If the student has the slightest
doubt about a finish he should run in another 5 c.c. of the cyanide and
again finish with silver nitrate. The second result will be the same as
the first. For example, if 40 c.c. of cyanide and 30 c.c. of silver
nitrate were required at the first titration, then the 45 c.c. of
cyanide in the second titration will require 35 c.c. of silver nitrate.
The difference between the quantities of the two solutions used in each
case will be 10 c.c. It is this difference in the readings of the two
burettes which measures the quantity of nickel present. Each c.c. of the
difference is equal to .0025 gram of nickel. But if the cyanide solution
is not exactly equal in strength to the silver nitrate, the quantity of
cyanide used should be calculated to its equivalent in silver nitrate
before making the subtraction.
The following experimental results illustrate the accuracy of the assay
and the effect upon it of varying conditions. A solution containing 1
gram of nickel sulphate (NiSO_{4}.6H_{2}O) in 100 c.c. was used. By a
separate assay the sulphate was found to contain 22.25 per cent. of
nickel. For the sake of simplicity the results of the experiments are
stated in weights of nickel in grams.
~Effect of varying excess of Cyanide Solution.~--In each experiment
there was 20 c.
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