The silica is washed by decantation two or three times with hydrochloric
acid and hot water, before being thrown on to the filter. The filtrate
is again evaporated to dryness, taken up with a little hydrochloric acid
and water and again filtered. The residue on the filter is silica. The
two lots of silica are washed free from chlorides with hot water, dried
on an air-bath, transferred to a platinum-crucible, ignited gently at
first, at last strongly over the blast or in a muffle, cooled in a
desiccator, and weighed.
The white powdery precipitate is silica (SiO_{2}), and its weight,
multiplied by 100, and divided by the weight of ore taken, gives the
percentage of silica in the sample. Where the percentage of silicon is
wanted, which is very rarely the case, it is got by multiplying this
result by 0.4667. It is always necessary to examine the purity of the
body weighed as silica. This is done by re-fusing the material weighed,
and re-determining the silica in it; or, better, by mixing a weighed
portion in a platinum-dish with a little strong sulphuric acid, covering
with hydrofluoric acid, and evaporating. In the latter case, the silica
will be converted into fluoride, which will be driven off, and the
impurities will be left behind as sulphates of barium, phosphate and
oxide of tin, titanium, &c.
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