Aqueous oxalic acid forms surface CrIII−oxalato
complexes with suspended chromium(III) oxide particles;
the
FTIR spectra demonstrate that both carboxylate groups of the ligand are
bound to surface CrIII. Surface
complexation is followed by changes in the surface redox potential
toward more negative values and by the
dissolution of the oxide. The dissolving steady state potential is
in the range −60 to −210 mV against SHE.
During surface conditioning, traces of oxidants at the interface
are reduced, and some reduced metal ions accumulate.
Minor amounts of dissolved CrII are generated and can
be collected at a vicinal ring electrode set at −60 mV.
In
agreement, dissolution kinetics suggest that generation of
CrII by ligand-to-metal charge transfer within the
surface
complexes produces a large increase in the rate of phase transfer, as
expected from the properties of CrIII and
CrII.
Added chromous salts also catalyze the dissolution through
intervalence charge transfer within an oxalato-bridged
CrIII−L−CrII surface dimeric complex.
The rate of dissolution at 65 °C follows a
Langmuir−Hinshelwood
dependence on oxalic acid concentration, a power law (order 0.31)
dependence on proton concentration, and an
a + b[CrII]0.64
dependence on [Cr(II)]. The Langmuir−Hinshelwood
parameters are interpreted in terms of the
stability constant of the surface CrIII−oxalato
complex.
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