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

Abstract

The reduction of K 2 Cr 2 O 7 solutions by H 2 O 2 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/T 1 and 1/T 2 of the solutions increase during the reduction of diamagnetic Cr(VI). This increase is proportional to the produced Cr(III) concentration. Using different initial H 2 O 2 concentrations, partially reduced Cr(VI) samples were prepared and studied by T 1 and T 2 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 measure-