The
residue, which in extreme cases may amount to 2 or 3 per cent. of the
rock, is weighed and deducted from the weight of the impure silica. It
is retained in the crucible.
~Alumina, &c.~--The filtrate from silica is treated by the basic acetate
method. That is, it is first treated by a cautious addition of a
solution of soda, almost to the point of producing a precipitate, in
order to neutralise the excess of acid; 2 or 3 grams of sodium acetate
are added, and the whole boiled for a minute or so. The precipitate is
filtered off and washed only slightly. Save the filtrate. The
precipitate is dissolved in hydrochloric, or, perhaps better, in nitric
acid; and is reprecipitated by adding an excess of ammonia and boiling.
The precipitate is filtered and washed with water containing 2 per cent.
of ammonium nitrate. Both filtrates are evaporated separately to a small
bulk, a drop or two of ammonia being added to the second towards the
finish. They are next filtered into a 6 or 8-ounce flask through a small
filter, the second filtrate coming after, and serving in a manner as
wash water for the first[113]. The two washed alumina precipitates are
dried and placed in the platinum crucible containing the residue from
silica after treatment with hydrofluoric acid. They are then ignited in
an oxidising atmosphere at a high temperature for about 10 minutes.
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