COMPROPORTIONATION AND DISPROPORTIONATION REACTIONS IN THE ELECTROCHEMICAL REDUCTION OF NITROPRUSSIDE AT A HANGING MERCURY DROP ELECTRODE IN ACIDIC SOLUTION

The appearance of (two) cathodic peaks in the region of the second red uction process (E similar to - 0.55 V) during positive-going potential scans in cyclic voltammograms, at the HMDE, of nitroprusside (NP) (1 x 10(-3) mol dm(-3)) at pH 5.5 is evidence of a comproportionation pro cess involving adsorption. This is supported by the fact that these in verted peaks are eliminated on the addition of surface active Triton X -100. The evidence from the present study indicates that the well-char acterised tetracyanonitrosylferrate(II) ion, [Fe(CN)(4)NO](2-), is the sole reductand responsible for the second one-electron reduction proc ess of nitroprusside at all pH values, and that the [Fe(CN)(5)NOH](2-) ion, the protonated form of the initial product ([Fe(CN)(5)NO](3-)) o f the first one-electron reduction of nitroprusside, is not formed. On the other hand, at lower (sub-micromolar) concentrations, the reactio n taking place at the potentials of the second reduction step of NP fo llows different pathways depending on the pH. The voltammetric and ads orptive characteristics of the ion [Fe(CN)(4)NO](2-) at pH 3.0 and 7.6 have been investigated by square-wave (SW) voltammetry and cyclic vol tammetry (CV). Integration of the current under the CV peak at - - 0.5 5 V allows the number of electrons involved in the reduction at this p otential to be determined: one at pH 7.6 and three at pH 3.0. A surfac e regenerative process induced by H+ and based on the disproportionati on of the product of the second reduction step of NP is proposed in or der to explain this. The disproportionation is favoured at high [H+]/[ NP] concentration ratios, and, under these solution conditions, the fi nal four-electron reduction product of NP, [Fe(CN)(4)NH2OH](2-), is pr oduced at this low potential, rather than at the much more negative po tentials required polarographically. Bulk electrolysis experiments con firmed the proposed mechanism. (C) 1998 Elsevier Science S.A. All righ ts reserved.