The
detection of 0.0001 grm. of metal can be fully relied on under all
conditions, and one hour is sufficient to completely precipitate such
small quantities.
_Mercury_.--Mercury is best separated from its nitric acid solution on
a small closely wound spiral of platinum wire. The solution to be
tested is acidified with nitric acid and electrolyzed with a current
of 4-5 c.c. (c.c. refer to c.c. of electrolytic gas per minute). The
deposition is effected in half an hour. The deposited metal is removed
from the spiral by heating the latter gently in a test tube, when the
mercury forms in characteristic globules on the upper portion of the
tube. As a confirmatory and very characteristic test, a crystal of
iodine is dropped into the tube, and the whole allowed to stand for a
short time, when the presence of mercury is indicated by the formation
of the red iodide. 0.0001 grm. of mercury in 150 c.c. of solution can
be clearly detected.
Wolff has applied this test under similar conditions, using a special
form of apparatus and a silver-coated iron anode (this Journal, 1888,
454).
_Lead_.--Lead is precipitated either as PbO_{2} at the anode from a
nitric acid solution or as metal at the cathode from an ammonium
oxalate solution.
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