i MILK 91 



(1) Ferrous ammonium sulphate solution containing 7 g. per litre, corre- 



sponding to 1 g. iron. 



(2) Potassium permanganate solution containing 0'5646 g. per litre, equiva- 



lent to 1 g. iron. Standardisation of the solution : 20 c.c. of solution (1) 

 are brought together with 5 c.c. pure sulphuric acid and 25 c.c. water 

 into a beaker, and the mixture titrated with the solution of permangan- 

 ate until the faint pink colour persists. 



For the actual estimation the ash of milk is brought into a 

 beaker with 25 c.c. water, then 5 c.c. of pure sulphuric acid are 

 added, and 20 c.c. of solution (1). The reduction of the chromic 

 acid follows immediately, and by titrating with permanganate 

 the excess of ferrous ammonium sulphate solution can be 

 determined. The number of cubic centimetres of permanganate 

 solution is subtracted from 20 c.c., and the difference represents 

 the amount of solution (1) required for the reduction of the 

 chromic acid. One c.c. ferrous ammonium sulphate solution is 

 equivalent to 0'000875 g. potassium bichromate. 



VII. ESTIMATION OF THE REMAINING CONSTITUENTS OF 



MILK. 



In addition to those constituents of milk already mentioned, 

 there are others, such as citric acid and lecithin, which 

 sometimes have to be determined. 



1. Estimation of Citric Acid. 



The latest and simplest method for estimating citric acid in 

 milk is M. Beau's 1 modification of that of M. Deniges. 2 The 

 principle underlying the method is that when citric acid is 

 oxidised with potassium permanganate, acetone dicarboxylic 

 acid, carbon dioxide, and water are formed. 



CH 2 .COOH CH 2 .COOH 



C(OH).COOH + O = CO .+ H 2 + C0 2 . 



CH 2 .COOH CH 2 .COOH 



Acetone dicarboxylic acid, on treatment with excess of 

 mercuric sulphate, gives a white precipitate of mercury 

 dicarboxysulphoacetone. 



CH 2 .COO V 



i \ 



CHj.COO 



1 Revue Generate du Lait, Vol. Ill, p. 385. 



8 Annales de Phys. et de Chim., Vol. XVIII, 1899. 



