Determination of the resonance frequency ratio of the 31P and 1H NMR resonance shift standards 85% orthophosphoric acid and tetramethylsilane

Abstract The NMR resonance frequency ratio of the 31 P NMR resonance shift standard 85% orthophosphoric acid (H 3 PO 4 ) and the 1 H standard tetramethylsilane (TMS) was determined by means of the ‘dispersed phase’ method. The Ξ value, equal to 10 8 times the resonance frequency ratio, was 40480737.0 ± 0.2 Hz at 25°C. A small dependence on temperature (−0.11 Hz °C −1 ) and on phosphoric acid concentration (−0.50 Hz % −1 ) was observed around 25°C. Traces of iron in phosphoric acid were found to have a negligible effect on Ξ values, although line widths increased. The determination of Ξ allows accurate referencing of 31 P NMR spectra relative to the 1 H resonance of internal TMS or of a secondary 1 H shift standard. The accuracy of the 31 P NMR resonance shift scale is primarily limited by the uncertainty of solvent‐induced 1 H resonance shifts of TMS.