NMR determination of the equilibrium constant for the liquid H2O–D2O mixture

The experimental analysis of the equilibrium between H2O, D2O and HOD in the liquid phase by Nuclear Magnetic Resonance (NMR) spectroscopy is tricky because of the difficulty in observing the HOD spectrum. This problem is primarily due to chemical exchange effects. Thus very few NMR methods have been suggested to study this equilibrium. A technique for studying it and determining the equilibrium constant K, is described. A direct measurement of K is carried out, without using additives, which could change the nature of the equilibrium. The described method is designed to measure the scalar coupling constant. A simple proton NMR experiment is recorded and the experimental data is used by a reconstitution program to generate a set of new signals, which are analyzed in the time-domain to obtain a resulting spectrum, where the HOD can be observed as well as the line of H2O. The relative areas corresponding to these multiplets are measured and the equilibrium constant K is determined. It was found to be K = 3.86 ± 0.01 at 298 °K, a value which perfectly agrees with those determined by theoretical calculations and other experimental techniques.