Series and Reference Electrodes

In halide melts, unlike the oxyanion systems, the gas/gas ion electrode corresponding to the melt anion, e.g., Ch/Cr for chloride melts, naturally suggests itself as the best primary reference point for these series.  

Among the secondary reference half cells which have been employed in work on many melt types, the Ag/Ag system has proved most popular, although the Pt/Pt is also useful in higher-melting media. Even in halide melts, however, these series may be slightly restricted because the metals are disposed to dissolve in their own salt melts, e.g., Na in NaCl. This causes a lowering of their thermodynamic activity, resulting in deposition at more noble potentials than that predicted on the basis of the defined standard state of unit activity. 

Care is needed in these media to ascertain that is the sole oxide ion present/  

A related phenomenon is the common occurrence of secondary deposition in molten salts. Thus, a metal may be deposited as a result of a chemical reaction between a metallic ion in the melt and another preferentially-deposited, reactive metal corresponding to the solvent cation(s). (Notably, there is still discussion as to whether sodium or aluminum is the primarily deposited metal during operation of the Hall-Heroult cell. ) Such occurrences can be rationalized in theoretical terms if the primary deposition of the solute metal is precluded by an observed limiting current exceeding that predicted, or if its deposition potential is displaced in a cathodic direction by activation overpotential. Some authors have preferred an explanation which involves underpotential deposition (vide infra). 

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