c. of water
and 10 c.c. of nitric acid; cover with a clock glass, and allow to
dissolve with moderate action; boil off nitrous fumes, dilute to 100
c.c., and electrolyse. The cylinder must be carefully weighed, and the
electrolysis allowed to proceed for 24 hours. The weight found will be
that of the copper and silver. The silver in it must be determined[53]
and deducted.
~Determination of Copper in Brass, German Silver, or Bronze.~--Treat in
the same manner as commercial copper. If nickel is present, the few
milligrams of copper remaining in the electrolysed solution should be
separated with sulphuretted hydrogen, the precipitated sulphide
dissolved in nitric acid, and determined colorimetrically.
VOLUMETRIC PROCESSES.
There are two of these in use, one based on the decolorising effect of
potassic cyanide upon an ammoniacal copper solution, and the other upon
the measurement of the quantity of iodine liberated from potassic iodide
by the copper salt. The cyanide process is the more generally used, and
when carefully worked, "on certain understood and orthodox conditions,"
yields good results; but probably there is no method of assaying where a
slight deviation from these conditions so surely leads to error. An
operator has no difficulty in getting concordant results with duplicate
assays; yet different assayers, working, without bias, on the same
material, get results uniformly higher or lower; a difference evidently
due to variations in the mode of working.
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