Decomposing and
Dissolving the Sample
CHAPTER 37
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, automated 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.
Courtesy of Aurora Biomed Inc., Vancouver, B.C.
A refractory substance is resistant
to heat and attack by strong chemical
agents.
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.
1
For 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.
37B Decomposing Samples with Inorganic Acids in Open Vessels 977
Sources of Error in Decomposition
37A 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 selenide, 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 produce 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 present 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 hightemperature fusions. Again, this source of error becomes of particular concern in
trace analyses.
Decomposing Samples with Inorganic
37B 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.
the reagent selected
❮ Ideally
should dissolve the entire
sample, not just the analyte.
978 CHAPTER 37
Decomposing and Dissolving the Sample
37B-1 Hydrochloric Acid
Concentrated 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 hydrochloric 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 Acid
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.
Hot 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 Acid
Many 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 Acid
Hot 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.
2
See A. A. Schilt, Perchloric Acid and Perchlorates, Columbus, Ohio: G. Frederick Smith Chemical
Company, 1979.
37C Microwave Decompositions 979
37B-5 Oxidizing Mixtures
More 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 oxidation of the organic material is complete. Severe explosions and injuries have resulted
from failure to observe this precaution.
37B-6 Hydrofluoric Acid
The 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 present 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 attacking 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 Decompositions
The use of microwave ovens for the decomposition of both inorganic and organic samples 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.
3
For 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. LuqueGarcia and M. D. Luque de Castro, Trends in Analytical Chemistry, 2003, 22, 90, DOI: 10.1016/
s0165-9936(03)00202-4.
980 CHAPTER 37
Decomposing and Dissolving the Sample
Vent tubing
Venting nut
Vessel cap
One of the main advantages of microwave decompositions compared with conventional 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 difficult 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 contaminants. A further advantage of decompositions of this type is loss of volatile
components of samples is virtually eliminated. Finally closed-vessel microwave decompositions are often easy to automate, thus reducing operator time required to
prepare samples for analysis.
Safety valve
37C-1 Vessels for Moderate-Pressure Digestions
Vessel body
Figure 37-1 A moderate-pressure
vessel for microwave decomposition.
(Courtesy of CEM Corp.,
Matthews, NC.)
Pressure screw
Screw cap
Relief disk
Sealer disk
Inner cover
O-ring
Sample cup
Bomb body
Bottom plate
Figure 37-2 A bomb for highpressure microwave decomposition.
(Courtesy of Parr Instrument Co.,
Moline IL.)
Microwave 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 attacked 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 Vessels
Figure 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
Unless otherwise noted, all content on this page is © Cengage Learning.
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 occurs, 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 useful 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 polymeric 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 Digestions
The 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 spectroscopic determinations.
37C-4 Microwave Ovens
Figure 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 received by each of the vessels is approximately the same.
37C-5 Microwave Furnaces
Recently, microwave furnaces have been developed for performing fusions and for
dry ashing samples containing large amounts of organic materials before acid dissolutions. These furnaces consist of a small chamber constructed of silicon carbide 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 usually operated continuously because of the elapsed time required to get them up to
temperature. Furthermore, with a microwave furnace there are no burned-out heating 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.
Unless otherwise noted, all content on this page is © Cengage Learning.
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.)
982 CHAPTER 37
Decomposing and Dissolving the Sample
37C-6 Applications of Microwave Decompositions
During 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 decompositions 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.
Combustion Methods for Decomposing
37D Organic Samples4
37D-1 Combustion over an Open Flame (Dry Ashing)
Dry ashing is a process of oxidizing an
organic sample with oxygen or air at
high temperature, leaving the
inorganic component for analysis.
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 until 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 uncertainty about the completeness of recovery of supposedly nonvolatile elements from
a dry-ashed sample. Some losses probably result from the entrainment of finely divided 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 Methods
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.
Several 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 available 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
4
For a thorough treatment of this topic, see T. S. Ma and R. C. Rittner, Modern Organic Elemental
Analysis, New York: Marcel Dekker, 1979.
5
Ibid., Chs. 2–4.
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 measurements (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 Container
A relatively straightforward method for the decomposition of many organic substances
involves combustion with oxygen in a sealed container. The reaction products are absorbed 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 oxidations has been suggested by Schöniger.6 It consists of a heavy-walled flask of 300to 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 combustion, the flask is shielded to minimize damage in case of explosion.
After cooling, the flask is shaken thoroughly and disassembled, and the inner surfaces are carefully rinsed. The analysis is then performed on the resulting solution.
This procedure has been applied to the determination of halogens, sulfur, phosphorus, fluorine, arsenic, boron, carbon, and various metals in organic compounds.
Sample
Sample in
holder
Ignition
point
Absorption
liquid
Sample
wrapped in
paper holder
6
Stopper with
S ground joint
W. 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.
Unless otherwise noted, all content on this page is © Cengage Learning.
Figure 37-4 Schöniger combustion
apparatus. (Courtesy of Thomas
Scientific, Swedesboro, NJ.)
984 CHAPTER 37
Decomposing and Dissolving the Sample
While they are very effective
solvents, fluxes introduce high
concentrations of ionic species
to aqueous solutions of the melt.
❯
Decomposing Inorganic Materials
37E 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 watersoluble 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 concentration 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 subsequent 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 Procedure
The 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 solidification, 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 Fluxes
With 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 Carbonate
Silicates 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.
37E Decomposing Inorganic Materials with Fluxes 985
Table 37-1
Common Fluxes
Flux
Na2CO3
Melting Point, °C
851
Na2CO3 1 an oxidizing
agent, such as KNO3,
KClO3, or Na2O2
LiBO2
NaOH or KOH
Type of Crucible
for Fusion
Pt
—
Pt (not with Na2O2), Ni
849
Pt, Au, Glassy carbon
318
380
Au, Ag, Ni
Na2O2
Decomposes
K2S2O7
300
Pt, porcelain
B2O3
577
Pt
CaCO3 1 NH4Cl
—
Ni
Fe, Ni
Potassium Pyrosulfate
Potassium 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 Metaborate
Lithium metaborate, LiBO2, by itself or mixed with lithium tetraborate finds considerable 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.
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Type of Substance
Decomposed
Silicates and silica-containing
samples, alumina-containing
samples, sparingly soluble
phosphates and sulfates
Samples requiring an oxidizing
environment, that is samples
containing S, As, Sb, Cr, etc.
Powerful basic flux for silicates,
most minerals, slags, ceramics
Powerful basic fluxes for
silicates, silicon carbide, and
certain minerals (main limitation
is purity of reagents)
Powerful basic oxidizing flux for
sulfides; acid-insoluble alloys of
Fe, Ni, Cr, Mo, W, and Li;
platinum alloys; Cr, Sn, Zr
minerals
Acidic flux for slightly soluble
oxides and oxide-containing
samples
Acidic flux for silicates and
oxides where alkali metals are to
be determined
On heating the flux, a mixture of
CaO and CaCl2 is produced;
used to decompose silicates for
determining alkali metals