Kinetic studies of the oxidation of transition metal(II) malate complexes by peroxomonosulphate

Abstract

The kinetics of oxidation of malic acid by peroxomonosulphate (PMS) in the presence of Cu(II) (2.50 × 10⁻⁴–5.00 × 10⁻³ M), Co(II) (2.00 × 10⁻⁶–1.00 × 10⁻⁵ M) and Ni(II) (5.00 × 10⁻⁴–6.00 × 10⁻³ M) were studied in the pH range 4.05–5.89. The oxidation of Ni(II) malate follows simple first-order kinetics with respect to both [PMS] and [Ni(II)], while the oxidations of Cu(II) malate and Co(II) malate show autocatalysis. There is an appreciable induction period in the Cu(II) malate oxidation, while Co(II) malate oxidation follows a simple curve. The initial oxidation product for all three systems was identified as malonic semialdehyde. Alcohol quenching studies suggest that, even in the Co(II) malate-PMS system, no radical intermediates such as ${\text{SO}}_{4}^{ - .}$ or ${\text{OH}}{}^{.}$ are detected. The malonic semialdehyde intermediate may react with M(II) malates to give a hemiacetal, which may be more reactive.