MECHANISMS OF REDUCTION OF CHLOROAQUOCHROMIUM(III) IONS IN ACID MEDIA

Abstract

The polarographic behavior of the hexaaquochromium(III) and dichlorotetraaquochromium(III) ions was observed in the absence of maxima suppressors, and in the presence of various concentrations of perchloric acid, hydrochloric acid, and neutral salt. The polarographic waves obtained for each Cr(III) species in the different media were analyzed by plotting the function log[i/(id − i)] vs. E; the plots generally consist of two straight lines intersecting at about the half-wave potential and can best be explained by assuming two different concurrent mechanisms for the electron transfer process: the direct transfer of an electron to the Cr(III) ion from the electrode at the mercury–solution interface, and the transfer of an electron to Cr(III) from a Cr(II) ion produced at that interface. Only for the dichlorotetraaquochromium(III) ion in 12 M HCl does the direct transfer process appear to be dominant. Polarography of Cr(III) in the latter medium before and after reaction with the medium shows no change in [Formula: see text] corresponding to the shift observed in the ultraviolet spectra; this is accounted for by the "catalytic" degradation, i.e., the loss of chloride of the higher chloro complexes due to the rapid anion transfer mechanism for the electron transfer process between halo Cr(III) and Cr(II) ions.