674 



DONNAN EQUILIBRIUM AND MEMBRANE POTENTIALS 



of the valency of anion in the case of gelatin-acid salt has been dis- 

 cussed in preceding papers. The question now arises: Does the 

 value of the difference in the pH inside minus pH outside vary simi- 

 larly as the values of the observed p.d. and the observed osmotic 

 pressure, and do we get values approximating the observed p.d, if 

 we multiply the value pH inside minus pH outside by 59? Table 

 III gives the values for pH inside and outside and also the difference, 

 pH inside minus pH outside, at the point of equilibrium (i.e., after 18 

 hours). 



The values for the p.d. calculated by multiplying the values for 

 pH inside minus pH outside by 59 agree remarkably well with the 

 observed values for p.d. 



TABLE III. 

 Influence of Concentration ofNazSOt on pH Inside Minus pH Outside at Equlibrium. 



We will consider as a third case the influence of a salt of the type 

 CaCl2 on the osmotic pressure and the p.d. of a gelatin chloride solu- 

 tion. It had been shown in a preceding paper that the depressing 

 effect of CaCl2 is always about twice as great as that of an equimolec- 

 ular concentration of NaCl and that hence the whole effect of CaCl2 

 is due exclusively to the CI ion. Fig. 2 shows that the depressing 

 effects of CaCl2 on the osmotic pressure and the p.d. run closely paral- 

 lel and that the depressing effect of CaClo on p.d. is about the same 

 as that of a NaCl of twice the molecular concentration. Table V 

 gives the influence of CaCl2 on the values of pH inside minus pH 

 outside, and shows that the agreement between the observed and 

 the calculated p.d. is quite close. 



