THE STABILITY OF PROTEIN SOLUTIONS 



245 



find the molecular concentration of different salts namely, 

 (NH 4 ) 2 SO 4 , Na 2 SO 4 , MgSO 4 , KC1, and MgCl 2 required for pre- 

 cipitation. Table XLV shows that regardless of the pH the 

 sulphates are better precipitants than the chlorides. Wherever 

 we are dealing with colloidal phenomena, i.e., phenomena regu- 

 lated by the Donnan equilibrium, we must expect that sulphates 

 will have a more depressing effect than chlorides when the protein 

 is on the acid side of the isoelectric point but no when it is on the 

 alkaline side or at the isoelectric point. But this is not true for 

 the influence of ions on the salting out of gelatin in aqueous 

 solution. 



TABLE XLV. MINIMAL MOLAR CONCENTRATIONS REQUIRED TO PRE- 

 CIPITATE 0.8 PER CENT SOLUTIONS OF GELATIN 



The question arises, How does it happen that sulphates are 

 better precipitants than chlorides? Some light is thrown on 

 this fact by experiments on the rate of solution. 



Powdered gelatin of not too small a size of grain (going through 

 sieve 30 but not through sieve 60) was rendered isoelectric in the 

 way described in Chap. II and about 0.8 gm. was put into 100 

 c.c. of each of a series of solutions of NaCl, CaCl 2 , or Na 2 SO 4 , 

 varying in concentration from M/4,096 to 2 M. The suspensions 

 of the powdered gelatin were frequently stirred and the time 

 required to practically completely dissolve all the grains of 

 powdered gelatin in suspension at 35C. was measured. The 

 ordinates in Fig. 73 are the solution times of isoelectric gelatin, 

 and the abscissae are the molecular concentrations of the salt 

 used. It is obvious that NaCl and still more CaCl 2 increase the 

 rate of solution of isoelectric gelatin in water, and the more the 

 higher the concentration of the salts added. There exists, 

 however, a striking discontinuity in the Na 2 SO< curve. As long 



