Kinetic Studies of the Oxidation Reaction of Arsenic(III) to Arsenic(V) by Peroxodisulphate Ion in Aqueous Alkaline Media

Article abstract of DOI:10.1039/DT9890000357

The arsenic(III) ion was oxidized to arsenic(V) by the peroxodisulphate ion in aqueous alkaline solutions of various pH (>8.6) at 20-40 deg C.The only reactive species at pH 8.6-11.6 was the deprotonated species H2AsO3(1-); the mechanisms (i) and the rate law (iii) are proposed to (i) (ii) (iii) account for the results obtained.The first acid dissociation constant K_a of H3AsO3 and the rate constant k for the reaction between the S2O8(2-) and H2AsO3(1-) ions were determined under various conditions.The activation enthalpy and entropy, ΔH<*> and ΔS<*>, for the reaction of H2AsO3(1-) with S2O8(2-) were 47.6 kJ mol^-1 and -130.4 J K^-1 mol^-1, respectively, whereas ΔH_a and ΔS_a for the acid dissociation of H3AsO3 were 2.8 kJ mol^-1 and -117.4 J K^-1 mol^-1, respectively.The addition of acrylamide and acrylonitrile as radical scavengers produced no polymer and did not affect the reaction rate.The radicals SO4^.(1-) and As(IV), the expected reaction intermediates, could not be detected by the addition of acrylamide and acrylonitrile monomers, and thus, the reaction between S2O8(2-) and As(III) is considered to be a two-electron transfer reaction.In strong basic medium (pH<*>12.0), the HAsO3(2-) ion as well as H2AsO3(1-) reacted with the peroxodisulphate ion with the rate constant (1.6+/-0.3)*10^-2 dm³ mol^-1 s^-1 at an ionic strenght of 0.1 mol dm^-3, at 30 deg C.

Full text of DOI:10.1039/DT9890000357

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