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 emf of cells Th/N ThXsub(n) + (1-N)MX/ MX//C, Xsub(2(g)) and Th/NThXsub(n) + (1-N)[Nsub(Y)MY + (1-N)sub(Y)MX]/MX/C,Xsub(2(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 ThXsub(n)(N) and MY(Nsub(Y)). From experimental observations Msub(Z)ThXsub(n+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 Msub(Z)ThClsub(2+Z) and especially Msub(Z)ThClsub(4+Z) decomposition potentials increase and ThClsub(n) 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 Msub(Z)ThXsub(n)Ysub(Z) formed by tetra (n = 4)- and divalent (n = 2) thorium. Equations are derived for their temperature dependence. (author)