Quantification of H2O2 concentrations in aqueous solutions by means of combined NMR and pH measurements

Abstract

The NMR transverse relaxation time T₂, determined by a CPMG multipulse sequence, of aqueous hydrogen peroxide (H₂O₂) solutions strongly depends on the rate of exchange of the spin-bearing protons between the H₂O₂ and H₂O molecules. For pulse separations exceeding the inverse exchange rate, this value becomes a constant only depending on proton exchange time and magnetic field strength. Since this exchange time depends in a non-analytical way on the concentration of H₂O₂ and on the pH value, a measurement of T₂ and pH allows the inversion of the data for the non-invasive determination of the H₂O₂ concentration. The generation of calibration data for a range of concentrations and pH values is presented and is applied to a heterogeneously catalyzed H₂O₂ decomposition involving commercial catalyst pellets. The evolution of the concentration during the course of reactions is monitored and the long-time deactivation of the pellet is demonstrated using this technique. The method is suggested as a means for contact-free and non-invasive on-line monitoring of H₂O₂ concentrations.