Formation of Calcium and Magnesium Molybdate Complexes in Dilute Aqueous Solutions

Abstract

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 ^cQ) for the formation of Ca and Mg molybdate ion pairs. Potentiometric titrations were performed at pH 7 and 25 °C in NaNO₃ ionic media. The log^cQ 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 (ΔG^o_f) values for the formation of CaMoO⁰₄ are 2.57 ± 0.08 and −1405.19 ± 1.60 kJ mol⁻¹, respectively. The mean log Q and ΔG^o_f values for the formation of MgMoO⁰₄ are 3.03 ± 0.13 and −1309.08 ± 2.06 kJ mol⁻¹, respectively.