Page 16 - MODERN ASPECTS OF ELECTROCHEMISTRY

A Critique of the Additivity Principle for Mixed Couples
the same as the value of E mix obtained from adding the current-potential 3
curves. However, it can be seen that the additivityprinciple, as normally
3
applied, is actuallya combination of two distinct premises :
1. Each couple present is assumed to act independently so that its
current-potential curve is unaffected by the presence of the other
couple.
2. At anygiven potential, the observed current is assumed to equal the
algebraic sum of the currents exhibited byeach of the couples
present. In particular, at E mix the algebraic current sum is zero.

This distinction will be seen to be important in the following sections.=

III. EARLIER TESTSA

The main system chosen by Wagner and Traud themselves was the
corrosion of zinc amalgam in aqueous HCl. They measured the current-
potential curves of
2+
Zn(Hg) → Zn +2e - (5)
in acetate buffer to avoid hydrogen evolution, and
-
+
2H +2e → H 2(g) (6)
in aqueous HCl at a mercury electrode to avoid zinc dissolution.=On the
assumption of the additivity of the curves of (5) and (6) in the mixture
situation, they determined I and hence by equation (4) predicted the rate
mix
of hydrogen evolution from the reaction
2+
+
Zn(Hg) + 2H → Zn +H 2 (g) (7)
in aqueous HCI. The predicted rate agreed well with the experimentally
measured rate of hydrogen evolution over a range of experimental condi-
tions.
Despite the good agreement, there are two reasons why this test was
4
flawed. First, the anodic half-reaction (5) had of necessity to be carried
out at a higher pH, and thus in a different medium from the overall reaction
(7). Second, and even more important, the electrode surface in the cathodic
half-reaction (6) was not the same as the amalgam surface in reaction (7).
Hydrogen evolution could be affected by the presence of zinc sites on the