P. Martinez

Oxidation of L-ascorbic acid by trisoxalatoferrate(III) in aqueous solution. Kinetic and spectroscopic evidence for the formation of an intermediate species

Abstract Kinetic and spectroscopic evidence is reported for the formation of an intermediate Fe(III) oxalate-ascorbate complex during the oxidation of L-ascorbic acid by trisoxalatoferrate(III). The kinetic data for the formation of the intermediate are characteristic of a substitution-controlled process, and exhibit an inverse acid-concentration dependence. This is followed by a relatively slow innersphere electron- transfer reaction which is independent of pH, ascorbic acid concentration, and ionic strength. A detailed discussion of the kinetic data and a comparison with closely related oxidation reactions are presented.

Kinetics and mechanism of the oxidation of L-ascorbic acid by trisoxalatocobaltate(III) in basic aqueous solution

The kinetics and mechanism of the oxidation of L- ascorbic acid by trisoxalatocobaltate(III) were studied as a function of pH, ascorbate concentration, ionic strength and temperature in a weakly basic aqueous solution. The pH dependence of the process can be ascribed to the oxidation of the doubly deprotonated ascorbate ion for which k = 20 M−1 s−1 at 25 °C, ΔH# = 34 ± 2 kJ mol−1 and ΔS# = −108 ± 7 J K−1 mol−1. The results are discussed in reference to literature data for this reaction in weakly acidic medium and for the oxidation by a series of other oxidants.

Kinetics of the oxidation of L-ascorbic acid by diaquatetraamminecobalt(III) in acidic aqueous solution. Application of the Fuoss model and the Marcus-Sutin cross-relationship for electron-transfer processes

Abstract The kinetics of the oxidation of L -ascorbic acid by diaquatetraamminecobalt(III) was studied as a function of pH, L -ascorbic acid concentration, ionic strength and temperature using a stopped-flow technique. The rate of the process was found to be first order with respect to both redox partners, whereas the [H+] concentration showed a retarding influence. The kinetic data are interpreted in terms of rate-determining oxidation of the deprotonated ascorbate anion, for which k=3.42±0.15)×10−2 M−1 s−1 at 25 °C, ▵H≠=81±6 kJ mol−1 and ▵S≠=1±20 J K−1 mol−1. The ion-pair formation theory of Fuoss and the Marcus-Sutin cross-relationship for electron transfer were applied to this redox process to estimate the ion-pair formation constant, the rate constant for electron transfer and the self-exchange rate constant for the Co(NH3)4(H2O)22+/3+ couple.

The oxidation of L-ascorbic acid by trisoxalatoferrate(III) in weakly acidic aqueous solution revisited. Evidence for parallel inner-sphere and outer-sphere electron-transfer reaction paths

Abstract The oxidation of L -ascorbic acid by trisoxalatoferrate(III) was reinvestigated as a function of pH, ascorbic acid and oxalate concentration, in terms of a multistep process that involves the formation of an intermediate ascorbate-oxalate complex. The kinetic data indicated rapid formation of the intermediate complex, followed by a slower electron-transfer process between the redox partners that occurs in two parallel reaction paths involving an inner-sphere and outer-sphere process. A direct comparison of the electron-transfer rate constants for these processes was possible following a calculation of the outer-sphere precursor formation constant. The self-exchange rate constant for the Fe(C 2 O 4 ) 3 3−/4− system was estimated to be 2.1x10 4 M −1 s −1 at 25 °C and is discussed in reference to similar data for related systems.

Kinetics and mechanism of the oxidation of glutathione by hexacyanoferrate(III) in aqueous solution

A detailed kinetic study of the oxidation of glutathione by Fe(CN)6(3)- was performed as a function of pH, temperature, and pressure. The pH profile indicates a maximum pH independent rate in the pH range 5 to 8, which is ascribed to the oxidation of the monoanionic form of glutathione. The activation parameters for the oxidation process in this pH range are characterized by significantly negative delta V# (-22 cm3 mol-1) values. The latter value is in good agreement with that calculated theoretically on the basis of the Marcus-Hush-Stranks relationships, and can be ascribed to a large volume collapse associated with the outer-sphere reduction of Fe(CN)6(3-) to Fe(CN)6(4-).