Oxidation rate constants ascorbic acid-ascorbate

This experiment describes the use of FIA for determining the stoichiometry of the Fe +-o-phenanthroline complex using the method of continuous variations and the mole-ratio method. Directions are also provided for determining the stoichiometry of the oxidation of ascorbic acid by dichromate and for determining the rate constant for the reaction at different pH levels and different concentration ratios of the reactants. 

Glassy carbon electrodes polished with alumina and sonicated under clean conditions show activation for the ferrl-/ ferro-cyanlde couple and the oxidation of ascorbic acid. Heterogeneous rate constants for the ferrl-/ ferro-cyanlde couple are dependent on the quality of the water used to prepare the electrolyte solutions. For the highest purity solutions, the rate constants approach those measured on platinum. The linear scan voltammetrlc peak potential for ascorbic acid shifts 390 mV when electrodes are activated. 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

Oxidation with ozone, under physiological conditions, follows the rate order uric acid ascorbic acid > glutathione. The amounts of ozone absorbed and antioxidant consumed have been simulated with a mathematical model and reaction rate constants of the oxidations have been evaluated.194 Various facets of transition metal-catalysed oxidation of benzylic compounds with ozone have been reported. The correlation of the effect of substituents with Hammett constants and steric factors has been discussed. The reaction seemed to proceed via a radical mechanism.195... 

Superoxide radicals are another factor in oxidative damage. They can be determined with nitrobluetetrazolium (NBT), which then forms the colourless formazan. When melanoidins scavenge the superoxide radicals, the colour of the NBT persists.490,491 The activity of a glucose-glycine melanoidin on superoxide radicals is equivalent to the effect of 16 units of superoxide dismutase. The effect of the HMM and LMM fractions of this melanoidin is almost the same. The reaction rate constant of the melanoidin was markedly higher than that of ascorbic acid. If this were due to the reductone structures embedded in the melanodin, it is difficult to explain why the reducing power of the melanoidins is only 0.7 that of ascorbic acid.490... 

Reactions of hydroquinone, catechol, and L-ascorbic acid with dicyanobis(l,10-phenan-thn>line)iion(III) were studied in dimethyl sulfoxide (DMSO). Application of the Marcus theory to the reactions of catechol and hydroquinone provided the electron exchange rate constant for the Fe(III/II) couple in DMSO. The self-exchange rate constant for the ascorbic acidAadical couple was estimated for the first time in DMSO. The one electron-oxidation process of L-ascorbic acid in an aprotic solvents such as DMSO may be completely different from that in aqueous solutions. 

The electron exchange rate constant of the iron(III) complex in DMSO was estimated from the cross reactions with hydroquinone and catechol, which was compared with the rate constant obtained electrochemically. The mechanism of the ascorbic acid oxidation reaction in DMSO is discussed based on the Marcus theory. 

Table II. Rate Constants for Oxidation of Ascorbic Acid/Ascorbate...

Ascorbic acid is quoted as an example of a first-order consecutive reaction. This substance is oxidized electrochemically to an unstable electro-active intermediate, transformed by a fast chemical reaction with rate constant k into the electro-inactive dehydroascorbic acid. It is assumed that the electro-inactive form is hydrated. The halfwave potential of the anodic wave is independent of ascorbic acid... 

Another application of Zinc oxide nanostructure is immobilization of uricace onto ZnO nanorod and fabrication a sensitive biosensor for uric acid detection [167], The biosensor successfully used for micromolar detection of uric acid in the presence serious interferences, glucose, ascorbic acid, and 1-cysteine. The apparent KM value for the uric acid biosensor is 0.238 mM, showing high affinity of the biosensor. Direct electron transfer of SOD at a physical vapor deposited zinc oxide nanoparticles surface was investigated [168], In comparison to SOD immobilized onto ZnO nanodisks [169], the electron transfer rate constant is small and a quasi- reversible electrochemical behavior observed. A novel... 

It is known that the redox potential of the Fe+3/Fe+2 pair can vary by complexing ligands (27). EDTA reduces the redox potential of Fe+2 (28) and this increases the rate constant transfer of the electron from Fe+2 to H202, which is formed during autooxidation of ascorbic acid (29), and decomposition of the latter to H0-. However, at low pH 3-4, EDTA was found to inhibit ascorbic acid oxidation by ferric ions (29). Thus, the form the metal chelate takes, as a function of pH, plays a key role in its effectiveness as a catalyst. Cupric ions are known to accelerate ascorbic acid oxidation however, EDTA inhibits its catalytic effect at both neutral and low pH (24). 

Figure 87. Effect of oxygen concentration on the apparent first-order rate constants for ascorbic acid oxidation at 25°C. (Reproduced from Ref. 177 with permission.)...

Ascorbate is most likely required by hydroxylases to maintain iron or copper at the active enzyme site in the reduced form, since it is necessary for hydroxylation. The semidehydroascorbate radical is not very reactive (Bielski and Richter, 1975 Rose, 1989). It decays by disproportionation to ascorbate and dehydroascorbate (the latter subsequently degrades to oxalic acid and L-threonic acid), rather than acting as a reactive free radical. Reaction of ascorbic acid with (OH) is rapid and diffusion-dependent (K 7.2 x 10 -1.3 x lO o M- s- ) (Cabelli and Bielski, 1983). (O2) oxidizes ascorbic acid with a rate constant of 10 -10 M s (Bielski et al., 1985). Besides direct scavenging of radicals, ascorbic acid is known to have a number of physiological effects (Padh,... 

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