Measurement and Thermodynamic Analysis of M ∕ MFn (M = Cu and Fe) Electrode Potentials in a Few Fluoride Melts Containing HF

Abstract

The potentials of copper and iron reference electrodes were measured in dehydrated melts of KF∙2HF, , and against the hydrogen evolution potential on Pt. The potential of a fresh copper reference electrode was unstable for the initial and then became constant, whereas those of an anodically oxidized and a thermally fluorinated copper reference electrode became stable in a shorter time. The potential of the thermally fluorinated copper reference electrode obtained experimentally in the KF∙2HF melt was vs at , and the values for the three kinds of copper electrodes were close to one another. Theoretical electrode potentials calculated from thermodynamic data were for and for against . Because the observed potentials on the copper electrodes existed between these values, it may be caused by coexistence of CuF and on the surface. These copper electrodes, which should be denoted as or , are useful as reference electrodes for electrochemical measurements in fluoride melts containing HF. The observed potentials of the fresh iron and the thermally fluorinated iron electrodes in the KF∙2HF melt at were and vs , respectively, and were not in agreement with each other. Both iron electrodes after immersion in the melt were covered with the film composed of and ; however, the theoretical potentials [ and vs ] were fairly lower than the experimentally observed values. These results suggested that the potential of iron electrodes was determined by other minor species on the surface and therefore, they cannot be used as reference electrodes in fluoride melts containing HF.