Symbol, CdI. Atomic weight, 182.
This salt is formed by heating filings of metallic cadmium with iodine, or by mixing the two together with addition of water. It is useful in iodizing collodion intended for keeping, since it does not become brown from liberation of free iodine with the same rapidity as the alkaline iodides.
Iodide of cadmium is very soluble both in alcohol and water; the solution yielding on evaporation large six-sided tables of a pearly lustre, which are permanent in the air. The crystalline form of this salt is a sufficient criterion of its purity.
Iodide of Iron.
Symbol, FeI. Atomic weight, 154.
Iodide of iron, in a fit state for photographic use, is easily obtained by dissolving a drachm of iodine in an ounce of proof spirit—that is, a mixture of equal bulks of spirits of wine and water—and adding an excess of iron filings. After a few hours, a green solution is obtained without the aid of heat. The presence of metallic iron in excess prevents the liberation of iodine and deposit of peroxide of iron which would otherwise speedily occur. It is very soluble in water and alcohol, but the solution rapidly absorbs oxygen and deposits peroxide of iron; hence the importance of preserving it in contact with metallic iron, with which the separated iodine may recombine. By very careful evaporation, hydrated crystals of protoiodide may be obtained, but the composition of the solid salt usually sold under that name cannot be depended on.
The periodide of iron, corresponding to the perchloride, has not been examined, and it is doubtful if any such compound exists.
Iodide of Potassium.
Symbol, KI. Atomic weight, 166.