Wet Ashing for Soil Samples

Mineral samples such as rock, soil, and sediments require more aggressive digestion. Total sample dissolution may be done by several methods, and 

A weighed 1-g sample was heated in a platinum crucible with 10 mL of concentrated HF and 7.5 mL of concentrated HCIO4. After evaporation, a second treatment with the same acids was carried out. Then 20 mL of 4% H3BO3 was added and evaporated. The residue was dissolved in 2 mL of concentrated HC1 and diluted to volume with distilled water. 

The sample was mineralized at 450°C for 8 hours and a 5-g sample was weighed into a platinum dish. Then 15 mL of HNO3 and 10 mL of HCIO4 were added and the sample was leached for 24 hours at room temperature, followed by heating to dryness. The sample was taken up in 25 mL of dilute HC1 and digested on a water bath for an hour. The silica residue was filtered olf and washed with 1% hot HC1 and diluted to 100 mL. 

Comparison of the results of analysis by FAAS of the solutions produced by each of these methods indicated that procedures 1 and 3 were preferable to the others with respect to precision and accuracy. Procedure 1 was less accurate for chromium than procedure 3. In addition, procedure 3, which employed microwave digestion, took considerably less time to complete. Method 4, the leaching process, produced acceptable results only for Fe, Mg, Zn, and Cu. 

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