Wet Digestion

Introduction

Wet digestion methods for elemental analysis involve the chemical degradation of sample matrices in solution, usually with a combination of acids to increase solubility. The various acid and flux treatments are carried out at high temperatures in specially designed vessels that help to minimize contamination of the sample with substances in the air, the local environment, and from the vessel walls. Losses from the sample may occur due to adsorption onto the vessel

walls, volatilization, and coextraction, but these can be reduced by procedural modifications. The use of closed systems, where the digestion reaction is completely isolated from the surroundings, may help to reduce both contamination and sample loss.

The Nature of the Sample Matrix

Many techniques employed for elemental analysis require the conversion of the sample matrix into a solution form. The selection of an appropriate treatment for sample dissolution depends on the nature of the sample, and different approaches are required for  predominantly inorganic and predominantly organic matrices. Geological, geochemical, and soil samples generally contain silicate, metal oxides, carbonates, and, in many cases, organic matter. Such samples

must be dried and ground to a fine powder to facilitate dissolution. Minerals and coal often have a nonuniform distribution of elements, while fly ash is very fine and is composed of metal silicates and oxides. Both these types of sample are difficult to solubilize. Similarly, alloys can be difficult to dissolve because of the strong bonds between metal atoms and their brittle nature. Solid and crystalline samples may possess interstitial water and water of crystallization,

so thorough drying of samples is necessary before and after grinding. Biological samples must be processed with great care, since the dissolution and total decomposition of all organic matter is required for the release of trace elements. However, the use of oxidizing acids to decompose organic matter can produce violent reactions and the alternative procedure of dry ashing may be more suitable in some cases. Environmental and water samples often contain mixtures of organic and inorganic substances, so dissolution techniques need to be modified to take this composition into account.

In particular, water samples may contain dissolved and suspended solids, colloids, and microorganisms. Elements embedded in such samples may be present both in dissolved and solid forms. The nature of the sample matrix must be given special attention during wet digestion.

Extraction of the Analyte

Solid samples generally contain some adsorbed and/ or absorbed water. In the case of inorganic materials, drying is carried out in an oven at 105–1101C for a few hours, although lower temperatures need to be used if the sample contains volatile components. On the other hand, higher temperatures may be required to remove water trapped within crystalline matrices. Frequently, the sample is not soluble in water and must be treated with acids or mixtures of acids to facilitate solubilization. The type of acid treatment must be given careful consideration, since particular acids may or may not oxidize the sample, and may be incompatible with certain elements. For example, sulfuric acid cannot be used to dissolve samples containing barium, while hydrochloric acid cannot be used to dissolve silver or samples containing lead and lead compounds. The choice of acid is also restricted by sample volatilization, e.g., hydrochloric acid should be avoided in samples containing arsenic since this is more volatile as a trichloride. Naturally occurring inorganic materials, such as ores, must be given special treatments to facilitate solubilization. The two most common methods employed in dissolving samples are treatments with hydrochloric, hydrofluoric, nitric, sulfuric, or perfluoric acids (or various combinations thereof) and fusion with an acidic or basic flux followed by treatment with water or an acid. Organic materials are usually decomposed by wet digestion with a boiling oxidizing acid or acid mixture, ultimately producing

carbon dioxide, water, and other volatile compounds that are driven off to leave behind salts or acids of the inorganic constituents of the sample. Wet digestions may be performed in open beakers on hot plates, but Kjeldahl flasks or specially designed containment vessels give results that are more satisfactory. Wet Digestion with Single Acids The solvent action of an acid depends on several factors:

1. The reduction of hydrogen ions by metals that are

more active than hydrogen, for example:

Zn(s)þ2Hþ-Zn2þ þH2(g).

2. The combination of hydrogen ions with anions of a weak acid, for example:

 CaCO3(s)þ2Hþ -Ca2þ þH2OþCO2(g).

3. The oxidizing properties of the acid anion, for example: 

3Cu(s)þ2NO3 _þ8Hþ-3NO(g)þ4H2O.

4. The tendency of the acid anion to form soluble complexes with the sample cation, for example:

Fe3þ þCl_-FeCl2þ.

Ideally, the chosen reagent should cause the complete dissolution of the sample. As a general guide it is useful to classify the more common acid treatments according to whether they oxidize the sample or not. The nonoxidizing acids include dilute hydrochloric, hydrofluoric, sulfuric, and perchloric acids, whereas the oxidizing acids include hot, concentrated nitric, sulfuric, and perchloric acids. Dissolution of metals by nonoxidizing acids is a process of hydrogen replacement.

Hydrochloric acid will dissolve metals above the standard reduction potential of hydrogen, salts of weak acids, and many oxides. Dilute sulfuric and perchloric acids are useful for metals above the standard reduction potential of hydrogen. Hot, concentrated sulfuric acid will often dissolve metals below the standard reduction potential of hydrogen. The most potent oxidizing conditions are obtained using hot concentrated perchloric acid, which will dissolve all common metals. Concentrated hydrochloric acid is an excellent solvent for many metal oxides as well as those metals that are more easily

oxidized than hydrogen. In addition, it is often a better solvent for oxides than the oxidizing acids. Hot, concentrated nitric acid will dissolve all common metals with the exception of aluminum and chromium, which are passive to the reagent as a result of surface oxide formation. Hot nitric acid also readily oxidizes many organic substances. Hot, concentrated sulfuric acid can be used to decompose and dissolve many substances in part because of its high boiling point (3401C), and it is particularly useful for the dehydration and oxidation of organic samples. Most metals and alloys are also attacked by this hot acid. Perchloric acid is a potent oxidizing agent that leads to the formation of highly soluble perchlorate salts. As with sulfuric acid, perchloric acid dehydrates and oxidizes organic samples very efficiently.

It also attacks iron alloys and stainless steel, which are resistant to other mineral acids. Care is required when using perchloric acid because it is explosive in contact with certain organic compounds and easily oxidized inorganic materials. Special chemical hoods are recommended. Perchloric acid, as a 72–74% solution, boils at 2031C. Hydrofluoric acid is a weak, nonoxidizing acid that is particularly useful for dissolving silicate samples since it removes the silicon quantitatively as volatile SiF4. In many cases, hydrofluoric acid dissolution can be achieved by adding sodium fluoride to

samples treated with hydrochloric acid. Wet Digestion with Acid Mixtures Acids in combination are preferred for certain

inorganic matrices and are generally more advantageous for the decomposition of organic compounds. Wet digestion procedures using acid mixtures can be divided into four types:

1. Total decomposition, usually with hydrofluoric acid and another mineral acid.

2. Strong attacks, for routine analysis but leaving a residue of certain minerals, particularly silicates. Carried out with various mixtures of sulfuric, nitric, and perchloric acids.

3. Moderate attacks, using weaker acid mixtures.

4. Partial digestions (acid leaching).

Both (3) and (4) are typically employed for environmental analysis where complete dissolution is either not required or is undesirable and the goal is to determine the presence of certain trace elements. For geochemical samples containing silicates, the matrix is decomposed by heating with hydrofluoric acid in combination with either nitric or perchloric acid, each of which has a higher boiling point than hydrofluoric acid. The presence of the second acid with a higher boiling point ensures that, once the hydrofluoric acid has been boiled off and the dry sample redissolved, sparingly soluble metal fluorides are converted to salts that are more soluble. As stated above, however, caution should be exercised with the

use of perchloric acid if the sample has a significant organic component. Perchloric acid is also more expensive than nitric acid, and can introduce chloride ions as contaminants. For organic samples, a widely used mixture is aqua regia (1:3 nitric acid–hydrochloric acid). The nitric acid acts as the oxidizing agent, while the hydrochloric acid provides the complexing properties. The addition of bromine or hydrogen peroxide can sometimes increase the solubilizing power of mineral

acids. Wet digestion is generally carried out in open flasks, covered loosely to avoid atmospheric contamination.

However, it is becoming increasingly common to use closed vessels, such as polytetrafluoroethylene (PTFE)-lined containers or ultrapure quartz vessels, especially for small samples. A 1:4 mixture of sulfuric and nitric acids is also widely employed for organic samples. The nitric acid decomposes the bulk of the organic matter but does not reach a temperature sufficient to destroy the last traces. However, as the nitric acid boils off, the sulfuric acid is left behind. Dense SO3 fumes evolve and begin to reflux in the flask, making the solution very hot and allowing the hot sulfuric acid to decompose the remaining organic matter. Because of the fumes produced in this method, it must be carried out under

a fume hood. More nitric acid may be added to prolong the digestion and eliminate any stubborn organic material. A very efficient acid mixture is nitric, sulfuric, and perchloric acid in a volume ratio of B3:1:1. For a typical 10 g sample of tissue or blood, 10 ml of this  solution is sufficient for complete dissolution. The samples are heated until the nitric acid boils off and perchloric acid fumes begin to appear. Heating continues until the perchloric acid boils off and SO3 fumes appear. There is little danger of perchloric acid explosions as long as sufficient nitric acid is present to decompose the bulk of the organic matter, and as long as sulfuric acid remains after the perchloric acid has evaporated to prevent the sample becoming dry. Perchloric acid should never be added directly to an organic sample. A mixture of nitric and perchloric acid may also be used. The availability of strong hydrogen peroxide solutions allows a combination of sulfuric acid and hydrogen peroxide to be used for the decomposition of organic matter. Hydrogen peroxide is a vigorous

oxidizing agent and is particularly useful for the degradation of resistant plastics. There is little danger of explosion if sulfuric acid is present in excess.Most elements can be recovered quantitatively in this procedure, with the exceptions of ruthenium, osmium, germanium, arsenic, and selenium. In the case of germanium and arsenic, loss is attributable to

volatilization of chlorides. Additionally, precipitated calcium sulfate may retain lead and silver if not solubilized.

After decomposition, the sulfuric acid solution should be diluted and boiled gently for 10 min to destroy any remaining hydrogen peroxide. 

Source : R M Twyman, University of York, York, UK & 2005, Elsevier Ltd. All Rights Reserved. This article is a revision of the previous-edition article by A D Sawant, pp. 4503–4510, & 1995, Elsevier Ltd.