292 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



ions due to the addition of another salt with a common ion. He finds, 

 in general, a correspondence between the two effects. Harned has also 

 pointed out that the neutral salt effect appears to be related to the hydra- 

 tion of the added salt. 



Akerlof 17 has measured the influence of acids on the rate of reaction 

 of ethyl acetate in water at 20 in the presence of varying concentrations 

 of salts having an ion in common with the acid. The activity of the 

 hydrogen ion in the presence of an added salt was determined by meas- 

 urement of the electromotive force of concentration cells. With hydro- 

 chloric and sulphuric acids, Akerlof found that, with increasing activity 

 of the hydrogen ion, the velocity constant increases. For the same con- 

 centration of the catalyzing acid, the velocity constant K r was found to 



increase approximately as the cube root of the activity of the hydrogen 

 ion; or, 



(87) K = 



where K f is the velocity constant of the reaction, A is a constant having 



the same value for different salts, and a is the activity of the hydrogen 

 ion in the mixture. In the case of a number of salts the value of A was 

 found to depend upon the nature of the salt as well as upon its concen- 

 tration. It is possible that these discrepancies are due to various sources 

 of error. With increasing acid concentration, the constant A was found 

 to increase, but apparently not in direct proportion to the concentration. 



Equation 87 is an empirical one, and, so long as it lacks a theoretical 

 foundation, the interpretation of the foregoing results remains uncertain. 

 It appears that, for a number of salts, the velocity constant varies in a 

 similar manner with the activity of the catalyzing ion; but, in view of 

 the possible exceptions which have been found, it would be unsafe to 

 generalize the results obtained. Further investigations along this line, 

 however, are of considerable interest. 



4. Optical Properties of Electrolytic Solutions. Among the various 

 optical properties of solutions, only the absorption spectra have been 

 determined with sufficient precision to make it possible to draw conclu- 

 sions with any degree of certainty. Since the optical properties are pri 

 marily dependent upon the number and arrangement of the electrons 

 it is not to be expected that the ions and the un-ionized molecules wil 

 exhibit any marked difference with respect to these properties. It is tru 

 that, in the case of a few solutions, such as the copper salts for example 

 marked changes take place in the. optical properties as the concentration 



"Akerlof, Ztschr. f. phys. Chem. 98, 360 (1921). 



