In working, a quantity of the sodium chloride solution
equal to 1 gram of silver is added at once to the assay; and, when the
solution has been rendered clear by shaking, the residual silver (which
should not exceed a few milligrams) is estimated with the help of a
weaker solution of sodium chloride. The success in working evidently
depends upon the accuracy with which the first addition of the salt
solution is made. On this account the standard solution is run in from a
special pipette capable of delivering a practically invariable volume of
solution. It is not so important that this shall deliver exactly 100
c.c. as that in two consecutive deliveries the volume shall not differ
by more than 0.05 c.c. The dilute salt solution is one-tenth of the
strength of that first run in, and 1 c.c. of it is equivalent to 1
milligram of silver. Ordinarily it is run in 1 c.c. at a time (and an
ordinary burette may be used for this purpose), shaking between each
addition until it ceases to give a precipitate. If many such additions
have to be made the operation not only becomes tedious, but the
solution also ceases to clear after shaking, so that it becomes
impossible to determine the finishing point.
If the assay contains less than one gram of silver the first addition of
the dilute salt solution of course produces no precipitate.
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