Page 217 - Modern Analytical Chemistry

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200 Modern Analytical Chemistry

7
Table .2 Acids and Bases Used for Sample Digestion
Solution
(≈ %w/w) Uses and Properties
o
HCl (37%) • dissolves metals more easily reduced than H 2 (E < 0)
• dissolves insoluble carbonates, sulfides, phosphates,
fluorides, sulfates, and many oxides
HNO 3 (70%) • strong oxidizing agent
• dissolves most common metals except Al and Cr
• decomposes organics and biological samples (wet ashing)
H 2 SO 4 (98%) • dissolves many metals and alloys
• decomposes organics by oxidation and dehydration
HF (50%) • dissolves silicates forming volatile SiF 4
HClO 4 (70%) • hot, concentrated solutions are strong oxidizing agents
• dissolves many metals and alloys
• decomposes organics (reactions with organics are often
explosive, use only in specially equipped hoods with a blast
shield and after prior decomposition with HNO 3 )
HCl:HNO 3 (3:1 v/v) • also known as aqua regia
• dissolves Au and Pt
NaOH • dissolves Al and amphoteric oxides of Sn, Pb, Zn, and Cr

Pressure relief valve opposed quarters are discarded. The remaining material is cycled through the
process of coning and quartering until the desired amount of sample remains.
Pressure
Temperature probe
probe
Bringing Solid Samples into Solution If you are fortunate, the sample with which
you are working will easily dissolve in a suitable solvent, requiring no more effort
than gentle swirling and heating. Distilled water is usually the solvent of choice for
inorganic salts, but an organic solvent, such as methanol, chloroform, or toluene, is
Cap used for organic materials. More often, one or more of the sample’s components re-
sist simple dissolution.
With samples that are difficult to dissolve, the first approach is usually to try di-
gesting the sample with an acid or base. Table 7.2 lists the most commonly used
acids and bases and summarizes their use. Digestion is commonly carried out in an
open container, such as a beaker, using a hot plate as a source of heat. The chief ad-
vantage of this approach is its low cost as it requires no special equipment. Volatile
reaction products, however, are lost, leading to a determinate error if analyte is in-
Vessel body
cluded among the volatile substances.
Many digestions are now carried out in closed containers using microwave ra-
diation as a source of energy for heating the solution. Vessels for microwave diges-
tion are manufactured using Teflon (or some other fluoropolymer) or fused silica.
Both materials are thermally stable, chemically resistant, transparent to microwave
Figure 7.10 radiation, and capable of withstanding elevated pressures. A typical microwave di-
Schematic diagram of a microwave digestion gestion vessel is shown in Figure 7.10 and consists of the vessel body and cap, a tem-
vessel.
perature probe, and a pressure relief valve. Vessels are placed in a microwave oven
(typically 6–12 vessels can be accommodated), and microwave energy is controlled
by monitoring the temperature or pressure within the vessels. A microwave diges-
tion has several important advantages over an open container digestion, including
higher temperatures (200–300 °C) and pressures (40–100 bar). As a result, diges-
tions requiring several hours in an open container may be accomplished in less than