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 formation constants of Pb-II at ionic strength 0.5 M and 25 degrees C with [2,2,2]cryptand in the presence of 0.5 M sodium nitrate. The equilibrium between Pb2+ and the ligand is established slowly on the polarographic time scale, so that two well-separated peaks are observed, that for the metal ion and for a nonlabile ML complex for which the stability 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 labile lead complexes are reported, namely ML(OH) and ML(OH)(2) for which stability constants (log beta) were found to be 20.51 and 23.75, respectively. 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 the presence of 0.5 M Na+ is reported and its value was found to be log beta(2), = 18.