302 EXTRACTION OF IODINE FROM DILUTE SOLUTIONS. 
the protiodide of copper, owing to the influence which the 
respective proportions of the solutions employed exert, I 
had recourse to sulphurous acid, a powerful reducing agent, 
and whose action upon the peroxide of copper, which it re- 
duces partially to the state of protoxide, was pointed out 
by M. Chevreul. A few words will suffice to render this 
kind of reaction intelligible. If 1 grm. of persulphate of 
copper be dissolved in 150 centigrms. of water, and to this 
solution 1 grm. of sulphate of soda be added, the liquid ac- 
quires a green colour, and becomes turbid. As the formation 
of a precipitate should be avoided, and at the same time the 
liquid decolorized, the requisite quantity of sulphurous acid 
to obtain this double result is added ; on letting fall a drop 
of a solution of iodide of potassium into it, it immediately 
becomes opalescent, the turbidness goes on increasing, and 
in the course of an hour a white, slightly-pinkish precipitate 
of the protiodide of copper is formed, which is readily col- 
lected by boiling the liquid for a few minutes, and then de- 
canting. 
Accordingly, in treating ioduretted waters, sulphurous 
gas should be passed into them until they exhale a faint 
odour, in order to convert all the iodine which may exist in 
the state of iodate into ioduretted hydrogen ; then to pre- 
vent the formation of the precipitate from the mutual action 
of the sulphite of soda and the persulphate of copper; and 
lastly, to cause the reduction of the peroxide of copper. 
For this purpose, therefore, there is successively dissolved 
in the liquid under treatment 1 part of persulphate of copper 
and 1 part of bisulphite of soda, calculating approximatively 
the amount of the first for the quantity of iodine supposed 
in solution, upon the fact that about 3 parts of the persul- 
phate of copper are required for 1 part of the iodide of po- 
tassium or sodium. The liquid is then left to itself or boiled, 
according to whether the precipitate is desired immediately 
or after a few hours. On letting the precipitate form in co- 
nical vessels, it is easy to collect it into a small volume ; in 
