Kinetics of oxidation of L-aspartic acid and L-glutamic acid by manganese(II1) ions have been studied in aqueous sulphuric acid, acetic acid, and pyrophosphate media. Manganese(II1) solutions were prepared by known electrolytidchemical methods in the three media. The nature of the oxidizing species present in manganese(II1) solutions was determined by spectrophotometric and redox potential measurements. The reaction shows a variable order in [manganese(III)1,: the order changes from two to one as the reactive oxidizing species changes from an aquo ionic form to a complex form. There is a first-order dependence of the rate on
Conditions have been established for the electrogeneration of Mn(III) from Mn(II) in sulfuric acid medium. Mn(III) formed was identified and characterized by UV spectra. Kinetics of Mn(III) oxidation of l-histidine in aqueous sulfuric acid medium has been studied spectrophotometrically. Oxidation followed second order kinetics in [Mn(III)], first order in [histidine] and increase in [H+] decreased the rate. The reduced product of the oxidant, Mn(II), retarded the rate of reaction. Evidence for the transient existence of the free radical reaction intermediate is given. Effects of ionic strength, solvent composition, HSO4− and certain complexing agents like P2O74−, Cl−, F−, have also been studied. Added complexing agents decreased the redox potential of the Mn(III)/Mn(II) couple, thereby reducing the oxidation rate. Dependence of reaction rate on temperature is examined and activation parameters computed from Arrhenius and Eyring plots. A mechanism consistent with kinetic results has been proposed in which a combined role of Mn(III)aq and MnOH2+aq was envisaged.
Mn m is stabilized by pyrophosphate in weakly acidic solutions. The nature of the complex formed was elucidated spectrophotometrically. The kinetics of Mn m oxidation of thiocyanate in pyrophosphate medium was investigated over the pH range 2-3. The oxidation followed first order kinetics with [Mnm]. The effects of varying [Mn'H], [NCS-], added Mn" and metal ions, pH, total [P2Ov 4-] and added C10~, C1-and SO 2-were studied. The order in [NCS-] was unity, and increasing [H +] increased the rate. Retardations with added P20~ -and Mn" were observed. Complexation of NCS-as KEZn(NCS)r decreased the reactivity without any change in overall mechanism. The dependence of the reaction rate on temperature was examined, and activation parameters were computed from Arrhenius and Eyring plots. A mechanism consistent with the results is proposed.
Conditions have been established for the electrogeneration of Mn1I1 from MnIt in a sulphuric acid medium. The MnIll formed was identified and characterized by ultraviolet spectra. The stability of standard solutions of manganese(iit) sulphate at various concentrations of H+, Mnll and acetic acid and in the presence of the complexing agents P2074-, CIand Fwas studied and conditions for the potentiometric determination of thiosemicarbazide (TSC), its metal complexes, M(TSC)2S04r and its thiosemicarbazone derivatives, propanal thiosemicarbazone (PTSC) and benzaldehyde thiosemicarbazone (BTSC), have been investigated. Thiosemicarbazide in H2SO4 underwent a six-electron oxidation. The M(TSC)*S04 complex participates in a 12-electron redox process, indicative of the number of TSC ligands present in the complex. The PTSC in an H2S04-HOAc (20% v/v) medium underwent oxidation in two stages, namely, six-electron oxidation of TSC with the regeneration of propionaldehyde and two-electron oxidation of the aldehyde to the corresponding carboxylic acid. The benzaldehyde regenerated from BTSC did not undergo further oxidation. Above 40% HOAc, a reproducible four-electron stoichiometry was achieved thereby showing the selectivity of Mn1I1 as an oxidizing agent. The formal redox potential of the Mntll-Mnlt couple with and without the complexing agents and at various concentrations of H+ was determined. Addition of complexing agents decreased the formal redox potential of the Mn1I1-Mnl1 couple, thereby reducing the oxidation rate, but did not alter the over-all stoichiometry of the electron transfer process.
Manganese(III) acetate was prepared by the oxidation of manganese(II) acetate in aqueous acetic acid by electrolytic method. The anodically generated Mn(III) species was characterised by spectroscopic and redox potential studies. Conditions for the study of kinetics of Oxidation ofα-amino acids by Mn(III) in aqueous acetic acid was investigated. Plots of log[Mn(III)] versus time for the first order process, or Mn(III) versus time for zero order process were nonlinear. The rate constants computed from [Mn(III)](1/2) versus time plots were independent of [Mn(III)]0 indicating that the reaction goes through competitive zero and first order path-ways in [Mn(III)]0. The kinetic order in [amino acid], [H(+)] and [Mn(II)] were found out. Effect of added anions like CH3COO(-), F(-), Cl(-) and ClO 4 (-) were investigated. Evidence for the transient existence of the free radical reaction intermediate is given. Dependence of reaction rate on temperature is explained and activation parameters computed from Arrhenius and Eyring plots. A mechanism consistent with the observed results is proposed and discussed.
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