ESTIMATION 19 



capacity, which is warmed over a gently boiling water bath ; 

 when the whole liquid has been evaporated to dryness, the 

 residue is heated in an air oven for an hour at 100°, and, after 

 cooling in a desiccator, is weighed with the glass stopper in- 

 serted to prevent the hygroscopic soap from absorbing any 

 moisture from the air. 



The amount of fat which corresponds to a given weight of 

 soap may be calculated as follows : — 



C^HsjCOOK — CH2 Ci,H,5COOCH2 



C^HasCOOK - 3K + — CH - Ci,H3sCOOCH 

 Ci,H35COOK — CH2 Ci,H35COOCH2 



Soap Fat 



From the above equation it will be seen that in order to 

 convert three molecules of soap into one molecule of fat, three 

 atoms of potassium, 3 X 39-1, have to be withdrawn from 

 three molecules of soap, and have to be replaced by 41 parts 

 of CH2 . CH . CH2 ; this is equivalent to deducting 39-1 from 

 one molecule of soap and adding V, or I3"6 ; or, in other 

 words, deducting 25-5. 



Hence, if " n " is the number of centimetres of N/io caustic 

 potash required for the titration, and since i c.c. N/io 

 KOH = -00391 gram K = -00136 gram C3H5, we have to 

 deduct from the weight of the soap Wg 



n X 00391 and add n x -00136 



which is equivalent to deducting n X -00255. 



Also, since i c.c. of phenolphthalein solution on evapora- 

 tion would leave 0-0 1 gram of solid, this quantity must be 

 deducted from the weight of the soap. 



Hence the percentage of fat may be calculated from the 

 relation 



F = rW.--OT-(nx -00255)1 ^ 

 I m J 



in which " m " is the weight of the sample taken. 



In estimating fat in flour or farinaceous grain by this 

 method, it is best to subject the substance to a preliminary 

 treatment by heating 5 grams of the sample for half an hour 

 with 30 c.c. of dilute sulphuric acid (i : lo), the mixture is 



