246 OILS AND [CH. XI 



residue obtained on evaporating off the solvent ; as soon 

 as the weight is constant, this may roughly be taken 

 as the weight of oils and fats. 



If any considerable amount of unsaponifiable residue 

 remains after treatment with alkali, this must be washed, 

 dried and weighed, and its weight subtracted from the 

 total residue before calculating as total oils and fats. 



Determination of free fatty acids. 



The weight of these may be obtained by weighing the 

 dried cake, or residue insoluble in water, after acidifying 

 the products of saponification. 



Determination of glycerin. 



A convenient and fairly accurate process, applicable in 

 these cases, is based on the power of glycerin to dissolve 

 cupric hydroxide in alkaline solution. 



The acid filtrate, from which free fatty acids have been 

 removed, is neutralised with soda, and then rendered 

 strongly alkaline by the addition of 10 c.c. of a strong 

 soda solution ; a dilute solution of copper sulphate is then 

 run in with constant agitation until a permanent precipi- 

 tate of cupric hydroxide is obtained. 



Similar experiments are then made with the same 

 quantities of water and soda to determine (by means of a 

 standard solution of pure glycerin) how much glycerin 

 corresponds to the solution of the amount of cupric 

 hydroxide noticed in the original experiment, which will 

 be the amount of glycerin in the products of saponi- 

 fication. 



[Various modifications of this process are used, but the 

 method described is one of the simplest, and is quite 



