Complexing in molten mixtures of thorium and alkali halides

Abstract

Complexing is studied by potentiometry in dilute thorium tetra- and dihalide solutions in molten alkali halides and some of their mixtures. The emf of cells Th|NThXn + (1 − N)MX|MX||C, X2(g) and Th|NThXn + (1 − N)[NYMY + (1 − NY)MX]|MX|C, X2(g) in which X is Cl, Br or I, Y is F, Cl or Br and M is Li, Na, K, Cs or Na + K is measured as a function of temperature and mole fraction of ThXn(N) and MY(NY). From experimental observations MZThXn+Z decomposition potentials and ThX4 and ThCl2 activity coefficients are calculated in the melts of XThX4 + (1 − X)ThX2 + MX (X is a share of tetravalent thorium in equilibrium with metallic thorium in molten alkali halides). Expressions are obtained for their temperature dependences MZThCl2+Z and especially MZThCl4 + Z decomposition potentials increase and ThCln and ThCl2 activity coefficients decrease as the ionic potential of cations diminish in the series of alkali chlorides from LiCl to CsCl. Numbers Z of addends are evaluated. Complexing reactions of addition are shown to take place in melts containing thorium and two different kinds of halide anions. Addend numbers Z and conditional stability constants are found for complex compounds MZThXnYZ formed by tetra (n = 4)- and divalent (n = 2) thorium. Equations are derived for their temperature dependence. The addend number reduces as temperature is raised. The complex thorium halide compound stability decreases as thorium valency, electronic polarizability of anions X− more ionic potential of anions Y− drop in the series of ThI4FZ−Z, ThBr4FZ−Z, ThCl4FZ−Z, ThI4ClZ−Z, ThI4BrZ−Z, ThBr4ClZ−Z, ThI2ClZ−Z, ThBr2ClZ−Z, ThI2BrZ−Z.