Equilibrium conditions of ytterbium/III/ citrate complexes

In order to be able to prepare 169Yb citrate complex (which is a tumour-finder material) it was necessary to study the equilibrium in the system of ytterbium(III) and citric acid and to determine the stability constants of the formed complexes. On the basis of experimental data on pH-titrimetric investigations and results on equilibrium calculations it has been established, that the Yb(III) forms protonated and non-protonated carboxylate- and alcoholate-type complexes having different (1:1 and 1:2) composition and different metal/ligand bond. Their relative amounts depend on the pH of the medium and their coexistence range is around the physiological pH value (pH=7.4). (K.A.)