Potentiometric and conductometric studies on the system acid-isopolytungstate and the formation of lanthanum tungstates

The stoichiometry of the polyanions formed by the action of nitric acid on sodium tungstate (≤ 0.01M) has been studied by means of electrometric techniques involving pH-potentiometric and conductometric titrations. The well defined inflections and breaks in the titration curves provide evidence for the existence of the polyanions, para-W12O41 and meta-W12O39 corresponding to the ratio of H:WO4 as 7:6 and 9:6 in the pH ranges 5.7-6.0 and 3.6-4.1, respectively. The interaction of lanthanum nitrate with sodium tungstate solutions, at specific pH levels 8.0, 5.9 and 4.0 was also studied by pH and conductometric titrations, 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-La2O3.3WO3, para-5La2O3.36WO3 and meta-La2O3.12WO3 tungstates in the vicinity of pH 6.3, 5.0 and 4.2, respectively. Analytical investigations on the precipitates formed confirm the results of the electrometric study.


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 [1].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 [2].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 [3,4].
Tungstate solutions can absorb considerable quantities of strong acid by mechanism leading to formation of isopolytungstates [5].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 [6].The existence of so many isopolyanions seems to be doubtful; moreover, there is a great variance in the results published by earlier workers which allows no satisfactory interpretation of the mechanism of the condensation process.A further verification of the tungstate system seems very desirable with a view to rationalizing the conflicting details of the previous workers by employing electrometric techniques, which have provided more conclusive evidences on such systems [7,8,9].After establishing for suitable conditions for the stability of different tungstate polyanions, the investigations for the formation of lanthanum tungastates as a function of pH were also carried out as no reference could be traced out in the literature about their formation.The same concentrations of reactants were employed in conductometric and glass electrode titrations for the sake of comparison.The curves were plotted between pH and corrected conductance vs. volume of HNO 3 added.As the inflections in pH titration curves were not strongly defined, differential graphs in dpH/dV were also drawn and end-points were marked by pronounced maxima in dpH/dV.A summary of the electrometric titrations of the acid system is given in Table 1.

Experimental details
The formation of lanthanum tungstates was investigated by the action of lanthanum nitrate with different vanadate anions at specific pH levels 8.0, 5.9 and 4.0 using different concentrations of the reactants.A series of pH and conductometric titrations was performed by direct and reverse methods, i.e. when lanthanum nitrate solution from the microburette was added to sodium tungstate solution and vice versa (Table 2).25.00 mL of ethanolic solution (20%) was taken in the cell, which was thermostated at 25.0±0.1 o C.
The precipitates obtained at the end-points of titrations between lanthanum nitrate and sodium tungstates were also analyzed to substantiate the electrometric results.The different lanthanum tungstates were precipitated by mixing stoichiometric amounts of lanthanum nitrate solutions with sodium tungstate solutions at specific pH levels 8.0, 5.9 and 4.0.The precipitates obtained were washed several times with 20% ethanolic solution and dried in a vacuum desiccator for 40 h.A known amount (ca. 2 g) of each of the precipitates was used for analysis tungsten and lanthanum.Tungsten [10a] was determined as oxinate and lanthanum [10b] with oxalate.From the proportions of lanthanum and tungsten in the compounds thus obtained their composition was established.The results are summarized in Table 3.The analytical results were also confirmed by atomic absorption spectroscopy.

Results and Discussions
The pH titrations of the acid against sodium tungstate are shown in Fig. 1 (curves 1 and 2).Two points of inflection were obtained; the first at H + :WO 4 2-as 7:6 corresponding to the for-Table 2. Summary of results of electrometric study on formation of lanthanum tungstates.Volume of titre solution taken in the cell = 25.00 mL.mation of paratungstate and the second at H + :WO 4 2-as 9:6 due to the formation of paratungstate anions.In the first stage of the formation, when H + :WO 4 2-is less than 7:6, the addition of the acid brings about a gradual change in pH with a rapid attainment of its equilibrium value.When the ratio of the acid to normal tungstate passed 7:6, the initial pH dropped sharply leading to the observed inflection due to the formation of paratungstate anion in the pH range 5.7-6.0.But in contrast to the earlier part of the titration, the pH value showed an increasing tendency on waiting after each addition of the titrant and took a long time for attaining the equilibrium value, leading to a partial disappearance of the paratungstate A (HW 6 O 21 5-) to a more stable ion, paratungstate Z (W 12 O 41 10-).The increase in pH value observed on standing should be the result of an increase in negative charge (from -5 on HW 6 O 21 5-to -10 on W 12 O 41 10-) on the formation of higher polymer of a weaker acid.The existence of these two species is in agreement with the ultracentrifuge results [11][12][13][14].The paratungstate formation may be represented in a simplified way as follows: During the region of the second inflection in the titration curves, corresponding to the formation of metatungstate isopolyanions, the tendency to increase the pH value on waiting was again observed, leading to a conclusion that the formation of the final metatungstate species, W 12 O 39 6-, is also a slow process and should be preceded by the formation of an unstable species HW 6 O 20 3-, psedo-metatungstate [5,6].The two stage reaction may be represented as follows: It was observed that after passing the second inflection the pH value got stabilized and did not give any more inflection even with an addition of a lot excess of the acid (H + :WO 4 2-› 10:1) and hence not confirming the formation of the tungstic acid [2b].
Conductometric titrations between HNO 3 and Na 2 WO 4 were also carried out employing the same concentrations of the reactants as in pH titrations for the sake of comparison of the results.The plots (Fig. 1, curve 3) of corrected conductance as a function of the volume of titrant yielded two well-defined breaks at the ratio H + :WO 4 2-as 7:6 and 9:6 corresponding to the formation of paratungstate and metatungstate, respectively, as suggested by the preceding pH study.In all these titrations the break corresponding to the addition of 1.5 equivalents of the acid per Na 2 WO 4 is strongly defined, followed by a sharp rise in conductance which was determined to be the same as expected for the addition of free acid to the system.
When a basic tungstate solution containing WO 4 2-and Na + is acidified, the tungstate ions have been found to condense in definite steps, as evidenced by the foregoing electrometric experiments, to form different isopolytungstate anions.The overall condensation process may be considered formally at least to begin with partial neutralization of the tungstate ions, and then expanding its coordination sphere from four to six by water molecules followed by the formation of a highly aggregated isopolyanion at lower pH value.The condensation process may be represented by the following general equation, which is almost similar to the one proposed by MacInnis and Kim [15]: with the values for a and b as represented in Table 4.
It is quite probable that the highly charged species given in Table 4 [17].
The results of a careful pH and conductometric study on the acid tungstate system provided definite evidence for the formation of para-W 12 O 41 10-(via HW 6 O 21 5-) and meta-W 12 O 39 6- (via HW 6 O 20 3-) in the pH ranges 5.7-6.0 and 3.6-4.1,respectively.The observation of MacInnis and Kim [15]

Formation of lanthanum tungstates
The above studies show that the addition of acid to Na 2 WO 4 solution under suitable condition causes the formation of paratungstate W 12 O 41 10- and metatungstate W 12 O 39 6-anions.Hence it was considered of interest to ascertain whether similar salts of heavy metals may be precipitated as a result of double decomposition.The reaction between lanthanum nitrate and different alkali tungstates has therefore been studied by means of pH and conductometric titrations.The solution of Na 2 WO 4 was prepared in deionized distilled water.The solutions of sodium para-and meta-tungstates were prepared by adding 7 and 9 mol of HNO 3 to 6 mol of Na 2 WO 4 .

Lanthanum normal tungstate
Figure 2 illustrates the curves of the pH and conductometric titrations performed between the solutions of the normal tungstate and lanthanum nitrate.In direct titrations (curve 1), when La(NO 3 ) 3 solution (pH 4.3) 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 La 3+ :WO 4 2-as 2:3 in the vicinity of pH 6.3, corresponding to the stoichiometry for the formation of lanthanum tungstate, La 2 O 3 .3WO3 .This sharp fall in pH occurs because of the presence of unreacted acidic (pH 4.3) La(NO 3 ) 3 in the cell just after completion of precipitation of lanthanum tungstate.In reverse titrations (curve 3), when Na 2 WO 4 solution was added to the lanthanum 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: 2La(NO 3 ) 3 + 3Na 2 WO 4 = La 2 O 3 .3WO3 + 6NaNO 3 (11) Employing similar concentrations of the reactants, both direct (curve 2) and reverse (curve 3) conductometric titrations between the solutions of La(NO 3 ) 3 and Na 2 WO 4 gave welldefined breaks at 2:3 molar ratio of La 3+ :WO 4 2-, confirming the stoichiometry for formation of the cerium tungstate La 2 O 3 .3WO3 , as suggested by the pH study.

Lanthanum 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. Figure 3 illustrates the changes occurring in pH and conductance when La(NO 3 ) 3 solution (pH 4.3) is treated with Na 10 W 12 O 41 solution (pH 5.9).In direct titrations (curve 1) when La(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 La 3+ :W 12 O 41 10-as 10:3, suggesting the formation of lanthanum paratungstate 5La 2 O 3 .36WO3 in the vicinity of pH 5.0.In case of reverse titrations (curve 3) the first addition of Na 10 W 12 O 41 solution to La(NO 3 ) 3 caused a slight decrease in pH till about half the volume of titrant required for the precipitation of lanthanum paratungstate was added.This initial lowering in pH value was due to the presence of hydrolyzed acid from the lanthanum 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 lanthanum paratungstate.Employing similar concentrations of the reactants a series of direct (curve 2) and reverse (curve 4) conductometric titrations were performed between the solutions of La(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 La 3+ :W 12 O 41 10-is 10:3 (Table 2), thus confirming formation of the paratungstate as suggested by the pH study.The

Lanthanum metatungstate
Sodium metatungstate, Na 6 W 12 O 39 , solution was prepared by addition of nitric acid to Na 2 WO 4 in the molar ratio 3H:2W.Using different concentrations of the solutions of La(NO 3 ) 3 and Na 6 W 12 O 39 , a series of direct and reverse conductometric titrations was carried out.The nature of these titration curves (Figure 4) is similar to those of the paratungstate.The curves provide breaks at molar ratio 2:1 of La 3+ :W 12 O 39 6- corresponding to the stoichiometry for formation of lanthanum  The pH titrations of this system did not provide reliable results for the formation of lanthanum metatungstate, which may be ascribed to very close pH values of the reactants involved.
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.

Conclusions
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 3).

Na 2
WO 4 .2H 2 O, La(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 [10a].pH measurements were carried out on Metrohm Herisau pH-meter using Scott Gerate glass combination electrode.Conductance values were recorded by employing a Metrohm conductometer.The sodium tungstate solution (25.00 mL) was placed in the cell each time and thermostated at 25.0±0.1 o C. Using different concentrations of HNO 3 and Na 2 WO 4 , a series of glass electrode and conductometric titrations have been carried out.