If the
chlorine is passed into a solution of potassium iodide, an equivalent of
iodine will be set free.[85] This is apparently a very indirect way of
determining how much of the dioxide is present; but the reactions are
very sharp, and the final determination of the iodine is an easy one.
[Illustration: FIG. 60.]
The finely-powdered sample of dioxide is placed in a small flask
provided with an exit tube leading into a solution of potassic iodide
(fig. 60). On adding hydrochloric acid and boiling, the chlorine evolved
is driven into the iodide solution and there absorbed; the boiling is
continued till the steam and hydrochloric acid fumes have driven the
last portions of the chlorine out of the flask and into the solution. In
this experiment there is a strong tendency for the iodide solution to
rush back into the flask. This tendency is overcome by avoiding draughts
and regulating the heat; or by placing a lump of magnesite in the flask,
which acts by evolving carbonic acid and so producing a steady outward
pressure. When the distillation is finished the tube containing the
iodine is detached and washed out into a beaker. If the solution is
strongly acid it should be almost neutralised by the cautious addition
of dilute ammonia. If crystals of iodine have separated, potassium
iodide must be added in quantity sufficient to dissolve them.
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