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 x 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 degreesC the formation constants of the polyoxoanions, denoted by beta(pq), where the subscripts p and q refer to the stoichiometric coefficients of [MoO4](2-) and H- in the reactions. have the values: log beta(78) = 52.86. log beta(79) = 57.25, log beta(68) = 49.53, log beta(8.11) = 67.90. log beta(8.12) = 71.52, log beta(13.21) = 119.49 and log beta(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 DeltaH(78) = -258, DeltaH(79) = -248, DeltaH(68) = -218, DeltaH(8.11) = -275. DeltaH(8.12) = -285, DeltaH(13.21) = -508 and DeltaH(18.32) = -782 kJ mol(-1). (C) 2002 Published by Elsevier Science B.V.