Reduction of potassium chromate by tannins

Abstract

The reduction of potassium chromate has been carried out with several tannin substances, including gallic acid, pyrogallol, ethyl gallate, polygalloylglucose, and withered oak-leaf tannin. Each reaction was followed by colorimetry, EPR spectroscopy, electrochemistry, and molecular-sieve chromatography. The progress of the chromate reductions was monitored by measuring the VIS and EPR spectra as a function of time. The observed preudo-first-order reaction rate constants for Cr(III) formation were derived based on the intensities of the Cr(III) signals of the EPR spectra. Cyclic voltammograms of the simple reductants and their final Cr(III)-containing products formed from the reductions of chromate have also been measured. The formation of Cr(III)-tannin complexes was indicated by separation on a Sephadex G-25 column. The results obtained from VIS, EPR, and cyclic voltammetry were found to be mutually dependent, and showed close correlations among the data. The rate of reduction for Cr(VI) revealed the following trend: gallic acid > pyrogallol > ethyl gallate > polygalloylglucose > witheredoak-leaf tannin. The results obtained in the present study suggest that the rates of reduction for Cr(VI) in the tannins depend upon their peak oxidation potentials and stabilities for Cr(III)-tannin complexes.