UNSAPONIFIABLE RESIDUE 15 



even though that salt may have none of the characteristic pro- 

 perties of a soap. 



The saponification of a fat on a small scale * in the labora- 

 tory may be effected as follows : boil the fat with about three 

 or four times its weight of alcoholic potash under a reflux 

 condenser. The alcoholic potash is prepared by dissolving 

 caustic potash in about twice its weight of water and mixing 

 the solution with twice its volume of alcohol. The heating 

 should be continued until on pouring a little of the solution 

 into a large volume of water an opalescent solution free from un- 

 decomposed fat results. The time required for this may vary 

 from a few minutes to half an hour or more. 



When the saponification is complete, the contents of the 

 flask should be heated in an evaporating basin over a water 

 bath, and thoroughly stirred to get rid of the alcohol. If 

 the free fatty acids are required sufficient sulphuric acid is 

 then added to make the solution strongly acid, whereupon 

 the fatty acids separate out and rise to the surface. 



The aqueous layer contains the glycerol together with the 

 excess of sulphuric acid and potassium sulphate. 



CHOLESTEROL AND PHYTOSTEROL. 



In addition to the trihydric alcohol glycerol, all fats contain 

 a small quantity of the monohydric alcohols cholesterol and 

 phytosterol f which form what is known as the unsaponifiable 

 residue of fats. 



These substances may be isolated from fats according to 

 the following method devised by Kossel and Obermiiller.J 



An ethereal solution of the fat is mixed with a solution 

 of sodium in alcohol ; saponification takes place in the cold 

 and the soap which is precipitated from solution can be filtered 

 off; the filtrate, which is a mixture of alcohol and ether, 

 contains the glycerol together with the so-called unsaponifiable 

 residue consisting of phytosterol or cholesterol which may be 

 obtained by evaporating the solvent. 



* For commercial soap manufacture, see p. 5. 



f The term phytosterol though employed by many authors to indicate a single 

 definite substance is beginning to be used as a generic term for a whole group of 

 closely allied substances the number of which is rapidly increasing as the investi- 

 gation of vegetable fats proceeds. 



+ Kossel and Obermiiller : "'Zeit. physjol, Chem.," 1890, 14, 599; 1891, 15, 321. 



