MILK 



123 



Peters 1 with benzidino (paradiamidodiphenyl). An alcoholic 

 solution of bcnzidine acetate gives a deep blue coloration with 

 the oxygen freed from the hydrogen peroxide. 



When 4 drops of an alcoholic solution of benzidine 

 (4 per cent.) and two drops of acetic acid are added to 10 c.c. 

 milk, the presence of hydrogen peroxide is denoted by a blue 

 coloration. Investigations made by the author show that by 

 this method 0'005 g. hydrogen peroxide in 100 c.c. milk 

 easily be detected. 



(b) Quantitative Estimation. 



For the quantitative estimation of hydrogen peroxide in milk 

 the following method can be used. It has been tested by the 

 author, 2 and very good results have been obtained. 



0'5 c.c. of dilute sulphuric acid (1 : 3) are added to 25 c.c. of 

 milk, and after the coagulated casein has been allowed to settle 

 completely it is filtered off, and 5 c.c. of the filtrate measured 

 out into a bottle with a glass stopper. Then 10 c.c. of a 

 10 per cent, solution of potassium iodide and 0'5 c.c. of sulphuric 

 acid (1:3) are added, the flask shaken, kept in the dark for 

 four hours, the shaking being repeated frequently and the free 

 iodine afterwards titrated with sodium thiosulphate 3 solution, 

 starch paste being used as an indicator. One c.c. of the 

 thiosulphate solution = 0'0017 g. hydrogen peroxide. The 

 reactions take place as follows : 



I 2 + 2Na 2 S 2 O 3 = 2NaI + Na 2 S 4 O 6 . 



The method, which is thus based upon the power of hydrogen 

 peroxide to free iodine, cannot be used with the milk itself, for 

 otherwise a compound of iodine and milk would be formed. 

 After coagulation of the casein the serum which remains has 

 no action upon the iodine. 



1 ZeitAchrifb fur Untersuch. der Nahr- und Genussmittel, 1908, Vol. XVI, 

 p. 172. 



2 Revue Gen. du Lait, Vol. II, p. 289. 



'' For the preparation of this solution, see p. 154. 



