Carboxyl groups of citric acid in the process of complex formation with bivalent and trivalent metal ions in biological systems.

Binary complexes of citric acid (H3L - protonated form, H2L and HL - partly protonated forms, L - fully deprotonated) with d- and f-electron metal ions were investigated. The studies have been performed in aqueous solution using the potentiometric method with computer analysis of the data, electron paramagnetic resonance, infrared, visible as well as luminescence spectroscopies. The overall stability constants of the complexes were determined. Analysis of the equilibrium constants of the reactions and spectroscopic data has allowed determination of the type of coordination and effectiveness of the carboxyl groups in the process of complex formation. On the basis of potentiometric titration for d-electron were found dimeric and monomeric type of complexes and for f-electron four type of complexes: MHL, ML, ML(OH) and ML(OH)2.