Page 21 - MODERN ASPECTS OF ELECTROCHEMISTRY

Michael SpiroA
8
-3
out in the same medium containing not only0.1 M (M = mol dm ) HCl
+0.8 M KCl but also 0.1 M KI, the additivityprinciple held very well for
both the catalytic rate and potential. However, when the anodic Ti(III)
curve had been determined in a medium without KI, v mix was much greater
than v ca t while E mix was much less than E cat. This illustrated once again
that the adsorbed iodide, which was part of one of the couples, strongly
influenced the electrochemical behavior of the other couple.= The results
also demonstrated that the current-potential curves werð additive provided
they had been obtained under the same conditions.=
In contrast, the rate of the platinum-catalyzed reaction
3+ -
2+
Fe +I → Fe +½I 2 (19)
3
was found byCreeth and Spiro to be greater than expected from adding
3+
the current-potential curves of the reduction of Fe and the oxidation of
-
I . Once again adsorption of iodide on the reduced platinum surface was
identified as the culprit.= When a very small concentration of iodide ions
3+
had been added to the Fe solution, the currents in the cathodic Fe 3+
voltammogram became significantlylarger. Superposition of this curve
-
with the anodic voltammogram for I then yielded values for v mix and E mix
that were in good agreement with v cat and E cat ,respectively. These results,
too, confirmed the additivitypostulate underlying the Wagner and Traud
principle, but again did not bear out the premise that the couples acted
completely independently.
Some other types of interaction were reported by Garnica Meza and
17
Spiro in their studyof the platinum-catalyzed reaction
2+
Ce(1V)+ ½Hg 2 → Ce(III) +Hg 2+ (20)
The additivity principle was well obeyed on adding the voltammograms
of the two redox couples involved even though the initially reduced
platinum surface had become covered by a small number of underpoten-
tial-deposited mercury monolayers.= With an initially anodized platinum
di=the catalytic rates were much smaller, although the decrease was less
if the Hg(I) solution had been added to the reaction vessel before the
Ce(1V) solution.= The reason was partial reduction by Hg(I) of the ox -
ide/hydroxide layer, so partly converting the surface to the reduced state
on which catalysis was greater.=
An industrial application is provided by a reaction important in
photographic processing. On exposure to light, the silver halide in photo-