Cyclic voltammetric studies of hydrogen peroxide on a polycrystalline gold electrode have permitted to point out that the oxidation of the molecule may occur at two distinct potentials depending on the sample composition and cleanliness and on the redox state of the electrode. At a clean gold surface, the oxidation of hydrogen peroxide gives only one wave at +490mV (vs. Ag/AgCI) at pH 7.4, besides the gold oxidation wave at + 820mV. Species in solution which adsorb readily on gold (trace impurities originating because of sonication or solution stimng), gold oxide and acidic pH values, inhibit this wave and oxidation of hydrogen peroxide is occurring at a higher potential, i t . , + 870 mV in phosphate buffer pH 7.4.Cyclic voltammograms show these two waves, depending on the degree of inhibition of hydrogen peroxide. At oxidized gold electrodes or contaminated by surface active species, oxidation of hydrogen peroxide gives only one wave at + 870 mV. From these data it may be inferred that hydroxy species adsorbed on clean gold favor the formation of the first wave. Both waves may be advantageously exploited for quantitative purpose. The latter is not affected by surface active species and permits quantification of hydrogen peroxide down to 20 pM with a detection limit of 4 pM.

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Different types of PVC-matrix ion-selective electrodes (ISEs) that respond to the cationic form of zopiclone are described. Four ion-pairing agents, namely tetraphenylborate (TPB), reineckate (RW), phosphotungstate (PT) and silicotungstate (ST), and different plasticizers are investigated. The best PVC polymeric membrane contains nitrophenyloctylether (NPOE) as plasticizer and zopiclone-TPB as active material. This electrode exhibits a Nernstian response (60 mV per decade) in the range 1 x lo-' to 1 X 10-' M and a fast dynamic response time, this response being not affected by pH between 1.9 and 5.5. Direct potentiometn. was used to determine zopiclone in drug formulations using this new zopiclone-selective electrode. The results are comparable to those obtained by differential pulse (DP) and square-wave (SW) polarographic techniques using both direct and standard addition methods.

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