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

The NMR transverse relaxation time T-2, 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 T-2 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.