CH. VI.] PROPERTIES OF FATS. 155 



Since the fatty acid is only slightly ionised, and is in- 

 soluble in water, dilution of a clear solution of sodium 

 stearate causes the liberation of sodium hydroxide and of 

 the fatty acid, the latter causing the solution to become 

 opalescent. This reaction is not so well marked with the 

 oleates. This hydro ly tic dissociation is checked by the 

 addition of alcohol. For this reason it is essential to have 

 alcohol present to the extent of 50 per cent, at the end of a 

 titration of fatty acids with aqueous alkalies. 



The calcium, magnesium, barium and lead salts or 

 soaps are insoluble in water. 



The glycerides formed by these three acids are known as 

 tripalmitin, tristearin and triolein respectively. The 

 melting points are 66, 71 and 5C. In the body they 

 are found mixed in different proportions, and possibly 

 some of the compounds have more than one fatty acid in 

 the molecule. They are hydro lysed by boiling acids and 

 alkalies, by superheated steam, and by certain enzymes 

 called lipases or steapsins. If an alkali be used as the 

 hydrolytic reagent, the fatty acid liberated combines with 

 the alkali to form a soap. This special form of hydrolysis 

 is therefore called saponification. The most rapid method 

 of effecting this hydrolysis in the laboratory is by boiling 

 the fat with an alcoholic solution of soda or potash. The 

 fat being soluble in the alcohol the formation of ethyl esters 

 of the fatty acids proceeds rapidly. The ethyl esters are 

 themselves hydrolysed by the water present, and so the fat 

 is completely converted into glycerol and soap. 



Various methods have been devised for the identifica- 

 tion of the fats and oils, amongst them being. 



1. The melting point, or the solidifying point. 



2. The saponification value. A known weight of the 

 fat is heated with a given volume of a standardised alco- 

 holic solution of potash. The mixture is then titrated with 

 standard hydrochloric acid against phenol phthalein, 

 alcohol being added to maintain a concentration of 50 per 

 cent. The number of milligrammes of potassium hydroxide 



