Molybdenum(VI) equilibria in different ionic media. Formation constants and thermodynamic quantities

Molybdenum(VI) equilibria have been investigated in LiClO4 (2.0 M) and NaCl (1.0 and 2.0 M) media by potentiometry, molybdenum-95 NMR and calorimetry. In the case of 2.0 M Li(ClO4) and 2.0 M Na(ClO4) media the molybdenum concentration was 1.0 M and the pH range covered was 2–7.5. For the potentiometric measurements in 1.0 M NaCl the molybdenum concentration was varied from 0.1–5.0×10−4 M and the pH range covered 1.0–6.5. Reaction models, based on computer treatment of the potentiometric data, have been established which are consistent with NMR information. These reaction models differ from most of those previously published in that the protonated heptamolydates [H2Mo7O24]4− and [H3Mo7O24]3− are not included. In 1.0 M NaCl at 25 °C the formation constants of the polyoxoanions, denoted by βpq, where the subscripts p and q refer to the stoichiometric coefficients of [MoO4]2− and H+ in the reactions, have the values: log β78=52.86, log β79=57.25, log β68=49.53, log β8,11=67.90, log β8,12=71.52, log β13,21=119.49 and log β18,32=171.43. Enthalpy and entropy changes for the formation of the polyions were calculated from the calorimetric data using these constants. The enthalpy values are ΔH78=−258, ΔH79=−248, ΔH68=−218, ΔH8,11=−275, ΔH8,12=−285, ΔH13,21=−508 and ΔH18,32=−782 kJ mol−1.