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A self-assembled monolayer (SAM) of 6-thioguanine (6TG) was formed on the hanging mercury drop electrode, under open circuit potential, from 6TG solutions. The SAM has been characterized by cyclic voltammetry under both in situ and ex situ conditions and the experimental data reveal that the film is a densely packed structure of chemisorbed (mercury-thiol) molecules. The presence of this SAM has no influence on the rate of outer-sphere electrode reactions but strongly inhibits inner-sphere processes. The electrode reaction involving the Fe(CN) 3À=4À 6 couple appears reversible in a hanging mercury drop electrode coated by the 6TG-SAM.

A polarographic method for the determination of pyrithioxine in pharmaceutical preparations is described. The method is based on the first cobalt(I1) catalytic pre-wave given by the pyrithioxine in a solution that is 1 x M in cobalt (11), 0.05 M in ammonia, 0.1 M in ammonium chloride and 2 x 1 0-6 M in tetrabutylammonium hydroxide. I t is rapid and sensitive, and its precision and accuracy are satisfactory.

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