Wash the precipitated metal back
into the beaker, and dissolve in 10 c.c. of dilute nitric acid,
evaporate off the excess of acid; dilute, boil, and filter. Wash, dry,
ignite strongly in a porcelain crucible, and weigh.
In the absence of antimony the above separation works very well, but if
this metal is present in quantity the metals precipitated on the zinc
must be covered with hydrochloric acid and treated with a few drops of
nitric. It is then warmed with iron wire until no more of the latter
dissolves. The antimony is precipitated as metal, and the tin remains in
solution as stannous chloride. The antimony is filtered off, and may be
washed with alcohol, and weighed, whilst the tin in the filtrate is
precipitated with zinc, and treated as already described.
GRAVIMETRIC METHOD.
If the tin is not already in the metallic state it is reduced to this
condition by the method given (precipitation by zinc). Treat the
finely-divided metal (washed free from chlorides) in a four-inch
evaporating dish with 10 c.c. of dilute nitric acid, cover with a
clock-glass, and apply a gentle heat until the precipitate appears of a
white colour and the metal is completely attacked. Evaporate nearly to
dryness on a water-bath; then add 50 c.c. of water, heat to boiling, and
filter. Wash with hot water, dry, transfer to a weighed porcelain
crucible, add the filter-ash, ignite strongly, and weigh.
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