Electroanalytical Investigations on Formation of Thallium ( I ) Molybdates as a Function of pH

A formacao de molibdatos de talio obtidos a partir de interacao de nitrato de talio(I) e molibdato de sodio nos valores especificos do pH 7,6; 5,5 e 4,1 foi estudada atraves de tecnicas eletrometricas envolvendo titulacoes potenciometricas e condutometricas, em meios aquoso e alcoolico, com cada reagente usado alternadamente como titulante. As inflexoes e degraus bem definidos nas curvas de titulacao fornecem evidencias incontestaveis sobre a formacao de molibdatos normal-Tl2O.MoO3, para-3Tl2O.7MoO3 e octa-2Tl2O.8MoO3 de talio(I) nas vizinhancas de pH 6,8; 5,9 e 5,2 respectivamente. Estudos analiticos tambem foram realizados sobre os precipitados de molibdatos de talio(I) confirmando os resultados obtidos pelas tecnicas eletrometricas.


Introduction
The chemistry of molybdenum is very prominent in both biological and industrial systems. 1,2Recent studies have shown that certain molybdates have antiviral, including anti-AIDS, and antitumor activity. 3Although a large number of studies have been done in the field of molybdate chemistry, the chemical state of isopolymolybdates, obtained on acidification of a molybdate solution, is not well understood because of the complexity in polymerization.Jander   [10][11][12] On account of the complexity of the relation of equilibrium between the polyanions or due to the experimental difficulty in early works, the conclusions of earlier workers seem to be overstrained and hence it was considered worthwhile to make a careful and precise study on the formation of molybdates as a function of pH by electrometric techniques, which have provided more conclusive evidence on the condensation process of vanadate, 13 antimonite 14 thiotungstate 15 and tungstate anions. 16In an earlier publication 17 Prasad and Gonçalves have reported the effect of pH change on composition of thorium molybdate.The results on formation of thallium molybdates as a function of pH are presented here.

Experimental
All the reagents including TlNO 3 , Na 2 MoO 4 .2H 2 O, HNO 3 and ethanol of extra-pure grade were used, and their solutions were prepared with deionized distilled water.Concentration of sodium molybdate solutions was further verified by determining molybdenum with oxime as MoO 2 (C 9 H 6 ON) 2 . 18The variations of pH of Na 2 MoO 4 solutions were obtained by progressive additions of determined quantities of nitric acid.pH measurements were carried out using a Metrohm Herisau pH-meter and Schott Gerate glass combined 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.For each experiment, 25 mL of solution were taken in the cell and thermostated at 25±0.1 o C. The same concentrations of reactants were employed in both techniques for the sake of comparison of results.The potentiometric and conductometric titration curves are plotted together in the same figure for similar reasons and also for the sake of brevity.The titrations were performed both by direct and reverse methods at three different concentrations.The electrometric titration results on formation of different thallium molybdates are summarized in Table 1.
Analytical investigations on precipitates were also carried out with a view to substantiate the electrometric results.Different molybdates of thallium(I) were prepared by mixing stoichiometric amounts of thallium nitrate solution with the respective sodium molybdate solutions at specific pH levels 7.6, 5.5 and 4.1.The precipitates obtained were washed several times with aqueous 30% (v/v) ethanolic solution and dried in a vacuum dessicator for 36 h.A known amount (ca. 2 g) of each of the above precipitates was dissolved in a minimum quantity of nitric acid and then analyzed quantitatively for molybdenum 18 with dithiol and for thallium 18 with thionalide.The results are summarized in Table 2.

Results and Discussion
When nitric acid is gradually added to Na 2 MoO 4 solution, it changes to para-molybdate Mo 7 O 24 6-and octamolybdate Mo 8 O 26 4-polyanions around pH 5.5 and 4.1, respectively.Figure 1 illustrates the curves of pH and conductometric titrations of Na 2 MoO 4 solution with nitric acid.The titration curves of both the techniques show two inflections at 7Mo:8H and 8Mo:12H corresponding to the formation of the polyanions para-Mo 7 O 24 6-and octa-Mo 8 O 26 4-, respectively (see Figure 1, points A, B and A', B').The results are similar to those obtained for the interaction of hydrochloric acid with sodium molybdate 17 and are also in conformity with the results of the temperature-jump studies by Honing and Kustin 19 and the Raman spectra studies by Ozeki et al. 20 The stepwise condensation of MoO 4 2-by gradual addition of nitric acid can be represented by the following equations: 17

Thallium normal-molybdate
Using different concentrations of Na 2 MoO 4 (pH 7.6) and thallium(I) nitrate (pH 6.1) solutions, a series of potentiometric titrations was carried out.In direct titrations (Figure 2, curve 1), when Na 2 MoO 4 solution was used as titrate, a gradual decrease in pH value 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 observed (see point A in Figure 2) when the molar ratio of Tl + :MoO 4 2-is 2:1 corresponding to the formation of thallium normal-molybdate, Tl 2 O.MoO 3 , in the neighborhood of pH 6.8.In case of inverse titrations (Figure 2, curve 3), the pH at first gradually increases till in the vicinity of stoichiometric end-point when the last traces of thallium ions have been removed by precipitation, further addition of alkali molybdate causes a marked upward jump in pH and the inflection corresponds to the molar ratio for the formation of Tl 2 O.MoO 3 according to the reaction: Employing similar concentrations of the reactants, direct (Figure 2, curve 2) and reverse (Figure 2, curve 4) conductometric titrations between the solutions of thallium nitrate and sodium molybdate gave well-defined breaks at 2:1 molar ratio of Tl + :MoO 4 2-confirming the formation of the same compound Tl 2 O.MoO 3 as indicated by the pH study.

Thallium para-molybdate
Sodium para-molybdate solution was prepared by addition of nitric acid to Na 2 MoO 4 in molar ratio 8H:7Mo.Using different concentrations of sodium para-molybdate (pH 5.5) and thallium(I) nitrate (pH 6.1) solutions, a series of potentiometric titrations was carried out.In direct titrations (Figure 3, curve 1), when the para-molybdate solution was used as titrate, a gradual change in pH value was observed till at the stoichiometric end-point a sharp upward jump in pH was observed when the molar ratio of Tl + :Mo 7 O 24 6-is 6:1 corresponding to the formation of thallium para-molybdate, 3Tl 2 O.7MoO 3 , in the neighborhood of pH 5.9.In case of inverse titrations (Figure 3, curve 3), when thallium nitrate solution was used as titrate, a gradual decrease in pH was observed till at the stoichiometric point a downward jump in pH was noted and the inflection corresponds to the molar ratio for formation of 3Tl 2 O.7MoO 3 .The formation of the paramolybdate can be represented as follows: 6TlNO 3 + Na 6 Mo 7 O 24 = 3Tl 2 O.7MoO 3 + 6NaNO 3 (4)   Employing similar concentrations of the reactants a series of direct (Figure 3, curve 2) and reverse (Figure 3, curve 4) conductometric titrations was performed between the solutions TlNO 3 and Na 6 Mo 7 O 24 .The titration curves provide well-defined breaks at a point where the molar ratio of Tl + :Mo 7 O 24 6-is 6:1 (Table 1), thus confirming formation of para-molybdate, as suggested by the pH study.

Thallium octa-molybdate
Sodium octa-molybdate solution was prepared by addition of nitric acid to Na 2 MoO 4 in molar ratio 3H:2Mo.Using different concentrations of the solutions of TlNO 3 and Na 4 Mo 8 O 26 , a series of potentiometric and conductometric titrations was carried out.The slope and nature of these titration curves (Figure 4) are similar to those of the para-molybdate.The curves provide well-defined inflections at molar ratio 4:1 of Tl + :Mo 8 O 26 4-(see points A, B and A', B' in Figure 4) corresponding to stoichiometry for formation of thallium octa-molybdate 2Tl 2 O.8MoO 3 in the vicinity of pH 5.2, according to the reaction: 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 the neighborhood of the equivalence point has a favorable effect.The presence of ethanol (30%) improves the position of end-point and increase the magnitude of the jump in pH curves as it decreases the solubility of the precipitates formed and minimizes hydrolysis and adsorption.The results obtained were precise.The relative standard deviation of the results reported in Table 1 was <1%.

Quantitative analysis of precipitates
The results of the quantitative elemental analyses 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 electroanalytical investigations and the quantitative analysis of the precipitates confirm the formation and precipitation of three thallous molybdates viz.normal-Tl 2 O.MoO 3 , para-3Tl 2 O.7MoO 3 and octa-2Tl 2 O.8MoO 3 in the vicinity of pH 6.8, 5.9 and 5.2, respectively.As the structure of these compounds are not known they are represented as double oxides, the manner which is usually adopted for such compounds. 21,22
7veston etal.9bycentrifugedata could only tell that in the range studied, the species probably contain more than 6 and less than 9 Mo atoms.Sasaki et al. proposed the presence of large isopolymolybdate anions of the order of 20 Mo in the solution of Z >1.5.7Numerous species such as HMoO 4 5In 1959, Sasaki et al. deduced from potentiometry that the main complex formed is Mo

Table 1 .
Summary of results of electrometric study on formation of thallium molybdates.Volume of titre solution taken in the cell = 25 mL

Table 2 .
Summary of analytical results of the precipitates of thallium molybdates Direct -Thallium nitrate solution added to sodium molybdate solution.Reserve -Sodium molybdate solution added to mercuric nitrate solution. *