Electrometric investigations on the system acid-molybdate and the formation of heavy metal molybdates

Article

Article

Electrometric Investigations on the System Acid-Molybdate and the Formation of Heavy Metal Molybdates

Shiva Prasad*, and Thalma L.M. Guimarães

Departamento de Engenharia Química, Centro de Ciências e Tecnologia, Universidade Federal da Paraíba, 58109-970 Campina Grande - PB, Brazil

Received: September 18, 1997

A natureza precisa dos poliânions formados pela ação de ácido clorídrico sobre soluções de molibdato de sódio foi estudada por meio de titulações de pH e condutométricas. As inflexões e degraus nas curvas de titulações indicaram a formação de ânions polimolibdatos para-Mo₇O₂₄^6- e octa-Mo₈O₂₆^4- ao redor do pH 5,5 e 4,1, correspondendo às razões 8H:7Mo e 3H:2Mo, respectivamente. Titulações semelhantes de pH e condutométricas entre Hg²⁺ e MoO₄^2-, Mo₇O₂₄^6- e Mo₈O₂₆^4- forneceram evidências incontestáveis sobre a formação de molibdato normal HgO.MoO₃, paramolibdato 3HgO.7MoO₃ e octamolibdato 2HgO.8MoO₃ de mercúrio(II) nas vizinhanças de pH 4,9; 4,2 e 3,7 respectivamente. Estudos analíticos também foram realizados sobre precipitados de molibdatos de mercúrio(II) confirmando os resultados obtidos pelas técnicas eletrométricas.

The precise nature of the polyanions formed by the action of hydrochloric acid on sodium molybdate solutions has been studied by pH and conductometric measurements involving glass electrode and conductometric titrations. The inflections and breaks in the titration curves indicate the formation of para-Mo₇O₂₄^6- and octa-Mo₈O₂₆^4- polymolybdate anions around pH 5.5 and 4.1, corresponding to the molar; ratio 8H:7Mo and 3H:2Mo, respectively. Similar pH and conductometric titrations between Hg^{2 +} and MoO₄^2-, Mo₇O₂₄^6-, and Mo₈O₂₆^4- provide definite evidence for the formation of normal molybdate HgO.MoO₃, paramolybdate 3HgO.7MoO₃ and octamolybdate 2HgO.8MoO₃ of mercury(II) in the vicinity of the pH 4.9, 4.2 and 3.7, respectively. The analytical investigations of the precipitates of mercuric molybdates have also been carried out, which substantiate the results of the electrometric study.

Keywords: molybdate anions, mercuric molybdates, electrometric techniques

Introduction

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 activity³. Altough 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 Mo₃O₁₁^4-, HMo₃O₁₁^3-, HMo₆O₂₁^5-, H₂Mo₆O₂₁^4-, H₃Mo₆O₂₁^3-, H₇Mo₁₂O₄₁^3-, H₇Mo₂₄O₇₈^5- and H₉Mo₂₄O₇₈^3- from diffusion and optical experiments⁴. Bye claimed the existence of Mo₇O₂₄^6-, Mo₆O₂₀^4-, Mo₄O₁₃^2-, and HMo₆O₂₀^3- by cryoscopic study⁵. In 1959, Sasaki et al. deduced from potentiometry that the main complex formed is Mo₇O₂₄^6-6. Subsequently mathematical analysis was applied to potentiometric equilibrium curves, and Sasaki et al. claimed the existence of Mo₇O₂₄^6-, HMo₇O₂₄^5-, H₂Mo₇O₂₄^4-, and H₃Mo₇O₂₄^3- up to a value of Z (average number of H⁺ being consumed by MoO₄^2-) 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 > 1.5⁷. Numerous species such as HMoO₄^-, H₂MoO₄, Mo₂O₇^2-, HMo₃O₁₁^3-, Mo₆O₁₉^2-, Mo₇O₂₄^6-, HMoO₂₄^5-, H₂Mo₇O₂₄^4-, Mo₈O₂₆^4-, HMo₈O₂₆^3-, Mo₁₂O₃₇^2-, H₇Mo₂₄O₇₈^5-, Mo₃₆O₁₁₂^8-, etc. have been reported in many recent publications^10-12.

On account of the complexity of the relation of equilibria 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 of the acid-molybdate system by electrometric techniques, which have provided more conclusive evidence on the condensation process of vanadate, antimonate, thiotungstate and tungstate anions¹³. After establishing the suitable conditions for the stability of different molybdate isopolyanions, the investigations of formation of molybdates of mercury, chromium and nickel as a function of the pH were also carried out.

Experimental

Na₂MoO₄.2H₂O, Hg(NO₃)₂, CrCl₃.6H₂O, NiCl₃.6H₂O, Na₂B₄O₇.10H₂O and hydrochloric acid of extra-pure grade were used, and their solutions were prepared in deionized distilled water. Concentration of sodium molybdate solutions was further verified by determining molybdenum with oxine as MoO₂(C₉H₆ON)₂^14a. The hydrochloric acid solutions were standardized with recrystallized sodium tetraborate decahydrate^14b.

The pH measurements were carried out on Metrohm Herisau (Switzerland) pH-meter using a Scott Gerate glass combination electrode. Conductivity values were recorded by employing a Metrohm conductometer. 25 mL of the titre solution was placed in the cell each time and thermostated at 25 + 0.1 °C. Using different concentrations of the reactants, a series of glass electrode and conductometric titrations was performed. The observed pH changes were plotted as a function of volume of titrant added. The inflections obtained in the curves were confirmed by the pronounced maxima in dpH/dV and zero in d²pH/dV² graphs. The breaks in the conductometric titrations were located by plotting corrected conductance as a function of volume of titrant added. The same concentrations of reactants were employed in the two techniques for the sake of comparison of results. The pH and conductometric titration curves are plotted together in the same figure for similar reasons and also for the sake of brevity. The electrometric titration results for the formation of molybdate polyanions and mercuric molybdates are summarized in Tables 1 and 2, respectively.

Job’s method of continuous variation was employed using electrical conductance measurements for determining the composition of the polyanions formed by the interaction of sodium molybdate with hydrochloric acid. This consists in plotting the differences in specific conductivities (the sum of observed specific conductivities of the constituent solutions minus the observed specific conductivities of the mixture) against the composition of the mixtures. From the maxima obtained in such plots, the stoichiometry of the compound formed was established.

The precipitates obtained at the end-points of titrations between mercuric nitrate and sodium molybdates were also analyzed to substantiate the electrometric results. The different mercuric molybdates were prepared by mixing stoichiometric amounts of mercuric nitrate solution with the respective sodium molybdate solutions. The precipitates obtained were washed several times with aqueous 20% (v/v) ethanolic solution and dried in a vacuum dessicator for 40 h. A known amount (2 g) of each of the above precipitates was dissolved in a minimum quantity of hydrochloric acid and then analyzed quantitatively for mercury^14c with ethylenediamine and molybdenum^14a with oxine. From the proportion of mercury and molybdenum in the compounds thus obtained their composition was established. The results are summerized in Table 3.

Results and discussion

Isopolymolybdate anions

Figure 1, curve 1 represents the changes occurring in H⁺ concentration on the addition of HCl to sodium molybdate solution. It is noted that the smallest addition of the acid in the beginning causes a sharp fall in the pH (not shown in the figure), whilst further reaction with HCl produces buffer action between pH 6.2 and 5.7 showing strong affinity for the hydrogen ions in this region; subsequent addition of HCl shows a sudden fall in pH at a ratio 8H:7Mo around pH 5.5 corresponding to the stoichiometry for the formation of para-Mo₇O₂₄^6- molybdate anions. Further addition of acid yields one more inflection indicating the consumption of 1.5 moles of HCl per mole of Na₂MoO₄ and suggesting the formation of octa-Mo₈O₂₆^4- molybdate anions in the vicinity of pH 4.1. The pH of such acidified solutions became steady after a lapse of some time indicating that the reaction proceeds by way of some intermediate species. Each time the pH value was recorded only after waiting enough for its stabilization. It was noted that in the case of moderately concentrated solutions, the curves were steeper than for dilute reactants. Out of the two inflections in the curves, the one corresponding to the conversion into octamolybdate is more pronounced.

Conductometric titrations between HCl and Na₂MoO₄ solutions were also carried out using similar concentrations as in the pH titrations. The observed breaks (Fig. 1, curve 2) correspond to the formation of the same polyanions, para-Mo₇O₂₄^6- and octa-Mo₈O₂₆⁴, as suggested by the preceding pH study. The slow increase in conductance values on addition of HCl to Na₂MoO₄ till the point 8H:7Mo may be ascribed to the formation of the highly charged Mo₇O₂₄^6- anions of the relatively weak acid. The break corresponding to the addition of 1.5 moles of HCl per mole of Na₂MoO₄ is strongly defined, after which a sharp rise in conductance was observed which was determined to be the same as expected for the addition of free HCl to the system.

The course of the isopolyanion formation of molybdate by the acid was also followed by employing Job’s method of continuous variation using electrical condutance measurements. The plot of difference in specific conductivity values against composition of mixtures produced a sharp peak (Fig. 2) at the molar ratio 3H:2Mo providing further evidence for the formation of the octamolybdate polyanions.

The formation of the polyanions, as suggested by the electrometric study, may be represented as follows:

8H⁺ + 7MoO₄^2- = Mo₇O₂₄^6- + 4H₂O

12H⁺ + 8MoO₄^2- = Mo₈O₂₆^4- + 6H₂O

The existence of Mo₇O₂₆^6- and Mo₈O₂₆^4- is in conformity with the results of the temperature-jump studies by Honing and Kustin¹⁵ and the Raman spectra studies by Ozeki et al.¹⁶ Murata et al.¹⁷, however, claimed that the octamolybdate underwent further reaction with acid (Z > 1.7) to form Mo₁₂O₃₇^2- and Mo₆O₁₉^6-, but this observation could not be confirmed by our results.

When a dilute solution of MoO₄^2- (< 10^-4 M) is acidified, it yields HMoO₄^- and “H₂MoO₄”. It has been confirmed^10,18 that the tetrahedral ion MoO₄^2- transforms into an octahedral especies at the stage of second protonation.

When a basic molybdate solution at millimolar or higher concentrations is acidified, the molybdate ions have been found to condense in definite steps, as evidenced by the foregoing electrometric study, to form different isopolymolybdate species. The condensation process may be considered as rapid aggregation of the protonated species. Six monoprotonated [MoO₃(OH)]^- can easily link to the dipronated species, each one sharing a weak corner of octahedral Mo(OH)₆ . And, when six tetrahedra have been accomodated, all of these peripheral tetrahedra can expand to octahedra simply by folding at the common corners to share the octahedral edge along with a considerable decrease in enthalpy^10,12. This negative enthalpy change can be expected to stabilize the resultant product Mo₇O₂₄^6- .

Mo(OH)₆ + 6[MoO₄(OH)]^- = Mo₇O₂₄^6- + 6H₂O

The condensation process, for formation of the unprotonated polyanions, may be represented by the following general equation:

aH⁺ + bMoO₄^2- = ( MoO₄^2- ) _b-a/2 ( MoO₃)_a/2 + ( a/2)H₂O

The values of a and b are 8 and 7 for heptamer and 12 and 8 for octamer, respectively.

Mercury molybdates

The solution of Na₂MoO₄ was prepared in deionized distilled water. Sodium paramolybdate (pH 5.5) and octamolybdate (pH 4.1), as suggested by the preceding studies, were prepared by progressive additions of hydrochloric acid to Na₂MoO₄ solutions in the molar ratios 8H:7Mo and 3H:2Mo, respectively.

8HCl + 7Na₂MoO₄ = Na₆Mo₇O₂₄ + 8NaCl + 4H₂O

12HCl + 8Na₂MoO₄ = Na₄Mo₈O₂₆ + 12NaCl + 6H₂O

Figure 3 illustrates the curves of the pH and conductometric titrations performed between the solutions of the normal molybdate and mercuric nitrate. In direct titrations (curve 1), when Hg(NO₃)₂ solution (pH 2.6) was added to the Na₂MoO₄ solution (pH 7.6) a sharp fall in pH was noted with an inflection at the molar ratio of Hg²⁺:MoO₄^2- as 1:1 in the vicinity of pH 4.9, corresponding to the stoichiometry for the formation of mercuric molybdate, HgO.MoO₃. This sharp fall in pH occurs because of the presence of unreacted acidic (pH 2.6) Hg(NO₃)₂ in the cell just after completion of precipitation of mercuric molybdate. In reverse titrations (curve 3), when Na₂MoO₄ solution was added to the mercuric 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:

Hg(NO₃)₂ + Na₂MoO₄ = HgO.MoO₃ + 2NaNO₃

Employing similar concentrations of the reactants, both direct (curve 2) and reserve (curve 4) conductometric titrations between the solutions of Hg(NO₃)₂ and Na₂MoO₄ gave well-defined breaks at 1:1 molar ratio of Hg²⁺:MoO₄^2-, confirming the formation of the mercuric molybdate HgO.MoO₃, as suggested by the pH study.

Figure 4 (curves 1 and 3) illustrates the changes occurring in H⁺ concentration when Hg(NO₃)₂ solution (pH 2.6) is treated with Na₆Mo₇O₂₄ solution (pH 5.5). In direct titrations, curve 1, when Hg(NO₃)₂ solution was added from the microburette to Na₆Mo₇O₂₄ 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 Hg²⁺:Mo₇O₂₄^6- as 3:1, suggesting the formation of mercuric paramolybdate 3HgO.7MoO₃ in the vicinity of pH 4.2. In the case of reverse titrations (curve 3) when Na₆Mo₇O₂₄ solution was used as titrant, the pH first changes slowly but at the stoichiometric end-point a marked jump in pH was observed, suggesting the formation of the same compound. Employing similar concentrations of the reactants a series of direct (Fig. 4, curve 2) and reverse (Fig. 4, curve 4) conductometric titrations were performed between the solutions of Hg(NO₃)₂ and Na₆Mo₇O₂₄ . The titration curves provide well-defined breaks at a point (Table 2), where the molar ratio of Hg²⁺:Mo₇O₂₄^6- is 3:1, confirming the formation of mercuric paramolybdate as suggested by the pH study. The reaction can be represented as follows:

3Hg(NO₃)₂ + Na₆Mo₇O₂₄ = (3HgO.7MoO₃) + 6NaNO₃

Figure 5 illustrates the changes occurring in the pH and conductance values when Hg(NO₂)₂ solution is added to sodium octamolybdate solution. The titration curves provide inflections and breaks at the point where the molar ratio of Hg²⁺:Mo₈O₂₆^4- is 2:1, which corresponds to the stoichiometry for the formation of 2HgO.8MoO₃ in the neighbourhood of pH 3.7. The reaction can be represented by the following equation:

2Hg(NO₃)₂ + Na₄Mo₈O₂₆ = (2HgO.8MoO₃) + 4NaNO₃

Reserve titrations on the formation of mercuric octamolybdate did not give dependable results which may be ascribed to a considerable solubility of the product in excess of mercuric nitrate.

It is 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 neighbourhood of the equivalence point has a favourable effect. The presnce of ethanol (20%) improves the position of the end-points and increases the magnitude of the jump in pH curves, as it decreases the solubility of the precipitates formed and minimises hydrolysis and adsorption. It was found that the presence of 20% ethanol checked the hydrolysis, as a concentration higher than this could not make any change in the results. Similar studies of this system were carried out in the presence of electrolytes such as NaCl and NaNO₃. But the presence of these salts did not have a significant effect on the position of the end points.

Similar investigations on the interaction of solutions of chromium chloride with sodium molybdate at different pH levels confirmed the precipitation of Cr₂O₃.3MoO₃ (pH 5.6), Cr₂O₃.7MoO₃ (pH 4.4) and Cr₂O₃.24MoO₃ (pH 3.8) molybdates¹⁹. Electrometric studies on interaction of nickel chloride and different sodium molybdate solutions did not suggest the formation of nickel molybdates which is in accord with the observations of Sleight and Chamberland²⁰.

As the structure of these compounds is not known they are represented as double oxides, the manner which is usually adopted for such compounds^21,22.

Conclusion

The results of the electrometric investigations on acid-molybdate system, at the studied concentration levels, suggest the formation of para-Mo₇O₂₄^6- and octa-Mo₈O₂₆^4- molybdate polyanions in the vicinity of pH 5.5 and 4.1, respectively. The electrometric and analytical investigations on the interaction of mercuric nitrate and alkali molybdates indicate the formation of HgO.MoO₃, 3HgO.7MoO₃ and 2HgO.8MoO₃ mercuric molybdates in the neighbourhood of pH 4.9, 4.2, and 3.7, respectively.

Acknowledgement

The authors wish to express their sincere thanks to CNPq for financial assistance.

References

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13. Prasad, S. An. Acad. bras Ci. 1981, 53, 471; Bull Electrochem. 1990, 6, 163; Can. J. Chem. 1981, 59, 563; Quim. Nova. 1994, 17, 31.

14. Vogel, A.I. In A Textbook of Quantitative Inorganic Analysis; Longmans, London, Third Edition, 1962, p. (a) 508, (b) 238, (c) 488.

15. Honig, D.S.; kustin, Inorg. Chem. 1972, 11, 65.

16. Ozeki, T.; Kihara, H.; Hikima, Anal. Chem. 1987, 59, 945.

17. Murata, K.; Ikeda, S. Spectrochim. Acta 1983, 39A, 787.

18. Cruywagen, J.J.; Rohwer, E.F.C.H. Inorg. Chem. 1975, 14, 3136.

19. Prasad, S. In 20^a Reunião Anual da Sociedade Brasileira de Química; Poços de Caldas-MG, 1997, EQ-48.

20. Sleight, A.W.; Chamberland, B.L. Inorg. Chem. 1968, 7, 1672.

21. Standen, A. ex. ed., Kirk-Othmer Encyclopedia of Chemical Technology; Interscience Publishers, New York, Second Edition, 1967, vol. 13, p. 782.

22. Brauer, G.; Ed.; Handbook of Preparative Inorganic Chemistry, Academic Press, New York, Second Edition, 1965, vol. 2, p. 1705.

Publication Dates

• Publication in this collection
  06 Nov 2002
• Date of issue
  May 1998

History

• Received
  18 Sept 1997