Transfer the lead sulphate to the
crucible; ignite gently, keeping the temperature below redness; cool,
and weigh. The precipitate will contain 73.6 per cent. of lead oxide or
68.3 per cent. of lead.
~Determination of Lead in Commercial Zinc.~--Take 10 grams of zinc, and
treat (without heating) with 60 c.c. of dilute hydrochloric acid. When
the zinc is nearly all dissolved, decant off the clear liquid, and
dissolve the residue in 2 c.c. of dilute nitric acid. Evaporate till
most of the acid is removed; dilute to 20 or 30 c.c. with water, and add
10 c.c. of dilute sulphuric acid. Filter off, and weigh the lead
sulphate. Ten grams treated in this way gave--0.1610 gram of lead
sulphate, equivalent to 1.10 per cent. of lead.
VOLUMETRIC METHOD.
This is based upon the reaction between chromate of potash and soluble
lead salts in neutral solutions, whereby an insoluble yellow chromate of
lead is produced.[60] An excess of the chromate is required to complete
the reaction, so that the point at which an indicator shows the presence
of undecomposed chromate cannot be satisfactorily taken as the finish.
Therefore an excess of the standard chromate must be run in, and such
excess determined.
Chromate of lead is not precipitated from strong nitric acid solutions,
and only incompletely from dilute ones.
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