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

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 normalCe2O3.3WO3 and para5Ce2O3.36WO3 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.


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 Cr 2 O 7 2-, and heptamolybdate, Mo 7 O 24 6-, are well-characterized examples 7 .There is no doubt that the formation of isopolytungstates on acidification of the solutions of normal tungstate, WO 4 2-, is the most complex and the least understood system 5 .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 + :WO 4 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 M 2 I O.nWO 3 .mH 2 O, differing in the value of n, have been reported from the solutions at different pH's 1 .
In earlier publications the authors have reported the effect of pH variation on tungstate anions 10 and formation of different chromium tungstates 12 .In view of the interesting results obtained it was considered worthwhile to investigate the composition of cerium tungstates obtained by action of Ce 3+ on different tungstate anions at specific pH levels by means of electrometric techniques, which have provided conclusive evidences on thiotungstates 9 , thiomolybdates 8 , molybdates 14 and vanadates 13 Experimental Na 2 WO 4 .2H 2 O, Ce(NO 3 ) 3 , HNO 3 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 Na 2 WO 4 solutions were obtained by progressive additions of determined quantities of nitric acid 10 .
The pH-meter, conductometer, electrodes and titration cells were used as reported earlier 11 .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 o C. 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 Na 2 WO 4 was prepared in deionized distilled water.When nitric acid is gradually added to Na 2 WO 4 solution, it changes to sodium paratungstate Na 10 W 12 O 41 and metatungstate Na 6 W 12 O 39 around pH 5.9 and 4.0 corresponding to the ratios 6W:7H and 6W:9H, respectively 10 .Employing similar concentrations of the reactants, both direct (Fig. 2) and reverse (Fig. 3) conductometric titrations between the solutions of Ce(NO 3 ) 3 and Na 2 WO 4 gave well-defined breaks at 2:3 molar ratio of Ce 3+ :WO 4 2-, confirming the for formation of the cerium tungstate Ce 2 O 3 .3WO3 , as suggested by the pH study.

Cerium paratungstate
The solution of sodium paratungstate, Na 10 W 12 O 41 , was prepared by addition of nitric acid to Na 2 WO 4 in the molar ratio 7:6.Figures 4 and 5 illustrate the changes occurring in pH and conductance when Ce(NO 3 ) 3 solution (pH 4.6) is treated with Na 10 W 12 O 41 solution (pH 5.9).In direct titrations (Fig. 4) when Ce(NO 3 ) 3 solution was added from microburette to Na 10 W 12 O 41 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 Ce 3+ :W 12 O 41 10-as 10:3, suggesting the formation of cerium paratungstate 5Ce 2 O 3 .36WO3 in the vicinity of pH 5.3.In case of reverse titrations (Fig. 5) the first addition of Na 10 W 12 O 41 solution to Ce(NO 3 ) 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(NO 3 ) 3 and Na 10 W 12 O 41 .The titration curves provide well-defined breaks at a point where the molar ratio of Ce 3+ :W 12 O 41 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(NO 3 ) 3 + 3Na 10 W 12 O 41 = (5Cr 2 O 3 .36WO3 ) + 30NaNO 3 The reaction between cerium nitrate and sodium metatungstate, Na 6 W 12 O 39 , has also been studied by pH and conductometric titrations but curves did not exhibit sharp inflections and breaks.This may be ascribed to the difference in pH values of the reactants and the presence of NaNO 3 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-Ce 2 O 3 .3WO3 and para-5Ce 2 O 3 .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 compounds 15 .

Fig. 1
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(NO 3 ) 3 solution (pH 4.6) was added to the Na 2 WO 4 solution (pH 8.0) a sharp fall in pH was noted with an inflection at molar ratio of Ce 3+ :WO 4 2-as 2:3 in the vicinity of pH 6.2, corresponding to the stoichiometry for the formation of cerium tungstate, Ce 2 O 3 .3WO3 .This sharp fall in pH occurs because of the presence of unreacted acidic (pH 4.6) Ce(NO 3 ) 3 in the cell just after completion of precipitation of cerium tungstate.In reverse titrations (Fig.3), when Na 2 WO 4 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: