A fully automated pH–NMR titration set-up for protonation studies

Abstract

A fully automated pH–NMR titration set-up, consisting of a Bruker 250 MHz NMR instrument and a potentiometric titration unit, has been built so that pH titrations with simultaneous recording of ¹H and ³¹P NMR spectra at each titration point can be run. The new set-up has been tested by studying the protonation of three diazacrown ethers having dangling phosphonate groups. From the fitting of the pH dependences of the chemical shifts of the ³¹P and ¹H signals the protonation constants as well as the chemical shifts of the individual protonated species were obtained using the program HYPNMR. The main advantages of the new set-up are the relatively small amount of substance (0.05 mmol) needed for a single titration and the fact that once started the system needs no operator during the whole titration time (about 20 h).