Low melting metals (Ga, In, Sn, Al) can find a number of applications in pyrochemical processes. For example, they can be used as liquid (so-called “active”) electrodes for electrochemical separation or selective extraction of components in spent nuclear fuel separation technologies. Considerable attention was paid to investigation of electrochemical behavior of fissile elements and a wide range of fission products on liquid electrodes but electrochemical properties of the low melting metals themselves were often overlooked. Relatively little attention was given so far to the electrochemistry of gallium in fused chlorides. The aim of the present work was therefore determining the electrode potentials of gallium in alkali chloride based melts. The experiments were performed in the melts based on two low melting eutectic mixtures, binary LiCl–KCl (at 450–600 ^oC) and ternary NaCl–KCl–CsCl (at 530–750 ^oC).

Preliminary experiments performed using transient techniques (cyclic and square wave voltammetry) showed that gallium ions of two oxidation states, +1 and +3, are present in the melt in contact with gallium metal. The ratio of Ga(I) and Ga(III) species depends on temperature and total gallium concentration in the melt.

Electrode potentials of gallium were then measured using electromotive force measurements between gallium working and chlorine reference electrodes: GaǀMCl–GaCl_nǁMClǀCl₂(C), where MCl is the alkali chloride mixture. Gallium chloride containing electrolytes with various gallium content were prepared by reacting gallium with hydrogen chloride in the corresponding melt. This method was chosen to ensure the absence of oxygen impurities in the electrolytes used for the electrochemistry measurements. Electrode potentials were measured using the electrolytes with different gallium concentrations. Experimental values of the electrode potentials measured at different temperatures were then used to calculate Ga/Ga(I) and Ga/Ga(III) formal standard electrode potentials. Examples of the results thus obtained are shown in the Figure. Electrode potentials of Ga/Ga(III) couple are more negative than those of Ga/Ga(I). Increasing temperature resulted in expected shift of the potentials to the positive values.

Figure. Formal standard electrode potentials of Ga/Ga(I) (1, 3) and Ga/Ga(III) (2, 4) couples in NaCl–KCl–CsCl (1, 2) and LiCl–KCl (3, 4) based melts. 2Cl^–/Cl₂ reference electrode.