Complexing in molten mixtures of thorium and alkali halides

Complexing is studied by potentiometry in dilute thorium tetra- and dihalide solutions in molten alkali halides and some of their mixtures. The emfof 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 Xis Cl, Br or I, Yis F, Cl or Br and Mis 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+Zdecomposition potentials and ThX4and ThCl2activity coefficients are calculated in the melts of XThX4+ (1 − X)ThX2+ MX(Xis a share of tetravalent thorium in equilibrium with metallic thorium in molten alkali halides). Expressions are obtained for their temperature dependences MZThCl2+Zand especially MZThCl4 + Zdecomposition potentials increase and ThClnand ThCl2activity coefficients decrease as the ionic potential of cations diminish in the series of alkali chlorides from LiCl to CsCl. Numbers Zof 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 Zand conditional stability constants are found for complex compounds MZThXnYZformed 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.