728 ACTIVITY COEFFICIENT OF THE HYDROGEN ION 



In all the above experiments the hydrogen ion concentration has 

 been expressed as that calculated by Nernst's formula from the po- 

 tential of the hydrogen electrode. It is certain that this value 

 does not really express the actual concentration in grams per liter 

 of hydrogen ions. It would probably be better to call it the "ac- 

 tivity coefficient" or "active concentration" as proposed by Lewis;^^ 

 that is, it represents that value which must be assigned to the hydrogen 

 ion concentration in order to have it obey the law of mass action 

 as assumed in Nernst's equation. In low concentration, how- 

 ever, the value so obtained does not differ very much from 

 that obtained by the conductivity method. In more concen- 

 trated solution the discrepancy becomes greater and may amount 

 to several hundred per cent in concentrated HCl. It seemed of 

 interest to determine which of these values is the significant one 

 for the hydrolysis of gelatin. A series of experiments was there- 

 fore made in which the concentration of acid was varied from 1.0 N 

 to about 4.1 N acid. The results are shown in Table V. The 

 total acidity was determined approximately by titration. The values 

 for the hydrogen ion concentration by the conductivity measurements 

 were interpolated from Kohlrausch's tables. The hydrogen ion con- 

 centrations as determined by the e. m. f. method agree fairly well 

 with those given by Noyes and Maclnnes^ except in the case of the 

 strongest HCl concentration in which it is too low. It is remark- 

 able that the rate of hydrolysis is in all cases nearly proportional to 

 the hydrogen ion concentration as determined from the e. m. f. 

 measurements. It is clear therefore that it cannot be the concen- 

 tration of hydrogen ions as expressed in grams per liter which deter- 

 mines the rate of the reaction, since in the case of 3.7 n HCl it would 

 be necessary to assume nearly 120 per cent dissociation. On the 

 other hand, the result cannot be ascribed to the undissociated acid 

 since it is hardly conceivable that the undissociated acid could effect 

 the hydrogen electrode. It seems necessary to conclude that it is 

 the "activity" of the hydrogen ion which determines the rate of 

 hydrolysis of gelatin as well as the potential of the hydrogen electrode. 



1^ Lewis, G. N., Proc. Am. Acad. Arts and Sc, 1907, xliii, 259. 



