Synergistic effect of Ni(ii) and Co(ii) ions on the sulfite induced autoxidation of Cu(ii)/tetraglycine complex

Abstract

The synergistic effect of Ni(II) and Co(II) on the sulfite induced autoxidation of Cu(II)/tetraglycine was investigated spectrophotometrically at 25.0 °C, pH = 9.0, 1 × 10⁻⁵ mol dm⁻³ ≤ [S(IV)] ≤ 8 × 10⁻⁵ mol dm⁻³, [Cu(II)] = 1 × 10⁻³ mol dm⁻³, 1 × 10⁻⁶ mol dm⁻³ ≤ [Ni(II)] or [Co(II)] ≤ 1 × 10⁻⁴ mol dm⁻³, [O₂] ≈ 2.5 × 10⁻⁴ mol dm⁻³, and 0.1 mol dm⁻³ ionic strength. In the absence of added nickel(II) or cobalt(II), the kinetic traces of Cu(III)G₄ formation show a large induction period (about 3 h). The addition of trace amounts of Ni(II) or Co(II) increases the reaction rate significantly and the induction period drastically decreases (less than 0.5 s). The effectiveness of Cu(III)G₄ formation becomes much higher. The metal ion in the trivalent oxidation state rapidly oxidizes SO₃²⁻ to SO₃˙⁻, which reacts with oxygen to produce SO₅˙⁻. The strongly generated oxidants oxidize Cu(II)G₄ to Cu(III).