Evaporate in platinum 100 to 1,000 cc. of the sample or sufficient if
possible to form a residue weighing 0.4 to 0.6 gram, and preferably
containing 0.1 to 0.2 gram of calcium. When the residue is nearly dry
add 1 cc. of hydrochloric acid (1 to 1) and, after moistening the sides
of the dish, evaporate to dryness. Dry at 180° C. and if much organic
matter is present char it in a radiator. Moisten the residue with dilute
hydrochloric acid and expel the excess of acid by heating on the water
bath. Add a few drops of hydrochloric acid, dissolve in hot water, and
filter. Wash the residue with hot water. Evaporate the filtrate to
dryness, repeat the filtration, and combine the two residues. If great
accuracy is not required the second evaporation with hydrochloric acid
may be omitted. Ignite and weigh the insoluble residue. Add 2 drops of
concentrated sulfuric acid and a little hydrofluoric acid, volatilize
the acids, ignite, and weigh again. Report the loss in weight as silica
(SiO_{2}). A weight of non-volatile matter exceeding 0.5 mg. should be
analyzed.