M ost analytical measurements are performed on solutions (usually aqueous) of the analyte. While some samples dissolve readily in water or aqueous solutions

of the common acids or bases, others require powerful reagents and rigorous treatment. For example, when sulfur or halogens are to be determined in an organic compound, the sample must be subjected to high temperatures and potent reagents to rupture the strong bonds between these elements and carbon. Similarly, drastic conditions are usually required to destroy the silicate structure of a siliceous mineral and free the ions for analysis.

The proper choice among the various reagents and techniques for decomposing and dissolving analytical samples can be critical to the success of an analysis, particularly where refractory substances are involved or where the analyte is present in trace amounts. In this chapter, we first consider the types of errors that can arise in decomposing and dissolving an analytical sample. We then describe four general methods of decomposing solid and liquid samples to obtain aqueous solution of analytes. The methods include (1) heating with aqueous strong acids (or occasionally bases) in open vessels; (2) microwave heating with acids; (3) high-temperature ignition in air or oxygen; and (4) fusion in molten salt media.1 These methods differ in the temperatures and the strengths of the reagents used.

A refractory substance is resistant to heat and attack by strong chemical agents.

Microwave digestion systems have become very popular for decomposing samples. The photo shows a closed-vessel microwave digestion system for high-pressure digestions. This device is capable of simultaneously running up to 10 high-pressure closed vessels, providing a fast, auto-mated method to digest samples. The system has one-step operation and full computer control of temperature, pressure, and power. Teflon sample vessels can be operated at temperatures up to 250°C and 800 psi. This chapter considers methods of decomposing and dissolving real samples. Acid decomposition, microwave, combustion, and fusion methods are considered.

Decomposing and Dissolving the Sample

1For an extensive discussion of this topic, see R. Bock, A Handbook of Decomposition Methods in Analytical Chemistry, New York: Wiley, 1979; Z. Sulcek and P. Povondra, Methods of Decomposition in Inorganic Analysis, Boca Raton, FL: CRC Press, 1989; J. A. Dean, Analytical Chemistry Handbook, Section 1.7, New York: McGraw-Hill, 1995.

CHAPTER 37

Courtesy of Aurora Biomed Inc., Vancouver, B.C.

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37B Decomposing Samples with Inorganic Acids in Open Vessels  977

37A

SourceS of error in DecoMpoSition AnD DiSSolution

We encounter several sources of error in the sample decomposition step. In fact, such errors often limit the accuracy that can be achieved in an analysis. The sources of these errors include the following:

1. Incomplete dissolution of the analytes. Ideally the sample treatment should dissolve the sample completely. Attempts to leach analytes quantitatively from an insoluble residue are usually not successful because portions of the analyte are retained within the residue.

2. Losses of the analyte by volatilization. An important concern when dissolving samples is the possibility that some portion of the analyte may volatilize and be lost. For example, carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen sel-enide, and hydrogen telluride are often emitted when a sample is dissolved in strong acid, while ammonia is often lost when a basic reagent is used. Similarly, hydrofluoric acid reacts with silicates and boron-containing compounds to pro-duce volatile fluorides. Strong oxidizing solvents often cause the evolution of chlorine, bromine, or iodine; reducing solvents may lead to the volatilization of such compounds as arsine, phosphine, and stibine.

A number of elements form volatile chlorides that are partially or completely lost from hot hydrochloric acid solutions. Among these elements are the chlorides of tin(IV), germanium(IV), antimony(III), arsenic(III), iron(III), and mercury(II). The oxychlorides of selenium and tellurium also volatilize to some extent from hot hydrochloric acid. The presence of chloride ion in hot concentrated sulfuric or perchloric acid solutions can cause volatilization losses of bismuth, manganese, molybdenum, thallium, vanadium, and chromium.

Boric acid, nitric acid, and the halogen acids are lost from boiling aqueous solutions. Certain volatile oxides can also be lost from hot acidic solutions, including the tetroxides of osmium and ruthenium and the heptoxide of rhenium.

3. Introduction of analyte as a solvent contaminant. Ordinarily, the mass of solvent required to dissolve a sample exceeds the mass of sample by one or two orders of magnitude. As result, the presence of analyte species in the solvent even at small concentrations may lead to significant error, particularly when the analyte is pres-ent in trace amounts in the sample.

4. Introduction of contaminants from reaction of the solvent with vessel walls. This source of error is often encountered in decompositions that involve high-temperature fusions. Again, this source of error becomes of particular concern in trace analyses.

37B

DecoMpoSing SAMpleS with inorgAnic AciDS in open VeSSelS

The most common reagents for open-vessel decomposition of inorganic analytical samples are the mineral acids. Much less frequently, ammonia and aqueous solutions of the alkali metal hydroxides are used. Ordinarily, a suspension of the sample in the acid is heated by flame or a hot plate until the dissolution is judged to be complete by the absence of a solid phase. The temperature of the decomposition is the boiling (or decomposition) point of the acid reagent.

Ideally the reagent selected should dissolve the entire sample, not just the analyte.

❮

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978  CHAPTER 37 Decomposing and Dissolving the Sample

37B-1 Hydrochloric AcidConcentrated hydrochloric acid is an excellent solvent for inorganic samples but finds limited application for decomposing organic materials. It is widely used for dissolving many metal oxides as well as metals more easily oxidized than hydrogen. Often, it is a better solvent for oxides than the oxidizing acids. Concentrated hydro-chloric acid is about 12 M, but on heating, HCl gas is lost until a constant-boiling 6 M solution remains (boiling point about 110°C).

37B-2 Nitric AcidHot concentrated nitric acid is a strong oxidant that dissolves all common metals with the exception of aluminum and chromium, which become passive to this reagent due to surface oxide formation. When alloys containing tin, tungsten, or antimony are treated with the hot reagent, slightly soluble hydrated oxides, such as SnO2 · 4H2O, form. After coagulation, these colloidal materials can be separated from other metallic species by filtration.

Hot nitric acid alone or in combination with other acids and oxidizing agents, such as hydrogen peroxide and bromine, is widely used for decomposing organic samples for determining their trace metal content. This decomposition process, which is called wet ashing, converts the organic sample to carbon dioxide and water. Unless carried out in a closed vessel, nonmetallic elements, such as the halogens, sulfur, and nitrogen, are completely or partially lost by volatilization.

37B-3 Sulfuric AcidMany materials are decomposed and dissolved by hot concentrated sulfuric acid, which owes part of its effectiveness as a solvent to its high boiling point (about 340°C). Most organic compounds are dehydrated and oxidized at this temperature and are thus eliminated from samples as carbon dioxide and water by this wet ashing treatment. Most metals and many alloys are attacked by the hot acid.

37B-4 Perchloric AcidHot concentrated perchloric acid, a potent oxidizing agent, attacks a number of iron alloys and stainless steels that are not affected by other mineral acids. Care must be taken in using the reagent, however, because of its potentially explosive nature. The cold concentrated acid is not explosive, nor are heated dilute solutions. Violent explosions occur, however, when hot concentrated perchloric acid comes into contact with organic materials or easily oxidized inorganic substances. Because of this property, the concentrated reagent should be heated only in special hoods, which are lined with glass or stainless steel, are seamless, and have a fog system for washing down the walls with water. A perchloric acid hood should always have its own fan system, one that is independent of all other systems.2

Perchloric acid is marketed as the 60% to 72% acid. A constant-boiling mixture (72.4% HClO4) is obtained at 203°C.

Wet ashing is a process of oxidative decomposition of organic samples by liquid oxidizing reagents, such as HNO3, H2SO4, HClO4, or mixtures of these acids.

2See A. A. Schilt, Perchloric Acid and Perchlorates, Columbus, Ohio: G. Frederick Smith Chemical Company, 1979.

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37C Microwave Decompositions  979

37B-5 Oxidizing MixturesMore rapid wet ashing can sometimes be obtained by the use of mixtures of acids or by the addition of oxidizing agents to a mineral acid. Aqua regia, a mixture containing three volumes of concentrated hydrochloric acid and one volume of nitric acid, is well known. The addition of bromine or hydrogen peroxide to mineral acids often increases their solvent action and hastens the oxidation of organic materials in the sample. Mixtures of nitric and perchloric acid are also useful for this purpose and less dangerous than perchloric acid alone. With this mixture, however, care must be taken to avoid evaporation of all the nitric acid before oxida-tion of the organic material is complete. Severe explosions and injuries have resulted from failure to observe this precaution.

37B-6 Hydrofluoric AcidThe primary use of hydrofluoric acid is for the decomposition of silicate rocks and minerals in the determination of species other than silica. In this treatment, silicon is evolved as the tetrafluoride. After decomposition is complete, the excess hydrofluoric acid is driven off by evaporation with sulfuric acid or perchloric acid. Complete removal is often essential to the success of an analysis because fluoride ion reacts with several cations to form extraordinarily stable complexes that then interfere with the determination of the cations. For example, precipitation of aluminum (as Al2O3 · xH2O) with ammonia is quite incomplete if fluoride is pres-ent even in small amounts. Frequently, it is so difficult and time consuming to remove the last traces of fluoride ion from a sample that the attractive features of HF as a solvent are negated.

Hydrofluoric acid finds occasional use in conjunction with other acids in attack-ing steels that dissolve with difficulty in other solvents. Because hydrofluoric acid is extremely toxic, dissolution of samples and evaporation to remove excess reagent should always be carried out in a well-ventilated hood. Hydrofluoric acid causes serious damage and painful injury when brought into contact with the skin. Its effects may not become evident until hours after exposure. If the acid comes into contact with the skin, the affected area should be immediately washed with copious quantities of water. Treatment with a dilute solution of calcium ion, which precipitates fluoride ion, may also be of help.

37c MicrowAVe DecoMpoSitionSThe use of microwave ovens for the decomposition of both inorganic and organic sam-ples was first proposed in the mid-1970s and by now has become an important method for sample preparation.3 Microwave digestions can be carried out in either closed or open vessels, but closed vessels are more popular because of the higher pressures and higher temperatures that can be achieved.

3For more detailed discussions of microwave sample preparation and commercial instrumentation, see H. M. Kingston and S. J. Haswell, Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation and Applications, Washington, DC: American Chemical Society, 1997; B. E. Erickson, Anal. Chem., 1998, 70, 467A–471A, DOI: 10.1021/ac981908z; R. C. Richter, D. Link, and H. M. Kingston, Anal. Chem., 2001, 73, 31A–37A, DOI: 10.1021/ac0123781; J. L. Luque-Garcia and M. D. Luque de Castro, Trends in Analytical Chemistry, 2003, 22, 90, DOI: 10.1016/s0165-9936(03)00202-4.

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980  CHAPTER 37 Decomposing and Dissolving the Sample

One of the main advantages of microwave decompositions compared with con-ventional methods using a flame or hot plate (regardless of whether an open or a closed container is used) is speed. Typically, microwave decompositions of even dif-ficult samples can be accomplished in five to ten minutes. In contrast, the same results require several hours when carried out by heating over a flame or hot plate. The difference is due to the different mechanism by which energy is transferred to the molecules of the solution in the two methods. Heat transfer is by conduction in the conventional method. Because the vessels used in conductive heating are usually poor conductors, time is required to heat the vessel and then transfer the heat to the solution by conduction. Furthermore, because of convection within the solution, only a small fraction of the liquid is maintained at the temperature of the vessel and thus at its boiling point. In contrast, microwave energy is transferred directly to all of the molecules of the solution nearly simultaneously without heating the vessel. Thus, boiling temperatures are reached throughout the entire solution very quickly.

As noted previously, an advantage of using closed vessels for microwave decompositions is the higher temperatures that develop as a result of the increased pressure. In addition, because evaporative losses are avoided, significantly smaller amounts of reagent are used, therefore reducing interference by reagent con-taminants. A further advantage of decompositions of this type is loss of volatile components of samples is virtually eliminated. Finally closed-vessel microwave de-compositions are often easy to automate, thus reducing operator time required to prepare samples for analysis.

37C-1 Vessels for Moderate-Pressure DigestionsMicrowave digestion vessels are constructed from low-loss materials that are transparent to microwaves. These materials must also be thermally stable and resistant to chemical attack by the various acids used for decompositions. Teflon is a nearly ideal material for many of the acids commonly used for dissolutions. It is transparent to microwaves, has a melting point of about 300°C, and is not attacked by any of the common acids. Sulfuric and phosphoric acids, however, have boiling points above the melting point of Teflon, which means that care must be exercised to control the temperature during decompositions. For these acids, quartz or borosilicate glass vessels are sometimes used in place of Teflon containers. Vessels of this type have the disadvantage, however, that they are at-tacked by hydrofluoric acid, a reagent that is often used for decomposing silicates and refractory alloys.

Figure 37-1 is a schematic of a commercially available closed digestion vessel designed for use in a microwave oven. It consists of a Teflon body, a cap, and a safety relief valve designed to operate at 120 6 10 psi. At this pressure, the safety valve opens and then reseals.

37C-2 High-Pressure Microwave VesselsFigure 37-2 is a schematic of a commercial microwave bomb designed to operate at 80 atm or about 10 times the pressure that can be tolerated by the moderate-pressure vessels described in the previous section. The maximum recommended temperature with this device is 250°C. The heavy-wall bomb body is constructed of a polymeric material that is transparent to microwaves. The decomposition is carried out in a

Vessel body

Vessel cap

Vent tubingVenting nut

Safety valve

figure 37-1 A moderate-pressure vessel for microwave decomposition. (Courtesy of CEM Corp., Matthews, NC.)

Pressure screw

Screw capRelief diskSealer disk

Inner coverO-ringSample cupBomb body

Bottom plate

figure 37-2 A bomb for high-pressure microwave decomposition. (Courtesy of Parr Instrument Co., Moline IL.)

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37C Microwave Decompositions  981

Teflon cup supported in the bomb body. The microwave bomb incorporates a Teflon O-ring in the liner cap that seats against a narrow rim on the exterior of the liner and its cap when the retaining jacket is screwed into place. When overpressurization oc-curs, the O-ring distorts, and the excess pressure then compresses the sealer disk that allows the gases to escape into the surroundings. The sample is compromised when this occurs. The internal pressure in the bomb can be judged roughly by the distance the pressure screw protrudes from the cap. This microwave bomb is particularly use-ful for dissolving highly refractory materials that are incompletely decomposed in the moderate-pressure vessel described previously.

When alloys and metals are digested in high-pressure microwave vessels, there is a risk of explosion caused by the production of hydrogen gas. Common poly-meric liner materials may not be capable of reaching the temperatures needed to fully decompose organic materials. Another limitation is that most high-pressure vessels are restricted in sample sizes to less than 1 g of material. It is also necessary to allow time for cool down and depressurization.

37C-3 Atmospheric-Pressure DigestionsThe limitations of closed-vessel microwave digestion systems just noted have led to the development of atmospheric-pressure units, often called open-vessel systems. These systems do not have an oven, but instead, they use a focused microwave cavity. They can be purged with gases and equipped with tubing to allow for the insertion and removal of reagents. There is no longer a safety concern from gas-forming reactions during the digestion process since the systems operate at atmospheric pressure. There are even flow-through systems available for on-line dissolution prior to introducing the samples in flames or ICPs for atomic spectro-scopic determinations.

37C-4 Microwave OvensFigure 37-3 is a schematic of a microwave oven designed to heat simultaneously 12 of the moderate-pressure vessels described in Section 37C-1. The vessels are held on a turntable that rotates continuously through 360 deg so that the average energy re-ceived by each of the vessels is approximately the same.

37C-5 Microwave FurnacesRecently, microwave furnaces have been developed for performing fusions and for dry ashing samples containing large amounts of organic materials before acid dis-solutions. These furnaces consist of a small chamber constructed of silicon car-bide that is surrounded by quartz insulation. When microwaves are focused on this chamber, temperatures of 1000°C are reached in two minutes. The advantage of this type of furnace relative to a conventional muffle furnace is the speed at which high temperatures are reached. In contrast, conventional muffle furnaces are usu-ally operated continuously because of the elapsed time required to get them up to temperature. Furthermore, with a microwave furnace there are no burned-out heat-ing coils such as are frequently encountered with conventional furnaces. Finally, the operator is not exposed to high temperatures when samples are introduced or removed from the furnace. A disadvantage of the microwave furnace is the small volume of the heating cavity, which only accommodates an ordinary size crucible.

Digesting vessel Turntable

figure 37-3 A microwave oven designed for use with 12 vessels of the type shown in Figure 37-1. (Courtesy of CEM Corp., Matthews, NC.)

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982  CHAPTER 37 Decomposing and Dissolving the Sample

37C-6 Applications of Microwave DecompositionsDuring the last 30 years, hundreds of reports have appeared in the literature regarding the use of closed-vessel decompositions carried out in microwave ovens with the reagents described in Section 37B. These applications fall into two categories: (1) oxidative de-compositions of organic and biological samples (wet ashing) and (2) decomposition of refractory inorganic materials encountered in industry. In both cases, this new technique is replacing older conventional methods because of the large economic gains that result from significant savings in time. Open-vessel digestions have also become popular in recent years, and applications are on the increase.

37D

coMBuStion MethoDS for DecoMpoSing orgAnic SAMpleS4

37D-1 Combustion over an Open Flame (Dry Ashing)The simplest method for decomposing an organic sample prior to determining the cations it contains is to heat the sample over a flame in an open dish or crucible un-til all carbonaceous material has been oxidized to carbon dioxide. Red heat is often required to complete the oxidation. Analysis of the nonvolatile components follows dissolution of the residual solid. Unfortunately, there is always substantial uncer-tainty about the completeness of recovery of supposedly nonvolatile elements from a dry-ashed sample. Some losses probably result from the entrainment of finely di-vided particulate matter in the convection currents around the crucible. In addition, volatile metallic compounds may be lost during the ignition. For example, copper, iron, and vanadium are appreciably volatilized when samples containing porphyrin compounds are ashed.

Although dry ashing is the simplest method for decomposing organic compounds, it is often the least reliable. It should not be used unless tests have demonstrated its applicability to a given type of sample.

37D-2 Combustion-Tube MethodsSeveral common and important elemental components of organic compounds are converted to gaseous products as a sample is pyrolyzed. With suitable apparatus, it is possible to trap these volatile compounds quantitatively, thus making them avail-able for the analysis of the element of interest. The heating is commonly performed in a glass or quartz combustion tube through which a stream of carrier gas is passed. The stream transports the volatile products to parts of the apparatus where they are separated and retained for the measurement; the gas may also serve as the oxidizing agent. Elements susceptible to this type of treatment are carbon, hydrogen, oxygen, nitrogen, the halogens, sulfur, and oxygen. The volatile products of pyrolysis can also be determined by mass spectrometry and/or gas chromatography.

Automated combustion-tube analyzers are now available for the determination of carbon, hydrogen, and nitrogen or carbon, hydrogen, and oxygen in a single sample.5

Dry ashing is a process of oxidizing an organic sample with oxygen or air at high temperature, leaving the inorganic component for analysis.

Pyrolysis is the thermochemical decomposition of organic compounds at elevated temperature in the absence of oxygen. Combustion is this process in the presence of oxygen. Pyrolysis decomposition can be combined with techniques such as mass spectrometry or gas chromatography to separate and determine the volatile compounds.

4For a thorough treatment of this topic, see T. S. Ma and R. C. Rittner, Modern Organic Elemental Analysis, New York: Marcel Dekker, 1979.

5Ibid., Chs. 2–4.

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37D Combustion Methods for Decomposing Organic Samples  983

The apparatus requires essentially no attention by the operator, and the analysis is complete in less than 15 min. In one such analyzer, the sample is ignited in a stream of helium and oxygen and passes over an oxidation catalyst consisting of a mixture of silver vanadate and silver tungstate. Halogens and sulfur are removed with a packing of silver salts. A packing of hot copper is located at the end of the combustion train to remove oxygen and convert nitrogen oxides to nitrogen. The exit gas, consisting of a mixture of water, carbon dioxide, nitrogen, and helium, is collected in a glass bulb. The analysis of this mixture is accomplished with three thermal-conductivity mea-surements (see Section 32A-4). The first is made on the intact mixture, the second is made on the mixture after water has been removed by passage of the gas through a dehydrating agent, and the third is made on the mixture after carbon dioxide has been removed by an absorbent. The relationship between thermal conductivity and concentration is linear, and the slope of the curve for each constituent is established by calibration with a pure compound such as acetanilide.

37D-3 Combustion with Oxygen in a Sealed ContainerA relatively straightforward method for the decomposition of many organic substances involves combustion with oxygen in a sealed container. The reaction products are ab-sorbed in a suitable solvent before the reaction vessel is opened; they are subsequently analyzed by ordinary methods.

A remarkably simple apparatus, shown in Figure 37-4, for performing such oxi-dations has been suggested by Schöniger.6 It consists of a heavy-walled flask of 300- to 1000-mL capacity fitted with a ground-glass stopper. Attached to the stopper is a platinum gauze basket that holds from 2 to 200 mg of sample. If the substance to be analyzed is a solid, it is wrapped in a piece of low-ash filter paper cut in the shape shown in Figure 37-4. Liquid samples are weighed into gelatin capsules, which are then wrapped in a similar fashion. The paper tail serves as the ignition point.

A small volume of an absorbing solution (often sodium carbonate) is placed in the flask, and the air in the flask is displaced by oxygen. The tail of the paper is ignited, the stopper is quickly fitted into the flask, and the flask is inverted to prevent the escape of the volatile oxidation products. The reaction ordinarily proceeds rapidly, being catalyzed by the platinum gauze surrounding the sample. During the combus-tion, the flask is shielded to minimize damage in case of explosion.

After cooling, the flask is shaken thoroughly and disassembled, and the inner sur-faces are carefully rinsed. The analysis is then performed on the resulting solution. This procedure has been applied to the determination of halogens, sulfur, phospho-rus, fluorine, arsenic, boron, carbon, and various metals in organic compounds.

6W. Schöniger, Mikrochim. Acta, 1955, 43, 123; 1956, 44, 869. See also the review articles by A. M. G. MacDonald, in Advances in Analytical Chemistry and Instrumentation, C. E. Reilley, ed., Vol. 4, p. 75, New York: Interscience, 1965.

Sample

Samplewrapped inpaper holder

Stopper withS ground joint

Sample inholder

Absorptionliquid

Ignitionpoint

figure 37-4 Schöniger combustion apparatus. (Courtesy of Thomas Scientific, Swedesboro, NJ.)

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984  CHAPTER 37 Decomposing and Dissolving the Sample

37e

DecoMpoSing inorgAnic MAteriAlS with fluxeS

Many common substances—notably silicates, some mineral oxides, and a few iron alloys—are attacked slowly, if at all, by the methods just considered. In such cases, recourse to a fused-salt medium is indicated. In this case, the sample is mixed with an alkali metal salt, called the flux, and the combination is then fused to form a water-soluble product called the melt. Fluxes decompose most substances by virtue of the high temperature required for their use (300°C to 1000°C) and the high concentra-tion of reagent brought in contact with the sample.

Where possible, we tend to avoid fluxes because of the possible danger as well as several disadvantages. Among these disadvantages is the potential contamination of the sample by impurities in the flux. This possibility is exacerbated by the relatively large amount of flux (typically at least ten times the sample mass) required for a successful fusion. Moreover, the aqueous solution that results when the melt from a fusion is dissolved has a high salt content, which may cause difficulties in the subse-quent steps of the analysis. In addition, the high temperatures required for a fusion increase the danger of volatilization losses. Finally, the container in which the fusion is performed is almost inevitably attacked to some extent by the flux leading again, to contamination of the sample.

For a sample containing only a small fraction of material that dissolves with difficulty, it is common practice to use a liquid reagent first. The undecomposed residue is then isolated by filtration and fused with a relatively small quantity of a suitable flux. After cooling, the melt is dissolved and combined with the major portion of the sample.

37E-1 Fusion ProcedureThe sample in the form of a very fine powder is mixed intimately with perhaps a tenfold excess of the flux. Mixing is usually accomplished in the same crucible to be used for the fusion. The time required for fusion can range from a few minutes to hours. The production of a clear melt signals completion of the decomposition, although often this condition is not obvious.

When the fusion is complete, the mass is allowed to cool slowly. Just before solidi-fication, the crucible is rotated to distribute the solid around the walls to produce a thin layer of melt that is easy to dislodge.

37E-2 Types of FluxesWith few exceptions, the common fluxes used in analysis are compounds of the alkali metals. Alkali metal carbonates, hydroxides, peroxides, and borates are basic fluxes used to attack acidic materials. The acidic fluxes are pyrosulfates, acid fluorides, and boric oxide. If an oxidizing flux is required, sodium peroxide can be used. As an alternative, small quantities of the alkali nitrates or chlorates can be mixed with sodium carbonate. The properties of the common fluxes are summarized in Table 37-1.

Sodium CarbonateSilicates and certain other refractory materials can be decomposed by heating to 1000°C to 1200°C with sodium carbonate. This treatment generally converts the cationic constituents of the sample to acid-soluble carbonates or oxides. The nonmetallic constituents are converted to soluble sodium salts. Carbonate fusions are normally carried out in platinum crucibles.

While they are very effective solvents, fluxes introduce high concentrations of ionic species to aqueous solutions of the melt.

❯

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37E Decomposing Inorganic Materials with Fluxes  985

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Potassium PyrosulfatePotassium pyrosulfate is a potent acidic flux that is particularly useful for attacking the more intractable metal oxides. Fusions with this reagent are performed at about 400°C. At this temperature, the slow evolution of the highly acidic sulfur trioxide takes place:

K2S2O7 S K2SO4 1 SO3(g)

Potassium pyrosulfate can be prepared by heating potassium hydrogen sulfate:

2KHSO4 S K2S2O7 1 H2O

Lithium MetaborateLithium metaborate, LiBO2, by itself or mixed with lithium tetraborate finds consid-erable use for attacking refractory silicate and alumina minerals, particularly for AAS, ICP emission, and X-ray absorption or emission determinations. These fusions are generally carried out in graphite or platinum crucibles at about 900°C. The glass that results on cooling the melt can be used directly for X-ray fluorescence measurements. It is also readily soluble in mineral acids. After solution of the melt, boric oxide is removed by evaporation to dryness with methyl alcohol and distillation of methyl borate, B(OCH3)3.

tABle 37-1Common Fluxes

Flux

Melting Point, °C

Type of Crucible for Fusion

Type of Substance Decomposed

Na2CO3 851 Pt Silicates and silica-containing samples, alumina-containing samples, sparingly soluble phosphates and sulfates

Na2CO3 1 an oxidizing agent, such as KNO3, KClO3, or Na2O2

— Pt (not with Na2O2), Ni Samples requiring an oxidizing environment, that is samples containing S, As, Sb, Cr, etc.

LiBO2 849 Pt, Au, Glassy carbon Powerful basic flux for silicates, most minerals, slags, ceramics

NaOH or KOH 318 380

Au, Ag, Ni Powerful basic fluxes for silicates, silicon carbide, and certain minerals (main limitation is purity of reagents)

Na2O2 Decomposes Fe, Ni Powerful basic oxidizing flux for sulfides; acid-insoluble alloys of Fe, Ni, Cr, Mo, W, and Li; platinum alloys; Cr, Sn, Zr minerals

K2S2O7 300 Pt, porcelain Acidic flux for slightly soluble oxides and oxide-containing samples

B2O3 577 Pt Acidic flux for silicates and oxides where alkali metals are to be determined

CaCO3 1 NH4Cl — Ni On heating the flux, a mixture of CaO and CaCl2 is produced; used to decompose silicates for determining alkali metals

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