CHEMICAL-STABILITY OF SOLUTIONS OF NIOBIUM-V IN MOLTEN NACL-KCL AT 750-DEGREES-C

Solutions of Nb(V), mainly as K2NbF7, in molten NaCl-KCl at 750 degrees C, were studied by potentiometry, linear sweep voltammetry, chemical and x-ray analysis. Voltammograms at tungsten electrodes showed that the use of silica parts in contact with the melt had to be strictly discarded. The variations of the open-circuit potentials of tungsten or Vitreous carbon electrodes, recorded during several weeks, showed that the Nb(V) species was not stable and underwent slow reduction to Nb(IV). This hypothesis was reinforced by chemical and x-ray analysis of the frozen melts. Reduction of Nb(V) to Nb(IV) was fast and complete when niobium metal was introduced in the melt.