It forms two well-defined series of salts, corresponding to
the oxides: arsenious oxide (As_{2}O_{3}), and arsenic oxide
(As_{2}O_{5}). These combine with bases to form arsenites and arsenates
respectively. Boiling with nitric acid converts the lower into the
higher oxide; and powerful reducing-agents, such as cuprous chloride,
have the opposite effect.
Arsenic may be detected by dissolving the substance in hydrochloric
acid, or in aqua regia (avoiding an excess of nitric acid), and adding a
little of this solution to the contents of a small flask in which
hydrogen is being made by the action of zinc and hydrochloric acid. The
ignited jet of hydrogen assumes a blue colour if arsenic is present, and
a cold porcelain dish held in the flame (fig. 67) becomes coated with a
dark deposit of metallic arsenic. Antimony produces a similar effect,
but is distinguished by the insolubility of its deposit in a cold
solution of bleaching-powder.
[Illustration FIG. 67.]
Arsenites are distinguished by the volatility of the chloride; by
decolorising a solution of permanganate of potassium, and by immediately
giving a yellow precipitate with sulphuretted hydrogen. Arsenates are
distinguished (after converting into soda salts by boiling with
carbonate of soda and neutralising) by giving with nitrate of silver a
red precipitate, and with "magnesia mixture" a white crystalline one.
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