FORMATION OF CALCIUM AND MAGNESIUM MOLYBDATE COMPLEXES IN DILUTE AQUEOUS-SOLUTIONS

In order to increase our understanding of the chemistry of Mo in soil, waste-affected soil, and waste-disposal environments, it is imperative to understand the processes controlling the aqueous and solid-phase distribution of Mo. Recent research shows the importance of alkaline earth metal ion pairs with molybdate in aqueous solutions. However, ion association constants for these species have not been rigorously determined. The objectives of this study were to measure conditional ion association constants (log (c)Q) for the formation of Ca and Mg molybdate ion pairs. Potentiometric titrations were performed at pH 7 and 25-degrees-C in NaNO3 ionic media. The log(c)Q values were converted to the zero-ionic-strength molar reference state (log Q) using the Davies equation. The mean log Q and standard free energy of formation (DELTAG(t)-degrees) values for the formation of CaMoO40 are 2.57 +/- 0.08 and - 1405.19 +/- 1.60 kJ mol-1, respectively. The mean log Q and AG, values for the formation of MgMoO40 are 3.03 +/- 0.13 and - 1309.08 +/- 206 kJ mol-1, respectively.