Electrodeless, accurate pH determination in highly basic media using a new set of (1)H NMR pH indicators

A set of indicator molecules was selected and applied to elaborate an NMR-based pH determination method, free of glass electrode errors in highly basic media. Accurate measurement of pH values and protonation constants was achieved by a successive build-up of overlapping, increasingly high pH solutions, using a collection of 8 compounds of appropriately incremented basicities. In order to verify the method, acid-base properties were quantified for two compounds with very high basicities in conflicting reports: two pharmaceutically important biguanidine drugs, metformin and phenformin.