Comparative e.m.f. study of CaF2 and ?-alumina cells with Ni/NiF2 and Fe/FeF2 or Cr/CrF2 electrodes

Abstract

The reliability of employing beta-alumina as electrolyte for fluorine potential measurement is examined by measuring the e.m.f.s of the galvanic cells with metal/metal fluoride electrodes and comparing with those obtained by using CaF2 as electrolyte under identical conditions. The results from both types of galvanic cell can be superimposed to give the following standard Gibbs energy of formation, ΔG f 0 , of FeF2 and CrF2 over extended ranges of temperature: $$\begin{gathered} \Delta G_f^0 (FeF_2 ) = - 702.0 + 0.125 20T (K) ( \pm 0.70) kJ mol^{ - 1} (506 - 1063K) \hfill \\ \Delta G_f^0 (CrF_2 ) = - 732.8 + 0.087 90T (K) ( \pm 0.64) kJ mol^{ - 1} (497 - 1063K) \hfill \\ \end{gathered} $$ The absence of significant temperature-dependent errors in both these measurements are verified by a third law treatment of the data yielding values of −716.8 and −777.4 kJ mol−1 for ΔH f.298 0 of FeF2 and CrF2, respectively. The feasibility of using beta-alumina electrolyte cells for e.m.f. measurements on other metal/metal fluoride systems is discussed in the light of the existence of a useful potential domain of beta-alumina. High sodium potential in the electrode system can lead to sodium depletion. Likewise, low sodium potential may result in oxidation of the metals in the electrodes. Both these limiting factors are also examined.