Art. LIV.—On the Proposed Substitution of Acetate for Sulphate of Copper in the Manufacture of Iodine.
[Read before the Wellington Philosophical Society, 23rd October, 1872.]
The precipitation of iodine from the residual liquors obtained in its manufacture is at present accomplished by Soubeiran's method, namely by the addition of sulphate of copper thereto, iodide of copper thus forming and precipitating, but it is found in practice that the precipitation is so incomplete that a notable quantity of iodine remains in solution, necessitating the application of after processes for the more complete removal of iodine from such cupreous liquors.
In connection with this I would desire to make it publicly known that from certain investigations I have made upon this subject it appears that by a slight modification of Soubeiran's method this loss of iodine may be prevented, or so nearly that the necessity of after processes will be avoided.
The particular agents most active in causing this retention of iodine in the liquor are sulphate of copper, free sulphuric acid, and alkaline sulphates and chlorides, since they exercise a considerable solvent action upon the iodide of copper formed in Soubeiran's process.
The modification therefore which I propose is the use of acetate of copper in place of the iodide, this salt, as also free acetic acid and alkaline acetates, dissolving the cupreous iodide to only a very slight extent.
For practical use this salt might be prepared from common sulphate of copper by adding thereto acetate of soda in quantity sufficient to allow of the whole of the sulphuric acid of the copper salt being interchanged for acetic acid, but for the more complete removal of iodine I should recommend the use of the acetate of copper alone.
I will only state further that the pure acetate of copper (acidified with acetic acid if necessary) is so delicate a test for iodine, if in the form of a soluble iodide, that it may very effectively and conveniently be used for this purpose in place of the expensive salt, chloride of palladium; indeed, by this process I have readily detected iodine in certain waters from the east coast of this (North) Island.