A Comprehensive Treatise on Inorganic and Theoretical Chemistry

114 INORGANIC AND THEORETICAL. CHEMISTRY
Lu Kahlenberg, E. S. Hedges and J. E. Myers, A. Giinther-Schulze, G. Tammann,
M. Thalinger and M. Volmer, K. Bennewitz, and G. Senter. E. Miiller observed
that in an electrolytic cell containing hydrochloric acid, a gradually increasing
anodic potential difference results in the current strength rising to a constant
value which is maintained for an interval—Fig. 16. It was assumed that the
ion in soln. primarily concerned in the electro­
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lysis is exhausted in the region corresponding
•with the flat part of the wire. R. Luther and
F. J. Brislee showed that this explanation cannot
be correct, and they suggest that here
Cl2"-ions are present in the soln. and that these
ions are in equilibrium with the Cl'-ions, and
the exhaustion of these ions is responsible for
the constant value of the current strength.
The condition of the platinum anode is the
most important factor. When the anodic potential
difference, after a gradual increase, is
gradually diminished, without break of current,
the corresponding variation of the current
strength is not always that given by the first
Potential Difference units potential difference-current curve. The current
Kio. 16.—Anodic Potential—Cur- strength may fall away rapidly almost to zero,
rent Curves of Tiydrorvhlorio Acid. the electrode having become ** passive." If,
while the electrode is still passive, the potential
difference is again increased, the horizontal portion of the potential difference-current
curve cannot now be obtained. This passive condition is due to a superficial change
of the platinum anode, and has nothing to do with the solution. It disappears
immediately if the current is broken, and if the anode potential difference is
allowed to fall below 1»6 volts, a passive electrode becomes spontaneously active.
Further, if the change of potential difference is reversed before *nm. is reached,
the passive condition does not set in. There are thus three states of the platinum
surface. The production of the passive condition does not depend on the presence
of Cl'-ions, but begins in acid solutions at about 1*9 volts and vanishes at about
1-6 volts.
C. Fredenhagen observed that platinum electrodes in alkaline soln. of mixtures
of potassium ferrocyanide and ferricyanide are non-polarizable ; and C. Grube,
that in the electrolytic oxidation of a ferrocyanide in neutral or alkaline soln., the
reaction FeCy6""" FeGy 6"'-|-@ is probably instantaneous. Passivity may be
produced by a thin film of oxide on the metal, or by adsorbed oxygen, dependent
on the conditions. G. Just also found that the accelerating action of platinum
on the reaction between potassium ferricyanide and iodide is connected -with the
oxidation and reduction of the metal. The anodic formation of a brown film of
oxide on platinum was observed by W. ,Beetz, and the subject was studied by
R. Ruer, S. Popoff and M. J. McHenry, W. Nernst and H. von Wartenberg, F. Haber
and L. Bruner, G. Pfleiderer, F. C. Frary, G. C. Schmidt, M. Ie Blanc, K. R. Koch,
M. Krouchkoll, K. Waitz, H. Hauser, R. Lorenz and co-workers, F. Forster,
C. Marie, L. Wohler, L. Wohler and F. Martin, L. Arons, G. Grube, L. Cailletet
and E. Collardeau, M. Berthelot, and H. N. Warren. According to C. Marie, the
brown colour observed by F. Kohlrausch on the anode during the electrolysis of
soln. of platinum chloride is due to a superficial oxidation of the platinum ; and
similarly with the electrolysis of soln. of sodium hydroxide, nitric acid, or hydrochloric
acid with platinum electrodes. E. P. Schoch showed that the observed
anodic potential of platinum is not usually the reversible potential of the oxygen,
but rather of a platinum oxide ; and C. M.Gordon and F. E. Clark,that the capacity
of a platinum electrode is conditioned by a film of oxide. G. Lippmann said that
the discharge of hydrogen or oxygen from a platinum plate in electrolysis does not