The method of working an assay may be gathered from the following
example:--In the determination of silver in some buttons left after
cupellation, it was assumed that these would contain 99.5 per cent. of
silver. For the assay it was necessary to take a quantity that should
contain a little more than 1.0075 grams of silver; then
99.5 : 100 :: 1.0075 : _x_
_x_ = 1.0125
To ensure a slight excess, there was taken 1.0150 gram of the buttons,
which was treated in exactly the same way as for the standardising. The
quantity of the weaker salt solution required was 7 c.c.; deducting 1.5
c.c., and dividing by 10, gives 100.55 c.c. of strong salt solution,
which is equivalent to 1.0130 gram of silver. This being obtained from
1.015 gram of alloy, is equal to 99.8 per cent., or 998.0 fine.
~The Effect of Temperature.~--The standardising and the assay must be
done at the same time, since a difference of 5° C. makes a difference of
0.1 c.c. in measuring the 100 c.c. of strong solution of salt. It is
always best to prepare a standard with each batch of assays.
~SULPHOCYANATE METHOD.~--Volhard's process is based upon the
precipitation of silver in nitric acid solutions with potassium
sulphocyanate, the finishing point being the development of a
reddish-brown colour, produced by the action of the excess of
sulphocyanate upon ferric sulphate.
Pages:
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235