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Abstract
In this work, complexation between lead ion and the ligands 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) and N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid (AMPSO), which are commercial pH buffers, is presented. Both ligands form complexes with lead in their pH buffer range (between pH 6.5 and 8.5 for DIPSO and between pH 8.0 and 9.0 for AMPSO). The final models and the overall stability constants, which are reported here, were determined by direct current polarography and glass electrode potentiometry [only for the Pb–(DIPSO)x–(OH)y system] at 25.0 °C and 0.1 M KNO3 ionic strength. For the Pb–(DIPSO)x–(OH)y system, the proposed final model contains PbL, PbL2, PbL2(OH), and PbL2(OH)2 with stability constants, as log β, of 3.4 ± 0.1, 6.35 ± 0.15, 12.8 ± 0.2, and 18.0 ± 0.3, respectively. For the Pb–(AMPSO)x–(OH)y system, the species observed are PbL, PbL(OH), and PbL(OH)2 with stability constants, as log β, of 2.9 ± 0.5, 9.4 ± 0.1, and 14.5 ± 0.2, respectively. For AMPSO, the possible adsorption of the ligand at the mercury electrode surface was evaluated by alternating current polarography through calculation of the capacitance of the double layer.
Acknowledgments
The authors thank Professor Ignacy Cukrowski from the University of Pretoria (South Africa) for polarographic modeling software (3D-CFC program) and Professor Carlos Gomes from the Faculty of Sciences/Porto University for the COPOTISY program.
Funding
This work has been supported by Fundação para a Ciência e a Tecnologia (FCT), from the Portuguese Government, through [grant number PEst-C/EQB/LA0006/2011]; Isabel Pinto and Georgina Alves acknowledge their grant scholarships [SFRH/BD/70450/2010 and SFRH/BD/46521/2008, respectively] financed by FCT.