The polarographic reduction of
tris(ethylenediamine)platinum(IV) and the hexammineplatinum(IV) ions has been
studied in potassium chloride, potassium nitrate, and potassium nitrate plus
ammonia solutions. Both ions were reduced irreversibly producing similarly
shaped waves, showing well-defined diffusion current regions corresponding to
two-electron reductions of the complexes. A linear relationship existed between
diffusion current and concentration within the range examined. In aqueous
potassium chloride and potassium nitrate media, the waves contained slight
inflexions at positions corresponding to one-electron additions. The phenomenon
suggested the transient presence of platinum(III) ions, and indicated that the
half-wave potential of the reduction of the complexes to the trivalent state
was very close to the half-wave potential of the reduction from platinum(IV) to
platinum(II). The values were so close together as to indicate the
improbability of isolating the trivalent complexes. Gelatin enhanced the
inflexion in the wave but shifted the wave in a more negative direction. An
increased concentration of supporting electrolyte also shifted the wave to a
more negative position. In all cases a continuous discharge began at about -1.3
V (v. S.C.E.). This discharge was so far removed from that of the potassium
ions of the supporting electrolyte that it was attributed to the discharge of
hydrogen. Since the initial reduction of the platinum complexes corresponded to
a two-electron change, it can be represented by reduction to a tetrammine ion.
It is postulated that at higher applied potentials (namely, -1.3 V v. S.C.E.)
the reduction proceeded further, producing platinum metal. This platinum metal
would be in an active state, insoluble in mercury, and being on the surface,
would lower the overvoltage of hydrogen leading to its discharge at a more
positive potential than on a pure mercury surface. This view was supported by
the fact that gas bubbles were observed at the dropping electrode when a
voltage greater than -1.3 V was applied to the electrode for some time. When
ammonia was added to the supporting electrolyte, a wave, without an inflexion,
and corresponding to an irreversible two-electron reduction, was obtained at
more negative potentials. The bivalent tetrammineplatinum(II) and bis(ethylenediamine)platinum(II)
ions also gave polarograms showing the continuous discharge of hydrogen.
The Diffusion Potential between Dilute Solutions and Concentrated Solutions of Potassium Chloride plus Potassium Nitrate.
