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

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).