Synonym. Potassae Hydras. Lond. Potassa. Edin.

Prep. Two pints of the solution of potash, are evaporated in a silver or clean iron vessel over a fire, until the ebullition being finished, the hydrate of potash liquefies: this is poured into proper moulds, and when it has solidified, and while it is still warm, put it into stoppered bottles.

Prop. & Comp. Caustic potash is usually moulded for medical purposes into small sticks about the size of a pencil, which should be white, but are often greenish, bluish, or reddish-brown from impurities; it quickly deliquesces when exposed to air, and, if pure, dissolves in rectified spirit; it dissolves animal tissues, forming a kind of soap with them; a watery solution acidulated by nitric acid, gives a yellow precipitate with bichloride of platinum, and scanty white precipitates with nitrate of silver and chloride of barium. Composition (HO, KO); the equivalent of water cannot be driven off by heat. Fifty-six grains dissolved in water leave only a trace of sediment, and require for neutralization at least 90 measures of the volumetric solution of oxalic acid, corresponding to 42.3 grains of potash.

Off. Prep. Liquor Potassae, described above, contains caustic potash, and in the British Pharmacopoeia is considered an officinal preparation of that substance.

Therapeuties. The hydrate of potash, and also its mixture with equal parts of lime (potassa cum calce), which is not now officinal, are used only externally, as caustics, for the formation of sloughs, for touching ulcers, etc.; the advantage of the latter depends on its being much less deliquescent: it is applied as a paste made with spirit.

Adulteration. The same as of liquor potassae; besides which, oxides of iron and alumina are often present; these are not soluble in spirit.