Page 20 - MODERN ASPECTS OF ELECTROCHEMISTRY

A Critique of the Additivity Principle for Mixed Couples
3- - 4- 7
Fe(CN) 6 +e → Fe(CN) 6 (15)

-
-
3I → I 3 +2e -
(16)
In such systems the researcher can electrochemically clean and precondi-
tion the metal electrode before each run to provide an identical surface for
the anodic and the cathodic half-reactions as well as for the catalytic
reaction between them.= Use of a rotating di= electrode/ckatalyst also
allows surface- and diffusion-controlled processes to be easily distin-
guished. 14
4,15
Experiments by Freund and Spiro, with the ferricyanide-iodide
system showed that the additivity principle held within experimental error
for both the catalytic rate and potential when the platinum di had been
anodically preconditioned, but not when it had been preconditioned
cathodically. In the latter case the catalytic rate was ca 25% less than the
value predicted from adding the current-potential curves of reactions (15)
and (16). This difference in behavior was traced to the fact that iodide ions
chemisorb= only on reduced platinum surfaces.= Small amounts of adsorbed
3-
iodide were found to decrease the currents of cathodic Fe(CN) 6 voltam-
mograms over a wide potential range.= The presence of the iodine couple
(16) therefore affected the electrochemical behavior of the hexacyanofer-
rate (II, III) couple (15).
Iodide adsorbed on reduced platinum surfaces was found to affect
several other systems. The most dramatic effect was shown when the
-
couples I 2/I and O /H O were considered together.= Addition of the
2 2
current-potential curves of these two couples indicated that platinum
should significantlycatalyze the reaction
_
-
+
½O + 2H + 3à → I 3 + H O (17)
2
2
3
However, no such catalysis was detected. The reason was that even very
small amounts of iodide adsorbed on the platinum electrode strongly
inhibited the electrochemical reduction of oxygen.= A less dramatic but
I6
more instructive example was provided by Xiao and Spiro’s study of the
reaction
-
2Ti(III) + I 3 → 2Ti(IV) +3I - (18)
catalyzed on a reduced platinum di. As long as the anodic current -po-
-
tential curve for Ti(III) as well as the cathodic curve for I 3 had been carried