Kinetic studies on the reaction between nickel(II)lactate and peroxomonosulphate ion—The effect of formaldehyde

The kinetics of the reaction between nickel(II)lactate and peroxomonosulphate ion (PMS) in the presence of formaldehyde was studied in the pH range 4.0–5.9. When formaldehyde concentration is greater than or equal to [Ni(II)] the self-decomposition of peroxomonosulphate is observed. The observed rate equation is −d[PMS]/dt=(k2⋅[Ni(II)]⋅[HCHO]⋅[PMS]2)/([H+]⋅(1+K⋅[Lactic acid]))−d[PMS]/dt=(k2⋅[Ni(II)]⋅[HCHO]⋅[PMS]2)/([H+]⋅(1+K⋅[Lactic acid])). The turnover number, the number of PMS decomposed for each molecule of HCHO, is found to be four. Nickel lactate reacts with formaldehyde to give a hemiacetal intermediate. The Ni(II)–hemiacetal forms an ion pair with SO52−. The hemiacetal formation protects the alpha hydroxy group of the lactate (Ni(II)) and hence the oxidation of nickel alpha hydroxy caboxylate through the hydroperoxide intermediate is prevented. Therefore the hemiacetal–Ni(II)–SO52− ion pair reacts with another PMS molecule to give oxygen. The kinetic and thermodynamic parameters are calculated and entropy of activation is found to be positive.

Graphical abstractAt pH 4.0–5.9, peroxomonosulphate (PMS) ion in the presence of nickel(II)lactate and formaldehyde undergoes self-decomposition with the evolution oxygen. The reaction is second order in [PMS]. Experimental results suggest that the hemiacetal of nickel(II)lactate catalyses the self-decomposition.Figure optionsDownload full-size imageDownload high-quality image (80 K)Download as PowerPoint slide