Specific-cation medium effects in the oxidation of ascorbic acid by hexachloroiridate(IV) and hexabromoiridate(IV)

Abstract

The kinetics of the oxidation of ascorbic acid (H2A) by hexachloroiridate(1V) and hexabromoiridate(1V) have been investigated at 20.0 OC and = 1.0 M (LiC104-HC104). These reactions are first order with respect to ascorbic acid concentration and first order with respect to iridium(1V) concentration. The acidity dependence is compatible with a rate law of the form -'/2d[Ir(IV)]/df = ( k , + kb[H+]-')[H2A] [Ir(IV)], where k, pertains to the reaction of undissociated ascorbic acid and k b to the reaction of the ascorbate monoanion. The effect of changing ionic strength provides additional evidence for a mechanism consistent with the proposed rate law. Results from this investigation are compared to previously published results obtained with NaC104-HC104 media. Excellent second-order rate constant agreement is obtained when the concentrations of LiC104 or NaC104 are 50.30 M. Higher concentrations of supporting electrolyte yield lower second-order rate constants when LiC104-HC104 media are used. The Li' vs. Na' specific-cation medium effect behavior affects the values of k, and kb, the appropriateness of reactivity parameter values used with Marcus theory interpretations of k, and kb, and the percentage reaction thought to occur via each pathway. The difference in the specific-cation behavior is correlated with changes in mean ionic activity coefficients for HC104, LiC104, and NaC104 as a function of concentration, and the superiority of Lit over Na+ as a substitute for Hf in aqueous perchlorate media is thereby reaffirmed. A new estimate of the reduction potential for the H2At./H2A couple is given.

Redox Probing for Chemical Information of Oxidative Stress

“…This stoichiometric value of 4.2 is similar to the value of ∼4.5 observed in Figure c (Figure S6 of Supporting Information provides further details of these calculations) . For the case of ascorbic acid (AA), Table shows a 2 electron transfer to Ir OX was reported, which is also consistent with our calculated value (∼2.2) observed in Figure c.…”

“… 33 Scheme 1 b shows that we selected K 2 IrCl 6 (IV) (designated Ir OX ) as our mediator. Ir Ox is a reasonably strong oxidant 34 , 35 and has been shown to accept electrons from a broad range of biologically relevant reductants 36 including GSH, 37 ascorbate, 38 and cysteine. 39 The transfer of electrons from reducing species in serum to the Ir OX mediator can generate both optical and electrochemical signals which are particularly convenient for rapid, point-of-care analysis.…”

Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)

“…For both AA and IAA, the kinetic characters observed in the present work bolster the conclusion that the redox reactions take place via an outer sphere electron-transfer mode [25,26]. It is clearly shown from the data in Table 2 that k 1 << k 2 << k 3 .…”

“…e antioxidative reactivity of AA and IAA towards reduction of [IrCl 6 ] 2has been investigated in this work. Previously, the kinetics of oxidation of AA by [IrCl 6 ] 2was studied [25,26], but only was carried out in a very narrow acidity region (namely, 0.20 M ≤ [HClO 4 ] ≤ 1.00 M). On the other hand, the redox reaction between [IrCl 6 ] 2and IAA has never been investigated.…”

“…Spectral Insight into the Reaction Courses. Although an outer-sphere electron transfer was suggested for the oxidation of AA by [IrCl 6 ] 2-, the reaction course was not exploited previously[25,26]. e rapid scan spectra were thus recorded for the oxidations of AA/IAA by [IrCl 6 ] 2in this work; Figure1displays such spectra in the case of AA.…”