The determination of some 2-thiobarbiturates by cathodic stripping voltammetry

Pulse polarography and cyclic voltammetry are employed in studies of the electrochemical behaviour of 5-ethyl-5'-(l-methylbutyl)-2-thiobarbituric acid (I), l-methyl-5-ethyl-5'-(l-methylpropyl)-2-thiobarbituric acid (II) and l,3-dimethyl-5-ethyl-5'-p-chlorophenyl)-2-thiobarbituric acid (III) in the pH range 4–12. All three compounds show anodic and cathodic waves or peaks in this pH range. Compounds (I) and (II) are oxidized at mercury indicator electrodes to produce mercury salts which can adsorb thereon and are thus amenable to cathodic stripping voltammetric analysis (c.s.v.) down to concentrations of the order of 10^-6 M, which is superior to the sensitivities obtained by differential pulse polarography (d.p.p.) based on a reduction peak. Compound (III) oxidizes to produce sulphur which is subsequently plated as HgS. Again the sensitivity of the c.s.v. method is of the order of lO^-6 M and analytically superior to d.p.p. The optimum pH for the three determinations is 8. The determination of (II) in the presence of its oxygenated analogue and metabolite, phemitone, and the effect of chloride ions are reported.