184 THE ESTIMATION OF PROTEINS. 



burette ; the flask is kept in a thermostat at 25, and constantly 

 shaken, and when the volume is constant, the burette is set at 

 zero, and the tube allowed to fall into the milk. The flask must 

 be kept shaken to ensure the regular liberation of the oxygen, 

 and the volume of the gas read at intervals. The velocity con- 

 stant (which is strictly proportional to the amount of catalase 

 present) is determined by the formula 



where t = time and a = half the number of c.c. of oxygen 

 liberated when the quantity of hydrogen peroxide used is treated 

 with permanganate, and x = the number of c.c. liberated in 

 time t. The value of a should lie between 15 and 50 c.c. 



The method is rather tedious, but gives the most accurate 

 estimation of catala e ; in fresh unneutralised milk the value of 

 K varies from 0-0025 to 0-0015. Acids retard the value, but 

 if these be neutralised the value rises as the milk gets older, and 

 may have a very large value in cases of mastitis and other un- 

 healthy conditions of the udder. Faitelowitz recommends that 

 if the value of K is above 0-01 the milk should be condemned. 



Revis recommends Koning's method, which consists in taking 

 two stoppered flasks of 250 c.c. capacity, and placing 5 c.c. of 

 milk in each ; to one 6 drops of hydrochloric acid (1 : 1) is 

 added to destroy the enzyme, and 5 c.c. of 1 per cent, hydrogen 

 peroxide is added to each, and the flasks kept at 38 C. for 

 two hours, 10 c.c. of strong hydrochloric acid is added, shaking 

 well, and then 10 c.c. of 10 per cent, potassium iodide solution, 

 and after 15 minutes 100 c.c. of water. The liberated iodine is 



N 

 titrated with y^ thiosulphate with starch as indicator ; owing 



to the adsorption of iodine by the casein the titration takes about 

 J hour. The difference between the two titrations calculated 



N 

 as c.c. r-7; per 100 c.c. of milk give the catalase activity, and the 



normal figure is less than 5. Roughly speaking, the Koning 

 figure is about 1,000 times that of Faitelowitz. A less accurate 

 catalase test may be made by treating milk with one-third of 

 its volume of 1 per cent, hydrogen peroxide solution, and 

 measuring the volume of gas which is given off. Lobeck's catalase 

 tube (Fig. 27) is very convenient for this, though any other form 

 of measuring apparatus may be used ; for the estimation, the 

 measuring cylinder is filled with water through the opening d, 

 the cover of which is then screwed down. The opening b (for 

 cleaning the tube) is closed with a rubber cork, and the chamber 

 A is charged, through the opening c, with 15 c.c. of milk and 



