Wash it into a beaker, clean the
filter-paper (if necessary) with a drop or two of dilute ammonia;
evaporate with 10 c.c. of dilute nitric acid to a small bulk; dilute;
and filter off the globules of sulphur. The filtrate contains the
arsenic as arsenic acid.
GRAVIMETRIC METHOD.
Having got the arsenic into solution as arsenic acid, and in a volume
not much exceeding 50 c.c., add about 20 c.c. of dilute ammonia and 20
c.c. of "magnesia mixture." Stir with a glass rod, and allow to settle
overnight. Filter, and wash with dilute ammonia, avoiding the use of
large quantities of wash water. Dry, transfer the precipitate to a
Berlin crucible, and clean the filter-paper thoroughly. Burn this paper
carefully and completely; and add the ash to the contents of the
crucible, together with 4 or 5 drops of nitric acid. Evaporate with a
Bunsen burner, and slowly ignite, finishing off with the blow-pipe or
muffle. Cool, and weigh. The ignited precipitate is pyrarsenate of
magnesia (Mg_{2}As_{2}O_{7}), and contains 48.4 per cent. of arsenic
(As).
Instead of igniting the precipitate with nitric acid, it may be
collected on a weighed filter-paper, dried at 100° C., and weighed as
ammonic-magnesic arsenate (2AmMgAsO_{4}.H_{2}O), which contains 39.5 per
cent. of arsenic. The results in this case are likely to be a little
higher.
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