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13.1 Introduction
The overall success of any analytical procedure depends upon many factors, including proper sample preparation, appropriate sample dissolution, and adequate separation and isolation of the target analytes. This chapter describes sample dissolution techniques and strategies. Some of the principles of dissolution are common to those of radiochemical separation that are described in Chapter 14 ( Separation Techniques ), but their importance to dissolution is reviewed here.
Sample dissolution can be one of the biggest challenges facing the analytical chemist, because most samples consist mainly of unknown compounds with unknown chemistries. There are many factors for the analyst to consider: What are the measurement quality objectives of the program? What is the nature of the sample; is it refractory or is there only surface contamination? How effective is the dissolution technique? Will any analyte be lost? Will the vessel be attacked? Will any of the reagents interfere in the subsequent analysis or can any excess reagent be removed? What are the safety issues involved? What are the labor and material costs? How much and what type of wastes are generated? The challenge for the analyst is to balance these factors and to choose the method that is most applicable to the material to be analyzed.
The objective of sample dissolution is to mix a solid or nonaqueous liquid sample quantitatively with water or mineral acids to produce a homogeneous aqueous solution, so that subsequent separation and analyses may be performed. Because very few natural or organic materials are water-soluble, these materials routinely require the use of acids or fusion salts to bring them into solution. These reagents typically achieve dissolution through an oxidation-reduction process that leaves the constituent elements in a more soluble form. Moreover, because radiochemists routinely add carriers or use the technique of isotope dilution to determine certain radioisotopes, dissolution helps to ensure exchange between the carrier or isotopic tracer and the element or radioisotope to be determined, although additional chemical treatment might be required to ensure exchange.
There are three main techniques for sample decomposition discussed in this chapter: fusion; wet ashing, acid leaching, or acid dissolution; and microwave digestion.
The choice of technique is determined by the type of sample and knowledge of its physical and chemical characteristics. Fusion and wet ashing techniques may be used singly or in combination to decompose most samples analyzed in radioanalytical laboratories.
Contents
13.1 Introduction.......................... 13- 13.2 The Chemistry of Dissolution............ 13- 13.3 Fusion Techniques.................... 13- 13.4 Wet Ashing and Acid Dissolution Techniques......................... 13- 13.5 Microwave Digestion................. 13- 13.6 Verification of Total Dissolution........ 13- 13.7 Special Matrix Considerations.......... 13- 13.8 Comparison of Total Dissolution and Acid Leaching........................... 13- 13.9 References.......................... 13-
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Leaching techniques are used to determine the soluble fraction of the radionuclide of interest under those specific leaching conditions. Different formulas for leaching agents will yield different amounts of leachable analyte. It should be recognized that the information so obtained leaves unknown the total amount of analyte present in the sample. Because recent advances in microwave vessel design (e.g., better pressure control and programmable temperature control) have allowed for the use of larger samples, microwave dissolution is becoming an important tool in the radiochemistry laboratory. Leaching and the newer closed-vessel microwave methods provide assurance that only minimal analyte loss will occur through volatilization.
Because of the potential for injury and explosions during sample treatment, it is essential that proper laboratory safety procedures be in place, the appropriate safety equipment be available, a safe work space be provided, and that the laboratory personnel undergo the necessary training to ensure a safe working environment before any of these methods are used. Review the Material Data Safety Sheets for all chemicals before their use.
Aspects of proper sample preparation, such as moisture removal, oxidation of organic matter, and homogenization, were discussed in Chapter 12, Laboratory Sample Preparation. Fundamental separation principles and techniques, such as complexation, solvent extraction, ion exchange, and co-precipitation, are reviewed in Chapter 14, Separation Techniques.
There are many excellent references on sample dissolution (e.g., Bock, 1979; Bogen, 1978; Dean, 1995; Sulcek and Povondra, 1989).
13.2 The Chemistry of Dissolution
In order to dissolve a sample completely, each insoluble component must be converted into a soluble form. Several different chemical methods may need to be employed to dissolve a sample completely; usually, the tracer is added to the sample at the time of sample dissolution. Initially the sample may be treated with acids yielding an insoluble residue. The residue may need to be dissolved using fusion or hydrofluoric acid (HF) and then combined with the original mixture or analyzed separately. In either case, the tracer/carrier should be added to the sample during the first step of chemical change (e.g., acid dissolution as above) so that the yield for the entire process may be determined accurately. An outline of the principles of these chemical methods is provided in this section, but a complete description is available in Chapter 14, where the principles are applied to a broader range of topics.
13.2.1 Solubility and the Solubility Product Constant, Ksp
The solubility data of many compounds, minerals, ores, and elements are available in reference manuals. Solubilities typically are expressed in grams of substance per 100 mL of solvent, although other units are sometimes used. The information is more complete for some substances
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H 2 CO 3 6 CO 2 + H 2 O
and the net reaction is as follows:
RaCO 3 + 2 HCl 6 RaCl 2 + CO 2 + H 2 O
Sodium pyrosulfate fusion, for example, converts zirconia (ZrO 2 ) into zirconium sulfate [Zr(SO 4 ) 2 ], which is soluble in acid solution by a simple (nonoxidative) rearrangement of oxygen atoms (Hahn, 1961; Steinberg, 1960):
ZrO 2 + 2 Na 2 S 2 O 7 6 2 Na 2 SO 4 + Zr(SO 4 ) 2
Many environmental samples contain insoluble silicates, such as aluminum silicate [Al 2 (SiO 3 ) 3 or Al 2 O 3 · 3SiO 2 ], which can be converted into soluble silicates by fusion with sodium carbonate:
Al 2 (SiO 3 ) 3 + 4 Na 2 CO 3 6 3 Na 2 SiO 3 + 2 NaAlO 2 + 4 CO 2
Dissolution of radium from some ores depends on the exchange of anions associated with the radium cation (sulfate for example) to generate a soluble compound. Extraction with nitric acid is partly based on this process, generating soluble radium nitrate.
13.2.3 Oxidation-Reduction Processes
Oxidation-reduction (redox) processes are an extremely important aspect of sample dissolution. The analyte may be present in a sample in several different chemical forms or oxidation states. As an example, consider a ground-water sample that contains 129 I as the analyte. The iodine may be present in any of the following inorganic forms: I!, I 2 , IO!, or IO 3 !. If the ground water has a high reduction potential or certain bacteria are present, the iodine also may be present as CH 3 I. It is of paramount importance to ensure that all of these different forms of iodine are brought to the same oxidation state (e.g., to iodate) at the time of first change in redox environment or change in sample composition. Furthermore, accurate assessment of chemical yield only can be determined if the tracer or carrier is added prior to a change in chemical form or oxidation state of the analyte at an initial point in the digestion process. This process is referred to as equilibration of the tracer/carrier and analyte. From this point on during the sample analysis, any loss that occurs to the analyte will occur to an equal extent for the tracer/carrier, thus allowing the calculation of a chemical yield for the process.
A redox reaction redistributes electrons among the atoms, molecules, or ions in the reaction. In some redox reactions, electrons actually are transferred from one reacting species to another. In other redox reactions, electrons are not transferred completely from one reacting species to another; the electron density about one atom decreases, while it increases about another atom. A complete discussion of oxidation and reduction is found in Section 14.2, Oxidation-Reduction Processes.
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Many oxidizing agents used in sample dissolution convert metals to a stable oxidation state displacing hydrogen from hydrochloric, nitric, sulfuric, and perchloric acids. (This redox process often is referred to as nonoxidative hydrogen replacement by an active metal, but it is a redox process where the metal is oxidized to a cation, usually in its highest oxidation state, and the hydrogen ion is reduced to its elemental form.) Dissolution of uranium for analysis is an example of hydrogen-ion displacement to produce a soluble substance (Grindler, 1962):
U + 8 HNO 3 6 UO 2 (NO 3 ) 2 + 6 NO 2 + 4 H 2 O
Prediction of the reactivity of a metal with acids is dependent on its position in the electromotive force series (activity series). A discussion of the series appears in Section 13.4.1, Acids and Oxidants. In general, metals with a negative standard reduction potential will replace hydrogen and be dissolved. Perchloric acid offers a particular advantage because very soluble metal perchlorate salts are formed.
Other important oxidizing processes depend on either oxidizing a lower, less soluble oxidation state of a metal to a higher, more soluble state or oxidizing the counter anion to generate a more soluble compound. Oxidation to a higher state is common when dissolving uranium samples in acids or during treatment with fusion fluxes. The uranyl ion (UO 2 +2) forms soluble saltssuch as chloride, nitrate, and perchloratewith anions of the common acids (Grindler, 1962). (Complex- ion formation also plays a role in these dissolutions; see the next section). Dissolution of oxides, sulfides, or halides of technetium by alkaline hydrogen peroxide converts all oxidation states to the soluble pertechnetate salts (Cobble, 1964):
2 TcO 2 + 2 NaOH + 3 H 2 O 2 6 2 NaTcO 4 + 4 H 2 O
13.2.4 Complexation
The formation of complex ions (see also Section 14.3, Complexation) is important in some dissolution processes, usually occurs in conjunction with treatment by an acid, and also can occur during fusion. Complexation increases solubility in the dissolution mixture and helps to mini- mize hydrolysis of the cations. The solubility of radium sulfate in concentrated sulfuric acid is the result of forming a complex-ion, Ra(SO 4 ) 2!^2. The ability of both hydrochloric and hydro- fluoric acids to act as a solubilizing agent is dependent on their abilities to form stable complex ions with cations. Refractory plutonium samples are solubilized in a nitric acid-hydrofluoric acid solution forming cationic fluorocomplexes such as PuF+3^ (Booman and Rein, 1962). Numerous stable complexes of anions from solubilizing acids (HCl, HF, HNO 3 , H 2 SO 4 , HClO 4 ) contribute to the dissolution of other elements, such as americium, cobalt, technetium, thorium, uranium, and zirconium (see Section 14.10, Analysis of Specific Radionuclides). The process of fusion with sodium carbonate to solubilize uranium samples is also based on the formation of UO 2 (CO 3 ) 2!^4 after the metal is oxidized to U+6^ (Grindler, 1962).
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material generally depends on the salt used for the fusion.
During fusion, samples are heated slowly and evenly to prevent ignition of the sample before the reaction with the molten salt can begin. It is especially important to raise the temperature slowly when using a gas flame because the evolution of water and gases is a common occurrence at the beginning of the fusion, and hence a source of spattering. The crucible can be covered with a lid as an added precaution. Sand and oil baths provide the most even source of heat, but they are difficult to maintain at very high temperatures. Muffle furnaces provide an even source of heat, but when using them it is difficult to monitor the progress of the reaction and impossible to work with the sample during the fusion. Burners are used often as a convenient heat source although they make it difficult to heat the sample evenly.
T ABLE 13.1 Common fusion fluxes Flux (mp, E C)
Fusion Temperature, E C
Type of Crucible Types of Sample Decomposed Na 2 S 2 O 7 (403E) or K 2 S 2 O 7 (419E) Up to red heat^
Pt, quartz, porcelain
For insoluble oxides and oxide-containing samples, particularly those of Al, Be, Ta, Ti, Zr, Pu, and the rare earths. NaOH (321E) or KOH (404E)
450-600E Ni, Ag, glassycarbon For silicates, oxides, phosphates, and fluorides.
Na 2 CO 3 (853) or K 2 CO 3 (903) 900-1,000E^
Ni Pt for short periods (use lid)
For silicates and silica-containing samples (clays, minerals, rocks, glasses), refractory oxides, quartz, and insoluble phosphates and sulfates. Na 2 O 2 600 E Ni; Ag, Au, Zr;Pt (<500 EC) For sulfides; acid-insoluble alloys of Fe, Ni, Cr, Mo,W, and Li; Pt alloys; Cr, Sn, and Zn minerals.
H 3 BO 3 250 E Pt For analysis of sand, aluminum silicates, titanite,natural aluminum oxide (corundum), and enamels.
Na 2 B 4 O 7 (878E) 1,000-1,200E Pt
For Al 2 O 3 ; ZrO 2 and zirconium ores, minerals of the rare earths, Ti, Nb, and Ta, aluminum-containing materials; iron ores and slags.
Li 2 B 4 O 7 (920E) or LiBO 2 (845E)
1,000-1,100E Pt, graphite
For almost anything except metals and sulfides. The tetraborate salt is especially good for basic oxides and some resistant silicates. The metaborate is better suited for dissolving acidic oxides such as silica and TiO 2 and nearly all minerals. NH 4 HF 2 (125E) NaF (992E) KF (857E) or KHF 2 (239E)
900 E Pt
For the removal of silicon, the destruction of silicates and rare earth minerals, and the analysis of oxides of Nb, Ta, Ti, and Zr.
Source: Dean (1995) and Bock (1979).
The maximum temperature employed varies considerably and depends on the sample and the flux. In order to minimize attack of the crucible and decomposition of the flux, excessive temperatures should be avoided. Once the salt has melted, the melt is swirled gently to monitor the reaction. The fusion continues until visible signs of reaction are completed (e.g., formation of
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gases, foaming, fumes). It is frequently difficult to decide when heating should be discontinued. In ideal cases, a clear melt serves to indicate the completeness of sample decomposition. In other cases, it is not as obvious, and the analyst must base the heating time on past experience with the sample type.
The melt sometimes is swirled during cooling to spread it over the inside of the crucible. Thin layers of salt on the sides of the crucible often will crack and flake into small pieces during cooling. These small fragments are easier to remove and dissolve.
After the sample has returned to room temperature, the fused material is dissolved. The solvent is usually warm water or a dilute acid solution, depending on the salt. For example, dilute acid typically would not be used to dissolve a carbonate fusion because of losses to spray caused by release of CO 2. The aqueous solution from the dissolution of the fusion melt should be examined carefully for particles of undissolved sample. If undissolved particles are present, they should be separated from solution by centrifugation or filtration, and a second fusion should be performed.
Several types of materials are used for crucibles, but platinum, other metals (Ni, Zr, Ag), and graphite are most common. Graphite crucibles are a cost-effective alternative to metal crucibles; they are disposable, which eliminates the need for cleaning and the possibility of cross-sample contamination. Graphite crucibles are chemically inert and heat-resistant, although they do oxidize slowly at temperatures above 430 EC. Graphite is not recommended for extremely lengthy fusions or for reactions where the sample may be reduced. Platinum is probably the most commonly used crucible material. It is virtually unaffected by most of the usual acids, including hydrofluoric, and it is attacked only by concentrated phosphoric acid at very high temperatures, and by sodium carbonate. However, it dissolves readily in mixtures of hydrochloric and nitric acids (aqua regia), nitric acid containing added chlorides, or chlorine water or bromine water. Platinum offers adequate resistance toward molten alkali metal, borates, fluorides, nitrates, and bisulfates. When using a platinum crucible, one should avoid using aqua regia, sodium peroxide, free elements (C, P, S, Ag, Bi, Cu, Pb, Zn, Se, and Te), ammonium, chlorine and volatile chlorides, sulfur dioxide, and gases with carbon content. Platinum crucibles can be cleaned in boiling HNO 3 , by hand cleaning with sea sand or by performing a blank fusion with sodium hydrogen sulfate.
Many kinds of salts are used in fusions. The lowest melting flux capable of reacting completely with the sample is usually the optimum choice. Basic fluxes, such as the carbonates, the hydroxides, and the borates, are used to attack acidic materials. Sodium or potassium nitrate may be added to furnish an oxidizing agent when one is needed, as with the sulfides, certain oxides, ferroalloys, and some silicate materials. The most effective alkaline oxidizing flux is sodium peroxide; it is both a strong base and a powerful oxidizing agent. Because it is such a strong alkali, sodium peroxide is often used even when no oxidant is required. Alternatively, acid fluxes are the pyrosulfates, the acid fluorides, and boric acids. Table 13.1 lists several types of fusions, examples of salts used for each type of fusion, and the melting points of the salts.
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minutes. The solidified melt dissolves readily in water; and therefore, this step may be carried out directly in the crucible, or alternatively in a nickel dish. Under no circumstances should the dissolution be carried out in a glass vessel because the resulting concentrated hydroxide solution attacks glass quite readily.
FUSION WITH SODIUM CARBONATE (Na 2 CO 3 ) is a common procedure for decomposing silicates (clays, rocks, mineral, slags, glasses, etc.), refractory oxides (magnesia, alumina, beryllia, zirconia, quartz, etc.), and insoluble phosphates and sulfates (Bogen, 1978). The fusion may result in the formation of a specific compound such as sodium aluminate, or it may simply convert a refractory oxide into a condition where it is soluble in hydrochloric acidthis is the method of choice when silica in a silicate is to be determined, because the fusion converts an insoluble silicate into a mixture that is easily decomposed by hydrochloric acid (M represents a metal in the equations below):
MSiO 3 + Na 2 CO 3 6 Na 2 SiO 3 + MCO 3 (or MO + CO 2 ),
followed by acidification to form a more soluble chloride salt,
Na 2 SiO 3 + MCO 3 + 4 HCl + x H 2 O 6 H 2 SiO 3 · x H 2 O + MCl 2 + CO 2 + H 2 O + NaCl.
Carbonate fusions provide an oxidizing melt for the analysis of chromium, manganese, sulfur, boron, and the platinum group metals. Organic material is destroyed, sometimes violently. Na 2 CO 3 generally is used because of its lower melting point. However, despite its higher melting point and hygroscopic nature, K 2 CO 3 is preferred for niobium and tantalum analyses because the resulting potassium salts are soluble, whereas the analogous sodium salts are insoluble.
The required temperature and duration of the fusion depend on the nature of the sample as well as particle size. In the typical carbonate fusion, 1 g of the powdered sample is mixed with 4 to 6 g of sodium carbonate and heated at 900 to 1,000 EC for 10 to 30 minutes. Very refractory materials may require heating at 1,200 EC for as long as 1 to 2 hours. Silica will begin to react at 500 EC, while barium sulfate and alumina react at temperatures above 700 EC. Volatility could be a problem at these temperatures. Mercury and thallium are lost completely, while selenium, arsenic, and iodine suffer considerable losses. Nonsilicate samples should be dissolved in water, while silicate samples should be treated with acid (Bock, 1979).
Platinum crucibles are recommended for fusion of solid samples even though there is a 1 to 2 mg loss of platinum per fusion. Attack on the crucible can be reduced significantly by covering the melt with a lid during the fusion process, or virtually eliminated by working in an inert atmos- phere. Moreover, nitrate is often added to prevent the reduction of metals and the subsequent alloying with the platinum crucibles. The platinum crucibles may be seriously attacked by samples containing high concentrations of Fe2+, Fe3+, Sn4+, Pb2+, and compounds of Sb and As, because these ions are reduced easily to the metallic state and then form intermetallic alloys with
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platinum that are not easily dissolved in mineral acids. This problem is especially prevalent when fusion is carried out in a gas flame. Porcelain crucibles are corroded rapidly and should be discarded after a single use.
13.3.2 Boron Fusions
Fusions with boron compounds are recommended for analysis of sand, slag, aluminum silicates, alumina (Al 2 O 3 ), iron and rare earth ores, zirconium dioxide, titanium, niobium, and tantalum. Relatively large amounts of flux are required for these types of fusions. The melts are quite viscous and require swirling or stirring, so they should not be performed in a furnace. Platinum crucibles should be used for these fusions because other materials are rapidly attacked by the melt, even though some platinum is lost in each fusion.
BORIC ACID (H 3 BO 3 ) can be used to fuse a number of otherwise inert substances such as sand, aluminum silicates, titanite, natural aluminum oxide (corundum), and enamels. Boric acid fusions generally require 4 to 8 times as much reagent as sample. Initially, the mixture should be heated cautiously while water is being driven off, then more strongly until gas evolution is completed, and then more vigorously if the sample has yet to be fully decomposed. Normally, the procedure is complete within 20 to 30 minutes. The cooled and solidified melt usually is dissolved in dilute acid. Additionally, boric acid has one great advantage over all other fluxes in that it can be completely removed by addition of methanol and subsequent volatilization of the methyl ester.
Because MOLTEN SODIUM TETRABORATE (Na 2 B 4 O 7 ) dissolves so many inorganic compounds, it is an important analytical tool for dissolving very resistant substances. Fusions with sodium tetra- borate alone are useful for Al 2 O 3 , ZrO 2 and zirconium ores, minerals of the rare earths, titanium, niobium, and tantalum, aluminum-containing materials, and iron ores and slags (Bock, 1979). Relatively large amounts of borax are mixed with the sample, and the fusion is carried out at a relatively high temperature (1,000 to 1,200 EC) until the melt becomes clear. Thallium, mercury, selenium, arsenic, and the halogens are volatilized under these conditions. Boric acid can be removed from the melt as previously described. By dissolving the melt in dilute hydrofluoric acid, calcium, thorium, and the rare earths can be separated from titanium, niobium, and tantalum as insoluble fluorides.
LITHIUM METABORATE (Li 2 B 4 O 7 ) is well-suited for dissolving basic oxides, such as alumina (Al 2 O 3 ), quicklime (CaO), and silicates. Platinum dishes are normally used for this type of fusion, but occasionally graphite crucibles are advantageous because they can be heated rapidly by induction, and because they are not wetted by Li 2 B 4 O 7 melts. The fusion melt typically is dissolved in dilute acid, usually nitric but sometimes sulfuric. When easily hydrolyzed metal ions are present, dissolution should be carried out in the presence of ethylenediamine tetracetic acid (EDTA) or its di-sodium salt in 0.01 M HCl (Bock, 1979).
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major portion of the sample but leave a minor fraction as residue. Whether or not this residue requires additional treatment (by wet ashing or fusion) depends on the amount of residue and whether it is expected to contain the radionuclides of interest. The residue should not be discarded until all of the results have been reviewed and determined to be acceptable.
13.4.1 Acids and Oxidants
Numerous acids are commonly used in wet ashing procedures. Table 13.2 lists several acids and the types of compounds they generally react with during acid dissolution. The electromotive force series (Table 13.3) is a summary of oxidation-reduction half-reactions arranged in decreasing oxidation strength and is also useful in selecting reagent systems (Dean, 1995).
T ABLE 13.2 Examples of acids used for wet ashing Acid Typical Uses
Hydrofluoric Acid, HF
Hydrochloric Acid, HCl
Hydrobromic Acid, HBr Hydroiodic Acid, HI
Sulfuric Acid, H 2 SO 4
Phosphoric Acid, H 3 PO 4
Nitric Acid, HNO 3
Perchloric Acid, HClO 4
Removal of silicon and destruction of silicates; dissolves oxides of Nb, Ta, Ti, and Zr, and Nb, and Ta ores. Dissolves many carbonates, oxides, hydroxides, phosphates, borates, and sulfides; dissolves cement. Distillation of bromides (e.g., As, Sb, Sn, Se). Effective reducing agent; dissolves Sn+4^ oxide and Hg +2^ sulfide. Dissolves oxides, hydroxides, carbonates, and various sulfide ores; hot concentrated acid will oxidize most organic compounds. Dissolves Al 2 O 3 , chrome ores, iron oxide ores, and slag. Oxidizes many metals and alloys to soluble nitrates; organic material oxidized slowly. Extremely strong oxidizer; reacts violently or explosively to oxidize organic compounds; attacks nearly all metals.
The table allows one to predict which metals will dissolve in nonoxidizing acids, such as hydro- chloric, hydrobromic, hydrofluoric, phosphoric, dilute sulfuric, and dilute perchloric acid The dissolution process is simply a replacement of hydrogen by the metal (Dean, 1995). In practice, however, what actually occurs is influenced by a number of factors, and the behavior of the metals cannot be predicted from the potentials alone. Generally, metals below hydrogen in Table 13.3 displace hydrogen and dissolve in nonoxidizing acids with the evolution of hydrogen. Notable exceptions include the very slow dissolution by hydrochloric acid of lead, cobalt, nickel, cadmium, and chromium. Also, lead is insoluble in sulfuric acid because of the formation of a surface film of insoluble lead sulfate.
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T ABLE 13.3 Standard reduction potentials of
selected half-reactions at 25 E C
- Ag2+ + e!^6 Ag+ 1.980 I! 3 + 3e!^6 3I! 0. Half-Reaction E^0 (volts) Half-Reaction E^0 (volts)
- S 2 O 8 2- + 2e!^6 2SO 42 - 1.96 I 2 + 2e!^6 2I! 0.
- Ce4+ + e!^6 Ce3+ 1.72 Cu + + e!^6 Cu 0.
- MnO 4! + 4H + + 3e!^6 MnO 2 (s) + 2H 2 O 1.70 4H 2 SO 3 + 4H + + 6e!^6 S 4 O 6 2- + 6H 2 O 0.
- 2HClO + 2H + + 2e!^6 Cl 2 + 2H 2 O 1.630 Ag 2 CrO 4 + 2e!^6 2Ag + CrO 42 - 0.
- 2HBrO + 2H + + 2e!^6 Br 2 + 2H 2 O 1.604 2H 2 SO 3 + 2H + + 4e!^6 S 2 O 3 2- + 3H 2 O 0.
- NiO 2 + 4H + + 2e!^6 Ni2+ + 2H 2 O 1.593 UO 2 + + 4H + + e!^6 U 4+ + 2H 2 O 0.
- Bi 2 O 4 (bismuthate) + 4H + + 2e!^6 2BiO + + 2H 2 O 1.59 Cu 2+ + 2e!^6 Cu 0.
- MnO 4! + 8H + + 5e!^6 Mn 2+ + 4H 2 O 1.51 VO 2+ + 2H + + e!^6 V 3+ + H 2 O 0.
- 2BrO 3! + 12H + + 10e!^6 Br 2 + 6H 2 O 1.478 BiO + + 2H + + 3e!^6 Bi + H 2 O 0.
- PbO 2 + 4H + + 2e!^6 Pb 2+ + 2H 2 O 1.468 UO 2 2+ + 4H + + 2e!^6 U 4+ + 2H 2 O 0.
- Cr 2 O 7 2- + 14H + + 6e!^6 2Cr 3+ + 7H 2 O 1.36 Hg 2 Cl 2 (s) + 2e!^6 2Hg + 2Cl! 0.
- Cl 2 + 2e!^6 2Cl! 1.3583 AgCl (s) + e!^6 Ag + Cl! 0.
- 2HNO 2 + 4H + + 4e!^6 N 2 O + 3H 2 O 1.297 SbO + + 2H + + 3e!^6 Sb + H 2 O 0.
- MnO 2 + 4H ++ 2e!^6 Mn 2+ + 2H 2 O 1.23 CuCl 3 2- + e!^6 Cu + 3Cl! 0.
- O 2 + 4H + + 4e!^6 2H 2 O 1.229 SO 4 2- + 4H + + 2e!^6 H 2 SO 3 + H 2 O 0.
- ClO 4! + 2H + + 2e!^6 ClO! 3 + H 2 O 1.201 Sn4+ + 2e!^6 Sn 2+ 0.
- 2IO 3! + 12H + + 10e!^6 I 2 + 3H 2 O 1.19 CuCl + e!^6 Cu + Cl! 0.
- N 2 O 4 + 2H + + 2e!^6 2HNO 2 1.07 TiO 2+ + 2H + + e- 6 Ti3+ + H 2 O 0.
- 2ICl! 2 + 2e!^6 4Cl! + I 2 1.07 S 4 O 6 2- + 2e!^6 2S 2 O 32 - 0.
- Br 2 (aq) + 2e!^6 2Br- 1.065 2H + + 2e!^6 H 2 0.
- N 2 O 4 + 4H + + 4e!^6 2NO + 2H 2 O 1.039 Hg 2 I 2 (s) + 2e!^6 2Hg + 2I! -0.
- HNO 2 + H + + e!^6 NO + H 2 O 0.996 Pb2+ + 2e!^6 Pb -0.
- NO 3! + 4H + + 3e!^6 NO + 2H 2 O 0.957 Sn2+ + 2e!^6 Sn -0.
- NO 3! + 3H + + 2e!^6 HNO 2 + H 2 O 0.94 AgI (s) + e!^6 Ag + I! -0.
- 2Hg2+ + 2e!^6 Hg 2 2+ 0.911 V 3+ + e!^6 V 2+ -0.
- Cu2+ + I! + e!^6 CuI (s) 0.861 Ni2+ + 2e!^6 Ni -0.
- OsO 4 (s) + 8H + + 8e!^6 Os + 4H 2 O 0.84 Co 2+ + 2e!^6 Co -0.
- Ag+ + e!^6 Ag 0.7991 PbSO 4 + 2e!^6 Pb + SO 42 - -0.
- Hg 2 2+ + 2e!^6 2Hg 0.7960 Cd 2+ + 2e!^6 Cd -0.
- Fe3+ + e!^6 Fe2+ 0.771 Cr 3+ + e!^6 Cr 2+ -0.
- H 2 SeO 3 + 4H + + 4e!^6 Se + 3H 2 O 0.739 Fe2+ + 2e!^6 Fe -0.
- HN 3 + 11H + + 8e!^6 3NH 4 + 0.695 H 3 PO 3 + 2H + + 2e!^6 HPH 2 O 2 + H 2 O -0.
- O 2 + 2H + + 2e -^6 H 2 O 2 0.695 U 4+ + e!^6 U 3+ -0.
- Ag 2 SO 4 + 2e!^6 2Ag + SO 42 - 0.654 Zn 2+ + 2e!^6 Zn -0.
- Cu 2+ + Br! + e!^6 CuBr (s) 0.654 Mn2+ + 2e!^6 Mn -1.
- 2HgCl 2 + 2e!^6 Hg 2 Cl 2 (s) + 2Cl! 0.63 Al3+ + 3e!^6 Al -1.
- Sb 2 O 5 + 6H + + 4e!^6 2SbO + + 3H 2 O 0.605 Mg2+ + 2e!^6 Mg -2.
- H 3 AsO 4 + 2H + + 2e!^6 HAsO 2 + 2 H 2 O 0.560 Na+ + e!^6 Na -2.
- TeOOH + + 3H + + 4e!^6 Te + 2H 2 O 0.559 K+ + e!^6 K -2.
- Cu 2+ + Cl! + e!^6 CuCl (s) 0.559 Li+ + e!^6 Li -3.
- 3N 2 + 2H + + 2e!^6 2HN 3 -3.
- MARLAP 13-14 JULY Source: Dean, 1995.
amount of water to prevent losses as dust and spray when the acid is added to the sample. After the addition of HF, the sample may be allowed to react overnight to dissolve the silicates. However, heating the solution to 80 EC will allow reaction to occur within 1-2 hours. Because it is such a strong complexing agent, excess fluoride ion can cause problems with many separation methods. Residual fluoride is usually removed by evaporation to fumes in a low-volatility acid (e.g., H 2 SO 4 , HNO 3 , HClO 4 ) or, in extreme cases, excess fluoride ion can be removed by fusing the residue with boric acid or sodium tetraborate. The fluorides are converted to BF 3 that is then removed by evaporation.
HYDROCHLORIC ACID (HCl) is one of the most widely used acids for sample dissolution because of the wide range of compounds it reacts with and the low boiling point of the azeotrope (110 EC); after a period of heating in an open container, a constant boiling 6M solution remains. HCl forms strong complexes with Au+3, Ti+3, and Hg+2. The concentrated acid will also complex Fe+3, Ga+3, In+3, and Sn+4. Most chloride compounds are readily soluble in water except for silver chloride, mercury chloride, titanium chloride, and lead chloride. HCl can be oxidized to form chlorine gas by manganese dioxide, permanganate, and persulfate. While HCl dissolves many carbonates, oxides, hydroxides, phosphates, borates, sulfides, and cement, it does not dissolve the following:
Most silicates or ignited oxides of Al, Be, Cr, Fe, Ti, Zr, or Th; Oxides of Sn, Sb, Nb, or Ta; Zr phosphate; Sulfates of Sr, Ba, Ra, or Pb; Alkaline earth fluorides; Sulfides of Hg; or Ores of Nb, Ta, U, or Th.
The dissolution behavior of specific actinides by hydrochloric acid is discussed by Sulcek and Povondra (1989):
The rate of decomposition of oxidic uranium ores depends on the U(VI)/U(+4) ratio. The so-called uranium blacks with minimal contents of U(+4) are even dissolved in dilute hydrochloric acid. Uraninite (UO 2 ) requires an oxidizing mixture of hydrochloric acid with hydrogen peroxide, chlorate, or nitric acid for dissolution. Uranium and thorium compounds cannot be completely leached from granites by hydrochloric acid. Natural and synthetic thorium dioxides are highly resistant toward hydrochloric acid and must be decomposed in a pressure vessel. Binary phosphates of uranyl and divalent cations, e.g., autunite and tobernite, are dissolved without difficulties. On the other hand, phosphates of thorium, tetravalent uranium, and the rare earths (monazite and xenotime) are only negligibly attacked, even with the concentrated acid.
As+3, Sb+3, Ge+3, and Se+4^ are volatilized easily in HCl solutions, while Hg+2, Sn+4, and Rh(VII)
MARLAP 13-16^ JULY 2004
are volatilized in the latter stages of evaporation. Glass is the preferred container for HCl solutions.
HYDROBROMIC ACID (HBr) has no important advantages over HCl for sample dissolution. HBr forms an azeotrope with water containing 47.6 percent by weight of HBr, boiling at 124.3 EC. HBr is used to distill off volatile bromides of arsenic, antimony, tin, and selenium. HBr can also be used as a complexing agent for liquid-liquid extractions of gold, titanium, and indium.
HYDROIODIC ACID (HI) is readily oxidized. Solutions often appear yellowish-brown because of the formation of the triiodide complex (I! 3 ). HI is most often used as a reducing agent during dissolutions. HI also dissolves Sn+4^ oxide, and complexes and dissolves Hg+2^ sulfide. HI forms an azeotrope with water containing 56.9 percent by weight of HI, boiling at 127 EC.
S ULFURIC ACID (H 2 SO 4 ) is another widely used acid for sample decomposition. Part of its effectiveness is due to its high boiling point (about 340 EC). Oxides, hydroxides, carbonates, and sulfide ores can be dissolved in H 2 SO 4. The boiling point can be raised by the addition of sodium or potassium sulfate to improve the attack on ignited oxides, although silicates will still not dissolve. H 2 SO 4 is not appropriate when calcium is a major constituent because of the low solubility of CaSO 4. Other inorganic sulfates are typically soluble in water, with the notable exceptions of strontium, barium, radium, and lead.
Non-fuming H 2 SO 4 does not exhibit oxidizing properties, but the concentrated acid will dissolve many elements and react with almost all organic compounds. Concentrated sulfuric acid is a powerful dehydrating agent. Its action on organic materials is a result of removing OH and H groups (to form water) from adjacent carbon atoms. This forms a black char (residue) that is not easily dissolved using wet-ashing techniques. Moreover, because of the high boiling point of H 2 SO 4 , there is an increased risk of losses because of volatilization. Iodine can be distilled quantitatively, and boron, mercury, selenium, osmium, ruthenium, and rhenium may be lost to some extent. The method of choice is to oxidize the organic substances with HNO 3 , volatilize the nitric acid, add H 2 SO 4 until charred, followed by HNO 3 again, repeating the process until the sample will not char with either HNO 3 or H 2 SO 4. Dissolution is then continued with HClO 4. Glass, quartz, platinum, and porcelain are resistant to H 2 SO 4 up to the boiling point. Teflon should not be used above 250 EC, and, therefore, it is not recommended for applications involving concentrated H 2 SO 4 that require elevated temperature.
Glass, quartz, platinum, and porcelain are resistant to H 2 SO 4 up to the boiling point. Teflon decomposes at 300 EC, below the boiling point, and, therefore, is not recommended for applications involving H 2 SO 4 that require elevated temperature.
P HOSPHORIC ACID (H 3 PO 4 ) seldom is used for wet ashing because the residual phosphates interfere with many separation procedures. H 3 PO 4 attacks glass, although glass containers are usually acceptable at temperatures below 300 EC. Alumina, chromium ores, iron oxide ores, and
JULY 2004 13-17^ MARLAP
Mixing HClO 4 with hypophosphates. Mixing HClO 4 with fats, oils, greases, or waxes. Evaporating solutions of metal salts to dryness in HClO 4. Evaporating alcoholic filtrates after collection of KClO 4 precipitates. Heating HClO 4 with cellulose, sugar, and polyhydroxy alcohols. Heating HClO 4 with N-heterocyclic compounds. Mixing HClO 4 with any dehydrating agent.
Perchloric acid vapor should never be allowed to contact organic materials such as rubber stoppers. The acid should be stored only in glass bottles. Splashed or spilled acid should be diluted with water immediately and mopped up with a woolen cloth, never cotton. HClO 4 should only be used only in specially designed fume hoods incorporating a washdown system.
Acid dissolutions involving HClO 4 should only be performed by analysts experienced in working with this acid. When any procedure is designed, the experimental details should be recorded exactly. These records are used to develop a detailed standard operating procedure that must be followed exactly to ensure the safety of the analyst (Schilt, 1979).
AQUA R EGIA. One part concentrated HNO 3 and three parts concentrated HCl (by volume) are combined to form aqua regia:
3HCl + HNO 3 6 NOCl + Cl 2 + 2H 2 O
However, the interaction of these two acids is much more complex than indicated by this simple equation. Both the elemental chlorine and the trivalent nitrogen of the nitrosyl chloride exhibit oxidizing effects, as do other unstable products formed during the reaction of these two acids. Coupled with the catalytic effect of Cl 2 and NOCl, this mixture combines the acidity and complexing power of the chloride ions. The solution is more effective if allowed to stand for 10 to 20 minutes after it is prepared.
Aqua regia dissolves sulfides, phosphates, and many metals and alloys including gold, platinum, and palladium. Ammonium salts are decomposed in this acid mixture. Aqua regia volatilizes osmium as the tetroxide; has little effect on rhodium, iridium, and ruthenium; and has no effect on titanium. Oxidic uranium ores with uraninite and synthetic mixed oxides (U 3 O 8 ) are dissolved in aqua regia, with oxidation of the U+4^ to UO 2 +2^ ions (Sulcek and Povondra, 1989). However, this dissolution procedure is insufficient for poor ores; the resistant, insoluble fraction must be further attacked (e.g., by sodium peroxide or borate fusion) or by mixed-acid digestion with HF, HNO 3 , and HClO 4.
Oxysalts, such as KMnO 4 (potassium permanganate) and K 2 Cr 2 O 7 (potassium dichromate), are commonly not used to solubilize or wet ash environmental samples for radiochemical analysis because of their limited ability to oxidize metals and the residue that they leave in the sample
JULY 2004 13-19^ MARLAP
mixture. These oxysalts are more commonly used to oxidize organic compounds.
P OTASSIUM P ERMANGANATE (KMnO 4 ) is a strong oxidizer whose use is limited primarily to the decomposition of organic substances and mixtures, although it oxidizes metals such as mercury to the ionic form. Oxidation can be performed in an acid, neutral, or basic medium; near-neutral or basic solutions produce an insoluble residue of manganese dioxide (MnO 2 ) that can be removed by filtration. Oxidation in acid media leaves the Mn+2^ ion in solution, which might interfere with additional chemical procedures or analyses. Extreme caution must be taken when using this reagent because KMnO 4 reacts violently with some organic substances such as acetic acid and glycerol, with some metals such as antimony and arsenic, and with common laboratory reagents such as hydrochloric acid and hydrogen peroxide.
P OTASSIUM DICHROMATE (K 2 Cr 2 O 7 ) is a strong oxidizing agent for organic compounds but is not as strong as KMnO 4. K 2 Cr 2 O 7 has been used to determine carbon and halogen in organic materials, but the procedure is not used extensively. K 2 Cr 2 O 7 is commonly mixed with sulfuric acid and heated as a strong oxidizing agent to dissolve carbonaceous compounds. The Cr+3^ ion remains after sample oxidation and this might interfere with other chemical procedures or analyses. K 2 Cr 2 O 7 can react violently with certain organic substances such as ethanol and might ignite in the presence of boron. Caution also must be observed in handling this oxidizing agent because of human safety concerns, particularly with the hexavalent form of chromium.
S ODIUM BROMATE (NaBrO 3 ) is an oxidizing agent for organic compounds but is not used for metals. Unlike KMnO 4 and K 2 Cr 2 O 7 , the bromate ion can be removed from solution after sample oxidation by boiling with excess HCl to produce water and Br 2. Caution must be observed when using this oxidizing agent because it can react violently with some organic and inorganic substances.
13.4.2 Acid Digestion Bombs
Some materials that would not be totally dissolved by acid digestion in an open vessel on a hotplate, can be completely dissolved in an acid digestion bomb. These pressure vessels hold strong mineral acids or alkalies at temperatures well above normal boiling points, thereby allowing one to obtain complete digestion or dissolution of samples that would react slowly or incompletely at atmospheric pressure. Sample dissolution is obtained without losing volatile elements and without adding contaminants from the digestion vessel. Ores, rock samples, glass and other inorganic samples can be dissolved quickly using strong mineral acids such as HF, HCl, H 2 SO 4 , HNO 3 , or aqua regia.
These sealed pressure vessels are lined with Teflon, which offers resistance to cross-contamina- tion between samples and to attack by HF. In all reactions, the bomb must never be completely filled; there must be adequate vapor space above the contents. When working with inorganic materials, the total volume of sample plus reagents must never exceed two-thirds of the capacity
MARLAP 13-20^ JULY 2004
