Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples309Br i n g i n g So l i d Sa m p l e s In t o So l u t i o nIf you are fortunate, your sample will easily dissolve in a suitable solvent,requiring no more effort than gently swirling and heating. Distilled water isusually the solvent of choice for inorganic salts, but organic solvents, suchas methanol, chloroform, and toluene are useful for organic materials.When a sample is difficult to dissolve, the next step is to try digestingit with an acid or a base. Table 7.2 lists several common acids and bases,and summarizes their use. Digestions are carried out in an open container,usually a beaker, using a hot-plate as a source of heat. The main advantageof an open-vessel digestion is cost because it requires no special equipment.Volatile reaction products, however, are lost, resulting in a determinate errorif they include the analyte.Many digestions are now carried out in a closed container using microwaveradiation as the source of energy. Vessels for microwave digestionare manufactured using Teflon (or some other fluoropolymer) or fusedsilica. Both materials are thermally stable, chemically resistant, transparentto microwave radiation, and capable of withstanding elevated pressures. Atypical microwave digestion vessel, as shown in Figure 7.12, consists of aninsulated vessel body and a cap with a pressure relief valve. The vessels areplaced in a microwave oven (typically 6–14 vessels can be accommodated)and microwave energy is controlled by monitoring the temperature or pressurewithin one of the vessels.Table 7.2 Acids and Bases Used for Digesting SamplesSolutionUses and PropertiesHCl (37% w/w) • dissolves metals more easily reduced than H2 (E o < 0)• dissolves insoluble carbonate, sulfides, phosphates, fluorides,sulfates, and many oxidesHNO 3 (70% w/w) • strong oxidizing agent• dissolves most common metals except Al, Au, Pt, and Cr• decomposes organics and biological samples (wet ashing)H 2 SO 4 (98% w/w) • dissolves many metals and alloys• decomposes organics by oxidation and dehydrationHF (50% w/w) • dissolves silicates by forming volatile SiF4HClO 4 (70% w/w) • hot, concentrated solutions are strong oxidizing agents• dissolves many metals and alloys• decomposes organics ( Caution: reactions with organics often areexplosive; use only in a specially equipped hood with a blast shieldand after prior decomposition with HNO 3 )HCl:HNO 3 (3:1 v/v) • also known as aqua regia• dissolves Au and PtNaOH• dissolves Al and amphoteric oxides of Sn, Pb, Zn, and Cr