Studies on Characterisation of Kinetically Active Cerium(IV) Species in Oxidation of α-Hydroxy Acids in Aqueous Micellar Sulfuric Acid Media

Kinetics of the oxidation of α-hydroxy acids [HA], namely malic acid, lactic acid and mandelic acid, by Ce(IV) in aqueous sulfuric acid media have been investigated both in the absence and presence of the cationic surfactant, N-cetylpyridinium chloride (CPC) and the anionic surfactant, sodium dodecyl sulfate (SDS). Under the conditions [HA]  [Ce(IV)], the reaction exhibits first-order kinetics both in [Ce(IV)] and [HA]. The rate retarding effect of HSO4- has been analysed and it has been noted that both Ce(SO4)2+ and Ce(SO4)2 are kinetically active but their relative contribution depends on the nature of the α-hydroxy acid and surfactant. The different rate constants in the presence and absence of surfactants have been determined with the corresponding activation parameters. SDS has been found to accelerate the rate process for all three α-hydroxy acids. CPC has been found to retard the oxidation of malic acid more than the other two α-hydroxy acids. In fact, for lactic acid, the rate is more or less insensitive to CPC. All these findings, including the micellar effects, have been interpreted in terms of the proposed reaction mechanism and partitioning behaviour of the different kinetically active species of Ce(IV) between the aqueous and pseudomicellar phases.