75 % of the metal. The quantity of white
arsenic taken, .1 or .2 gram, should contain about as much arsenic as
will be present in the assays. It is converted into sodium arsenate by
evaporating to a small bulk with nitric acid and neutralising with soda.
The precipitation and titration of the silver arsenate should be exactly
as in the assays.
The difficulty of the method is in the neutralising; which has to be
very carefully done since silver arsenate is soluble in even faintly
acid solutions; one drop of nitric acid in 100 c.c. of water is enough
to produce an absolutely worthless result; and an excess of acid much
less than this is still very prejudicial. The addition of a little
sodium acetate to the solution after the final neutralising has a good
effect.
~Arsenic in Mispickel.~--Weigh up .250 gram of the finely-powdered ore,
and place in a Berlin crucible about 1-1/4 or 1-1/2 inch in diameter.
Treat with 10 or 12 drops, one drop at a time, of strong nitric acid,
warm very gently, but avoid much heating. Put on a thin layer of nitre,
and rather more than half fill the crucible with a mixture of equal
parts of soda and nitre. Heat quickly in the blow-pipe flame, and when
the mass is fused and effervescing, withdraw and allow to cool. Boil out
with water, filter and wash. Insert a piece of litmus paper and
cautiously neutralise with nitric acid, using ammonia to neutralise any
accidental excess of the acid.
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