. In summary, it is not possible to describe a set of general methods for sampling all substances under all conditions. However, the aim in all cases is statistical sampling, the use of chemical common sense, and cooperation between the analyst and the individual providing the sample. Many sources
DRYING
69
are available to aid the analyst in selecting the sampling procedure. These include general procedures,* clinical procedures,** and environmental procedures for airf and water. J DRYING Once a suitable sample is obtained, a decision must be reached on whether the analysis is to be made on the sample as received or after it has been dried. Most samples contain varying amounts of moisture, either because the sample is hygroscopic or because water is absorbed in the surface. Analysis on an "as-received basis'' treats water as part of the sample's composition and in handling the sample it is necessary that no gain or loss of water occur prior to analysis. Analysis on a dried basis means that the water has been removed. This is usually done by heating in an oven, a muffle furnace, or by Bunsen or Meeker burners. Techniques for this are shown in the chapter on laboratory techniques. Since heat is used for drying, it is possible to decompose the sample or to remove volatile components from the sample. In some cases a dried basis implies that this is true, while in others, perhaps the majority, only water removal is implied. Frequently, drying procedures are such that only a fixed amount of moisture is removed. An example of this is the removal of water to form a stable stoichiometric hydrate by controlled heating. Continued heating at an elevated temperature would be needed to remove the remaining water of hydration. Drying to a known hydrated state is an example of producing reproducible dryness, while the complete removal of water is the production of a state of absolute dryness. Regardless of which state is used, the handling of the dried sample must be rapid and its storage must be in the absence of water. In addition, the heated sample must be cooled, usually in a desiccator, before weighing. * P. D. La Fleur, Ed., "Accuracy in Trace Analysis: Sampling, Sample Handling, Analysis," Vols. 1-2, NBS Publication 422, U.S. Government Printing Office, Washington, D.C. (1976). I. M. Kolthoff, P. J. Elving, E. B. Sandell, "Treatise on Analytical Chemistry," Parts I to III, Wiley-Interscience, New York, 1963. ** J. S. Annino, "Clinical Chemistry: Principles and Procedures," third ed., Little Brown and Co., Boston, 1964. M. M. Wintrobe, "Clinical Hematology," Lea and Febiger, Philadelphia, 1961. t P. O. Warner, "Analysis of Air Pollutants," J. Wiley & Sons, New York, 1976. A. C. Stern, Ed., "Air Pollution," Academic Press, New York, 1968. t "Standard Methods for the Examination of Water and Waste Water," 13th ed., American Public Health Association, New York, 1970. W. Leithe, "The Analysis of Organic Pollutants in Water and Waste Water," Ann Arbor Science Publishers, Inc., Michigan, 1973.
70
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
WEIGHING Weighing of the sample is described in detail in the chapter on laboratory techniques. In ordinary analysis this measurement is one of the most accurate ones that can be made in the laboratory. Samples are usually weighed in triplicate and each of these is independently carried through the experimental procedure. An alternate technique is to weigh one sample and dissolve it in a known volume in a volumetric flask. Aliquots of this solution can then be taken for the quantitative determination. The weight of sample that is actually taken will depend on the concentration level of the substances in the sample that is being determined.
DISSOLVING Usually, after weighing the sample the next step is dissolution. If the sample is soluble in water the problem is solved, although occasionally the sample slowly hydrolyzes in water to produce insoluble hydrous oxides. Organic materials are usually dissolved in organic solvents or mixtures of organic solvents and water, if water alone is not a suitable solvent. Although the most direct and easily used solvent is water, there are a variety of chemical and instrumental procedures which require the use of a certain solvent composition. In other cases, the dissolving process is not even required. For example, in atomic emission, where the sample is excited by an arc or spark and the resulting radiant energy emitted is instrumentally analyzed, a solid or liquid sample can be used directly. There are general guidelines concerning the solubility of salts in water. These are briefly summarized in Table 5-1. Modern technology has resulted in a vast number of alloys and inorganic and organic mixtures. With such samples, the dissolution step is not a simple case of following the solubilities of salts. If the organic part of a mixture is to be analyzed, the solvents and techniques of organic chemistry must be used. For inorganic analysis, if water is not suitable, the sample must be dissolved in an acid or by fusion with a flux. Acid Treatment. In using acids several questions must be asked. Is a nonoxidizing or oxidizing acid needed? What kind of anion is desirable or most appropriate in solution? What are the chemical properties of the sample? Is it necessary to get rid of the excess acid? Knowledge of inorganic reactions and chemical common sense are needed to answer these questions. For example, H 2 S0 4 would not be used to dissolve a sample containing barium metal or ion, or HC1 would not be used to dissolve a silver metal or silver salt sample. In other situations acid treatment should
DISSOLVING Table 5-1.
71
General Guidelines for t h e Solubility of Inorganic Salts
Acetates are soluble, except a few sparingly soluble ones like AgC2H 3 02 and Hg 2 (C2H 3 02)2. Arsenates, borates, and carbonates are insoluble, except those of N H 4 + , Na + , and K + . The bicarbonates of these three and of Ba 2+ , Ca 2+ , Fe 2 + , Mg 2+ , Mn 2 + , and Sr 2+ are soluble. Many borates are soluble in NH 4 C1 solution, and all are soluble in hot dilute acids. Any salts of these ions with tervalent or quadrivalent metals are so completely hydrolyzed that they may be considered as not existing. Fluorides are insoluble, except those of Na + , K + , N H 4 + , Ag+, and Sn 2+ . F e F 3 is slightly soluble, but like many other metals, iron forms a very soluble complex ion with excess F~. Oxalates are insoluble, except those of Na + , K + , N H 4 + , and Fe 3 + . A number of oxalate complexes of other metals are soluble. Chromates are insoluble, except those of Na+, K+, NH 4 +, Ca 2+ , Mg 2 + , Zn 2+ , and Fe 3 + . Sulfides are insoluble, except Na 2 S, K 2 S, and (NH 4 ) 2 S. Slightly soluble sulfides are BaS, SrS, CaS, and MgS. Phosphates are insoluble, except those of N H 4 + , Na + , and K + . Phosphates of alkaline earth metals are soluble in acid but insoluble in neutral or alkaline solution. Chlorides and bromides of Ag + and Hg 2 2 + and iodides of Ag + , Hg 2 2 + , Hg 2 + , and P b 2 + are insoluble. PbCl 2 , PbBr 2 , and HgBr 2 are slightly soluble. Some basic halides are insoluble; for example, SbOCl. Thiocyanates are soluble, except Ag + , Pb 2 + , Hg 2 2 + , and Hg 2 + . Sulfates are soluble, except BaS0 4 , Hg 2 S0 4 , PbS0 4 , and SrS0 4 . Ag 2 S0 4 and C a S 0 4 are slightly soluble. Some basic sulfates are insoluble; for example, (BiO) 2 S0 4 . Sulfites are generally similar to carbonates. M g S 0 3 is slightly soluble. Nitrates are soluble, except a few basic salts like BiON0 3 . Nitrates do not exist for arsenic or tin (IV). Nitrites are soluble, except AgN0 2 , which is slightly soluble. Nitrites are very unstable to oxidation or reduction.
not be used because the sample may be volatilized. For example, acid treatment of carbonate or sulfide samples can result in the loss of C 0 2 or H 2 S, unless arrangements are made to trap the gases. But if the analysis is not concerned with the sulfide or carbonate content of the sample, then there is no difficulty. In some cases acid cannot be used because it causes parts of the sample to become passive and not take part in the reaction, usually because an oxide coating forms. It is difficult to describe acid conditions for dissolving inorganic samples since not all samples are simple pure metals, metal oxides, or alloys. For example, a high-temperature alloy might contain 13 different metals, including such elements as Nb, Ta, W, Zr, and rare earths. Sometimes, one technique is used to dissolve one part of the sample and another to dissolve the residue. As a general guide it is useful to classify the more common acid conditions according to whether they are oxidizing or nonoxidizing. The nonoxidizing acids are HC1, dilute H 2 S0 4 , and dilute HC104, while the oxidizing acids are HN0 3 , hot concentrated H 2 S0 4 , and hot concentrated HC10 4 . Dissolution of metals by the nonoxidizing acids is a process of hydrogen
72
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
replacement. Therefore, any scheme of relating the ability of metals to displace hydrogen can also be used to qualitatively predict solubility. The Table of Standard Reduction Potentials (see Appendix IV and Chapter 10) is such a scheme, and metals below hydrogen in the series will dissolve in a nonoxidizing acid. There are exceptions and these are usually due to a presence of a passive condition, oxide film formation, or insoluble salt formation. Briefly, HCl will dissolve the metals above hydrogen, salts of weak acids, and many oxides. Dilute H 2 S0 4 and HC10 4 are useful for metals above hydrogen, the difference being the solubility of the salts that are formed. Heat and concentrated H 2 S0 4 will often dissolve metals below hydrogen. However, the problem of solubility of sulfate salts should be considered. Nitric acid will dissolve metals below and above hydrogen since its oxidizing power will vary according to whether the acid is dilute or concentrated. In general, the metal is oxidized to its highest oxidation state. The principle limitations are that some metals (Al and Cr) become passive while others (Sn, Sb, and W) form insoluble acids. Sulfide salts and salts of oxidizable anions are also dissolved by HN0 3 . Perhaps the most powerful oxidizing conditions are obtained by using hot, concentrated HC10 4 which will dissolve all the common metals. It is often an advantage to use acids in combination. The most familiar mixture is aqua regia (1:3 HNO3-HCI). In essence, the features of both acids are included since the H N 0 3 furnishes oxidizing power, while HCl furnishes complexing properties and strong acidity. Solubility of many metal ions is maintained only in the presence of complexing agents. In fact, this is a useful technique for solubilizing an otherwise insoluble ionic salt. Sometimes the addition of bromine or hydrogen peroxide to mineral acids increases their solvent action. An added advantage of such combinations is the hastened oxidation (and destruction) of organic materials in the sample. Hydrofluoric acid is special in that it is a weak acid as well as a nonoxidizing acid, but yet it is still useful for solubilizing certain samples. Silicate samples, where silica is not to be analyzed are readily decomposed by HF, and the silica is volatilized as S1F4. Hydrofluoric acid is better than HCl in that it furnishes a good complexing anion, F~~. In certain cases, the complex is very difficult to decompose and it can, therefore, affect future chemical steps. Since HF can cause serious injury upon contact with the skin, a convenient way of creating HF conditions is to add NaF to an HCl solution of the sample. Perchloric acid when hot and concentrated is a potent oxidizer. If a dilute solution is boiled and the water is slowly evaporated, the oxidizing power increases, gradually reaching a maximum at 72% acid; at this point an azeotropic mixture is distilled. Other advantages of perchloric acid are that very soluble perchlorate salts are formed, it acts as a dehydrating agent, and the hot concentrated acid readily oxidizes organic materials. The latter technique, which is generally referred to as wet ashing and employs a HNO3-
DISSOLVING
73
HCIO4 mixture, requires careful handling since an improper procedure can result in a violent explosion. Nitric acid is added first, heated, cooled, and then the HNO3-HCIO4 mixture is added. Thus, the H N 0 3 acts as a moderating force by oxidizing the more reactive compounds at lower temperatures. Minerals and rocks, such as silicates, sulfides, phosphates, carbonates, sulfates, and very refractory minerals and oxides often require special treatment. The difficulties are enhanced if analysis of the anion is also desired as some of the more easily used acids can result in volatilization losses of this portion of the sample. Flux Treatment. The second general dissolving technique, fusion with a flux, is more potent than acid treatment for two reasons. First, since a flux is a fused salt media, the temperatures required for creating the condition (300° to 1000°C) are much higher than possible by the acid treatment. Second, there is a greater concentration of reagent in contact with the sample. The advantage of oxidizing or nonoxidizing conditions is not lost since there are fluxes which yield both of these conditions. The difficulties in using the fluxes are several. Special containers that will withstand the temperature as well as the reaction conditions are needed. Pt, Ag, Ni, Au, and Fe are some of the more common crucible materials. However, before choosing the appropriate crucible the reactivity of the flux must be known. Upon completion of the fusion and after cooling, the solid material is usually dissolved in water or dilute acid. The solution that is obtained has a very high salt content which often must be considered in the following chemical or instrumental steps. Other possible difficulties are impurities introduced by the flux, volatilization due to the high temperature, and spattering losses as a result of the reaction. Table 5-2 lists the common fluxes according to their oxidizing or nonoxidizing properties and also some of their applications. The fluxes can also be divided according to whether they yield acidic or basic conditions. Carbonates, hydroxides, and peroxides provide basic conditions, while boric oxide and pyrosulfates provide acidic conditions. Decomposition of Organic Matter. Decomposition of organic material can be done in several ways. The method that is finally chosen will depend on whether analysis of the organic matter is desired or whether only organic removal is desired. The wet ashing procedure, already mentioned, in which HCIO4-HNO3 is used, completely destroys organic matter. Some other chemical agents that can be used are metallic sodium in liquid ammonia, fuming nitric acid, and dry-ashing. Dry-ashing is the simplest to carry out. Usually, the sample is heated in an open crucible or dish in the presence of air or oxygen to red heat until all the carbonaceous material has been oxidized. The nature of
PRELIMINARY OPERATIONS IN QUANTITATIVE ANALYSIS
74
Table 5-2. Common Fluxes Flux
Crucible
Application Nonoxidizing
Na 2 C0 3 NaOH KOH B203 CaC03-NH4Cl
Pt Au, Ni, Ag Au, Ni, Ag Pt Ni
Silicates, phosphates, sulfates Silicates, silicon carbides Silicates, silicon, carbides Silicates, oxides Silicates (known as J. Lawrence Smith method for analysis of alkali metals) Oxidizing
Na 2 C0 3 mixed with KN0 3 ,, KC103 or Na 2 0 2 Na 2 0 2 Ιν2ο2θ7
Pt, Ni Fe, Ni (not Pt) Pt porcelain
Common oxidizing conditions for readily oxidized samples such as Sb, S, Cr, Fe Sulfides, acid-insoluble alloys such as ferrochromium, ferrotungsten, Ni, Mo, W, and Pt alloys Insoluble oxides and oxide-containing samples, iron ore, and Zr, Hf, and Th phosphates
t h e method requires some control to prevent loss of the desired portion of sample b y spattering or volatilization.
Questions Explain why heterogeneous samples require a statistical sampling procedure while homogeneous samples do not. Suggest a procedure that is suitable for obtaining a representative sample of milk of magnesia (a suspension of MgO in water). What type of problems are encountered when obtaining representative samples of gases? of liquids? of aerosols? of suspensions? Lead occurs in the environment as particulate matter. What kind of procedure should be used to obtain a representative atmospheric sample for a lead determination? In sampling air with the devices shown in Fig. 5-1 why would the pump not be placed in front of the sample collector? Why is it necessary to carefully monitor the flow rate in sampling air? If air contains 1 ppm Pb per liter of air, how many liters of air would have to be passed through a lead trapping solution in order to obtain 1 g of lead? Explain why it is preferred to reduce a solid heterogeneous sample to a uniform particle size.
QUESTIONS
75
9. What is the significance of an analysis reported on an ''as-received" basis? 10. Criticize the statement, "Reproducible dryness is the same as absolute dryness." 11. Suggest several reasons why desiccants are useful to the analytical chemist. 12. Suggest a procedure for dissolving each of the following: a. b. c. d.
Fe(N0 3 ) 3 Na 2 C0 3 CaC0 3 BaS0 4
e. f. g. h.
Sodium silicate Iron ore Zn Brass (Cu-Zn-Pb-Sn)
i. Dried blood j . Wool