A Comprehensive Treatise on Inorganic and Theoretical Chemistry

PLATINUM 113
then more slowly, reaching a maximum in 10 to 20 mins. at a current density of
0*04 to 0*1 amp., and 2£ to 3 hrs. at 0*01 amp. After a short interruption of the
polarizing current, a different potential occurs, and, in general, the numerical
values obtained are not constant in different experiments, although the general
character of the curves is the same. These numerical values depend greatly on the
previous treatment of the electrode under observation (action of oxidizing or
reducing agents, cathode polarization, etc.). With a platinized electrode the
maximum is attained more slowly and the results are more constant ; short interruptions
of the current have no effect on the electrode potential. The difference
between the potential of the platinized and the solid electrode is 0*1 to 0*2 volt,
not 0*4 to 0*6 as observed by F. Forster. The results are explained by the interaction
of the surface of the electrode with the gas generated in the process of
electrolysis, the metal suffering a change from which it recovers only after some
time ; the question of whether an oxide of platinum or a solid soln. of the gas in
platinum is formed is an open one. J. A. V. Butler and G. Drever observed that
platinum is anodically polarized in acidic and alkaline soln., and an adsorbed layer
of oxygen is formed prior to the establishment of the oxygen overvoltage, but, as
in the case of iridium, there is no evidence of a slow formation of oxides of a
peroxidic character such as occurs with palladium and rhodium.
The depolarization potentials were found by V. V. Pitcheta to be inversely
proportional to the current density for both smooth and platinized platinum.
The depolarization of the platinum was studied by J. Billitzer, F. Weigert, and
E. Muller. G. Meissner found that platinum is not polarized by oxygen at ordinary
temp., but it is polarized at a red-heat ; ozone was found by G. Bodlander, and
CF. Schonbein to polarize platinum negatively. F. Kaufler and C. Herzog found
that with plain platinum electrodes in the best conducting mixture of sulphuric
acid and water, there is contact resistance of about 3 ohms per sq. cm. of electrode
surface with a current of 0*01 to 0-02 ampere, this decreases with an increase in the
current density. The subject was investigated by J. C. Poggendorff, R. Ruer,
O. Troje, W. W. H. Gee and H. Holden,and K. R. Koch and A. Wiillner. A. L,. Clark
studied the polarization capacity and the electrical double layers ; and K. R. Klein
studied the rate of anodic depolarization. G. Armstrong and co-workers, and
J. A. V. Butler and G. Armstrong observed some periodicities in the anodic polarization
of platinum electrodes in dil. sulphuric acid saturated with hydrogen ; and
A. Gunther-Schulze, the effect of platinum salts on valve metals.
According to C. F. Varley, 13 with two platinum plates, 6*45 sq. cm. surface,
the polarization capacity, C microfarads, with different e.m.f., E volts, is :
K . . 0-2