COMPLEXATION OF THORIUM(IV) WITH 2-THENOIC AND 2-FUROIC ACIDS IN AQUEOUS-SOLUTION AT LOW PH

The aqueous complexation chemistry of thorium(IV) with 2-thenoate and 2-furoate has been studied using precision analytical potentiometry (I = 0.5 mol dm(-3) NaClO4, 25 degrees C) in the range pH 1.7-3.0. The b est-fit model. which was tested by varying the data and pH range emplo yed, took into account the competitive formation of the conjugate liga nd acids and equilibrium hydroxothorium complexes. The formation const ants of the complexes obtained, denoted by beta(rho qr') where the sub scripts rho, q and r refer to the stoichiometric coefficients in the g eneral formula [L(rho)Th(q)H(r)]((-rho+4q+r)) (L = 2-thenoate or 2-fur oate) have the values: 2-thenoate, log beta(101) = 3.24 +/- 0.00. log beta(110) = 3.04 +/- 0.07, log beta(11-1) = 0.39 (invariant), log beta (210) = 5.69 +/- 0.05, log beta(312) = 14.16 +/- 0.05, log beta(412) = 18.00 +/- 0.03; 2-furoate, log beta(101) = 3.01 +/- 0.00, log beta(11 0) = 2.85 +/- 0.04, log beta(210) = 5.11 +/- 0.07, log beta(312) = 12. 78 +/- 0.06, log beta(410) = 1 10.07 +/- 0.03, log beta(412) = 15.14 /- 0.07. These results suggest that higher species of the type ThL(n)( x+) (n = 3 or 4) are partially protonated at the pH necessary to avoid hydrolytic precipitation of the actinide.