THE STABILITY OF PROTEIN SOLUTIONS 



271 



Fig. 80. The ordinates are the average 

 diameters of the particles after 1 and 24 hours 

 respectively. The abscissae are the concentra- 

 tions of NaCl. The depressing effect is 

 similar to that found in the case of the swell- 

 ing of a jelly of gelatin. After 24 hours the 

 particles had dissolved in the NaCl solutions 

 of a concentration below M/256, but not in 

 concentrations of NaCl higher than M/256. 



That the solution of casein chloride is thus 

 regulated to a large extent by the Donnan 

 effect was ascertained also by measurements 

 of the quantity of casein chloride dissolved 

 at 20C. at various pH of the solution. One 

 gram of isoelectric powdered casein was put 

 into 100 c.c. of solutions of HC1 of different 

 concentration and kept in these solutions in 

 one case for 1 hour, in a second case for 22 

 hours. The mass was then poured into grad- 

 uated cylinders and the undissolved part was 

 allowed to settle to the bottom for 2 and for 

 6 hours respectively at 20C. The super- 

 natant liquid was removed and the sediment 

 dried over night in an oven at about 100C. 

 Table LI gives the result. The dry weight 

 of 1 gm. of isoelectric casein was found to be 

 0.870 gm. and this weight diminished by the 

 dry weight of the sediment was the amount 

 dissolved. Table LI shows that the rate of 

 solution increases with diminishing pH from 

 4.36 to 2.18 where the solubility of casein 

 chloride is a maximum ; with a further decline 

 in pH the solubility diminishes again. This 

 is in agreement with the Donnan effect. 



In a similar way the depressing effect of 

 NaCl on the rate of solution of casein 

 chloride was ascertained (Fig. 80). Solutions 

 of 12.5 c.c. of 0.1 N HC1 in 100 c.c. and 

 containing 1 gm. of powdered, originally 



