SOAP, a chemical combination of fats and alkali, used as a detergent. The cleansing prop erties of these compounds have been known for a very long time; the manufacture of soap upon a large scale dates only from about 1823, in which year Chevreul published his famous re searches upon the animal fats. The natural fats palmitin, stearin, olein, etc. are acid ethers of glycerine. Glycerine is an alcohol containing three atoms of replaceable hydrogen, its formula is CH3 Os; in the fats these three hydrogen H atoms are replaced by the radicles of various acids, chiefly of palmate, stearic, margaric and oleic acid, and thus are obtained the glycerides of these fatty acids, or, as they are commonly called, the fats palmitin, Calla C3H3 ( )1 I ; ( C311,50) a ; CaR5 margarin, (CI :H=0) t 03; and olein, C3H3 t 03. If these fats be heated with (C31-1330) 2 caustic alkalis they are decomposed, a union taking place between the particular acid, the radicle of which exists in the fat, and the alkali while glycerine is at the same time produced; thus with stearin and caustic potash the re action may be formulated: (C A W O), 03-1-3KH0= ` t O.
1, Glycerin. Potassium Stearate, The salt of the fatty acid so produced (in this instance potassium stearate) is called a soap. Soa is fore, may general! d d ,s he to is sa is o er arty an t e makiNEMEIMENEEIMInt.m. .
.oug is Inc er e name soap all the metallic salts of the higher fatty acids, yet in common usage the name is limited to the sodium and potassium salts of these acids.
Soaps are also produced by certain oils with caustic alkalis; these oils are of the class known as fixed oils, so-called because they cannot be distilled without decomposing. Oils which may he distilled, as the essential oils and mineral oils, are not available for soap-making.
The essential parts of a soap then are (1) the alkaline metal and (2) the fatty acid which it contains; upon these two the quality of the soap depends. Speaking broadly, two kinds of soap arc made for the market: hard soap and soft soap. In the former type, soda is used as the combining alkali; in the latter, potash.
Materials. The oils and fats in economic use for the manufacture of soap may be divided for discussion into five groups or alliances: (1) coconut oil and palm-kernel oil; (2) olive oil, olive oil foods, cottonseed oil, peanut oil, sunflower seed oil and corn oil; (3) tallow, greases and palm oil; (4) castor oil and lin seed oil; (5) red oil and resin. The oils in the
first group saponify very quickly at ordinary temperatures upon the addition of cold con centrated caustic soda. During the action great heat is evolved. A larger .yield of firm soap is obtained from these oils than from any others. They are, however, rarely used alone, but are added to tallow and cottonseed oil, and this combination results in a superior grade of soap, better than can be made from either of the in gredients alone. Their glycerine content is higher than in any other group, amounting to 12 per cent. The second group is composed chiefly of glycerides of oleic acid. They yield a thin soap, very soluble in water, and make good toilet and textile soaps. The well-known green Castile soap, when pure, is made from olive oil. Their glycerine content is about 10 per cent. The third group are glycerides of (principally) stearin and palmitin, and yield a firm soap of good body, dissolving slowly in water; much improved by the addition of oils from the first and second groups. The oils of the fourth group saponify readily, yielding highly soluble soaps of a pale brownish color, useful in mak ing transparent soaps. The acids of the fifth group combine directly with caustic soda. They are used only in combination with other groups.
In the last few years the process known as hydrogenation has made oils which had been unavailable for soaps a very important factor in the raw materials market, which had been seri ously narrowed by the demand for all edible oils by the manufacturers of oleomargarine and "nut butter." By this process oleic acid is con verted into stearic acid by passing a current of hydrogen gas through it in the presence of re duced nickel, while at the temperature of about 550° F. This process is applied chiefly to cheap fish oils which lose their objectionable odor during the treatment.