This
reagent should be the purest obtainable, but its purity should be
checked, or rather its impurities should be determined by running a
"check" or "blank" assay with 10 grams of it through the stages of the
analysis; the impurities will be chiefly silica, alumina and lime, and
altogether they ought not to exceed 1 milligram. The crucible with the
mixture is heated at first gently over a Bunsen and afterwards more
strongly in an oxidising atmosphere in a muffle or over the blowpipe.
The fused mass is allowed to cool in the crucible, and is then dissolved
out in a basin with water and a small excess of hydrochloric acid. After
the removal and cleaning of the crucible, the liquor is evaporated
almost to dryness. Dr. Hillebrand advises stopping short of complete
dryness. The residue is taken up with a little hydrochloric acid and
water and filtered and washed. The liquor, including the washings, is
again evaporated and taken up with water and a little acid. Usually
about 1 per cent. of silica will be thus recovered. It is to be filtered
off and washed and added to the main silica. The filtrate is reserved.
The silica, thoroughly washed, is dried and ignited at a high
temperature for twenty or thirty minutes. It is then weighed in a
platinum crucible. After weighing it is treated with hydrofluoric acid
and a little sulphuric, carefully evaporated and ignited strongly.
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