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.

Keywords:

• aqueous solutions
• citrate
• Mo^{6 +}
• potentiometric titrations
• speciation diagram

Acknowledgments

This work was supported in part from NSF under Grant #HRD- 0411493. We also thank many faculty members, especially Dr. S. Painter at the division of Natural and Mathematical Sciences of LeMoyne-Owen College, for reading the manuscript.

Notes

^a The number of equivalents (Eq.) is defined as the number of millimoles of titrant per number of millimoles of Mo^{6 +}.

^b Each experiment has been repeated three times or more. The corresponding standard deviation of all runs is shown in a separate column.