From proton nuclear magnetic resonance spectra to pH. Assessment of 1H NMR pH indicator compound set for deuterium oxide solutions

In this study, a protocol for pH determination from D2O samples using 1H NMR pH indicator compounds was developed and assessed by exploring the pH-dependency of 13 compounds giving pH-dependent 1H NMR signals. The indicators cover the pH range from pH* 0 to 7.2. Equations to transform the indicator chemical shifts to pH estimates are given here for acetic acid, formic acid, chloroacetic acid, dichloroacetic acid, creatine, creatinine, glycine, histidine, 1,2,4-triazole, and TSP (2,2,3,3-tetradeutero-3-(trimethylsilyl)-propionic acid). To characterize the method in presence of typical solutes, the effects of common metabolites, albumin and ionic strength were also evaluated. For the ionic strengths, the effects were also modelled. The experiments showed that the use of pH sensitive 1H NMR chemical shifts allows the pH determination of typical metabolite solutions with accuracy of 0.01–0.05 pH units. Also, when the ionic strength is known with accuracy better than 0.1 mol dm−3 and the solute concentrations are low, pHnmr* (the NMR estimate of pH) can be assumed to be within 0.05 pH units from potentiometrically determined pH.