To separate the mercury, pass sulphuretted hydrogen in considerable
excess through the somewhat dilute solution. The precipitate should be
black, although it comes down at first very light coloured. It is
filtered, washed, and transferred back to the beaker, and then digested
with warm ammonic sulphide. The residue, filtered, washed, and boiled
with dilute nitric acid, will, in the absence of much lead, be pure
mercuric sulphide. If much lead is present, a portion may be
precipitated as sulphate, but can be removed by washing with ammonic
acetate. To get the mercury into solution, cover with nitric acid and a
few drops of hydrochloric, and warm till solution is effected. Dilute
with water to 50 or 100 c.c.
GRAVIMETRIC DETERMINATION.
This may be made by _electrolysis_. The same apparatus as is used for
the electrolytic copper assay may be employed, but instead of a cylinder
of platinum one cut out of sheet copper should be taken, or the platinum
one may be coated with an evenly deposited layer of copper. Fix the
spiral and weighed copper cylinder in position, couple up the battery,
_and when this has been done_ put the nitric acid solution of the
mercury in its place.[49] The student had better refer to the
description of the _Electrolytic Copper Assay_.
The mercury comes down readily, and the precipitation is complete in a
few hours: it is better to leave it overnight to make sure of complete
reduction.
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