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

• Published in: Inorganica Chimica Acta Vol. 331; no. 1; pp. 322 - 329
• Main Authors: Cruywagen, J.J, Draaijer, A.G, Heyns, J.B.B, Rohwer, E.A
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 28.03.2002
• Subjects: Equilibrium constants; Molybdenum(VI) equilibria; Molybdenum(VI) polyoxoanions; Thermodynamic quantities; Thermodynamic quantities; Equilibrium constants; Molybdenum(VI) polyoxoanions; Molybdenum(VI) equilibria
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(02)00700-4

Molybdenum(VI) equilibria have been investigated in LiClO 4 (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(ClO 4) and 2.0 M Na(ClO 4) 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 [H 2Mo 7O 24] 4− and [H 3Mo 7O 24] 3− are not included. In 1.0 M NaCl at 25 °C the formation constants of the polyoxoanions, denoted by β pq , where the subscripts p and q refer to the stoichiometric coefficients of [MoO 4] 2− and H + in the reactions, have the values: log β 78=52.86, log β 79=57.25, log β 68=49.53, log β 8,11=67.90, log β 8,12=71.52, log β 13,21=119.49 and log β 18,32=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 Δ H 78=−258, Δ H 79=−248, Δ H 68=−218, Δ H 8,11=−275, Δ H 8,12=−285, Δ H 13,21=−508 and Δ H 18,32=−782 kJ mol −1. Molybdenum(VI) equilibria have been investigated by potentiometry, 95Mo NMR and calorimetry. The proposed reaction model agrees with some previously published models in that the well-characterized [Mo 7O 24] 6− and [Mo 8O 26] 4− ions are included but differs by the absence of [H 2Mo 7O 24] 4− and [H 3Mo 7O 24] 3−. An unexpected polyanion of (10, 10) proton-molybdate stoichiometry has been identified at high molybdate concentration. Equilibrium constants are reported for different ionic media. Enthalpy and entropy changes for the formation of some polyanions in 1.0 M NaCl medium have been determined.