CHEMISTRY OF NIOBIUM CHLORIDES IN THE CSCL-NACL EUTECTIC MELT .1. ELECTROMOTIVE-FORCE MEASUREMENTS OF NBCL3 IN THE CSCL-NACL EUTECTIC MELT AT TEMPERATURES BETWEEN 600 AND 700-DEGREES-C

EMF-data of NbCl3 dissolved in the NaCl-CsCl eutectic melt have been o btained between 600 and 700-degrees-C using a niobium metal electrode and a mullite tube containing a silver wire dipping into an NaCl-CsCl eutectic melt containing 5 mol% AgCl as a reference electrode. At NbCl 3 concentrations below 0.2 mol%, the cell reaction: NbCl3(in (Na,Cs)Cl ) + 3Ag(s) = Nb(s) + 3AgCl(in (Na,Cs)Cl) was established in the galvan ic cell. At higher concentrations, a black deposit containing sodium, caesium, chloride and niobium was precipitated on the electrode, and a new and unknown cell reaction was established. The calculated mean va lence state of niobium in the precipitate was +3.5. In the dilute solu tions of NbCl3 studied, the entropy of mixing of liquid NbCl3 in the N aCl-CsCl eutectic was ideal. The partial enthalpy of mixing of hypothe tical liquid NbCl3 in the eutectic was below -10 kJ mol-1 for concentr ations of NbCl3 < 0.2 mol%. The hypothetical enthalpy and entropy of f usion for NbCl3 have been calculated and DELTA(fus)S(NbCl3) = 52 J K-1 mol-1 and DELTA(fus)H(NbCl3) greater-than-or-equal-to 47 kJ mol-1.