Ascorbic acid polarography

It is evident from these considerations that the use of a less hydrophobic redox species in the O phase makes the homogeneous ET occur more favourably. Another example of the IT mechanism has been found in the ET between L-ascorbic acid in W and chloranil (with Ko = 900) in NB or DCE. This has been confirmed using potential-controlled polarography [47], potential modulated reflectance spectroscopy [46], microflow coulometry [39], ECSOW system [38] and digital simulation of cyclic voltammograms [48]. 

Although polarography was introduced into pharmacopoeias in Czechoslovakia as early as in 1954, in the later edition from 1970 the number of articles prescribing a polarographic determination has been reduced (niacinamide, ascorbic acid, aureomycin and insulin). In the present edition, the method is only used for the determination of zinc in zinc-protamin insulin. More articles with a polarographic determination... 

Research on ascorbate and dehydroascorbate can be performed by conventional biochemical methods, but study of the relatively short-lived ascorbate free radical requires methods such as flow techniques with rapid mixing or pulse radiolysis coupled with polarography, ESR, or spectrophotometry. Ascorbate free radicals have been generated preferentially by oxidation of ascorbic acid [enzymatic (14-17), chemical (18-20), radiation chemical (21-26), and photochemical (27)] because ascorbic acid is easily available in a high purity grade but dehydro-ascorbic acid is not. 

Other Chemical and Physical Methods. Polarography has been tried in special investigations, such as studies of the bound form of ascorbic acid. But because of limited specificity, the procedure has not seen wide application (82,83). Ascorbic acid is oxidized at the dropping mercury electrode, the basis of the polarographic determination. Dehydroascorbic acid is not measured, however, since it is not reducible at the dropping mercury electrode. Mason et al. (84) have developed a method for the determination of ascorbic acid based on electrochemical oxidation at the tubular carbon electrode that has been modified to measure water-... 

Figure 20.4 Pulsed polarography. NPP and DPP techniques. The diagram shows at which instants the measurements are made (see the arrows a and h). Examples of measurements. The measurement of ascorbic acid (vitamin C) in a fruit juice (see conditions) corresponds to an oxidation on the working electrode.

Polarography has had limited use in the estimation of ascorbic acid, since early investigations disclosed no advantage over the simpler methods of chemical titration, even as regards specificity. A recent application of this method to vegetables and fruits was reported by Krauze and Boxyk (K16). No dehydroascorbic acid was found in the samples after reduction by H2S. 

In short, polarography can be used for the analysis of C—N, C—O, N—O, O—O, S—S, and C—S groups and for the analysis of heterocyclic compounds. Also, many important biochemical species are electroactive, such as vitamin C (ascorbic acid), fumaric acid, vitamin B factors (riboflavin, thiamine, niacin), antioxidants such as tocopherols (vitamin E), A-nitrosamines, ketose sugars (fructose and sorbose), and the steroid aldosterone. 

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