EFFECT OF pH AND SOLVENT VARIATION ON FORMATION OF BARIUM TUNGSTATES: AN ELECTROMETRIC STUDY

Shiva Prasad (PQ), Ana C. S. Muniz (PG) and Joelma B. Brito (IC)

Departamento de Engenharia Química, CCT, UFPB, Campina Grande, PB

Key words: molybdates, barium, electrometry

In aqueous solutions molybdates, tungstates, vanadates, and to smaller extent, niobates, tantalates, and chromates, undergo complex hydrolysis-polymerization reactions upon acidification. The major species generated by such reactions are isopolyanions, of which Cr₂O₇^2- is well-characterized example. There is no doubt that the formation of isopolytungstates on acidification of the solutions of normal tungstate, WO₄^2-, is the most complex and the least understood system¹. It is reported² that even in a solution initially containing only ortho-tungstate and acid may exist eight types of species with H⁺:WO₄^2- ratio as 1:3, 2:3, 7:6, 3:2 and 2:1. The degree of aggregation in solutions increases as the pH is lowered, and numerous tungstates M₂^IO.nWO₃.mH₂O, differing in the value of n, have been reported from the solutions at different pH’s³. As there is a great variance in the results published by earlier workers it was considered to make a careful and precise study on the system.

In earlier publications the authors have reported the effect of pH change on solutions of Na₂WO₄⁴ and composition of chromium tungstates⁵. In view of the interesting results obtained it was considered worthwhile to investigate the composition of barium tungstates obtained by action of Ba²⁺ on different tungstate anions at specific pH levels by means of electrometric techniques.

Experimental. All reagents used were analytical grade chemicals and the solutions were prepared with deionized distilled water. The variations of pH of Na₂WO₄ solutions were obtained by progressive additions of determined quantities of nitric acid⁴. pH measurements were carried out using a Metrohm Herisau pH-meter and Schott Gerate glass combination electrode. Stoichiometric points were obtained from the sharp inflections in the titration curves. The conductometric measurements were performed on a Metrohm conductometer. Conductivity values after correcting for dilution effect were plotted as a function of ml of titrant solution added and the end-points were judged from the breaks in titration curves. 25 ml of solution was taken in the cell each time and thermostated at 25±0.1^oC. The same strengths of solutions were employed in both the techniques for the sake of comparison of results. The effect of different concentrations of ethanol on titration curves was also studied.

Analytical investigations were also carried out with a view to substantiate the electrometric results. Different tungstates of barium were prepared by mixing solutions of barium nitrate with solutions of sodium molybdate at specific pH levels 8.0, 5.9 and 4.0. Tungsten^6a was determined by dithiol and barium as sulfate^6b.

Results and Discussion. The results of electrometric study showed that the stoichiometry of the compounds formed by the interaction of barium nitrate with sodium tungsate at specific pH levels 8.0, 5.9 and 4.0 can be successfully followed by pH and conductometric titrations. The sharp inflection in the pH titration curves was obtained only in the presence of ethanol (Fig. 1) which may be ascribed to the decrease in solubility and hydrolysis of the precipitate by its presence. The end-points obtained from the sharp inflections in the titration curves, in presence of 40% ethanol, provide definite evidence for the formation of barium normal-BaO.WO₃ and para-5BaO.12WO₃ tungstates in the vicinity of pH 7.4 and 5.8, respectively, according to the reactions:

Na₂WO₄ + Ba(NO₃)₂ = BaO.WO₃ + 2NaNO₃

Na₁₀W₁₂O₄₁ + 5Ba(NO₃)₂ = 5BaO.12WO₃ + 10NaNO₃

Fig. 1 - Direct and reverse pH titrations. (A) M/20 Ba(NO₃)₂ added to 25 mL of

M/250 Na₂WO₄, (B) M/20 Na₂WO₄ added to 25 mL of M/250 Ba(NO₃)₂

The analysis of the precipitates confirms the results obtained by the electrometric techniques. The formation of barium normal-tungstate in presence of 40% ethanol was found to be almost quantitative and in view of the simplicity, rapidity and accuracy of the titrations, the potentiometric method may be recommended for the determination of barium as tungstate.

References

1. D.L. Kepert, in: A.F. Trotman-Dickenson (ex. ed.) Comprehensive Inorganic Chemistry, vol. 4, Pergamon, Oxford, 1973, pp. 641, 642, 646.

2. M. Hudson, Chem. Br. 6, 438 (1982).

3. F.A. Cotton and G. Wilkinson, Adv. Inorg. Chem., Interscience, New York, 1972.

4. S. Prasad, Quím. Nova. 17, 31 (1994).

5. S. Prasad, André B. Brito and Joelma B. Brito, Oriental J. Chem. 15(3), (1999).

6. A.I. Vogel, A Textbook of Quantitative Inorganic Analytical Chemistry, Longmans, London, 1968. (a) 805, (b) 554. CNPq