Electrometric investigations on the formation of cerium(III) tungstates as a function of PH

Investigações eletrométricas sobre a formação de tungstatos de cério(III) em função do pH

ELECTROMETRIC INVESTIGATIONS ON THE FORMATION OF CERIUM(III) TUNGSTATES AS A FUNCTION OF pH

Shiva PRASAD^*

André Luiz Fiquene de BRITO^*

Ana Cristina Silva MUNIZ^*

Valderí Duarte LEITE^**

ABSTRACT: The course of reaction between cerium(III) nitrate and different sodium tungstates (Na₂WO₄, Na₁₀W₁₂O₄₁ and Na₆W₁₂O₃₉) has been followed by means of pH and conductometric titrations between the reactants at different pH levels, in aqueous and alcoholic media, with each of the reagents alternatively used as titrant. The electrometric experiments provide definite evidence of the formation of normal-Ce₂O₃.3WO₃ and para- 5Ce₂O₃.36WO₃ tungstates of cerium in the vicinity of pH 6.2 and 5.3. The formation of normal tungstate is almost quantitative and the pH titrations offer a simple means for determination of cerium(III) or tungstate solutions at suitable concentrations and pH range.

KEYWORDS: Tungstates, cerium(III), electrometry.

Introduction

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-, and heptamolybdate, Mo₇O₂₄^6-, are well-characterized examples⁷. 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 system⁵. The difficulties are at least three fold. Firstly there is the instability to obtain well-defined crystalline salts with many of the isopolytungstate anions, and hence the structural basis remains somewhat speculative. Secondly many workers have used solutions containing buffers, many of which contain anions that preferentially form complexes with the tetrahedrally coordinated normal tungstate rather than the octahedrally coordinated polytungstates. Thirdly the rates of formation of some isopolytungstates are very slow^2,3.

Despite extensive investigations the problem of number and composition of the polymeric species has not been completely solved. It is reported^4,6 that even in a solution initially containing only orthotungstate 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’s¹.

In earlier publications the authors have reported the effect of pH variation on tungstate anions¹⁰ and formation of different chromium tungstates¹². In view of the interesting results obtained it was considered worthwhile to investigate the composition of cerium tungstates obtained by action of Ce³⁺ on different tungstate anions at specific pH levels by means of electrometric techniques, which have provided conclusive evidences on thiotungstates⁹, thiomolybdates⁸, molybdates¹⁴ and vanadates¹³

Experimental

Na₂WO₄.2H₂O, Ce(NO₃)₃, HNO₃ and ethanol of extra-pure grade were used, and their solutions were prepared with deionized distilled water. Concentration of the sodium tungstate solution was further verified by tannin-antipyrine method^16a. The variations of pH of Na₂WO₄ solutions were obtained by progressive additions of determined quantities of nitric acid¹⁰.

The pH-meter, conductometer, electrodes and titration cells were used as reported earlier¹¹. Stoichiometric points in pH titrations were obtained from the sharp inflections in the titration curves. 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. 25.00 mL of solution was taken in the cell each time and thermostated at 25.0± 0.1^oC. The same strengths of solutions were employed in both the techniques for the sake of comparison of results. The electrometric titration results are summarized in Table 1.

Analytical investigations were also carried out with a view to substantiate the electrometric results. Different cerium tungstates were precipitated by mixing solutions of sodium tungstate at specific pH levels 8.0, 5.9 and 4.0 with cerium nitrate. Tungsten^16a was determined as oxinate and cerium^16b volumetrically. From the proportions of cerium and tungsten in the compounds thus obtained their composition was established. The results are summarized in Table 2.

Results and discussion

The solution of Na₂WO₄ was prepared in deionized distilled water. When nitric acid is gradually added to Na₂WO₄ solution, it changes to sodium paratungstate Na₁₀W₁₂O₄₁ and metatungstate Na₆W₁₂O₃₉ around pH 5.9 and 4.0 corresponding to the ratios 6W:7H and 6W:9H, respectively¹⁰. Fig. 1 illustrates the curves of pH and conductometric titrations of Na₂WO₄ solution with nitric acid. The pH titration curve shows two inflections at 6W:7H and 6W:9H corresponding to the formation of Na₁₀W₁₂O₄₁ and Na₆W₁₂O₃₉, respectively. The position of the inflections is further checked by the curve of dpH/dV. Similar results are obtained by conductometric titrations (Fig. 1). The stepwise degradation of Na₂WO₄ by the gradual addition of HNO₃ can be represented by the following equations¹⁰:

12Na₂WO₄ + 14HNO₃ = Na₁₀W₁₂O₄₁ + 14NaNO₃ + 7H₂O

12Na₂WO₄ + 18HNO₃ = Na₆W₁₂O₃₉ + 18NaNO₃ + 9H₂O

Cerium normal tungstate

Figures 2 and 3 illustrate the curves of the pH and conductometric titrations performed between the solutions of the normal tungstate and cerium nitrate. In direct titrations (Fig. 2), when Ce(NO₃)₃ solution (pH 4.6) was added to the Na₂WO₄ solution (pH 8.0) a sharp fall in pH was noted with an inflection at molar ratio of Ce³⁺:WO₄^2- as 2:3 in the vicinity of pH 6.2, corresponding to the stoichiometry for the formation of cerium tungstate, Ce₂O₃.3WO₃. This sharp fall in pH occurs because of the presence of unreacted acidic (pH 4.6) Ce(NO₃)₃ in the cell just after completion of precipitation of cerium tungstate. In reverse titrations (Fig. 3), when Na₂WO₄ solution was added to the cerium nitrate solution, the pH first changed very slowly, but at the end-point it jumped upwards corresponding to the formation of the same compound in accordance with the following equation:

2Ce(NO₃)₃ + 3Na₂WO₄ = Ce₂O₃.3WO₃ + 6NaNO₃

Employing similar concentrations of the reactants, both direct (Fig. 2) and reverse (Fig. 3) conductometric titrations between the solutions of Ce(NO₃)₃ and Na₂WO₄ gave well-defined breaks at 2:3 molar ratio of Ce³⁺:WO₄^2-, confirming the stoichiometry for formation of the cerium tungstate Ce₂O₃.3WO₃, as suggested by the pH study.

Cerium paratungstate

The solution of sodium paratungstate, Na₁₀W₁₂O₄₁, was prepared by addition of nitric acid to Na₂WO₄ in the molar ratio 7:6. Figures 4 and 5 illustrate the changes occurring in pH and conductance when Ce(NO₃)₃ solution (pH 4.6) is treated with Na₁₀W₁₂O₄₁ solution (pH 5.9). In direct titrations (Fig. 4) when Ce(NO₃)₃ solution was added from microburette to Na₁₀W₁₂O₄₁ solution, a gradual change in pH was observed till at the stoichiometric end-point (the stage at which the reaction ends if simple double decomposition takes place), a sharp fall in pH was noted with the inflection corresponding to the molar ratio of Ce³⁺:W₁₂O₄₁^10- as 10:3, suggesting the formation of cerium paratungstate 5Ce₂O₃.36WO₃ in the vicinity of pH 5.3. In case of reverse titrations (Fig. 5) the first addition of Na₁₀W₁₂O₄₁ solution to Ce(NO₃)₃ caused a slight decrease in pH till about half the volume of titrant required for the precipitation of cerium paratungstate was added. This initial lowering in pH value was due to the presence of hydrolyzed acid from the cerium salt. Later on, with the progress of the reaction, pH began to rise and a pronounced upward jump was obtained at the stoichiometric point corresponding to the formation of cerium paratungstate. Employing similar concentrations of the reactants a series of direct (Fig. 4) and reverse (Fig. 5) conductometric titrations were performed between the solutions of Ce(NO₃)₃ and Na₁₀W₁₂O₄₁. The titration curves provide well-defined breaks at a point where the molar ratio of Ce³⁺:W₁₂O₄₁^10- is 10:3 (Table 1), thus confirming formation of the paratungstate as suggested by the pH study. The appreciable increase in conductance values in the initial stage of the inverse titration (Fig. 5) provides strong support to the observation noted in the reverse pH titrations. The formation of the paratungstate can be represented as follows:

10Ce(NO₃)₃ + 3Na₁₀W₁₂O₄₁ = (5Cr₂O₃.36WO₃) + 30NaNO₃

The reaction between cerium nitrate and sodium metatungstate, Na₆W₁₂O₃₉, has also been studied by pH and conductometric titrations but curves did not exhibit sharp inflections and breaks. This may be ascribed to the small difference in pH values of the reactants and the presence of NaNO₃ in appreciable amounts preventing the occurrence of breaks in the conductometric titration curves.

It was noted that after each addition of the titrant, it takes a little time for the pH and conductance values to become steady. A thorough stirring in neighborhood of the equivalence point has a favorable effect. The presence of ethanol (20%) improves position of the end-point and increases magnitude of the jump in pH curves, as it decreases solubility of the precipitates formed and minimizes hydrolysis and adsorption. For this reason all the titrations were performed in presence of 20% ethanol. The precipitation of normal tungstate was found to be almost quantitative. The titrations are simple and rapid and offer a quantitative method for determination of cerium(III) or tungstate solutions at suitable concentrations and pH range.

Analytical results

The results of the quantitative elemental analysis of the precipitates were used to calculate the proportions of the elements present in the compounds. From these proportions, the composition of the compounds was established which was found to be the same as obtained by the electrometric techniques (see Table 2).

The present electrometric and analytical investigations confirm the formation and precipitation of two cerium tungstates, viz. Normal-Ce₂O₃.3WO₃ and para-5Ce₂O₃.36WO₃ in the vicinity of pH 6.2 and 5.3, respectively.

As structure of these compounds is not known they are represented as double oxides, the manner which is usually adopted for such compounds¹⁵.

Acknowledgement

The authors are indebted to the CNPq for financial assistance.

PRASAD, S. et al. Investigações eletrométricas sobre a formação de tungstatos de cério(III) em função do pH. Ecl. Quím (São Paulo), v.25, p. 2000.

RESUMO: Através de técnicas eletrométricas envolvendo titulações potenciométricas e condutométricas entre os reagentes, foi estudada a reação do nitrato de cério(III) com diferentes tungstatos de sódio (Na₂WO₄, Na₁₀W₁₂O₄₁ e Na₆W₁₂O₃₉) em níveis diferentes do pH, em meios aquoso e alcoólico, com cada reagente usado alternadamente como titulante. Os resultados obtidos fornecem definitivas evidências de formação dos normal-Ce₂O₃.3WO₃ e para-5Ce₂O₃.36WO₃ tungstatos de cério nas vizinhanças de valores de pH iguais, respectivamente, 6,2 e 5,3. A precipitação do tungstato normal de cério foi quase quantitativa e as titulações potenciométricas oferecem um método simples para determinação de cério(III) ou tungstato nas soluções a concentrações e faixa do pH apropriadas.

PALAVRAS-CHAVE: Tungstatos; cério(III); eletrometria.

Recebido em 23.2.2000

Aceito em 16.3.2000

** Departamento de Química, Centro de Ciências e Tecnologia, Universidade Estadual da Paraíba, CEP 58100-000 Campina Grande, PB

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*

Departamento de Engenharia Química, Centro de Ciências e Tecnologia, Universidade Federal da Paraíba, Campus II, Cx. Postal 10108, CEP 58109-970 Campina Grande, PB.

Publication Dates

• Publication in this collection
  26 Nov 2002
• Date of issue
  2000

History

• Accepted
  16 Mar 2000
• Received
  23 Feb 2000