Mix about 500 mg of the sample with about 200 mg of anhydrous sodium
carbonate and 2 g of anhydrous potassium carbonate, and heat the mixture in
a platinum or nickel crucible until it melts completely. Cool, add 5 ml of water,
and allow to stand for 3 min. Heat the bottom of the crucible gently, detach the
melt, and transfer it to a beaker with the aid of about 50 ml of water. Add
gradually hydrochloric acid until no effervescence is observed, then add 10 ml
more of the acid, and evaporate the mixture on a steam bath to dryness. Cool,
add 20 ml of water, boil and filter the mixture through an ash-free filter paper.
An insoluble residue of silica remains. (Note. Retain the filtrate for the test for
aluminium). Transfer the gelatinous residue into a platinum dish, and
cautiously add 5 ml of hydrofluoric acid (warning: toxic, corrosive, must not
contact skin; work under fume hood). The precipitate dissolves. (If it does not
dissolve, repeat the evaporation with hydrofluoric acid.) Heat and hold in the
vapours a glass stirring rod with a drop of water on the tip. The drop becomes
turbid.