REDUCTION-MECHANISM OF IMINOMETHYL-5-HYDROXYMETHYL-1,2-DIMETHYLPYRIDINIUM IODIDE IN AN ACIDIC MEDIUM

The reduction mechanism of the selected oxime, iminomethyl-5-hydroxyme thyl-1,2-dimethylpyridinium iodide (HS10), has been studied by normal pulse polarography (NPP) from a Britton-Robinson buffer of pH 2.6, pre dominantly on a static mercury drop electrode (SMDE). At pH 2.6, 99% o f HS10 is present in the undissociated hydrophobic form. The limiting reduction current is diffusion controlled (diffusion coefficient D = 1 .5 x 10(-6) cm2/s; half-wave potential E1/2 = -0.355 V vs. Ag/AgCl/KCl (sat) reference electrode). The NPP wave of HS10 at pH 2.6 has the cha racteristic features of a reduction complicated by reactant adsorption . The reduction process is totally irreversible; the average an = 0.97 5. Considering the uptake of 4e- during the reductive cleavage of the azomethine group (-C=N-) the estimated cathodic transfer coefficient a = 0.234. During the prepulse period t0 the reactant adsorbs at the me rcury electrode, following the linear Henry type of isotherm. For 1.01 , 4.05 and 8.10 x 10(-5) mol l-1 bulk concentrations of HS10, the surf ace excesses. GAMMA0 on SMDE at t = t0 are 0.5, 2.1 and 5.8 x 10(-11) mol cm-2 respectively. For the same concentrations of HS10, according to the Henry isotherm the adsorption constant K has been calculated as 4.9, 5.2 and 7.2 x 10(-4) cm, which indicates weak reactant adsorptio n during the pre-pulse period t0.