PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 5 AUGUST 15. 1919 Number 8 
SOME APPLICATIONS OF THE VARIATION OF HYDROGEN 
OVERVOLTAGE WITH THE PRESSURE 
By D. A. MacInnes and A. W. Contieri 
Research Laboratory of Physical Chemistry, Massachusetts Institute of 
Technology 
Communicated by A. A. Noyes, June 2, 1919 
In a previous communication^ it was shown that the hydrogen over- 
voltage at a given electrode can be expressed, at low current densities, 
with a considerable degree of approximation, by the equation: 
pr 
in which E, r, p, and y are, respectively, the overvoltage, the radius 
of the hydrogen bubbles, the pressure, and the surface tension. The 
value of r was found experimentally to be independent of the pressure, 
and a plot of the variation of the overvoltage with the pressure was 
found to follow, very nearly at least, the hyperbola required by the 
above expression. 
It next became interesting to investigate some chemical processes in 
which hydrogen overvoltage is intimately involved in order to see 
whether changes in these processes, produced by variations in the ex- 
ternal pressure, are also in the direction predicted by the theory. 
Three such processes are: (1) the corrosion of metals in acid solutions, 
(2) reduction in acid solutions by metals, and (3) the electrodeposition 
of metals. 
1. Metal Corrosion. — When a metal above hydrogen in the electro- 
motive series is placed in a solution of an electrolyte there is a tendency 
for the metal to ionize; i.e., to split into ions and electrons; for instance, 
with iron, the following reaction tends to take place 
Fe = Fe++ + 2e, (1) 
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