With minerals, mattes, &c., solution is thus effected:--The
finely-powdered substance is mixed (in a large platinum or porcelain
crucible) with from six to ten times its weight of a mixture of equal
parts of carbonate of soda and nitre. The mass is then heated gradually
to fusion, and kept for a few minutes in that state. When cold, it is
extracted with warm water, and filtered from the insoluble residue. The
solution, acidified with nitric acid and boiled, contains the arsenic as
sodium arsenate. With mispickel, and those substances which easily give
off arsenic on heating, the substance is first treated with nitric acid,
evaporated to dryness, and then the residue is treated in the way just
described.
When the arsenic is present as arsenite or arsenide, distillation with
an acid solution of ferric chloride will give the whole of the arsenic
in the distillate free from any metal except, perhaps, tin as stannic
chloride. With arsenates, dissolve the substance in acid and then add an
excess of soda. Pass sulphuretted hydrogen into the solution; warm, and
filter. Acidulate the filtrate, and pass sulphuretted hydrogen. Decant
off the liquid through a filter, and digest the precipitate with ammonic
carbonate; filter, and re-precipitate with hydrochloric acid and
sulphuretted hydrogen. Allow to stand in a warm place, and filter off
the yellow sulphide of arsenic.
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