Dissolving Samples for Analysis

The laboratory sample is usually dissolved for analysis. It is important to dissolve the entire sample, or else we cannot be sure that all of the analyte was dissolved. If the sample does not dissolve under mild conditions, acid digestion or fusion may be used. Organic material may be destroyed by combustion (also called dry ashing) or wet ashing (oxidation with liquid reagents) to place inorganic elements in suitable form for analysis. 

HF causes excruciating burns. Exposure of just 2% of your body to concentrated (48 wt%) HF can kill you. Flood the affected area with water for 5 min and then coat the skin with 2.5% calcium gluconate gel kept in the lab for this purpose, and seek medical help. If the gel is not available, use whatever calcium salt is handy. HF damage can continue to develop days after exposure. 

HC1 37% 1.19 g/mL Nonoxidizing acid useful for many metals, oxides, sulfides, carbonates, and phosphates. Constant boiling composition at 109°C is 20% HC1. As, Sb, Ge, and Pb form volatile chlorides that may be lost from an open vessel. 

HBr 48-65% Similar to HC1 in solvent properties. Constant boiling composition at 124 C is 48% HBr. 

Most fusions use lithium tetraborate (Li2B407, m.p. 930°C), lithium metaborate (LiB02, m.p. 845°C), or a mixture of the two. A nonwetting agent such as KT can be added to prevent the flux from sticking to the crucible. For example, 0.2 g of cement might be fused with 2 g of Li2B407 and 30 mg of KI. 

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