Analytical Chem istry - DePauw University

310 Analytical Chemistry 2.0(a)(b)pressure relief valvedigestion vesselFigure 7.12 Microwave digestion unit. (a) View of the unit’s interior showing the carousel holding the digestionvessels. (b) Close-up of a Teflon digestion vessel, which is encased in a thermal sleeve. The pressure relief value,which is part of the vessel’s cap, contains a membrane that ruptures if the internal pressure becomes too high.A microwave digestion has several important advantages over an openvesseldigestion, including higher temperatures (200–300 o C) and pressures(40–100 bar). As a result, digestions requiring several hours in anopen-vessel may need no more than 30 minutes when using a microwavedigestion. In addition, a closed container prevents the loss of volatile gases.Disadvantages include the inability to add reagents during the digestion,limitations on the sample’s size (typically < 1 g), and safety concerns dueto the high pressures and corrosive reagents.Inorganic samples that resist decomposition by digesting with acidsor bases often can be brought into solution by fusing with a large excessof an alkali metal salt, called a flux. After mixing the sample and the fluxin a crucible, they are heated to a molten state and allowed to cool slowlyto room temperature. The melt usually dissolves readily in distilled wateror dilute acid. Table 7.3 summarizes several common fluxes and their uses.Fusion works when other methods of decomposition do not because of thehigh temperature and the flux’s high concentration in the molten liquid.Table 7.3 Common Fluxes for Decomposing Inorganic SamplesFluxMeltingTemperature ( o C) Crucible Typical SamplesNa 2 CO 3 851 Pt silicates, oxides, phosphates, sulfidesLi 2 B 4 O 7 930 Pt, graphite aluminosilicates, carbonatesLiBO 2 845 Pt, graphite aluminosilicates, carbonatesNaOH 318 Au, Ag silicates, silicon carbideKOH 380 Au, Ag silicates, silicon carbideNa 2 O 2 — Ni silicates, chromium steels, Pt alloysK 2 S 2 O 7 300 Ni, porcelain oxidesB 2 O 3 577 Pt silicates, oxides