Electrochemical Measurements in Molten Fluorides

The nickel (II) / nickel couple, contained in a boron nitrade compartment, was shown to be a useful reference electrode in fluoride melts. From emf measurements no nickel (II) / nickel concentration cells, the nickel (II) / nickel couple was shown to obey the Nernst equation in molten LiF-NaF-KF (46.5-11.5-42.0 mole per cent) and LiF-BeF₂-ZrF₄ (65.6-29.4-5.0 mole per cent). It was found that in some cases the reference electrode had a significant junction potential across the boron nitride wall.

Kinetics of the charge transfer reaction Ni(II) + 2e = Ni were investigated by the voltage-step method. Apparent adsorption of nickel (II) at the microelectrode surface complicated the interpretation of the data.

Employing the nickel (II) / nickel reference electrode as one half-cell, several standard electrode potentials were determined from emf measurements on galvanic cells. The standard electrode potential of the nickel (II) / nickel couple was arbitrarily set at 0.000 V. The following values were determined in LiF-BeF₂-ZrF₄ at 500°: beryllium (II) / beryllium, -2.120 V; zirconium (IV) / zirconium, -1.742 V; uranium (IV) / uranium (III), -1.480 V; chromium (II) / chromium, -0.701 V; chromium (III) / chromium (II), -0.514 V; iron (II) / iron, -0.410 V; nickel (II) / nickel, 0.000 V; iron (III) / iron (II), 0.166 V. The following values were determined in LiF-NaF-KF at 500°: iron (II) / iron, -0.390 V; iron (III) / iron (II), -0.200 V; nickel (II) / nickel, 0.000 V.

A simple voltammetric method was developed for the determination of hight U (IV) / U (III) ratios. The method involves the measurement of the equilibrium potential of the melt with respect to the voltammetric equivalent of the standard electrode potential, the polarographic E_1/2, of the uranium (IV) / uranium (III) couple.