Study of reduction of cobalt(II) in sodium azide aqueous medium by pulse polarography / Estudo da redução do cobalto(II) em meio aquoso de azoteto de sódio por polarografia de pulso

AUTOR(ES)

Claudia Giovedi Motta

DATA DE PUBLICAÇÃO

1994

RESUMO

The electroreduction of Co(ll) in perchloric media gives origin to a polarographic wave about -1.4 V VS. SCE at 25°C. With the addition of the azide ligand, N3-, that wave shifts towards more positive potentials. The higher value of the potential shift, 320 mV, is recorded for the concentration 0,600 M of ligand. The shift can be explained by the adsorption of the azide ion on mercury electrodes, bridging the electrode and the metal ion, and promoting, then, the electron transfer more easily. With intermediary ligand concentrations the reduction wave of the cobalt ion breaks in two curves having walf-wave potentials around -1.1 and -1.4 V. Taking into account the distribution curve of the complexes species in equilibrium, it is concluded that the first wave corresponds to the azide complex of cobalt(lI) (already present in the solution or formed on the electrode surface), and the second one is related to the Co(lI) aquo íon. The wave correspondent to the reduction of azide complex, in the differential pulse polarography, too shows a splitting in two curves. This behavior dependents of the ligand concentration can be interpreted in terms of a stepwise charge transfer reaction, with the possible production of a labil Co(l) intermediate. Coulometric experiments performed at the potential where the breaks occurs, confirmed the supposition of the presence of Co(l) species, which could be oxidized to Co(ll) by HN3 yielding NH4+ and N2. Two linear regions exhibited by polarization curves in the solution 0,160 M azide and the Ic/la current ratio equal to 1, obtained by reverse pulse polarography with the initial potential coinciding with the potential at which break occurs, gave further evidences of the possible formation of Co(l) species.

ASSUNTO(S)

azoteto de sódio cobalto(ii) polarografia de pulso pulse polarography polarography methods cobalt(ii) analytical chemistry sodium azide química analítica método polarográfico

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