CONTRIBUTIONS FROM THE CHEMICAL LABORATORY 

 OF HARVARD COLLEGE. • 



GALVANIC POLARIZATION ON A MERCURY CATHODE. 

 By Gilbert Newton Lewis and Richard Fay Jackson. 



Presented by T. W. Richards. Received July 4, 1905. 



Our knowledge concerning the real mechanism of chemical reactions 

 is very meagre, even in the case of those occurring in homogeneous sys- 

 tems. Still less do we know of what the processes really are that occur 

 in or near the surface of discontinuity in the case of a heterogeneous 

 system. To the latter class belong all electrolytic reactions, and here 

 the phenomenon of galvanic polarization offers an especially promising 

 field for investigation. Unfortunately, most studies in polarization have 

 been made under conditions too complicated to permit a ready interpre- 

 tation of their results. Even in cases where the polarization of a single 

 electrode has been studied alone, the essential phenomenon has been fre- 

 quently obscured by the presence of substances produced at the other 

 electrode and the existence of secondary reactions due to other im- 

 purities, notably atmospheric oxygen. In cases also where slow diffu- 

 sion processes occur, — such, for example, as a diffusion of gases into 

 platinum,* — the previous history of the cell is an important and usually 

 an unknown factor. 



It has been our purpose in the present work to choose a single definite 

 electrolytic reaction of a simple character, and, eliminating with the 

 greatest care secondary reactions of all kinds, to study the polarization 

 accompanying this reaction under varying conditions. The reaction 

 chosen was the deposition of hydrogen from normal sulphuric acid upon 

 a mercury cathode. In other words, we have studied the phenomenon 

 known as the over-voltage (Ueberspannung) of hydrogen. Mercury 

 was chosen for the cathode metal because of the high over-voltage upon 

 it, because of its small capacity for absorbing hydrogen, because it is the 

 only metal which is not liable to accidental variations in surface, and 

 finally because we believed that with this metal, if with any, we would 



* See Cottrell, Zeit. phys. Chem., 42, 385 (1903). 



