Characters. - In colourless, generally opaque, cubic crystals.

Preparation. - By mixing iodine and solution of potassa, when iodide and iodate of potassium are formed, 6KHO + 6I = KIO3 + 5KI + 3H2O. The iodate is then reduced to iodide by roasting with charcoal, 5KI + KIO3 + 3C - 6KI + 3CO.

Solubility. - It is readily soluble in water, and in a less degree in spirit.

Reactions. - It commonly has a feeble alkaline reaction; its solution gives the reactions of potassium (p. 603) and an iodide (p. 594).

Impurities. - Iodate from imperfect reduction, chlorides, sulphates, carbonates. Iodate is the most important impurity, since the dilute acid of the gastric juice will form hydriodic acid from the iodide, and this will liberate free iodine from the iodate in the stomach, and thus give rise to such gastric irritation that the iodide cannot be borne in doses where pure iodide would be readily tolerated.

Tests. - The addition of tartaric acid, B.P., or dilute sulphuric acid, U.S.P., and mucilage of starch, B.P., or gelatinised starch, U.S.P., to its watery solution does not develop a blue colour (absence of iodate). If iodate be present the acid liberates hydriodic acid, and this re-acting on the iodate forms free iodine, 6HI + KI03 = 3H20 + KI + I6. Solution of nitrate of silver added in excess forms a yellowish-white precipitate, which, when agitated with ammonia, yields on standing a clear liquid in which excess of nitric acid causes no turbidity, B.P. (absence of chloride). Iodide of silver is insoluble in ammonia, but chloride is readily soluble, so the chlorides, if present, would be taken up by the ammonia and re-precipitated on the addition of acid. Its aqueous solution is only faintly precipitated by the addition of saccharated solution of lime.

Dose. - 2 to 10 grains.

Preparations Containing Iodide Of Potassium

B.P.

Strength.

Linimentum Iodi (p. 516)....................................................

22 grs. in 1 fl. oz. nearly.

" Potassii lodidi cum Sapone (p. 516)

54 1/2 grs. in 1 fl. oz. nearly.

Liquor Iodi.............................................................................

33 grs. in 1 fl. oz.

Tinctura Iodi (Dose, 1-5 min.)...............................................

11 grs. in 1 fl oz. nearly.

Unguentum Iodi......................................................................

16 grs. in 1 oz. nearly.

" Potassi Iodidi......................................................

1 part in 8 3/4, nearly.

U.S.P.

Dose.

Unguentum Potassi Iodidi...............................................

Liquor Iodi Compositus...................................................

2-6 min. (0.10-0.40 c.c.).