COMPLEX-FORMATION OF TRACE-ELEMENTS IN GEOCHEMICAL SYSTEMS .1. POTENTIOMETRIC STUDY OF FLUORO COMPLEXES OF RARE-EARTH ELEMENTS IN FLUORITE BEARING MODEL SYSTEMS

With regard to the assumption that thermodynamic processes such as complex formation and distribution equilibria of the rare earth elements (REE) are responsible for their fractionation during the mineralization of fluorites, the formation of the fluoro complexes of the REE has been studied. The investigation was carried out in 1 M NaCl-aqueous medium at 25°C and pH-rang ≤ 5. From all the ligands to be considered at this pH-range only SO42- participate to the complex formation. Within the REE-series the stability constant of the monofluoro complexes has a maximum at Tb and the decrease Tb to La is more than its decrease Tb to Lu. The stability constant of difluoro complexes, on the contrary, increases gradually from La to Lu. © 1979.