136 



THEORY OF COLLOIDAL BEHAVIOR 



If Donnan's membrane equilibrium is the cause of the influence 

 of pH on the P.D. (and on the other physical properties) of 

 protein solutions, we must be able to show that equation (2) is 

 actually fulfilled. 



This consequence of Donnan's theory was put to a test and 

 some of the experiments described in the preceding part of this 

 chapter were selected for this purpose. Inside the collodion 

 bags were 1 per cent solutions of gelatin chloride of different 

 pH; outside, water. After 18 hours equilibrium was established 

 between inside and outside solutions and the pCl as well as the 

 pH was ascertained. The pCl was determined in two different 

 ways in the two experiments; in one experiment it was deter- 

 mined with the calomel electrode, in the other it was determined 

 in the gelatin chloride solution by titration with NaOH according 

 to the method described in the fourth chapter. Both methods of 

 determining the pCl led to the result that the value pCl outside 

 minus pCl inside was for the same solution at the point of equilib- 

 rium equal to the value pH inside minus pH outside (within 

 the limits of accuracy of the experiments) . The pCl outside was 

 identical with the pH outside since the outside solution contained 

 only free HC1. The values of pH were all determined potentio- 

 metrically (Table XVI). 



TABLE XVI 

 Experiment 1. pCl determined by titration 



Experiment 2. pCl determined electrometrically 



