Deuterium isotope effect in concentrated aqueous solutions

The acid dissociation constants have been determined by potentiometric titration in H2O and D2O for the zinc(II) complexes of 2,2'-ethylenedi-iminobis(ethylamine), 2,2'-trimethylenedi-iminobis(ethylamine) and 2,2',2''-nitrilotris(ethylamine) and by 13C n.m.r. spectroscopy in D2O for ethylenediamine, glycine, L-alanine, acetic acid and N-2-aminoethyl-propane-1,3-diamine. The equilibrium deuterium isotope effects are similar, pKsub(a)(D2O)-pKsub(a)(H2O) = 0.63 +- 0.07. This result supports the treatment of the protonated forms of polyamines as 'hybrid' ions in respect of their contribution to ionic strength and indicates that the coordination number of Znsup(II) in the low-pH forms of the three complexes is at least five. (author)