Kinetics of ascorbic acid

A leant of investigators at the Department of Food Science and Human Nutrition. Michigan State University, studied the kinetics of ascorbic acid... 

Several modifications of the OPDA procedure have been reported for automated analysis. Kirk and Ting (16) described a continuous flow analysis in which DCIP was substituted for Norit in the oxidation step. TAA and DHA can be determined directly RAA is calculated as the difference between TAA and DHA. Good agreement between the manual and automated procedures was achieved, with a considerable decrease in analytical time for the automated assay. Kirk and coworkers (17 19) have used the continuous flow analysis for studies of the kinetics of ascorbic acid degradation in model systems. 

Kinetics of Ascorbic Acid Destruction During Processing and Storage... 

To predict nutrient deterioration, knowledge of the reaction rate as a function of temperature of storage or processing is needed. The kinetics of ascorbic acid destruction have been examined most extensively in model systems, with particular attention being given to intermediate moisture foods (17, 71,78,79). Most of the data available for vitamin C losses in actual food systems are insuflBcient to calculate the kinetic parameters needed to predict losses during heat treatment or storage. 

According to Lenz and Lund (72), kinetic models for destruction of food components are needed to improve products by minimizing quality changes for new product development and to predict shelf life during storage. Numerous reports and reviews of the kinetics of ascorbic acid destruction can be found in the literature (68-88). A brief overview is presented here to indicate the need for further research in this area. 

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 ... 

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. 

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... 

Regna and Caldwell [/. Am. Chem. Soe., 66 (246), 1944] have studied the kinetics of the acid-catalyzed transformation of 2-ketopolyhydroxy acids into ascorbic acids and other products. 

Other workers have used the tristimulus parameters to study the kinetics of decomposition reactions. The fading of tablet colorants was shown to follow first-order reaction kinetics, with the source of the illumination energy apparently not affecting the kinetics [49]. The effect of excipients on the discoloration of ascorbic acid in tablet formulations has also been followed through determination of color changes [50]. In this latter work, it was established that lactose and Emdex influenced color changes less than did sorbitol. 

The kinetic results reported by Jameson and Blackburn (11,12) for the copper catalyzed autoxidation of ascorbic acid are substantially different from those of Taqui Khan and Martell (6). The former could not reproduce the spontaneous oxidation in the absence of added catalysts when they used extremely pure reagents. These results imply that ascorbic acid is inert toward oxidation by dioxygen and earlier reports on spontaneous oxidation are artifacts due to catalytic impurities. In support of these considerations, it is worthwhile noting that trace amounts of transition metal ions, in particular Cu(II), may cause irreproducibilities in experimental work with ascorbic acid (13). While this problem can be eliminated by masking the metal ion(s), the masking agent needs to be selected carefully since it could become involved in side reactions in a given system. 

According to ESR measurements, the semiquinone radical forms at nM concentration levels and its steady-state concentration was reported to increase by increasing the total concentration of ascorbic acid. The kinetic role of O was confirmed by the inhibitory effect of... 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

In alkaline solution (pH 11), the complex is present as a p-oxo dimer and ascorbic acid is fully deprotonated. In the absence of oxygen, kinetic traces show the reduction of Fe(III) to Fe(II) with a reaction time on the order of an hour at [H2A] =5xlO-3M. The product [Fen(TPPS)] is very sensitive to oxidation and is quickly transformed to Fe(III) when 02 is added. This leads to a specific induction period in the kinetic traces which increases with increasing [02]. The net result of the induction period is the catalytic two-electron autoxidation of ascorbic acid in accordance with the following kinetic model (23) ... 

The kinetic models for these reactions postulate fast complex-formation equilibria between the HA- form of ascorbic acid and the catalysts. The noted difference in the rate laws was rationalized by considering that some of the coordination sites remain unoccupied in the [Ru(HA)C12] complex. Thus, 02 can form a p-peroxo bridge between two monomer complexes [C12(HA)Ru-0-0-Ru(HA)C12]. The rate determining step is probably the decomposition of this species in an overall four-electron transfer process into A and H202. Again, this model does not postulate any change in the formal oxidation state of the catalyst during the reaction. 

The presence of ascorbic acid as a co-substrate enhanced the rate of the Ru(EDTA)-catalyzed autoxidation in the order cyclohexane < cyclohexanol < cyclohexene (148). The reactions were always first-order in [H2A]. It was concluded that these reactions occur via a Ru(EDTA)(H2A)(S)(02) adduct, in which ascorbic acid promotes the cleavage of the 02 unit and, as a consequence, O-transfer to the substrate. While the model seems to be consistent with the experimental observations, it leaves open some very intriguing questions. According to earlier results from the same laboratory (24,25), the Ru(EDTA) catalyzed autoxidation of ascorbic acid occurs at a comparable or even a faster rate than the reactions listed in Table III. It follows, that the interference from this side reaction should not be neglected in the detailed kinetic model, in particular because ascorbic acid may be completely consumed before the oxidation of the other substrate takes place. 

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,... 

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 oxidation of more complicated organic molecules follows much the same course initially. The oxidation of ascorbic acid (AH2) appears to follow the following kinetic scheme ... 

Kabanov et al.116 studied the oxidation of ascorbic acid by the Cu(II) complexes of poly(4-vinylpyridine) partially alkylated by bromoacetic acid. It was considered from kinetic and thermodynamic data that the higher catalytic activity of the polymer-Cu complex was caused by binding of the substrate to the catalytic site, represented as 48. 

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