732 ACTIVITY COEFFICIENT OF THE HYDROGEN ION 



urated KCl calomel electrode (in which case the hydrogen electrode 

 in the NaCl-HCl is negative), and second, against another hydrogen 

 electrode immersed in 1.0 n acid; a saturated KCl bridge was used 

 in both cases. In this case the electrode in the NaCl solution is 

 positive. Any diffusion potential therefore would tend to increase 

 the measured potential in one case and decrease it in the other, so that 

 the potential between the two hydrogen electrodes when measured 

 directly should differ from the difference in potential of the two 

 measured against the same calomel electrode by twice the value 

 of the diffusion potential. The experiment showed, however, that 

 an identical value was obtained by either method. 



The experiments summarized in Table VI show that the effect 

 of neutral salts can be accounted for by the increase of the hydrogen 

 ion concentration. Whatever discrepancy is found is due to the 

 fact that the hydrogen ion concentration apparently increased more 

 than the rate of hydrolysis. It is evidently unnecessary to assume 

 any activity of the unionized acid in this case at least. 



Experiments in Low Concentration of Acid: — The above experiments 

 were repeated with 0.02 n HCl (at 65°C.). In this case the addition 

 of KCl has a very slight depressing effect on the hydrogen ion con- 

 centration and little if any influence on the rate of hydrolysis. This 

 experiment also shows that the unionized gelatin chloride must hy- 

 drolyze at approximately the same rate as the ionized. It was shown 

 by conductivity measurements^ that gelatin chloride is practically 

 completely ionized at pH 2.4, and that the addition of 1.0 n CI ion 

 reduces the ionization to a very small amount. It also decreases 

 the rate of hydrolysis by pepsin very markedly and in direct pro- 

 portion to the decrease in the conductivity of the gelatin chloride. 

 The hydrolysis of gelatin by acid and by pepsin differs markedly in 

 this respect. In pepsin hydrolysis the rate of digestion is decreased 

 by increasing the amount of acid or salt beyond 0.01 N whereas in 

 the acid hydrolysis the rate is unaffected by the addition of salt and 

 increases in direct proportion to the hydrogen ion concentration 

 with the addition of more acid. These facts agree with the hy- 

 pothesis that the rate of hydrolysis of the ionized and non-ionized 

 gelatin salt by hydrogen ions is the same, but that the ionized gelatin 

 salt hydrolyzes much more rapidly than the non-ionized in the pres- 

 ence of pepsin. 



