220 POTENTIAL DIFFERENCES AND ANOMALOUS OSMOSIS 



valent anion {e.g., CI), acid is driven from the side where the salt 

 is added to the side originally free from salt. This difference in the 

 pH inside minus pH outside gives rise to a potential difference which 

 was discussed in a preceding paper. ^ In this paper it was shown that 

 when we separate a gelatin chloride solution from a solution of HCl 

 (without gelatin) both having at the beginning the same hydrogen ion 

 concentration, acid is forced from the gelatin solution into the out- 

 side solution. The same happens when solid jelly of gelatin chloride is 

 separated from a HCl solution. In this case there arises a p.d. between 

 the two phases; and the writer has been able to show that this p.d. 

 can be calculated with a good degree of accuracy from the difference 

 of the hydrogen ion concentration inside and outside, on the basis 

 of Nernst's well known formula. By multiplying the value (pH 

 inside minus pH outside) by 58 we get the value for the p.d. actually 

 observed at 18°C. in terms of millivolts. '^ 



This unequal distribution of acid inside and outside is due to a pecu- 

 liar membrane equilibrium the theory of which was developed by Don- 

 nan.^ Our new experiments show that such an equilibrium condi- 

 tion is produced also between a gelatin membrane and a solution of 

 HCl free from gelatin. The coating of gelatin on the collodion 

 membrane behaves like a solid jelly of gelatin, the gelatin being 

 transformed into gelatin chloride when in contact with HCl. The 

 Donnan equilibrium demands that the concentration of free HCl 

 inside the gelatin membrane be less than the concentration of free 

 HCl of the solution bounding the membrane. This gives rise to 

 the P.D. The value of pH inside minus pH outside is diminished 

 when a neutral salt is added — in accordance with Donnan's 

 theory — and the fact that a salt is added in our diffusion experi- 

 ments on one side of the membrane but not on the other is the cause 

 of the fact that the pH of the acid solution containing the salt solu- 

 tion (inside solution) becomes higher than the pH of the solution 

 containing no or less salt (outside solution). The reader will find 

 the experiments proving this in the writer's paper referred to.^ 



^ Loeb, J., /. Gen. Physiol., 1920-21, iii, 667. 



^ Donnan, F. G., Z. Elekirochem., 1911, xvii, 572. Donnan, F. G., and Harris, 

 A. B., /. Chem. Soc, 1911, xcix, 1554. Donnan, F. G., and Garner, W. E., /. 

 Chem. Soc, 1919, cxv, 1313. 



