Complexation of thorium(IV) with 2-thenoic and 2-furoic acids in aqueous solution at low pH
Abstract
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 °C) in the range pH 1.7–3.0. The best-fit model, which was tested by varying the data and pH range employed, took into account the competitive formation of the conjugate ligand acids and equilibrium hydroxothorium complexes. The formation constants of the complexes obtained, denoted by βpqr, where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [LpThqHr](–p+4q+r)(L = 2-thenoate or 2-furoate) have the values: 2-thenoate, log β101= 3.24 ± 0.00, log β110= 3.04 ± 0.07, log β11 – 1= 0.39 (invariant), log β210= 5.69 ± 0.05, log β312= 14.16 ± 0.05, log β412= 18.00 ± 0.03; 2-furoate, log β101= 3.01 ± 0.00, log β110= 2.85 ± 0.04, log β210= 5.11 ± 0.07, log β312= 12.78 ± 0.06, log β410= 10.07 ± 0.03, log β412= 15.14 ± 0.07. These results suggest that higher species of the type ThLnx+(n= 3 or 4) are partially protonated at the pH necessary to avoid hydrolytic precipitation of the actinide.