Oxidation of ascorbic acid

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. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

FIGURE 18.30 The physiological effects of ascorbic acid (vitamin C) are the result of its action as a reducing agent. A two-electron oxidation of ascorbic acid yields dehy-droascorbic acid. 

Low-molecular semiconductors and, in particular, phthalocyanines are known to exhibit photosensitizing properties268. The photosensitizing activity of these substances has been studied in most detail for the reaction of oxidation of ascorbic acid. 

Polypyrrole shows catalytic activity for the oxidation of ascorbic acid,221,222 catechols,221 and the quinone-hydroquinone couple 223 Polyaniline is active for the quinone-hydroquinone and Fe3+/Fe2+ couples,224,225 oxidation of hydrazine226 and formic acid,227 and reduction of nitric acid228 Poly(p-phenylene) is active for the oxidation of reduced nicotinamide adenine dinucleotide (NADH), catechol, ascorbic acid, acetaminophen, and p-aminophenol.229 Poly(3-methylthiophene) catalyzes the electrochemistry of a large number of neurotransmitters.230... 

Benzenediamine (358) and 5-(l,2-dihydroxyethyl)tetrahydro-2,3,4-furane-trione (359) (prepared in situ by oxidation of ascorbic acid with p-benzoqui-none) gave either 3-(2,3,4-trihydroxybutyryl)-2(17i)-quinoxalinone (360)... 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

Identical kinetics are found for the uranyl ion-catalysed aerobic oxidation of ascorbic acid and a similar mechanism has been put forward These results and others afford a sequence of catalytic activity for the aerobic oxidation of ascorbic acid ... 

The ferricyanide oxidation of ascorbic acid at pH 1.1 follows kinetics ... 

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. 

A typical result for DPV In Fig. 4a shows the presence of two redox couples with peak potentials of 0.25 V and 0.19 V ( lOmV). Similar results have also been obtained with SWV. The relative Intensities of the two peaks vary from sample to sample but are always present with activated electrodes. The similarities between the potentials found for the surface species and for the oxidation of ascorbic acid suggest that an ec catalytic mechanism may be operative. The surface coverage of the o-qulnone Is estimated to be the order of 10 mol cm . It Is currently not possible to control the surface concentration of the o-qulnone-llke species or the oxygen content of the GCE surface. 

Figure 6. Simulated cyclic voltammogram for the oxidation of ascorbic acid without Inclusion of ec catalysis by the surface qulnone functionalities. Filled circles represent the simulated data and an experimental curve Is shown with a line for comparison. A scan rate of 100 mV s was assumed for experimental and simulated data.

Cabelli, D.E. and Bielski, B. (1983). Kinetics and mechanism for the oxidation of ascorbic acid (ascorbate by HO2/O2 radicals. A pulse radiolysis and stopped-flow photolysis study. J. Phys. Chem. 87, 1809. 

Fessenden, R.W. and Verma, N.C. (1978). A time-resolved electron spin resonance study of the oxidation of ascorbic acid by the hydroxyl radical. Biophys. J. 24, 93. 

A chemical reaction subsequent to a fast (reversible) electrode reaction (Eq. 5.6.1, case b) can consume the product of the electrode reaction, whose concentration in solution thus decreases. This decreases the overpotential of the overall electrode process. This mechanism was proposed by R. Brdicka and D. H. M. Kern for the oxidation of ascorbic acid, converted by a fast electrode reaction at the mercury electrode to form dehydro-ascorbic acid. An equilibrium described by the Nernst equation is established at the electrode between the initial substance and this intermediate product. Dehydroascorbic acid is then deactivated by a fast chemical reaction with water to form diketogulonic acid, which is electroinactive. 

Polymer-supported catalysts often have lower activities than the soluble catalysts because of the intraparticle diffusion resistance. In this case the immobilization of the complexes on colloidal polymers can increase the catalytic activity. Catalysts bound to polymer latexes were used in oxidation reactions, such as the Cu-catalyzed oxidation of ascorbic acid,12 the Co-catalyzed oxidation of tetralin,13 and the CoPc-catalyzed oxidation of butylphenol14 and thiols.1516 Mn(III)-porphyrin bound to colloidal anion exchange resin was... 

Khan and Martell [J. Am. Chem. Soc., 91 (4668), 17, 1969] have reported the results of a kinetic study of the uranyl ion catalyzed oxidation of ascorbic acid. The stoichiometric equation for this reaction mav be represented as... 

F. Li and S. Dong, The electrocatalytic oxidation of ascorbic acid on Prussian blue film modified electrodes. Electrochim. Acta 32, 1511—1513 (1987). 

S.F. Wang, M.A. Jiang, and X.Y. Zhou, Electrocatalytic oxidation of ascorbic acid on nickel hexacyanoferrate film modified electrode. Gaodeng Xuexiao Huaxue Xuebao 13, 325-327 (1992). 

It is not clear either how the Ru center can accommodate four ligands simultaneously. The crowded coordination sphere around the metal center in the Ru(EDTA)-ascorbate complex is expected to hinder the coordination of other ligands as was proposed earlier (24,25). The contradiction between the two sets of results reported in Refs. (24,25) and (148) is obvious. While the Ru(EDTA)(H2A)(02) complex was not considered in the kinetic model proposed for the oxidation of ascorbic acid,... 

Needless to say, there is a definite possibility that, if reactions as just described participate in the transformations of polyuronide polysaccharides, enzymatic control of these may exist by systems leading to the oxidation of ascorbic acid or producing peroxide-type intermediaries. 

The oxidation of ascorbic acid AH2 by Ag(I) involves the rate determining step as... 

The oxidation of ascorbic acid in the presence of an excess of oxygen (saturation) was assumed to follow a first order reaction [2, 3,5]. The reaction rate can be expressed as ... 

Fig. 35.5 Kinetic data for Cu(II) ion-catalyzed (0.785 x 10 M) oxidation of ascorbic acid in acetate buffered PVP solution (0.4%) at pH = 4.5 [Ascorbic acid concentrations (A 2.85, B 5.67,...

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. ... 

The most important point during sample preparation is to prevent oxidation of ascorbic acid. Indeed, it is easily oxidized by an alkaline pH, heavy metal ions (Cu and Fe ), the presence of halogens compounds, and hydrogen peroxide. The most suitable solvent for this purpose is metaphosphoric acid, which inhibits L-ascorbic oxidase and metal catalysis, and it causes the precipitation of proteins. However, it can cause serious analytical interactions with silica-based column, e.g., C18 or amino bonded-phases [542] and it could co-elute with AA. 

Ascorbic acid oxidase (MW = 1,40,000 8 Cu). It is widely distributed in plants and micro-organisms. It catalyses oxidation of ascorbic acid (vitamin C) to dehydro ascorbic acid. 

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