Redox and complexation chemistry of the Cr-VI/Cr-V-D-galacturonic acid system

Abstract

The oxidation of D-galacturonic acid by Cr-VI yields the aldaric acid and Cr-III as final products when a 30-times or higher excess of the uronic acid over Cr-VI is used. The redox reaction involves the formation of intermediate Cr-IV and Cr-V species, with Cr-VI and the two intermediate species reacting with galacturonic acid at comparable rates. The rate of disappearance of CrVI, Cr-IV and Cr-V depends on pH and [substrate], and the slow reaction step of the Cr-VI to Cr-III conversion depends on the reaction conditions. The EPR spectra show that five-coordinate oxo-Cr-V bischelates are formed at pH less than or equal to 5 with the uronic acid bound to Cr-V through the carboxylate and the alpha-OH group of the furanose form or the ring oxygen of the pyranose form. Six-coordinated oxo-Cr-V monochelates are observed as minor species in addition to the major five-coordinated oxo-Cr-V bischelates only for galacturonic acid : Cr-VI ratio less than or equal to 10 : 1, in 0.25-0.50 M HClO4. At pH 7.5 the EPR spectra show the formation of a Cr-V complex where the vic-diol groups of Galur participate in the bonding to Cr-V. At pH 3-5 the Galur-Cr-V species grow and decay over short periods in a similar way to that observed for [Cr(O)(alpha-hydroxy acid)(2)](-). The lack of chelation at any vic-diolate group of Galur when pH less than or equal to 5 differentiates its ability to stabilise Cr-V from that of neutral saccharides that form very stable oxo-Cr-V(diolato)(2) species at pH > 1.