The general character of interconversions of tungsten(VI) ions was analyzed in detail in the monograph [1]. In the present study, some refinements were made with the aim of solving, though only partially, disputable problems in estimation of the behavior of tungsten(VI) anions in aqueous solutions. Tungsten(VI) anions in solutions are known to occur mainly as para-, meta-, and decatungstate anions. However, some questions still remain unanswered, e.g., about the effect of the concentration of tungsten in solution on the ranges of existence of its particular ions, the degrees of polycondensation of tungsten ions in the alkaline range, and possible intermediate ionic forms existing in the ranges between the known forms.At the same time, the solution of these problems would help to optimize the synthesis conditions and the thermodynamic parameters of complexation could be used to predict the compositions of the compounds obtained. However, such important parameters as Gibbs energy D G and thermodynamic equilibrium constants K 0 are unavailable from the literature. The concentration constants of formation K c of tungsten(VI) polyanions were reported only in [2].
EXPERIMENTALThe states of tungsten polyanions in aqueous solution were studied by pH-potentiometric titration at 25 ± 0.1°ë on an I-160 ionometer. Aqueous solutions of Na 2 WO 4 (analytical grade), HNO 3 , and NaNO 3 (reagent grade) were prepared from distilled ëé 2 -free water (carbon dioxide was removed according to ). Precise concentrations of solutions were determined by chemical analysis: by gravimetry for tungsten (weighed form is WO 3 , δ = ± 0.5% ) and by titration of a weighed sample of borax for δ = ± 0.8% for HNO 3 . The concentration of tungsten was varied from 8 × 10 -3 to 2 × 10 -2 mol/l. The ionic strength was maintained constant by addition of a required amount of 0.1 to 0.5 M NaNO 3 . The amount of the titrant acid Z was calculated as Z = ( H + )/(where ( H + ) is the total concentration of ç + in solution) with the increment ∆ Z = 0.05 for Z = 0 to 3.00. The ion product of water was calculated by the formula K w = , where = 10 -14 and the f ± is the average activity coefficient ( ç + and éç -) that considers the ionic strength of solution given by the supporting electrolyte (NaNO 3 ).Mathematical modeling of processes occurring in solution was performed with the CLINP program [3] designed to calculate equilibrium constants and the physicochemical parameters of reagents from composition vs. property plots. The calculated concentration constants and anion concentrations were used to refine the ranges of existence of individual tungsten(VI) isopolyanions in aqueous solution over a sufficiently wide Z range.In recent years, Pitzer's method [4-6] has become increasingly popular in the thermodynamics of aqueous solutions of electrolytes for bringing data on the formation constants to the universally accepted standard (solution at infinite dilution; i.e., I = 0). According to this method, the relation between the concentrationAbstract -Interactions...
We report Peacock–Weakley complexes, Na9[Ln(W5O18)2]∙35H2O, formed with Tm(III), 1, and Yb(III), 2. Their syntheses, physico-chemical characterizations, crystal structures, and magnetic properties are described. Ab initio calculations are also reported. These polyoxometalate (POM) complexes were obtained using original synthetic conditions where acidification was performed with a stoichiometric amount of nitric acid to an acidity of Z = ν(H+)/ν(WO42–) = 8/10 = 0.80. Both the Tm(III) and Yb(III) derivatives were found to exhibit field-induced slow relaxation of their magnetization likely controlled by Raman and Orbach relaxation processes. 1 is a rare example of a Tm(III)-based single-molecule magnet (SMM) and is a consequence of the oblate tetragonal anti-prismatic symmetry of the coordination sphere.
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