ELECTROMETRIC STUDIES ON FORMATION OF SILVER AND MANGANESE MOLYBDATES AS A FUNCTION OF pH
Shiva Prasad (PQ)1, André L.F. Brito (PG)1 and Valderí D. Leite (PQ)2
1Departamento de Engenharia Química, CCT, UFPB, Campina Grande, PB
2Departamento de Química, CCT, UEPB, Campina Grande, PB
Key words: molybdates, silver, manganese
The chemistry of molybdenum is very prominent in both biological and industrial systems1,2. Recent studies have shown that certain molybdates have antiviral, including anti-AIDS, and antitumor activity3. 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. Numerous species such as HHMoO4-, H2MoO4, Mo2O72-, HMo3O113-, Mo7O246-, HMo7O245-, H2Mo7O244-, Mo8O264-, HMo8O263-, H7Mo24O785-, Mo36O1126-, etc have been proposed4.
On account of the complexity of the reaction of equilibria between the polyanions or due to the experimental difficulty in the early works, the conclusions of the earlier workers seem to be overstrained and hence it is 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 processes of tungstate, thiotungstate and vanadate anions5,6. In earlier publications the authors have reported the effect of pH change on solutions of Na2MoO47 and composition of thorium molybdates8. The results obtained on formation of molybdates of silver and manganese are presented here.
Experimental. All reagents used were analytical grade chemicals and the solutions were prepared with deionized distilled water. The variations of pH of Na2MoO4 solutions were obtained by progressive additions of determined quantities of nitric acid7. pH measurements were carried out using a Metrohm Herisau pH-meter and Schott Gerate glass combination 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. 25 ml of solution was taken in the cell each time and thermostated at 25(0.1oC. The same strengths of solutions were employed in both the techniques for the sake of comparison of results.
Analytical investigations were also carried out with a view to substantiate the electrometric results. Different molybdates of silver and manganese were prepared by mixing solutions of silver nitrate and manganese sulfate, respectively, with solutions of sodium molybdate at specific pH levels 7.8, 5.5 and 4.1. Molybdenum9a was determined by dithiol, mangenese9b by periodate and silver9c as chloride.
Results and Discussion. The results of electrometric study showed that the stoichiometry of the compounds formed by the interaction of silver nitrate with sodium molybdate at specific pH levels 7.8, 5.5 and 4.1 can be successfully followed by pH and conductometric titrations. The end-points obtained from the sharp inflections in the titration curves provide definite evidence for the formation of silver normal-Ag2O.MoO3, para-3Ag2O.7MoO3 and octa-2Ag2O.8MoO3 molybdates in the vicinity of pH 5.6, 4.7 and 4.0, respectively, according to the reactions:
Na2MoO4 + 2AgNO3 = Ag2O.MoO3 + 2NaNO3
Na6Mo7O24 + 6AgNO3 = 3Ag2O.7MoO3 + 6NaNO3
Na4Mo8O26 + 4AgNO3 = 2Ag2O.8MoO3 + 4NaNO3
Similar electrometric studies on the interaction of manganese sulfate with sodium molybdate confirm the stoichiometry for formation of manganese normal-MnO.MoO3 and para-3MnO.7MoO3 molybdates in the neighborhood of pH 6.2 and 5.3, respectively. The reactions can be represented as follows:
Na2MoO4 + MnSO4 = MnO.MoO3 + Na2SO4
Na6Mo7O24 + 3MnSO4 = 3MnO.7MoO3 + 3Na2SO4
It was noted that presence of ethanol (20%) improves the position of end-point and increases the magnitude of the jump in pH curves as it decreases the solubility of the precipitates formed and minimizes hydrolysis and adsorption. The studies on formation of manganese octamolybdate failed to give any dependable results. The analysis of the precipitates confirms the results obtained by the electrometric techniques.