Molybdenum(VI) equilibria in different ionic media. Formation constants and thermodynamic quantities
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 [Mo7O24]6− and [Mo8O26]4− ions are included but differs by the absence of [H2Mo7O24]4− and [H3Mo7O24]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.
Introduction
Most of the previous work on molybdenum(VI) equilibria has been summarized in a recent review [1]. At very low concentrations (<10−4 M) mononuclear species predominate and equilibrium constants for the formation of [HMoO4]−, MoO3(H2O)3, and [MoO2(OH)(H2O)3]+ have been determined under different conditions. At higher concentrations numerous investigations have shown the existence of the heptamolybdate [Mo7O24]6− and the octamolybdate [Mo8O26]4− ions in aqueous solution. For the determination of equilibrium constants of the various polyoxoanions in solution, accurate pH measurements with computer treatment of the data proved to be one of the best methods. In the case of good data, the determination of a reaction model that gives the best fit between calculated and experimental points usually serves as a reliable method for the identification of at least all the major species in the solution. Equilibrium constants obtained by different research groups under the same conditions mostly agree remarkably well, e.g. the values for the formation of [Mo7O24]6− in 3.0 M NaClO4 are log β78=57.74 [2] and 57.70 [3], [3](a), [3](b) and in 1.0 M NaCl log β78=52.80 [4] and 52.79 [5], [5](a), [5](b). Although such results show that the proposed reaction models are based on reliable and reproducible data, differences among these models reveal that an unambiguous selection of species is not always possible. The [Mo7O24]6− ion, its protonated forms and the octamolybdate ion [Mo8O26]4− are included in most models. In some cases, [H3Mo7O24]3− is preferred to [Mo8O26]4−, but in some models both these polyanions are included. A recent 95Mo and 17O NMR study of aqueous Li2MoO4 at high concentrations confirmed the existence of [Mo7O24]6− and β-[Mo8O26]4− but only the monoprotonated form of heptamolybdate [HMo7O24]5− was found to occur [6]. The latter species, rather than being further protonated, is converted into a different polyanion, probably another octamolybdate, [H3Mo8O28]5−. These results cast doubt on well-established reaction models as far as the inclusion of doubly or triply protonated heptamolybdates are concerned.
In the present investigation potentiometric data have been collected at high concentration and in different ionic media to enable a meaningful comparison with NMR results. We also report results of a potentiometric and 95Mo NMR investigation study at lower concentrations in 1.0 M NaCl medium. A new reaction model which is consistent with NMR evidence is presented and used as the basis for a subsequent calorimetric investigation. Enthalpy and entropy changes for the formation of the various polyoxoanions could be calculated from the data.
Section snippets
Reagents and solutions
All reagents were of analytical grade (Merck, BDH, Aldrich) and solutions were prepared with water obtained from a Millipore Milli-Q system. Sodium molybdate and lithium molybdate stock solutions were prepared from the salts Na2[MoO4]·2H2O and Li2[MoO4] and standardized gravimetrically as described previously [7]. Lithium perchlorate was prepared by the neutralization of Li2CO3 with HClO4. Hydrochloric acid and perchloric acid solutions were standardized indirectly against potassium
Potentiometry
The protonation and condensation reactions that can occur upon acidification of molybdate are represented by the following equationFor brevity a species with overall formation constant βpq will often be described by simply using the stoichiometric coefficients which defines its composition, for example (7, 8) for the heptamolybdate polyanion [Mo7O24]6− and (8, 12) for the octamolybdate [Mo8O26]4−.
The main purpose of these potentiometric measurements was to find the
Conclusion
The results of potentiometric and NMR investigations show that in addition to the mononuclear species a reaction model comprising the polyoxoanions [Mo7O24]6−, [HMo7O24]5−, [H2Mo6O21]4− (at lower concentrations), [H3Mo8O28]5−, [Mo8O26]4−, [Mo18O56(H2O)8]4− and some minor species (depending on conditions) give a satisfactory description of molybdenum(VI) equilibria at concentrations up to 1.0 M and pH down to 1 irrespective of the ionic medium. This model agrees with various models previously
Acknowledgements
The authors are indebted to the late Mr. H.S.C. Spies for recording the NMR spectra.
References (22)
- et al.
J. Chem. Soc., Dalton Trans.
(1985) - et al.
Aust. J. Chem.
(1984) - et al.
Talanta
(1994) Adv. Inorg. Chem.
(2000)- et al.
Acta Chem. Scand.
(1964) - et al.
Inorg. Chem.
(1986)et al.Acta Chem. Scand.
(1985) - et al.
Inorg. Chem.
(1964) Inorg. Chem.
(1980)et al.Inorg. Chem.
(1985)- et al.
J. Chem. Soc., Dalton Trans.
(1990) - et al.
J. Chem. Soc., Dalton Trans.
(1986)
Talanta
Cited by (72)
All-in-one portable microsystem for on-site electrochemical determination of phosphate in turbid coastal waters
2022, Microchemical JournalCitation Excerpt :However, the peak current showed a slowly increase rate when the molybdate concentration was higher than 10 mM. Moreover, the ratio of molybdate and protons is vital for Mo(VI) speciation, which determines the “degree of protonation” of molybdate in solution [38]. Previous studies have reported that silicate has a similar formation process to phosphate under molybdate and acidic conditions, and silicate concentrations are in excess of phosphate concentrations in coastal waters [39].
Oxidation vs. coordination chemistry of the {Mo<sup>V</sup><inf>2</inf>O<inf>4</inf>}<sup>2+</sup> species: A structural study
2022, Journal of Molecular StructureExtraction equilibrium of molybdenum(VI) and tungsten(VI) in aqueous solutions containing hydrogen peroxide by synergistic solvent extraction with TRPO and TBP
2022, HydrometallurgyCitation Excerpt :Therefore, the realization of complexation and transformation of W species is the key to reduce W loss by eliminating the third phase. The speciation and composition of W and Mo in solution is influenced by pH value, concentration of metal ions and H2O2 (Cruywagen et al., 2002; Andersson et al., 1994; Taube et al., 2002a; Pettersson et al., 2003; Howarth, 2004). The concentration of hydrogen peroxide is decisive because when the molar ratio of H2O2 to metal exceeds 2, the polymerization of species goes no further than the dimer even at low pH (Taube et al., 2002a; Pettersson et al., 2003; Howarth, 2004).
Separation of V (V) and Mo (VI) in roasting-water leaching solution of spent hydrodesulfurization catalyst by co-extraction using P507 - N235 extractant
2020, Separation and Purification TechnologyPreparation and characterization of poly nano-cerium chloride for <sup>99</sup>Mo production based on neutron activation reactions
2020, Applied Radiation and IsotopesChemical forms of molybdenum ion in nitric acid solution studied using liquid-phase X-ray absorption fine structure, Ultraviolet–Visible absorption spectroscopy and first-principles calculations
2019, Chemical Physics LettersCitation Excerpt :In the previous studies, many chemical species of Mo complexes in solution have been found. Especially, the predominant species such as [MoO4]2−, [HMoO4]−, [Mo7O24]6−, [HMo7O24]5−, [H2Mo7O24]4−, and [Mo8O26]4− strongly depend on the ionic strength, Mo concentration, and pH [7,12–21]. However, there have been a few reports on Mo complexes in strong acid solutions so far [7,19].