730 ACTIVITY COEFFICIENT OF THE HYDROGEN ION 



///. Effect of Neutral Salts. 



It was noted by Arrheniusi^ in 1899 that the addition of neutral 

 salts to a weak acid increased the rate of hydrolysis of methyl acetate 

 by the acid, which is just the opposite of the effect expected from 

 the law of mass action. Arrhenius originally assumed that the salt 

 actually increased the concentration of hydrogen ions. This view 

 was criticised, however, and the hypothesis was put forward inde- 

 pendently^" by Dawson, and by Senter and Acree that the observed 

 effect was due to the undissociated acid and a large number of ex- 

 periments were performed which strengthened this view. It was 

 found by Nelson and Fales,-^ however, that under certain conditions 

 the effect of neutral salts on the activity of invertase could be as- 

 cribed entirely to the effect of the salt on the hydrogen ion concen- 

 tration as measured by the hydrogen electrode. Since that time 

 evidence has accumulated to show that the addition of neutral 

 salts to a strong acid causes an increase in the hydrogen ion concen- 

 tration as measured by the hydrogen electrode. A series of experi- 

 ments was therefore performed to determine whether the addition 

 of salts would influence the hydrolysis of gelatin. The results of 

 this series are given in Table VI. It will be seen that the addition 

 of 1.5 N NaCl to 1.0 N HCl solution increases the hydrogen ion con- 

 centration by nearly 50 per cent and that the rate of hydrolysis is 

 increased practically the same amount. That is, if the effect of 

 the neutral salt on the hydrogen ion concentration is taken into ac- 

 count, there is no effect on the rate of hydrolysis. In the case of 

 NaCl and CaCl2 the result is the same up to 1.0 n but above that 

 the hydrogen ion concentration is increased more than the rate of 

 hydrolysis. It was thought at first that this might be due to diffusion 

 potentials caused by the NaCl or CaCl2, since it will be noted that 

 NaCl and CaCl2 increase the hydrogen ion concentration more than 

 the same concentration of KCl, whereas the effect on the rate of 

 hydrolysis is the same for all the salts. (This is the result obtained 



^^ Arrhenius, S., Z. physik. Chem., 1899, xx.xi, 197. 



2'' For a review of this question see Lewis, W. C. McC, A system of physical 

 chemistry, London, 1918-19, i. 



21 Fales, H. A., and Nelson, J. M., J. Am. Chem. Soc, 1915, xxxvii, 2769. 



