ANODIC VOLTAMMETRY AND ITS ANALYTICAL APPLICATION TO DETECTION AND SIMULTANEOUS DETERMINATION OF HYPOXANTHINE, XANTHINE, AND URIC-ACID

The voltammetric oxidation of hypoxanthine, xanthine, and uric acid was investigated by means of linear-sweep voltammetry, cyclic voltammetry, and controlled-potential coulometry. Hypoxanthine, xanthine, and uric acid each gave a single oxidation peak at a different potential, with a stationary glassy carbon electrode, over the wide pH range of 0-13. Hypoxanthine was initially oxidized in a two-electron step to xanthine, which was then further oxidized in a two-electron step to uric acid, as well as the enzymatic oxidation. All three substances were strongly adsorbed on the surface of the glassy carbon electrode, so that the concentration vs. anodic peak current curves were not linear. The adsorption on the electrode was very dependent on the pH, and especially at pH values around neutrality the anodic peaks were ill-defined. However, in such acid solutions as H3PO4 and H2SO4, the voltammetric oxidation was a diffusion-controlled process and the differences between the peak potentials were most pronounced (ca. 400 mV). Consequently, it was possible to determine simultaneously three hydroxypurine bases by using 1 M H2SO4 or 1 M H3PO4 as a supporting electrolyte, without prior treatment or separation procedures.