Animal or curd soap is prepared by heating purified animal fat, consisting chiefly of stearin, with sodium hydroxide and water; on adding sodium chloride to the liquid the soap separates as a curd. It occurs as a white or light greyish, odourless solid, which is dry to the touch, though containing about 30 per cent. of water. It becomes plastic on heating, and horny when dried in warm air, being then readily powdered. The aqueous solution becomes alkaline on heating, owing to partial hydrolysis of the soap. The alcoholic solution, however, should be neutral. The fatty acids extracted by treating a solution with excess of sulphuric acid should have a melting-point between 45° and 50°. This test is of value in identifying the fatty substance from which the soap is made. A soap made with olive oil would yield fatty acids melting at about 25°. The limit of moisture (30 per cent.) allowed by the B.P. in this soap is rather high, since there is no difficulty in obtaining soaps containing only about 20 per cent. of water. Soaps prepared with sodium silicate, may contain as much as 50 per cent. of water.
Sparingly soluble in water, more soluble in boiling water (1 in 1.5), partially soluble in alcohol, and almost entirely soluble in boiling alcohol (1 in 2).
Constituents.—The chief constituent of animal soap is sodium stearate. Glycerin is formed as a by-product in making it, and remains in the liquid left on "salting out" the soap.
Uses.—Curd soap is employed in the preparation of Linimentum Potassii Iodidi cum Sapone. It is a useful pill excipient for resinous substances and volatile oils. Stearin soaps are preferred for the preparation of medicated soaps, as they are less readily soluble than olein soaps.
- Emplastrum Saponis Fuscum, B.P., 1885.—BROWN SOAP PLASTER. Syn.—Emplastrum Cerati Saponis.
- Curd soap, in powder, 10; lead oxide, 15; malt vinegar, 160; yellow beeswax, 12.5; olive oil, 20. Heat the vinegar with the lead oxide on a steam-bath until combined, then add the soap and heat again until the moisture is evaporated, next add the beeswax and olive oil, melted together, and evaporate to a suitable consistence.
- Linimentum Saponis Camphoratum, B.P.C.—CAMPHORATED SOAP LINIMENT. Syn.—Solid Opodeldoc.
- Curd soap, 8; camphor, 2; oil of thyme, by weight, 0.4; oil of rosemary, by weight, 0.6; solution of ammonia, by weight, 5; alcohol, by weight, 84. After mixing thoroughly by the aid of heat, this liniment is poured into small, dry warm phials, which should immediately be corked and allowed to cool.
The British Pharmaceutical Codex, 1911, was published by direction of the Council of the Pharmaceutical Society of Great Britain.