c. to 1 pint) of stannous chloride sol. to
fix any dissolved oxygen, is placed in the funnel, and let into the
bottle by opening the stopcock when required. Care must be taken to let
the hydrogen escape for some time before starting the reduction.
[Illustration: FIG. 33.]
~Gold~, Au.--Gold, obtained by cupelling and "parting," is for most
purposes sufficiently pure. It is best kept in the shape of foil. When
the purer metal is required, gold should be dissolved in aqua regia, the
solution evaporated to a paste, diluted, allowed to stand, and filtered.
The filtered solution is acidified with hydrochloric acid, warmed, and
precipitated with sodium sulphite. The precipitate is collected, washed,
and fused on charcoal.
~Iron~, Fe.--The soft wire (thin) is used for standardising. Rods are
used in dry assays as a desulphurising agent. Steel must not be used,
since it is not pure, and contains a variable amount of iron.
~Lead~, Pb.--Granulated lead or lead-foil is used in the dry assay for
silver and gold, and in the preparation of lead salts. It can be
obtained very pure, but always contains more or less silver, 1 or 2
milligrams in 100 grams. The amount of silver it contains must be
determined and recorded.
~Lead Acetate~ (Pb[=A=c]_{2}.3H_{2}O, or
Pb(C_{2}H_{3}O_{2})_{2}.3H_{2}O) is used as a test, specially for the
detection and estimation of sulphuretted hydrogen.
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