✍ Scribed by Milenko V. Šušić; Miljan G. Pješčić
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Despite the rather complex polarographic behaviour of As(III), several favourable conditions have been found, especially in acidic media, where arsenic can be determined by polarography I -6. Opinions regarding the mechanism of the electrode reaction differ considerably. Meites 7 believes that the first of the two waves, obtained in rather acidic solutions, corresponds to a reduction to elemental arsenic, while the second wave is a consequence of a reduction to arsine. Our earlier coulometric determination 8 of the number of electrons involved, using the method described by Vinogradova 9, showed that in acidic medium, pH 0.8, three electrons are taking part, one for the first wave, and two for the second. However, Vasilyeva et al. 1° recently proposed another mechanism where the reaction is supposed to go to arsine with formation of mercury arsenide.
Data on the polarographic behaviour of arsenic(III) in alkaline media are rather scant. According to Lingane 3 there is no reduction, while Kolthoffand Probst 11 showed only an oxidation to As(V) at -0.25 V, forming a well developed anodic wave which was later used for the determination of arsenic 12-14. However, Swannet al. i4~ consider this reaction to be an early decomposition of the supporting electrolyte in the presence of arsenic and not a reduction. Geyer and Geissler 15 found a well developed cathodic wave at pH values between 7 and 9. According to the authors of ref. 17, As(III) is reduced in alkaline media with six electrons taking part, provided the concentration of LiOH is below 0.2 M, but there is no reduction in more alkaline solutions; arsenic(III) is not oxidized to As(V), as claimed by Kolthoff and Probst 11, but only mercury is oxidized, thus forming sparingly soluble mercury arsenite.
Considering the data known to date, an attempt was made to reinvestigate the reduction of As(III) in solutions of low alkalinity in order to clarify the mechanism of the electrode process. EXPERIMENTAL Measurements were made on a Radiometer P04d polarograph and a Radiometer PHM22r pH-meter. The rate of the cathodic polarization was 0.4 V min-1 at a flow of mercury of 2.93 mg s -1 and a drop life-time of 2.8 s. The determinations of pH were made after the registering of the waves. All chemicals used were of reagent grade purity.
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