The stability of sodium complexes with poly-carboxylic and polyamino-carboxylic acids is investigated with ion-selective electrode-Na+ potentiometry, working at strictly constant ionic strength. It is observed that the formation constants of the Na+ complexes with monoligand stoichiometry (ML) increase with the number of charges on the ligand. For example, in poly-carboxylic acids this dependency is linear and is well captured by an experimetal equation. A different behaviour is observed for the poly-amino carboxylic acids, which show higher complexation capabilities reaching a plateau of the binding energy past a specific ligand size. The experimental results are discussed qualitatively using ab initio calculations based on DFT B3LYP, and identifiy the principal electronic caractheristics of the ligands under investigation. As a result of the flexibility imparted by the long chains of polyamino-carboxylic ligands, both experimental and theorethical models demonstrate that nitrogen atoms in proximity of Na+ ions can participate to the metal coordination, thus providing further stabilization for the complexes. Moreover, by increasing the ligand size the stabilization gained in terms of ΔG reached a plateau for EDTA, in agreement with experimental observations.

Assessing the formation of weak sodium complexes with negatively charged ligands

BERTO, Silvia;CHIAVAZZA, ENRICO;PRENESTI, Enrico;DANIELE, Pier Giuseppe
2016

Abstract

The stability of sodium complexes with poly-carboxylic and polyamino-carboxylic acids is investigated with ion-selective electrode-Na+ potentiometry, working at strictly constant ionic strength. It is observed that the formation constants of the Na+ complexes with monoligand stoichiometry (ML) increase with the number of charges on the ligand. For example, in poly-carboxylic acids this dependency is linear and is well captured by an experimetal equation. A different behaviour is observed for the poly-amino carboxylic acids, which show higher complexation capabilities reaching a plateau of the binding energy past a specific ligand size. The experimental results are discussed qualitatively using ab initio calculations based on DFT B3LYP, and identifiy the principal electronic caractheristics of the ligands under investigation. As a result of the flexibility imparted by the long chains of polyamino-carboxylic ligands, both experimental and theorethical models demonstrate that nitrogen atoms in proximity of Na+ ions can participate to the metal coordination, thus providing further stabilization for the complexes. Moreover, by increasing the ligand size the stabilization gained in terms of ΔG reached a plateau for EDTA, in agreement with experimental observations.
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http://pubs.rsc.org/en/content/articlepdf/2016/cp/c6cp00192k
sodium complexes, poly-carboxylic acid, stability constants, potentiometry, DFT
Berto, Silvia; Chiavazza, Enrico; Canepa, Pieremanuele; Prenesti, Enrico; Daniele, Pier Giuseppe
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2318/1561022
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