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 β_pqr, where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [L_pTh_qH_r](^–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 β_{11 – 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 ThL_n^x+(n= 3 or 4) are partially protonated at the pH necessary to avoid hydrolytic precipitation of the actinide.