Redox and complexation chemistry of the CrVI/CrV–d-galacturonic acid system

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 Cr^VI, 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 ≤ 5 with the uronic acid bound to Cr^V through the carboxylate and the α-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 ≤ 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)(α-hydroxy acid)₂]⁻. The lack of chelation at any vic-diolate group of Galur when pH ≤ 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.