A kinetic study of the reduction of chromium(VI) to chromium(III) by thiourea

The reduction of chromium(VI) to chromium(III) by thiourea is a key aspect of the chromium(VI)–thiourea–polyacrylamide gel polymer system used in oil recovery processes. A study was undertaken to develop a kinetic model for the reduction reaction. Reaction mixtures were prepared and chromium(VI) concentrations were determined from the mixturesʹs visible absorbance. The reaction rate was found to depend on the concentrations of chromium(VI), thiourea, and polyacrylamide and on solution pH. A kinetic model for the reactions was developed using the experimental data and previously proposed reaction mechanisms. The model accurately predicts the reaction rate in solutions that do not contain polyacrylamide and predicts a rate that is approximately correct for solutions containing polyacrylamide.