NMR-Based Determination of pH, Free of Electrodes and Reference Compounds.

A novel method was elaborated, and a set of 1H NMR-pH indicator molecules was selected to develop an NMR-based pH determination method, which does not need any glass or other electrodes or any separate reference compound, being thus devoid of the usual acid and base errors of the glass and combined electrodes. The method utilizes organic compounds of accurately known basicity and two or more carbon-bound protons, whose chemical shifts respond differently to pH changes. Accurate determination of protonation constants in pH extrema was achieved by the method of relative basicities using compounds of sequentially incremented protonation constants. The entire pH scale can be covered by six indicator molecules. The method was developed for two ionic strengths, 1.00 and 0.15 M.