Nuclear magnetic resonance relaxometry to monitor chromium (VI) reduction by hydrogen peroxide, ascorbic acid, and aluminum powder

Abstract

AbstractThe reduction of K2Cr2O7 solutions by H2O2 was studied by nuclear magnetic resonance (NMR) relaxometry and UV–vis spectroscopy in HCl/KCl buffer (pH 2), NaCl/glycine/HCl buffer (pH 3), and sodium acetate/acetic acid buffer (pH 4). Because of Cr(III) paramagnetism, 1/T1 and 1/T2 of the solutions increase during the reduction of diamagnetic Cr(VI). This increase is proportional to the produced Cr(III) concentration. Using different initial H2O2 concentrations, partially reduced Cr(VI) samples were prepared and studied by T1 and T2 relaxometry and by UV–vis spectroscopy. The correlation between the relaxation rates and the concentration of Cr(VI) remaining in the sample, measured by spectroscopy, was excellent. It was possible, thanks to the measurement of T2, to study the kinetics of the reduction of K2Cr2O7 by H2O2 in the pH 3 and pH 4 buffers. The reduction of Cr(VI) by ascorbic acid was successfully monitored by NMR relaxometry in the pH 2 buffer. The presence of complexing molecules/ions was shown to drastically influence the nuclear magnetic relaxation dispersion profiles of reduced K2Cr2O7 solutions: Both relaxation rates are divided by ~5 when citrate or acetate ions are present and by ~3 in the presence of ascorbic acid. Therefore, the comparison of relaxation results obtained in different reaction mixtures must be done carefully. When all the solutions are set to pH 0, which prevents any complexation, the longitudinal and transverse relaxation rates of all samples become comparable. Finally, as a proof of concept for a turbid solution, the kinetics of the reduction of a K2Cr2O7 solution by aluminum powder in the pH 2 buffer was successfully monitored.