Comparative e.m.f. study of CaF₂ and β-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 ⁰_f , 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 ⁰_f.298 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.