Mercury forms two series of salts, mercurous and mercuric, but for the
purposes of the assayer the most important property is the ease with
which it can be reduced to the metallic state from either of these.
Mercury itself is soluble in nitric acid, forming, when the acid is hot
and strong, mercuric nitrate. Cinnabar is soluble only in aqua regia.
Mercurous salts are generally insoluble, and may be converted into
mercuric salts by prolonged boiling with oxidising agents (nitric acid
or aqua regia). The salts of mercury are volatile, and, if heated with a
reducing agent or some body capable of fixing the acid, metallic mercury
is given off, which may be condensed and collected.
Mercury is separated from its solutions by zinc or copper, or it may be
thrown down by stannous chloride, which, when in excess, gives a grey
powder of metallic mercury, or, if dilute, a white crystalline
precipitate of mercurous chloride. Nitric acid solutions of mercury
yield the metal on electrolysis; and, if the pole on which the metal
comes down be made of gold or copper, or is coated with these, the
separated mercury will adhere thereto. It may then be washed and
weighed.
The best tests for mercury next to obtaining globules of the metal are:
(1) a black precipitate with sulphuretted hydrogen from acid solutions,
which is insoluble in nitric acid; and (2) a white precipitate with
stannous chloride.
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