28 FATS, OILS, AND WAXES 



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 1 00, 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 : 



CH 



- 3 K + CH = C 17 H 35 COOCH 



C^H^COOK CH 2 C 17 H 35 COOCH 2 



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 atoms of potassium, 3 x 39*1, have to be withdrawn from 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 CH 2 * CH CH 2 ; this is equivalent to deducting 39-1 from 

 one molecule of soap and adding ^ or 13*6, or, in other 

 words, deducting 25-5. 



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

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

 KOH = -00391 gram K = -00136 gram C 3 H 5 , we have to 

 deduct from the weight of the soap W s 



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 O-QI 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 = |W S - -oi -(nx -00255)1 x 

 V. 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 : 10), the mixture is 

 then diluted with 50 c.c. of 50 per cent, caustic potash. Finally 



