The gelling process in the aqueous lanthanum-cobalt-citric acid system was studied using UV-VIS and IR spectroscopic methods, as well as viscosity measurements, to establish the gelling mechanism of transition metal ions (La3+ and Co2+) in the presence of chelating agent. Lanthanum and cobalt nitrates and lanthanum and cobalt acetates, respectively, were used as starting reagents, and the citric acid was used as chelating agent. The gelation process was investigated at room temperature and at 80°C. At elevated temperature, the gels were obtained after 30 h, whereas at room temperature, these were obtained in 1 month. Complex gels were formed for the both studied systems. The Co2+ ions mainly present an octahedral symmetry during the whole gelation process, whereas the citric acid acts as monodentate or bridging ligand depending on the metal precursors used. The mechanism of formation of the La- Co-CA based gels with the coordinative structures was proposed.
L. Predoana, A. Jitianu, B. Malic, M. Zaharescu “Study of the gelling process in the La-Co-citric acid system "Journal of American Ceramic Society, 95 (3), 2012, 1068-1076.