MEMBRANE POTENTIALS 137 



Nernst's formula leads therefore to the same theoretical P. D. 

 regardless of whether we calculate the P.D. on the basis of 

 the difference pH inside minus pH outside or on the basis of 

 the difference pCl outside minus pCl inside. It is also obvious 

 that both assumptions lead to the same sign of charge of the 

 gelatin chloride solution. If we assume that the P.D. is deter- 

 mined by differences in the hydrogen ion concentration, the 

 outside solution is concentrated and the inside solution dilute; 

 if the P.D. is determined by differences in the concentration of 

 the Cl ions, the inside solution is concentrated and the outside 

 solution dilute. Since the common ion is positive in the former 

 and negative in the latter case, the gelatin solution becomes 

 positive in both cases. 



OUTSIDE SOLUTION INSIDE SOLUTION 



H+ dilute + 



H + concentrated 



~, ,., , membrane 



Cl- dilute 



Cl~ concentrated 



The facts contained in this section of this chapter prove that 

 the equation x 2 = y(y + z) is the correct expression for the 

 Donnan membrane equilibrium between acid-salts of proteins 

 with monovalent anion and water, and that the Donnan equilib- 

 rium accounts for the P.D. observed. We wish to point out 

 that we get the same result whether we determine pCl by titra- 

 tion or potentiometrically. The agreement with the theory is 

 the same in both cases though the accuracy of the determination 

 of pCl is less than that of pH. 



THE P.D. OF Na GELATINATE 



The Donnan theory demands that when a solution of Na 

 gelatinate contained in a collodion bag is in equilibrium with a 

 watery solution free from gelatin, free NaOH should be forced 

 from the inside gelatin solution through the membrane into the 

 outside watery solution free from gelatin. As a result the pH 

 inside will now be less than pH outside, and the value pH 

 inside minus pH outside will be negative for Na gelatinate while 

 it was positive for gelatin chloride. If the Donnan equilibrium 

 determines the P.D. (as it does) the sign of charge of Na gela- 

 tinate must be the reverse from what it was for gelatin chloride. 



