Electroanalytical investigations on formation of thallium(I) molybdates as a function of pH

Journal of the Brazilian Chemical Society

Brito, André L.F.¹  Souza, Antônio G.¹  Universidade Estadual da Paraíba, Campina Grande, Brazil¹  Prasad, Shiva¹  Universidade Federal da Paraíba, Campina Grande, Brazil¹  Leite, Valderí D.¹  Universidade Federal da Paraíba, João Pessoa, Brazil¹ 

关键词: molybdates;    thallium(I) molybdates;    electrometry;   

DOI  :  10.1590/S0103-50532002000400013

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The chemistry of molybdenum is very prominent in both biological and industrial systems.1,2 Recent studies have shown that certain molybdates have antiviral, including anti-AIDS, and antitumor activity.3 Although 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 et al. claimed existence of Mo3O114-, HMo3O113-, HMo6O215-, H2Mo6O214- , H3Mo6O213- , H7Mo12O413- , H7Mo24O785- and H9Mo24O783- from diffusion and optical experiments.4 Bye claimed the existence of Mo7O246-, Mo6O204-, Mo4O132- and HMo6O203- by cryoscopic study.5 In 1959, Sasaki et al. deduced from potentiometry that the main complex formed is Mo7O246-.6 Subsequently mathematical analysis was applied to potentiometric equilibrium curves, and Sasaki et al. claimed the existence of Mo7O246-, HMo7O245-, H2Mo7O244- and H3Mo7O243- up to a value of Z (average number of H+ being consumed by MoO42-) of around 1.4.7,8 Aveston et al.9 by centrifuge data 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 >1.5.7 Numerous species such as HMoO4-, H2MoO4 , Mo2O72-, HMo3O113-, Mo6O192-, Mo7O246-, HMoO245-, H2Mo7O244- , Mo8O264-, HMo8O263-, Mo12O372-, H7Mo24O785- , Mo36O1128- etc. have been reported in many recent publications.10-12 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 antimonite14 thiotungstate15 and tungstate anions.16 In an earlier publication17 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 TlNO3, Na2MoO4.2H2O, HNO3 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 MoO2(C9H6ON)2 .18 The variations of pH of Na2MoO4 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±0.1oC. 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 molybdenum18 with dithiol and for thallium18 with thionalide. The results are summarized in Table 2.    Results and Discussion When nitric acid is gradually added to Na2MoO4 solution, it changes to para-molybdate Mo7O246- and octa-molybdate Mo8O264- polyanions around pH 5.5 and 4.1, respectively. Figure 1 illustrates the curves of pH and conductometric titrations of Na2MoO4 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-Mo7O246- and octa-Mo8O264- , 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 molybdate17 and are also in conformity with the results of the temperature-jump studies by Honing and Kustin19 and the Raman spectra studies by Ozeki et al.20 The stepwise condensation of MoO42- by gradual addition of nitric acid can be represented by the following equations:17  Thallium normal-molybdateUsing different concentrations of Na2MoO4 (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 Na2MoO4 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+:MoO42- is 2:1 corresponding to the formation of thallium normal-molybdate, Tl2O.MoO3, in the neighborhood of pH 6.8. In

【 授权许可】

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