Page 19 - MODERN ASPECTS OF ELECTROCHEMISTRY

Michael SpiroA
6
potential, and then rapidlyreplaced the plating mixture with a solution in
which either cupric ions or formaldehyde was missing. If the additivity
theory was correct, they reasoned, the current after such an exchange
should be equivalent to the plating rate before the exchange and moreover
this current should be constant with time.= What they found was that on
2+
rapid removal of Cu from the solution, the resulting formaldehyde
oxidation current was indeed equal to the current corresponding to the
plating rate.= This proved that there was no non -electrochemical electron
transfer to cupric ions.= However, the formaldehyde current decreased
gradually with time because copper ion reduction had activated the surface
for HCHO oxidation.=
A more dramatic change occurred when the plating solution was
rapidly replaced by a copper solution without formaldehyde: here the
cupric ion reduction current quickly plummeted by an order of magnitude
from the current equivalent to the plating rate. This was completely
contrary to the prediction of the additivity theory. The authors concluded
from this and other experiments that a catalytic organic species had been
chemisorbed on the copper surface, and the pH dependence pointed to the
-
active species being methanediolate, H 2CO(OH) . Several other sub-
stances with a similar structure, or ones known to absorb= specifically on
2+
copper, were also found to catalyze Cu reduction from alkaline EDTA
solutions.= Thus the partial reactions (12) and (13) were not independent
of each other as the principle demanded; indeed, the rate of one was
increased by the other.=

V.A HETEROGENEOUSACATALYSISAOFAREDOXIREACTIONSA

The underlying problem in testing the validity of the additivity principle
in corrosion, mineral extraction, and electroless plating is that the elec-
trode metal itself forms part of one of the half-reactions involved, e.g.,=
zinc in equation (5) and copper in equations (8) and (12). A much better
test system is provided by the interaction of two couples at an inert metal
3,4
electrode that does not form a chemical part of either couple. A good
example is the heterogeneous catalysis by platinum or a similar inert metal
of the reaction

3- 3 - 4- 1 -
2
Fe(CN) 6 + - I → Fe(CN) 6 + - I 3
2
(14)
arising from the interaction between the couples