ABSTRACT
The analytical potential of the redox reactions between N, N-dimethyl- or N, N, N', N'-tetramethyl- p-phenylenediamine (DMPPD or TMPPD, respectively) and peroxodisulfate in acidic media for kinetic spectrophotometric determination of trace amounts of ascorbic acid (AA) has been investigated. The goal was to explore reaction conditions ensuring an excess or a deficit of S(2)O(8)(2-), and to identify some kinetic features of the global process for the purpose of calibration. Because the induction period of the overall reaction increases significantly in the presence of micromolar amounts of AA, the most suitable calibration graphs are generated by plots of induction period against the analyte concentration. The best sensitivity was achieved for oxidation of DMPPD with a deficit of S(2)O(8)(2-). The calibration line obtained has an RSD of 2.6% for 4 x 10(-6) mol L(-1) AA (n=3) and the detection limit is 4 x 10(-8) mol L(-1) (7 micro g L(-1)), a value comparable with the best mentioned by the literature. No systematic study of interferents was performed. Instead, the effect of buffer reagents (phosphate, phthalate, citrate) and dissolved oxygen on the results is discussed. Experiments were performed under closely controlled conditions. This is the first report of ascorbic acid analysis in which "contamination" by environmental oxygen was prevented by rinsing all vessels and saturating all solutions with argon, an approach which enables determination of the actual ascorbic acid content. Because of the effort involved, however, the method might not be suitable for routine analysis. Analysis of a commercially available vitamin C pellet gave good results when a "special" calibration graph, obtained in the presence of all other constituents of the sample except AA, was employed. The AA was "removed" from the pellet by oxidation with environmental air. Although it seems rather elaborate, this procedure did not require prior preparation of the sample nor extraction and/or concentration of ascorbic acid from it.
