PULSE POLAROGRAPHY STUDY OF THE COMPLEXES OF LEAD WITH AZACROWN [2,2,2]CRYPTAND IN THE PRESENCE OF AN EXCESS OF COMPETING SODIUM-ION

Differential pulse polarography (DPP) has been used to determine the f ormation constants of Pb-II at ionic strength 0.5 M and 25 degrees C w ith [2,2,2]cryptand in the presence of 0.5 M sodium nitrate. The equil ibrium between Pb2+ and the ligand is established slowly on the polaro graphic time scale, so that two well-separated peaks are observed, tha t for the metal ion and for a nonlabile ML complex for which the stabi lity constant was found to be log K-1 = 12.9. The equilibrium between the ML complex and hydroxide ion appeared to be fast and a substantial shift in the peak potential of the ML complex was observed. Two labil e lead complexes are reported, namely ML(OH) and ML(OH)(2) for which s tability constants (log beta) were found to be 20.51 and 23.75, respec tively. The Lingane equation has been modified to allow calculation of formation constants for labile complexes of lead with two ligands, [2 ,2,2]cryptand and OH-. A single, second, overall protonation constant of [2,2,2]cryptand at an ionic strength of 0.5 M and 25 degrees C in t he presence of 0.5 M Na+ is reported and its value was found to be log beta(2), = 18.