Solutions to exercises
83
Two phenomena may be observed:
2 The conductivity increases when the oxygen activity decreases. This
behavior may be explained by the fact that the ionic mobility is based
on vacancies. When a O
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2
decreases, the proportion of oxygen vacancies, which are the charge carriers, increases, thereby increasing the
conductivity.
2 For strong oxygen activity, the conductivity increases with a more
gentle slope, which may be explained by the eventual formation of
interstitial oxygen.
Solution 2.5 – Determination of cationic transport number
by dilatocoulometry
1. Figure 35 shows the principle of dilatocoulometry.
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Figure 35 – Schematic diagram illustrating the displacement of the interphases
Me,MX upon passage of current (a) before electrolysis and (b) after electrolysis.
Consider the following electrochemical chain:
X 2 , Me / MX / Me, X 2
where X 2 is the molecular form for the gaseous element X, Me is an inert
metallic electrode, and MX is the ionic compound under study.
When an electric current I runs through this chain, matter is transported
because of a partial cationic conduction in MX. This transport leads to a
displacement of the interphases Δx, which is followed by dilatometry.