Joaquin F. Perez-Benito

A kinetic study of the reaction between soluble (colloidal) manganese dioxide and formic acid

Abstract The kinetics of the reaction between a soluble form of colloidal manganese dioxide (obtained by reduction of potassium permanganate with sodium thiosulfate) and formic acid in aqueous perchloric acid solution has been investigated by UV—Vis spectrophotometry. The rate law is r = (k+k ′ [H + ] 1.67 )[MnO 2 ][HCOOH] 0.45 The apparent values found for the activation parameters are Ea = 53.6 ± 0.8 kJ mole−1, ΔH≠ = 51.1 ± 0.8 kJ mole−1, and ΔS≠ = −140 ± 3 J K−1 mole−1. The reaction seems not to be very sensitive toward addition of sodium perchlorate to the solution, but is noticeably retarded by addition of gum arabic acting as a protective colloid. A mechanism in accordance with the experimental results is proposed for the reaction taking place at the colloid surface.

Occurrence of colloidal manganese dioxide in permanganate reactions

Abstract It is shown that there exist several forms of colloidal MnO2 stable in both aqueous and dichloromethane solutions. Stabilization of the colloid may occur in two different ways: decreasing the polarity of the particles at the colloid surface and endowing the colloidal particles with a negative electrostatic charge. The manganese species identified in other works as a manganese(IV)—phosphate complex or as a cyclic manganate(V) diester (among others) are actually different forms of colloidal MnO2. A mathematical model for the UV—Vis spectrum of this species, taking into account both the light-absorbing and light-scattering contributions to the total absorbance, is proposed. From both nephelometric and spectrophotometric measurements, it has been estimated that the average size of the colloidal particles of several forms of colloidal MnO2 is in the range N = (0.41–2.93) × 106 (MnO2 units)/particle. The contribution of a single MnO2 unit to the polarizability of a colloidal particle has been estimated to be α 0 = 26.7 ± 1.4 A 3 . The ways for reduction and coagulation of colloidal MnO2 to occur are also discussed.

Permanganate Oxidation of α-Amino Acids: Kinetic Correlations for the Nonautocatalytic and Autocatalytic Reaction Pathways

The reactions of permanganate ion with seven α-amino acids in aqueous KH(2)PO(4)/K(2)HPO(4) buffers have been followed spectrophotometrically at two different wavelengths: 526 nm (decay of MnO(4)(-)) and 418 nm (formation of colloidal MnO(2)). All of the reactions studied were autocatalyzed by colloidal MnO(2), with the contribution of the autocatalytic reaction pathway decreasing in the order glycine > l-threonine > l-alanine > l-glutamic acid > l-leucine > l-isoleucine > l-valine. The rate constants corresponding to the nonautocatalytic and autocatalytic pathways were obtained by means of either a differential rate law or an integrated one, the latter requiring the use of an iterative method for its implementation. The activation parameters for the two pathways were determined and analyzed to obtain statistically significant correlations for the series of reactions studied. The activation enthalpy of the nonautocatalytic pathway showed a strong, positive dependence on the standard Gibbs energy for the dissociation of the protonated amino group of the α-amino acid. Linear enthalpy-entropy correlations were found for both pathways, leading to isokinetic temperatures of 370 ± 21 K (nonautocatalytic) and 364 ± 28 K (autocatalytic). Mechanisms in agreement with the experimental data are proposed for the two reaction pathways.

Catalysis by zinc ion in the reactions of carcinogenic chromium(VI) with thiols

Abstract The reactions of Cr(VI) with nine biological and nonbiological thiols (RSH) in aqueous acetate buffer (pH 5.60) have been followed by an spectrophotometric method, and a relatively stable intermediate (the thioester RSCrO3−) has been observed. The rate constants corresponding to the formation (k1), decomposition (k−1), and redox transformation (k2) of that intermediate have been obtained in both the absence and presence of zinc ion. This ion acts as a true catalyst for both the formation and decomposition of the observed intermediate. The equilibrium constants for the formation of RSCrO3− from Cr(VI) and RSH for the different thiols have been determined. A mechanism is proposed for the formation of RSCrO3− in the presence of catalyst according to which the rate-determining step would be the bimolecular reaction between a zinc–thiolate complex and an acetyl–chromate ester, involving an interchange of ligands.

Coagulation of colloidal manganese dioxide by divalent cations

Abstract The coagulation of colloidal MnO 2 by several divalent cations, both in the absence and presence of a complexing ligand ( l -histidine), has been studied. The efficiency as coagulating agents increases in the order Mg 2+ 2+ 2+ 2+ 2+ 2+ . For a given cation, the coagulating efficiency increases with increasing pH and with increasing temperature, and decreases with increasing concentration of l -histidine. Apparent values for the equilibrium constants of complexation of divalent cations by l -histidine have been obtained. The apparent equilibrium constant of formation of the complex increases in the order Mg 2+ 2+ 2+ 2+ 2+ . The value corresponding to Co 2+ could not be determined because l -histidine acted as a catalyst for the reduction of colloidal MnO 2 by that cation.

Mutual catalyst inhibition in the chromium(VI)–copper(II)–hydrogen peroxide reacting system

Cr(VI) and Cu(II) are both efficient catalysts for the decomposition of H2O2. However, in their joint presence, the two catalysts exert a mutual inhibition effect on the catalytic activity of the other, and the study of these inhibition effects has provided important information on the mechanisms of the Cr(VI)/H2O2 and Cu(II)/H2O2 reactions. The inhibition effect produced by Cr(VI) on the catalytic activity of Cu(II) is the more pronounced (a [Cr(VI)]/[Cu(II)] ratio as low as 2×10-4 is enough to provoke a 10% decrease in the decomposition rate), the limit being around 90% inhibition. On the contrary, higher concentrations of Cu(II) are necessary to produce a noticeable inhibition on the catalytic effect of Cr(VI), the limit being 50% inhibition. Besides, Cu(II) prevents the reduction of Cr(VI) to Cr(III) by H2O2, so that when [Cu(II)] is high enough the catalyst Cr(VI) is recovered unaltered at the end of the reaction. The inhibition effect produced by Cu(II) on the Cr(VI)/H2O2 reaction can be ascribed to its ability to catalyze the dismutation of superoxide free radicals, thus preventing the reduction of Cr(V) to Cr(IV) provoked by them, whereas the inhibition effect produced by Cr(VI) on the Cu(II)/H2O2 reaction can be ascribed to its ability to act as an oxidative scavenger for Cu(I), thus preventing its participation as the initiator of a free-radical chain mechanism leading to the decomposition of H2O2.

Kinetics and mechanisms of the oxidation by permanganate of L-alanine

The oxidation of L-alanine by permanganate ion in aqueous phosphate buffers is autocatalysed by the inorganic reaction product, a solube form of colloidal manganese dioxide temporarily stabilized in solution by adsorption of phosphate ion on its surface. The rate of the non-catalytic reaction pathway is first-order in both the oxidizing and reducing agent, is not affected by potassium chloride addition to the solution, and increases with the pH of the medium, its associated activation energy being 74.0 kJ mol–1. The rate of the catalytic reaction pathway is first order in both the oxidizing and autocatalytic agent, follows the Freundlich adsorption isotherm as far as the reducing agent concentration is concerned, is not affected by potassium chloride addition to the solution, and increases with the pH of the medium, its associated activation energy being 67.2 kJ mol–1. Mechanisms consistent with the experimental data are proposed.

The pyrophosphate-assisted reduction of chromium(VI) by manganese(II) and its reverse reaction

We report the discovery of a novel reaction that behaves in an unexpected way. Although Cr(VI) and Mn(II) do not react in the absence of stabilizing ligands, they react in aqueous perchloric acid provided that pyrophosphate [a stabilizing ligand for Mn(III)] is present in the medium. The reaction [whose stoichiometry is Cr(VI) + 3 Mn(II) → Cr(III) + 3 Mn(III)] takes place in a reversible manner, but the kinetic plots are not as those expected for conventional reversible reactions. On the contrary, a period of decay of the Cr(VI) concentration [and formation of Mn(III)] is followed by a period in which that reactant is actually regenerated (and the product consumed). During the course of the reaction, pyrophosphate is slowly hydrolyzed to phosphate. This hydrolysis constitutes the driving force for the backward reaction. A mechanism involving three consecutive 1-electron redox steps is proposed for the reduction of Cr(VI) to Cr(III), in each step a Mn(II) being oxidized to Mn(III), whereas colloidal manganese dioxide might be a key intermediate for the reoxidation of Cr(III) to Cr(VI).

Kinetics and mechanism of the reduction of chromium(VI) by D-ribose

The reaction between CrVI and D-ribose in aqueous perchloric acid, studied by application of the initial-rates method, is first order in both oxidant and reductant, whereas the apparent order of H+ is a little lower than 2. At [HClO4]=0.366 M and 25.0°C the reaction with monomeric CrVI is 12.7 times faster than that with dimeric CrVI. Under the same acidity conditions, the apparent activation parameters are ΔH0,=40.1±0.6 kJ mol-1 and ΔS0,=-155±2 J K-1 mol-1. At low [CrVI]0 the reaction is catalyzed by MnII. The catalytic reaction pathway is zero order in CrIV and first order in both D-ribose and H+. At high [CrVI]0 the reaction is inhibited by MnII because it traps the intermediate CrVI. In the presence of MnII the formation of MnIII as an intermediate has been detected. Mechanisms for the uncatalyzed and catalyzed reaction pathways are proposed.

Evidence for the involvement of chromium(II) as an intermediate in the reduction of chromium(VI) to chromium(III) by formaldehyde

The reaction between chromium(VI) and formaldehyde in aqueous perchloric acid is inhibited by manganese(II) ion, according to the relation k1=a/(1 +b[Mn2+])+c; manganese(II) ion and formaldehyde compete to reduce the intermediate CrIV, and CrII is an intermediate.