IODIDE OF POTASSIUM. 
113 
is to precipitate the whole of the oxide or carbonate of zinc 
or of iron, without having an excess of the precipitant. It 
is preferable, however, to have a slight excess of the latter, 
than that there should be an excess of either hydriodate of 
zinc or of iron. If potassa has been used as the precipitant, 
and there should happen to be an excess of this, the greater 
part of the iodide of potassium may be separated by crystal- 
lization. If the carbonate of potassa has been used, and there 
be an excess of this, the best mode of proceeding will be as 
follows: — Evaporate the solution sufficiently far to allow a 
part of the iodide of potassium to crystallize; separate these 
crystals and wash them with a small quantity of water; eva- 
porate the remaining solution, together with the washings, to 
dryness; then dissolve the residue in alcohol by the aid of 
heat, and lastly, having poured off the clear solution from the 
insoluble portion, evaporate it to dryness or crystallize. 
If the hydriodate of zinc or of iron be in excess, the iodide 
of potassium cannot be procured pure by crystallization; for 
in evaporating the solution, the hydriodates of these metals 
will be wholly or partly decomposed, and the crystals will be 
contaminated with the oxide of zinc or of iron,and perhaps,also, 
with free iodine. From these considerations it appears that 
carbonate of potassa is, on the whole, better adapted for form- 
ing the iodide in this way, than the pure alkali. 
Another process which has been proposed for obtaining 
iodide of potassium, consists in adding to a solution of sul- 
phuret of potassium, a sufficient quantity of iodine to precipitate 
the whole of the sulphur. By this means we obtain a solution 
of iodide of potassium which must be evaporated in order to 
procure the iodide in a solid state. For obtaining the iodide 
in this manner, it is necessary that the sulphuret of potassium 
should be perfectly pure, not containing any sulphate of 
potassa, an impurity always present when the sulphuret is 
prepared as directed by the Pharmacopoeias. The labour and 
cost of forming a pure sulphuret, by the decomposition of 
sulphate of potassa would render this process less eligible than 
some of the former methods. 
VOL. III. NO. II. 15 
