The following experiments show the effect of variation in the conditions
of the assay. Use a solution of copper sulphate containing 39.38 grams
of copper sulphate crystals (CuSO_{4}.5H_{2}O) in the litre. 100 c.c.
equal 1.00 gram of copper.
~Effect of Varying Temperature.~--The assay after the addition of the
potassic iodide must be kept cold, else iodine may be volatilised.
~Effect of Varying Potassium Iodide.~--In various descriptions of the
process the amount of iodide required is variously stated at from "a few
crystals" to as much as 10 grams. The proportion required by theory for
1 gram of copper is a little over 5 grams: an excess, however, is
required to keep the liberated iodine in solution. On economic grounds
this excess should not be extravagant; if the student uses 10 parts of
the iodide for each part of copper in the assay he will have sufficient.
In the experiments there were used 20 c.c. of the copper sulphate, with
varying amounts of potassic iodide, and the following results were
got:--
Potassic iodide added 1.5 gram 3 grams 5 grams
"Hypo" required 20.0 c.c. 20.0 c.c. 20.0 c.c.
In these the iodide was added direct to the solution containing the
copper, which was afterwards diluted to 100 c.c. and titrated. In
another series the iodide was added after the dilution to 100 c.
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