Page 14 - MODERN ASPECTS OF ELECTROCHEMISTRY

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A Critique of the Additivity Principle for MixedA

Couples

Michael Spiro

Department of Chemistry, Imperial College of Science, Technology and Medicine,
London SW7 2AY United Kingdom

I. INTRODUCTION

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Wagner and Traud, in their seminal paper in 1938, clearly enunciated the
principle of adding current-potential curves when two (or more) couples
are present together. Since then the additivityprinciple, sometimes called
the superposition or mixed potential theory, has been applied throughout
electrochemistry and has often been accepted almost as an article of faith.=
It has proved to be of particular importance in understanding such indus-
trial processes as corrosion, mineral extraction, froth flotation, electroless
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plating, and photographic development. It is surprising, then, that few
unambiguous tests of the validity of the additivity principle have been
available until recently. Evidence obtained in the past few years has now
revealed cases in which previously unsuspected interactions occur be-
tween the couples or with the electrode.= This evidence, described below,
has now led to a reformulation of the additivity principle that extends its
validity.

II. THE ADDITIVITY PRINCIPLE
An electrode in a system containing two couples automatically adopts a
mixed potential (or more correctly, a mixture potential) E . At this
mix

Modem Aspects of Electrochemistry, Number 34, edited by John O'M. Bockris et al.
Kluwer Academic / Plenum Publishers, New YoÀ,=2001.

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