Prep. The mode of preparing this salt consists in adding iodine to a solution of potash, when the following changes ensue (6 KO + I6 = 5 KI + KO, Io5). The mixture of the two salts, namely, the iodide of potassium and iodate of potash, with a little charcoal in fine powder, is heated to redness, by which means the iodate of potash is converted into iodide of potassium, the charcoal facilitating the deoxidation.

Another method, formerly employed, is to form an iodide of iron, by bringing together iodine and iron with water, at a moderate heat; then adding carbonate of potash to the solution, when carbonate of iron and iodide of potassium are formed; the former salt being separated by filtration, the solution by evaporation yields the latter in a crystallized state.

Prop. & Comp. Iodide of potassium forms white semi-transparent cubic crystals; without odour if pure, and of a saline taste; as met with in commerce, it occasionally has some odour of free iodine; very soluble in water, and in about six or eight parts of rectified spirit; the solutions should be neutral; and the watery solution, mixed with mucilage of starch, gives a blue colour on the addition of a minute quantity of chlorine. Iodide of potassium gives a crystalline precipitate with tartaric acid; but tartaric acid and starch should not develope a blue colour; should they do so, it indicates the presence of iodate of potash; for iodic acid, being then liberated by the tartaric acid, acts as an oxidizing agent upon the hydriodic acid which is formed at the same time, and sets free the iodine. Iodide of potassium, when treated with acetate of lead, gives a yellow precipitate of iodide of lead, soluble in boiling water; but it does not alter lime-water, or chloride of barium; with nitrate of silver a pale yellow iodide falls, insoluble in solution of ammonia, and the ammoniacal liquid gives with excess of nitric acid no turbidity, showing a freedom from chlorides. Composition (KI).

Off. Prep. Unguentum Potassii Iodidi. Iodide of Potassium Ointment. (Iodide of potassium, sixty-four grains; boiling distilled water, one fluid drachm; prepared lard, one ounce. Dissolve the iodide in the water, then mix with the lard.)

Iodide of potassium is contained also in linimentum iodi, tinc-tura iodi, and unguentum iodi compositum; but in all these preparations it is introduced more on account of its solvent than its therapeutic powers.

Therapeutics. The action and uses of this drug have been described under Iodine; it does not possess the local irritant properties of free iodine, and hence is more adapted for internal administration. The ointment may be used when the slow action of the iodide upon a diseased part is desired.

Dose. 1 gr. to 10 gr., or more.

Adulteration. The salt may be damp, from the presence of water; it may also contain many impurities, as carbonate of potash, chlorides of sodium and potassium, iodate of potash, free iodine, etc, all detectable bythe tests above given. When iodate of potash exists in the salt, from the imperfect ignition of the mixed iodide and iodate, the ointment is apt to become yellow, owing to the decomposition of the iodic acid by the animal matter.