THE STABILIZATION OF MANGANESE(III) BY AZIDE IONS IN AQUEOUS-SOLUTION

The evidences of spontaneous oxidation of Mn(II) by the dissolved oxyg en in azide buffer medium, which is dependent on the N-3(-)/HN3 concen tration, suggested a formation of stable Mn(III) complexes due to mark ed colour changes. Spectrophotometric studies combined with coulometri c generation of Mn(III), in presence of large excess of Mn(II), showed a maximum absorbance peak at 432 nm. The molar absorptivity increases with azide concentration (0.44-3.9 mol l(-1)) from 3100 to 6300 mol(- 1) 1 cm(-1), showing a stepwise complex formation. Potential measureme nts of the Mn(III)/Mn(II) system in several azide aqueous buffers solu tions: 1.0 x 10(-2) mol l(-1) HN3, (0.50-2.0 mol l(-1)) N-3(-), and 5. 0 x 10(-2) mol l(-1) Mn(II) and constant ionic strength 2.0 mol l(-1), kept with sodium perchlorate, leads to the conditional potential, E-0 'x, in several azide concentrations at 25.0 +/- 0.1 degrees C. Conside ring the overall formation constants of Mn(II)/N-3(-), from former stu dies, and the potential, E(0')s = 1.063 V versus SCE, for Mn(III)/Mn(I I) system in non-complexing media, it was possible to calculate the Fr onaeus function, F-0(L), and the following overall formation constants : beta(1) = 1.2 x 10(5) M-1, beta(2) = 6.0 x 10(8) M-2, beta(3) = (2.4 +/- 0.7) X 10(11) M-3, beta(4) = (1.5 +/- 0.5) X 10(11) M-4 and beta( 5) = (9.6 +/- 0.8) x 10(11) M-5 for the Mn(III)/N-3(-) complexes. Thes e data give important support to understand the importance of Mn(II) a nd Mn(III) synergistic effect on the analytical method of S(IV) determ ination based on the Co(IT) autoxidation. (C) 1997 Elsevier Science B. V.