Ice. -^ acid = .11315 grams casein, or in other words .11315 grams of 



casein is capable of neutralizing as much alkali as 1 ce. of t * acid, so if we take 

 11.315 grams of milk we see from the relation above that every ee. of 

 - N acid used equals 1 per cent, casein. B\' using differenl quantities of milk 

 we need only change the normality of our acid. 



If by using 11.315 grams of milk (or 11 ec.) where each ee. of placid 

 corresponds to 1 per cent., by using a greater or larger quantity of milk the 

 normality would have to be correspondingly less or greater. When we use 

 8.75 ce. or 9 grams of milk the normality would not be f 6 but 795 ee. 

 i N o acid plus water to make 1,000 ee. which equals i_ Upon the above 

 facts the volumetric method of Van Slyke and Bosworth is based. 



Procedure in carrying out in detail the volumetric estimation of casein: 

 "A given amount of milk, diluted with water, is made neutral to phenolpthalein 

 by the addition of a solution of sodium hydroxide. The casein is then completely 

 precipitated by the addition of the standardized acetic acid; the volume of the 

 mixture is then made up to 200 ce. by the addition of water, thoroughly shaken 

 and then filtered. Into 100 ee. of the filtrate a standard solution of sodium 

 hydroxide is run until neutral to phenolpthalein. The solutions are so stand- 

 ardized that 1 ee. is equivalent to 1 per cent, of casein when a definite amount of 

 milk is used. The number of cc. standard acid used, divided by two (since 

 only 100 ce. of the 200 ec. is used), less the standard alkali used in the last 

 titration gives the pei'centage of casein in the milk examined." When 17.5 

 or 18 grams of milk are used the strength of acetic acid and alkali are made 

 by diluting 795 cc. of ~ to 1,C00 ec. The same normality as was derived 

 above. Since only 100 cc. of the 200 ee. were titrated this then represents 

 the acid required to liberate the casein in 8.75 cc. or 9 grams of milk. Like- 

 wise by using 22 cc. cr 22.6 grams of milk treated as above, then 1 ec. of 

 ^ acid equals 1 per cent of casein. By the use of a factor any con- 

 venient quantity can be used. Example, by the use of 20 cc. of milk and 

 ^ solution, adjustment is made by multiplying the final result by 1.0964. 



Apparatus and reagents necessary to carry on the volumetric estimation 

 of casein in milk are, first, two 50 ee. burettes, graduated to 1/10 cc. or better 

 1/20 cc, these must be accurate. One of the burettes should be supplied 

 with a glass stop cock for the acid, and one with a pinch cock for the alkaline 

 solution. Second, flasks, volumetric, holding 200 ce. At least two of these 

 are needed and where a number of estimations are to be made more are 

 required to do rapid work; ten to twelve are necessary for rapid work. The 



