Speciation of Molybdenum(VI)-Citric Acid Complexes in Aqueous Solutions

Abstract

The chemistry of molybdenum(VI) (Mo 6+ ) encounters very complex pathways even when reacting with the simplest of ligands (the aqua ligand). Citric acid (Cit) is considered to be a simple organic ligand. From our efforts to study citric acid with a variety of metal ions (Hamada et al. 2003 [41] and Hamada et al. 2006) a di-hydrolytic complex (or dihydroxo complex) of the Mo 6 + : Cit system has been observed using both a speciation computer program and potentiometric titrations in aqueous solutions at 25°C. The speciation diagrams show that the percentage of formation of this di-hydrolytic complex species overshadows the percentage of formation of the individual free citric acid species. We have taken into account the presence of the following species: the mononuclear species Mo(H −1 Cit), Mo(H −1 Cit)(OH), and Mo(H −1 Cit)(OH) 2 , and the di-nuclear Mo 2 (H −1 Cit)(OH) 2 complex. Among all complexes taken into account, only the di-hydrolytic complex Mo(H −1 Cit)(OH) 2 has been detected in appreciable percentages. The UV-vis titrations performed at different pH values are in a good agreement with the chemistry literature. Further experimental and theoretical studies are underway in this area.