The neutral oxygen donor in complexes of lead and cadmium: A differential pulse polarographic, potentiometric and calorimetric study

Abstract

Differential pulse polarography has been used to determine the formation constants of Pb II with ethylenediamine (en) at ionic strength 0.1 M and 5, 25 and 50°C. The equilibria between Pb 2+ and the ligands en and OH − are established rapidly on the polarographic time scale for all complexes studied, so that one peak is observed, that for the metal ion and its labile complexes. At 5°C the following complexes were found: M(HL), ML, ML 2 and ML 2(OH), for which the stability constants (logβ) were found to be 12.97, 5.62, 9.70 and 13.45, respectively. At 25 and 50°C, the complexes ML, ML 2, ML 2(OH) and ML 2(OH) 2 were found and their stability constants were found to be 5.05 and 4.43, 8.67 and 7.37, 12.6 and 11.75, 15.29 and 14.8, respectively. The Lingane equation was modified to allow calculation of formation constants for labile complexes of lead with the two ligands, en and OH −. The two stepwise protonation constants of en at an ionic strength of 0.1 M and temperatures of 5 and 50°C are reported and their values were found to be log K 1 = 10.44 and 8.98 and log K 2 = 7.39 and 6.53, respectively. From the polarographic data, thermodynamic quantities were derived for the overall complex formation reactions of Pb II with en and the values of ΔH° (kJ mol −1) for ML, ML 2, ML 2(OH) and ML 2(OH) 2 were found to be −44.7, −90.6, −65.7 and −39.1, respectively. The stepwise protonation constants of N, N, N′, N′-tetrakis(2-hydroxypropyl)ethylenediamine (THPED) at an ionic strength of 0.5 M and 25°C are reported and their values (log K) wer found to be 8.85 and 4.38. At 25°C and an ionic strength of 0.5 M, the complexes ML, M(HL) and ML(OH) of Cd II and Pb II with THPED were found and their cumulative stability constants (log β) were found to be 7.98 and 7.51, 11.75 and 11.3, 11.14 and 13.00, respectively. The values of ΔH° (kJ mol −1) for ML complexes of Cd II and Pb II with THPED were found to be −48.0. Values of ΔG°, ΔH° and ΔS° for the ligand displacement reaction in which en is displaced by THPED in ML complexes of Pb II and Cd II are also reported and discussed. It is suggested that the coordination numbers of Pb IIin the complexes Pb(en) 2+ and Pb(THPED) 2+ in solution probably differ.