Cooxidation of Formic Acid and Oxalic Acid by Chromium(VI) in Aqueous Acid Media: A Kinetic Study

Reduction behavior of chromium(VI) with oxalic acid in aqueous solution

The direct Cr(VI) reduction process by oxalic acid was conducted. The existence of Cr(VI) in the reaction medium was measured by software Visual MINTEQ and the concentration of Cr(VI) was measured by ICP-OES. The results showed that the Cr(VI) was efficiently reduced by oxalic acid at high reaction temperature and high dosage of oxalic acid. The reduced product, Cr(III), was easily generated stable complex compounds (Cr(HC2O4)3) with oxalate, which displayed a negative effect on the reduction process. The high reaction temperature and high acidic medium could destroy the stable structure of a complex compound to release oxalate, and facilitate the reduction of Cr(VI). Generally, the results showed in this paper provided a versatile strategy for Cr(VI) reduction and exhibited a bright application future for real wastewater treatment.

Micellar Effect on the Catalytic Co-Oxidation of Dimethyl Sulfoxide and Oxalic Acid by Chromium(VI) in Aqueous Acid Media: A Kinetic Study

The CrVI oxidation of a mixture of dimethyl sulfoxide (DMSO) and oxalic acid (OXH2) in aqueous acid media occurs much faster than that of either of the two substrates alone (DMSO reacts extremely slowly under the experimental conditions). In the mixture, both substrates undergo oxidation simultaneously in a ternary complex of CrVI through a three electron transfer step (i.e. CrVI → CrIII), two electrons from DMSO and one electron from oxalic acid. Hitherto there has been a debate regarding the existence of a 3e transfer in a single step. The micellar effect may be considered as a probe for 3e transfer in a single step.

1,10-Phenanthroline-Catalysed Cr(Vi) Oxidation of D-Sorbitol and D-Mannitol in Aqueous Micellar Media

Chromic acid oxidation of two polyols (D-sorbitol and D-mannitol) to their corresponding aldohexoses in the presence of phenanthroline (phen) involves as the active oxidant, a Cr(VI)-phen complex formed in a rapid pre-equilibrium step. The observed micellar effect (rate acceleration by an anionic surfactant and rate retardation by a cationic surfactant) conforms with the proposed reaction mechanism.

Oxidation of d-glucose in the presence of 2,2′-bipyridine by CrVI in aqueous micellar media: a kinetic study

The kinetics of Cr(VI) oxidation of D-glucose to the corresponding lactone in the presence and absence of 2,2'-bipyridine (bipy) has been carried out under the conditions, [D-glucose](T) >> [Cr(VI)](T) at different temperatures in aqueous micellar media. The monomeric Cr(VI) species has been found to be kinetically active in the absence of bipy whereas in the bipy-catalysed path, the Cr(VI)-bipy complex has been found to be the active oxidant. In the bipy-catalysed path, the Cr(VI)-bipy complex undergoes nucleophilic attack by the substrate to form a ternary complex. The ternary complex spontaneously experiences a redox decomposition (through two-electron transfer) in the rate-determining step leading to the product lactone and Cr(IV)-bipy complex. The Cr(IV)-bipy complex then takes part in faster steps in the further oxidation of D-glucose and is ultimately converted into a Cr(III)-bipy complex. In the uncatalysed path, the Cr(VI)-substrate ester experiences acid catalysed redox decomposition (two-electron transfer) in the rate-determining step. The uncatalysed path shows a second order dependence on [H(+)] and a first order dependence on each of the reactants [D-glucose](T) and [Cr(VI)](T). In contrast, the bipy-catalysed path shows a first order dependence on each of the reactants [H(+)], [D-glucose](T) and [Cr(VI)](T). The bipy-catalysed path is first order in [bipy](T). These observations remain unaltered in the presence of externally added surfactants. The effect of the cationic surfactant, N-cetylpyridinium chloride (CPC) and anionic surfactant, sodium dodecyl sulfate (SDS) on both the uncatalysed and bipy-catalysed path has been studied. CPC inhibits both the uncatalysed and bipy-catalysed path, while SDS catalyses these reactions. The observed micellar effects have been explained by considering hydrophobic and electrostatic interactions between the surfactants and reactants.