Page 22 - MODERN ASPECTS OF ELECTROCHEMISTRY

9
A Critique ofthe Additivity Principle for Mixed Couples
graphic film forms tinysilver specks called the latent image. These need
to be greatlyintensified byadding a developing agent such as N,N-di-
methyl-p-phenylenediamine (DMPPD):
- +
AgBr + DMPPD → Ag + Br +S (21)
-
+
+
AgBr + S → Ag + Br +T +H + (22)
where S is the corresponding p-semiquinonediimine and T is the p-qui-
+
+
+
14
nonediimine. For color photography, T couples with certain organic
compounds incorporated into the film to produce colored dyes. The dye
+
+
intensity can be enhanced by forming more S , and then more T through
the reaction
2+
2+
DMPPD + CO(NH ) CI →S +Co + 5NH +Cl - (23)
+
3 5
3
All these redox processes [(21)–(23)] are heterogeneouslycatalyzed
18
by particles of silver. To test the electrochemical model of the catalysis,
19
Farchmin, Nickel, and Spiro measured the rate of reaction (23), as well
as the voltammograms of the couples involved, on rotating dis of silver,
gold, and platinum. On the silver surface, the values of v mix and E mix
obtained from the anodic current-potential curve of DMPPD and the
cathodic curve for CO(NH3)5Cl2+ agreed well with the values of v cat and
E cat found directly from reaction (23). On gold and platinum surfaces, on
the other hand, v cat was much less than v mix . This was shown to be due to
adsorption of DMPPD on these metal surfaces, which decreased the
2+
cathodic current for the reduction of Co(NH ) Cl . To allow for this
3 5
2+
effect, cathodic curves for CO(NH ) Cl were measured in the presence
3 5
of micromolar amounts of DMPPD. When these new curves were then
combined with the anodic DMPPD curves, good agreement was obtained
for both rates and potentials.= Thus the additivity premise held, provided
the interaction between the couples had been allowed for by superimpos-
ing onlycurves determined under circumstances that corresponded to
those in the reaction mixture.=
VI. CONCLUSIONSA
Although the original additivity principle of Wagner and Traud has been
an immensely useful concept with applications in numerous fields, 2
carefully designed studies in receiit years have revealed a number of
exceptions.= These have been described above and are summarized in