This salt is usually formed by dissolving iodine in solution of potash until it begins to acquire a brown color; a mixture of iodide of potassium and iodate of potash (KO IO{5}) is thus formed; but by evaporation and heating to redness, the latter salt parts with its oxygen, and is converted into iodide of potassium.
Properties.—It forms cubic and prismatic crystals, which should be hard, and very slightly or not at all deliquescent. Soluble in less than an equal weight of water at 60°; it is also soluble in alcohol, but not in ether. The proportion of iodide of potassium contained in a saturated alcoholic solution, varies with the strength of the spirit,—with common spirits of wine, sp. gr. ·836, it would be about 8 grains to the drachm; with alcohol rectified from carbonate of potash, sp. gr. ·823, 4 or 5 grains: with absolute alcohol, 1 to 2 grains. The solution of iodide of potassium is instantly colored brown by free chlorine; also very rapidly by peroxide of nitrogen; ordinary acids, however, act less quickly, hydriodic acid being first formed, and subsequently decomposing spontaneously.
Iodide of potassium, as sold in the shops, is often contaminated with various impurities. The first and most remarkable is carbonate of potash. When a sample of iodide of potassium contains much carbonate of potash, it forms small and imperfect crystals, which are strongly alkaline to test-paper, and become moist on exposure to the air, from the deliquescent nature of the alkaline carbonate. Sulphate of potash is also a common impurity; it may be detected by chloride of barium.
Chloride of potassium is another impurity; it is detected as follows:—Precipitate the salt by an equal weight of nitrate of silver, and treat the yellow mass with solution of ammonia; if any chloride of silver is present, it dissolves in the ammonia, and after nitration is re-precipitated in white curds by the addition of an excess of pure nitric acid. If the nitric acid employed is not pure, but contains traces of free chlorine, the iodide of silver must be well washed with distilled water before treating it with ammonia, or the excess of free nitrate of silver dissolving in the ammonia would, on neutralizing, produce chloride of silver, and so cause an error.
Iodide of potash is a fourth impurity often found in iodide of potassium: to detect it, add a drop of dilute sulphuric acid, or a crystal of citric acid, to the solution of the iodide; when, if much iodate be present, the liquid will become yellow from liberation of free iodine. The rationale of this reaction is as follows:—The sulphuric acid unites with the base of the salt, and liberates hydriodic acid (HI), a colorless compound; but if iodic acid (IO{5}) be also present, it decomposes the hydriodic acid first formed, oxidizing the hydrogen into water (HO), and setting free the iodine. The immediate production of a yellow color on adding a weak acid to aqueous solution of iodide of potassium is, therefore, a proof of the presence of an iodate. As iodate of potash is thought to render collodion insensitive (?), this point should be attended to.
Iodide of potassium may be rendered very pure by recrystallizing from spirit, or by dissolving in strong alcohol of sp. gr. ·823, in which sulphate, carbonate, and iodate of potash are insoluble. The proportion of iodide of potassium contained in saturated alcoholic solutions varies with the strength of the spirit.
Solution of chloride of barium is commonly used to detect impurities in iodide of potassium; it forms a white precipitate if carbonate, iodate, or sulphate be present. In the two former cases the precipitate dissolves on the addition of pure dilute nitric acid, but in the latter it is insoluble. The commercial iodide, however, is rarely so pure as to remain quite clear on the addition of chloride of barium, a mere opalescence, therefore, may be disregarded.
Iodide of Silver. (See Silver, Iodide of.)
Iron, Protosulphate of.