Untersuchungen zur Anwendbarkeit der Ionenchromatographie für die Bestimmung vonStabilitätskonstanten von Komplexverbindungen mit dreiwertigen Metallionen

In this work the species distributions of metal ions (chromium, iron, gallium, indium and scandium) in the oxidation state +III are studied in the presence of different organic ligands, like biogenic amines, amino acids, oxalate, and ethylenediamine tetraacetate (EDTA). Aqueous nitrate solutions with an acidic pH (3-4) are chosen as matrix. The separation of the charged species is performed by ion exchange chromatography, and for detecting the metal species an ICP-AES is used. A new method is developed with which the stability constants of one complex ion can be calculated from the measured species distributions. As a model system, the combination of a trivalent metal ion and the ligand EDTA is chosen. The hexa-dentate ligand EDTA, together with the metal ions (Me), forms a [MeEDTA]−-complex. Due to the high stability of the complexes (logK for the tested metal complexes are between 23.4-27.5) not all concentrations at equilibrium can be measured. Therefore, models are investigated with equimolar amounts of two metal ions that act as competitors for ligand ions. From the law of mass action an equation can be deduced with which the stability constant of a [MeEDTA]−-complex can be determined, when the stability constant of the second complex is known. An advantage of this method is that the factors (pH, ionic strength) are included in the calculation because of the formation of a ratio of the stability constants. In particular, a consideration is that the surfaces of the total integrals of the metal ions are functions of the concentration of ligand in the solutions. Free metal ions are bound by adsorption on the polymer surface of the PE tubes. The model solutions, containing iron(III) ions, show a photolytic degradation of the [FeEDTA]−-complex. Therefore, the model solutions should be stored in opaque HDPE containers and it is necessary to measure an acidified metal standard for the determination of the total integral. The ions apparant in the model system can be separated by anion- and cation-IC, thus offering the possibility of determine the stability constant with two independent systems to verify the results.