38 Dr. H. J. S. Sand on the Measurement of the 



low values of the potential represent the most conspicuous 

 feature of these experiments. As the electrode is not smooth, 

 the time values are not comparable with those of fig. 7. 



Alkaline Solutions of Nitrobenzene. 



Fig. 11 represents results obtained with the alkaline 

 solution of nitrobenzene. As the solution at the cathode 

 becomes heavier during electrolysis in an alkaline electrolyte, 

 owing to accumulation of alkali, the cathode was placed 

 horizontally at the foot of the cell in these experiments (fig. 2, 

 p. 26). At the points of maximum voltage hydrogen begins 

 to be evolved. The differences between the experiments with 

 copper and platinum electrodes are similar to those observed 

 in acid solution. The time which elapses until hydrogen is 

 evolved by a current of given density is considerably smaller 

 than in acid solution. This points to the conclusion that the 

 diffusion coefficient of nitrobenzene is considerably smaller in 

 alkaline than in acid solution, and possibly also that the 

 reduction does not proceed quite so far before hydrogen is 

 evolved. 



Example of Application of ltesults. 



In conclusion, we have to show how experiments carried 

 out according to the method described may be employed in 

 order to arrive at an estimate whether results that have been 

 obtained in electrochemical investigations are due simply to 

 concentration changes at the electrode that take place so 

 rapidly that they cannot be neutralized by stirring, or whether 

 they are really properties of the solution originally made up 

 by the experimenter. As an example, we shall investigate 

 whether concentration changes can have taken place during 

 the experiments by Haber, referred to in the introduction, 

 with sufficient rapidity to explain his results. 



We may approach such questions from two points of view. 

 Firstly, we may make an arbitrary assumption regarding the 

 time that must elapse until artificial stirring can completely 

 renew the layer of liquid touching the electrode. For ex- 

 ample, we might assume that such a renewal takes place 

 completely after the lapse of one second, or of one minute, 

 and then calculate what is the maximum change of concen- 

 tration that can have taken place. Secondly, we may assume 

 that owing to stirring the concentration of the electrolyte is 

 always left unaltered at some definite distance, say ^ (T mm., 

 from the electrode, and then calculate the maximum change 

 of concentration that is conceivable on this assumption. 



