Specificity of Terbium(III) Oxidation by Persulfate Ions in the Presence of Unsaturated Phosphotungstate K₁₀P₂W₁₇O₆₁

Abstract

The oxidation of Tb(III) by persulfate ions in solutions of unsaturated phosphotungstate K₁₀P₂W₁₇O₆₁ (L) was studied by spectrophotometry. The accumulation of Tb(IV) is observed at pH < 3. The yield of Tb(IV) depends on pH and [L]. The maximum yield is observed in solutions with pH 1.5–2.5 and [L] > 6×10^–4 M. The accumulation of Tb(IV) proceeds as a zero-order reaction with respect to [Tb(III)] and a first-order reaction with respect to [S₂O₈^2–]. The reaction rate increases with the increase in pH from 0.91 to 1.5–1.9; an increase in pH to 2.44 decreases the rate. The activation energy (42–60°С) is estimated at 140 kJ/mol. The reaction mechanism includes the decomposition of S₂O₈^2– ions into SO₄^– radical ions, which are converted into OH˙ radicals, which oxidize Tb(III) bound in a complex with L (1 : 1). In parallel, the catalytic decomposition of S₂O₈^2– ions occurs, which leads to the formation of Caro’s acid H₂SO₅. Its hydrolysis generates Н₂О₂, a reducing agent for Tb(IV). In addition, Tb(IV) is reduced by water. It was not possible to oxidize thulium(III) under the conditions of terbium(IV) formation.