In working with pure solutions, the two finishing points give the same
results; and this is true even when there is much difference in the
degree of dilution. The finishing point with the iodide, however, has
an advantage in precision. Moreover, it is but little affected by
variations in alkalinity, which render the other finishing point quite
useless. The great difference between the two is shown when zinc is
present in the solution. In this case, when working without the iodide,
the first appearance of a turbidity is less distinct; the turbidity
increases on standing and as a finishing point is unsatisfactory. It can
be determined with precision only by very systematic working and after
some experience. The turbidity is due to the separation of an insoluble
zinc compound. A most important point (to which reference has already
been made) is that less silver nitrate is required to give this
turbidity and, consequently, a lower strength in cyanide is reported. On
the other hand, as much silver nitrate is required to give the yellow
turbidity due to silver iodide as would be required if no zinc were
present.
Unfortunately the difference in the two titrations does not depend
merely on the quantity of zinc present; as it is also influenced by the
extent of dilution, the degree of alkalinity of the solution, and the
quantity of cyanide present.
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