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Abstract
This paper describes an experimental investigation in aqueous solution of the equilibria between gallium (III) metal ions and acetate ions as a model of low-molecular-weight ligand using potentiometric techniques under physiological conditions of temperature 37 °C and ionic strength 0.15 mol.dm-3 NaCl. Potentiometric measurements were monitored with the help of a glass electrode calibrated daily in hydrogen ions concentrations. Several metal to ligand ratios were used and the respective titrations data obtained in the pH range of 2.3-5.3 were treated using the SUPERQUAD computer program to determine global stability constants. Different species were considered during the calculation procedure and the following hydroxides have been characterized: , and coexist with one mononuclear complex Ga(CH3COO)+2. Speciation calculations based on the determined constants were then used to simulate the species distribution as a function of pH.
Highlights
Gallium is a semimetal that occupies the third position of Group 13 of the Periodic Table
In aqueous solution gallium is more stable in its 3+ oxidation state and is classified according to Pearson [1] as a hard Lewis acid, giving rise to thermodynamically stable complexes with ligands that are hard Lewis bases
The important shifts observed for increased gallium ions concentrations, which reflect the release of protons from complexing ligand and/or water molecules, suggest the formation of stable gallium acetate complexes and/or metal ion hydroxides
Summary
Gallium is a semimetal that occupies the third position of Group 13 of the Periodic Table. The coordination chemistry of gallium (III) ions has been largely studied in the last decades [3,4,5,6,7,8,9] where many ligands with oxygen and/or nitrogen donor atoms (such as carboxylate, phosphonate, phenolate, hydroxamate and amine groups) were used as chelating agents. Due to the analogy with Fe(III) in terms of electronegativity and ionic radius [10], electron affinity [1], and coordination geometry, Ga(III) is considered as a good mimetic of Fe(III) [11]. These properties suggest that Ga(III) ion follows biochemical routes similar to those found in iron metabolism. In the United States, this metal ion was approved by the FDA for i.v. administration and is used in its 67Ga(III) salts or citrate complex to diagnose tumors [14]
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