Formation of Nickel Vanadates as a Function of pH

A natureza precisa da reação entre ácido clorídrico e orto-vanadato de sódio foi estudada por técnicas eletrométricas envolvendo titulações potenciométricas e condutométricas. As inflexões e degraus bem definidas nas curvas de titulações confirmaram a existência de ânions, pyro-V 2 O 7 , meta-VO 3 – e poly-H 2 V 10 O 28 4– correspondendo às razões de VO 4 :H como 1:1, 1:2 e 1:2,6 na vizinhança do pH 10,5; 7,4 e 3,6, respectivamente. A interação entre soluções de cloreto de níquel e vanadato de sódio a específicos níveis de pH 12,4; 10.5; 7,4 e 3,6 também foi estudada por titulações potenciométricas e condutométricas entre os reagentes. Os pontos finais obtidos a partir de inflexões nítidas nas curvas de titulações forneceram evidências incontestáveis sobre a formação de vanadatos ortho-3NiO.V 2 O 5 , pyro-2NiO.V 2 O 5 e meta-NiO.V 2 O 5 de níquel nas proximidades dos valores de pH 8,9; 8,0 e 6,8; respectivamente. Investigações analíticas sobre os precipitados formados confirmam os resultados do estudo eletrométrico.


Introduction
The chemistry of vanadium is very prominent in both biological and industrial systems. 1 Besides this, a new interest in the chemistry of vanadium has developed during the last decade. 1,2This has arisen in part from antiviral, including anti-AIDS activity of vanadates and their interaction with biological molecules like proteins. 3,4anadium also exhibits catalytic properties in an extensive variety of chemical reactions.These include the use of vanadium oxide as catalyst in the following reactions: sulfur dioxide to the trioxide, the sulfonation of aromatic hydrocarbons and of pyridine, the reduction of olefines; the oxidation of hydroiodic acid by hydrogen peroxide, of sugar by nitric acid, of alcohol by air, of stannous salts by nitric acid, of cyclic organic compounds by hydrogen peroxide, of naphthalene by air, and the reduction of aromatic hydrocarbons by hydrogen. 5][8][9][10] The structure of the vanadia is a very important factor. 9,10Khodakov et al. 11 have recently determined that the oxidative dehydrogenation rates of propane increase as the size of poly-vanadate domain increases.[8][9][10][11][12][13][14][15] In highly alkaline solution, pH >14, vanadium(V) exists as a tetrahedral VO 4 3-anion. 16On acidification the aqueous solutions turn from colorless to orange-yellow polymeric species.A survey of the literature suggests the occurrence of a series of alkali metal vanadates having the ratios of Na 2 O:V 2 O 5 as 3:1, 2:1, 1:1, 1:2, 1:2.5, 1:3, 2:3 etc in solution under different conditions.The existence of so many polymeric species 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.8][19] The knowledge on the formation of different species under different conditions may help in the explanation of the catalytic performance of vanadium and may be a key to understanding the catalytic mechanism.In an earlier publication the results of pH change by HNO 3 on solution of Na 3 VO 4 , and composition of lead vanadates have been reported. 20The results obtained on pH change of Na 3 VO 4 by HCl, and on formation of different vanadates of nickel are presented here.

Experimental
V 2 O 5 , NaOH, NiCl 2 .6H 2 O, HNO 3 , ethanol and hydrochloric acid of extra-pure grade were used and their solutions were prepared with deionized distilled water.The solution of sodium ortho-vanadate Na 3 VO 4 was prepared by digesting one mol of V 2 O 5 in boiling solution of NaOH containing six mol of it.
Potentiometric measurements were carried out on Metrohm Herisau pH-meter using Scott Gerate glass combination electrode.Conductance values were recorded by employing a Metrohm conductometer.A series of potentiometric and conductometric titrations was carried out between sodium ortho-vanadate in concentrations ≥ 10 -4 mol L -1 and hydrochloric acid using same strength of the reactants in each technique.All observations were taken at the state of chemical equilibrium.For attaining the equilibrium state the titrations were performed by heating the solution after each addition of titrant and cooling to 25 o C before taking observations.The achievement of constant values of pH and conductance required about 40 s boiling after each addition of the titrant for the formation of pyro-vanadate, whereas the time needed for attaining the state of chemical equilibrium for the formation of meta-and poly-vanadate was 1 min and 2.5 min, respectively.The curves were plotted between pH and corrected conductance versus volume of acid used.As the inflections of pH curves were not strongly defined, pronounced maxima in dpH/dV were used to locate the end-points.
The formation of nickel vanadates was investigated by the action of nickel chloride with different vanadate anions at specific pH levels 12.4, 10.5, 7.4 and 3.6 using different concentrations of the reactants.For this purpose, the variations of pH of Na 3 VO 4 solutions were obtained by progressive additions of determined quantities of hydrochloric acid.A series of potentiometric and conductometric titrations was performed by direct and reverse methods, i.e. when nickel chloride solution from the microburette was added to sodium vanadate solution and vice-versa.25 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 nickel chloride and different sodium vanadates were also analyzed to substantiate the electrometric results.The different nickel vanadates were prepared by mixing stoichiometric amounts of nickel chloride solution with the respective sodium vanadate solutions.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 above precipitates was dissolved in a minimum quantity of nitric acid and then analyzed quantitatively for nickel by dimethylglyoxime 21 and vanadium as silver vanadate. 21From the proportions of nickel and vanadium in the compounds thus obtained their composition was established.

Results and Discussion
In an earlier publication 20 Prasad et al. have shown that the addition of HNO 3 to Na 3 VO 4 solutions at room temperature causes the formation of various poly-anions of uncertain compositions, but when the solutions were heated after each addition of titrant the three different vanadates viz.Pyro (Na 4 V 2 O 7 ), meta (NaVO 3 ) and poly (Na 4 H 2 V 10 O 28 ) are formed in the pH ranges 10.1-10.7,7.0-7.5 and 3.4-3.8,respectively.In the present study related to the formation of different nickel vanadates as a function of pH, by the interaction of nickel chloride with alkali vanadates at specific pH levels, the use of HCl for pH variation was considered more appropriate.Hence it was considered of interest to ascertain whether similar alkali vanadate species are formed by the action of HCl with Na 3 VO 4 solutions.A series of glass electrode and conductometric titrations of Na 3 VO 4 solution with HCl were therefore performed.A typical titration curve between pH observed and volume of the acid added is demonstrated in Figure 1 (curve 1).The three inflections in the titration curve at the molar ratios of H + :VO 4 3-as 1, 2 and 2.6 corresponding to the stoichiometry for the formation of pyro-V 2 O 7 , meta-VO 3 -and poly-H 2 V 10 O 28 4-anions confirm the formation of the same three vanadate species as obtained by the previous study with HNO 3 . 20The position of these inflections is further checked by dpH/dV graphs (curve 2).The conductometric titrations between the acid and Na 3 VO 4 also confirm the formation of the same species (Figure 1, curve 3).The existence of V 6 O 16 2-as reported by Sen Gupta 22 and Russel and Salmon, 23  The stepwise condensation of ortho-vanadate to polyvanadate can be represented by the following set of equations: The above studies show that the addition of hydrochloric acid to sodium ortho-vanadate under suitable conditions causes the formation of three different sodium vanadates containing the anions Pyro-V 2 O 7 4-, meta-VO

Formation of nickel vanadates
A series of direct and reverse electrometric titrations between the solutions of nickel chloride (pH 6.0) and different sodium vanadates at specific pH levels 12.4, 10.5, 7.4 and 3.6 were realized and the results obtained from the stoichiometric end-points are summarized in Table 1.Only three figures illustrating the formation of nickel orthovanadate (Figure 2), pyro-vanadate (Figure 3) and metavanadate (Figure 4) by direct titrations have been given for the sake of brevity.The precipitates obtained at the end-points of the electrometric titrations between nickel chloride and sodium vanadates were also analyzed to substantiate the electrometric results.The analytical results are presented in Table 2.
Nickel ortho-vanadate.Using different concentrations of nickel chloride (pH 6.0) and Na 3 VO 4 (pH 12.4) a series of potentiometric titrations was carried out.Figure 2 (curve 1) shows the changes occurring in H + ion concentration when the solution of Na 3 VO 4 was titrated with nickel chloride solution.It may be noted that the first addition of nickel chloride solution to the alkaline vanadate results   in gradual decrease in pH to about 11.0.Further addition of the titrant brings about steep fall in pH value at a point where the molar ratio of Ni 2+ :VO 4 3-is 3:2 (see Table 1), corresponding to the stoichiometry for the precipitation of nickel ortho-vanadate 3NiO.V 2 O 5 in the vicinity of pH 8.9.The reaction can be represented as follows: In the case of conductometric titrations of the solution of Na 3 VO 4 with nickel chloride (Figure 2, curve 2) the conductance value decreases gradually in the beginning of the titration (due to removal of the VO 4 3-ions in the form of a precipitate), but after completion of the reaction, conductance starts rising with the increase in ionic concentration at the ratio Ni 2+ :VO 4 3-as 3:2, which coincides with the stoichiometry for the formation of nickel ortho-vanadate as indicated by the potentiometric study.
Nickel pyro-vanadate. Figure 3 (curve 1) illustrates the changes occurring in pH when nickel chloride solution (pH 6.0) was added from the microburette to the solution of sodium pyro-vanadate (pH 10.5).The titration curve shows a well-defined inflection at the equivalence point, where the molar ratio Ni 2+ :V 2 O 7 4-is 2:1, corresponding to the stoichiometry for the formation of nickel pyro-vanadate 2NiO.V 2 O 5 in the neighborhood of pH 8.0.The reaction can be represented by the following equation: Employing similar concentrations of the reactants a series of conductometric titrations between the solution of nickel chloride and sodium pyro-vanadate was carried out.Well-defined breaks in the titration curves (Figure 3, curve 2) were obtained at 2:1 molar ratio of Ni 2+ :V 2 O 7 4-, which confirm the formation of the identical compound, nickel pyro-vanadate 2NiO.V 2 O 5 .
Nickel meta-vanadate.Using different concentrations of nickel chloride (pH 6.0) and sodium meta-vanadate (pH 7.4) a series of potentiometric and conductometric titrations (Figure 4) was carried out.The breaks and inflections in the titration curves at the stoichiometric end-point corresponding to the molar ratio Ni 2+ :VO 3 -as 1:2, suggest the formation of nickel meta-vanadate NiO.V 2 O 5 around pH 6.8, according to the equation: The feeble breaks obtained in the conductometric titration curves (Figure 4, curve 2) may be ascribed to the presence of sodium chloride in appreciable amount in metavanadate solutions.
Similar studies using the different sodium vanadate solutions as titrant (reverse titrations) were also realized (Table 1).The breaks and inflections in titration curves confirmed the results obtained by the direct titrations.As the curves were normal in shape and nature the figures of these titrations are not presented for the sake of brevity.
The reaction between nickel chloride and sodium polyvanadate Na 4 H 2 V 10 O 28 (pH 3.6) was also studied, but the curves did not provide conclusive evidence for the formation of nickel poly-vanadate.
The precipitates obtained at the end-points of the titrations of nickel chloride with different sodium vanadates were analyzed by classical methods.Nickel was determined by dimethylglyoxime and vanadium as silver vanadate, and oxygen was calculated from the difference 4. Nickel meta-vanadate titrations.25 mL of 8.00 x 10 -3 mol L -1 NaVO 3 titrated with 5.00 x 10 -2 mol L -1 NiCl 2 .

3.
Nickel pyro-vanadate titrations.25 mL of 2.00 x 10 -3 mol L -1 Na 4 V 2 O 7 titrated with 5.00 x 10 -2 mol L -1 NiCl 2 . in the percentage.From the proportions of nickel, vanadium and oxygen in the compounds thus obtained, their composition was established.The results obtained (Table 2) confirm those obtained by the electrometric study.
It was noted that the presence of ethanol in nickel vanadate titrations slightly improves the end-points and gives better results as it decreases solubility of the precipitates formed and minimizes hydrolysis and adsorption.20% ethanolic medium was therefore employed for the entire course of the study.A thorough stirring in the vicinity of the end-point had a favorable effect.
As the structure of these compounds is not known these are represented as double oxides, the manner, which is usually adopted for such compounds. 26,27

Conclusions
The results of the electrometric investigations on the system hydrochloric acid and sodium ortho-vanadate, at the specific concentration level of ≥ 10 -4 mol L -1 , suggest the formation of pyro-V 2 O 7 , meta-VO 3 -and poly-H 2 V 10 O 28 4vanadate anions in the neighborhood of pH 10.5, 7.4 and 3.6, respectively.The electrometric and analytical investigations on the interaction of nickel chloride with sodium vanadate at specific pH levels 12.4, 10.5 and 7.4 provide cogent evidence for the formation and precipitation of ortho-3NiO.V 2 O 5 , pyro-2NiO.V 2 O 5 and meta-NiO.V 2 O 5 vanadates of nickel around pH 8.9, 8.0 and 6.8, respectively.The composition of nickel vanadates can thus be represented by the general formula nNiO.V 2 O 5 , where the proportion of nickel oxide in the vanadate obtained decreases with the pH of the medium.
HV 10 O 28 5-as reported by Bystrom and Evans24 and Naumann and Hallada25 could not be confirmed by this study.
Therefore it was considered suitable to use hydrochloric acid for pH variation of the alkali vanadate for the study on formation of different nickel vanadates as a function of pH.To carry out this study, a solution of sodium ortho-vanadate was prepared by digesting one mol of V 2 O 5 in boiling solution of NaOH containing six mol of it.The solutions of sodium pyro-, meta-and poly-vanadates were prepared by adding 1, 2 and 2.6 mol of HCl to 1mol of Na 3 VO 4 at 100 o C.

Table 1 .
Summary of the results of electrometric study on the formation of nickel vanadates.Volume of titrated solution taken in the cell = 25 mL

Table 2 .
Summary of analytical results of the precipitates of nickel vanadates a Nickel chloride solution added to sodium ortho-vanadate solution; b Sodiun ortho-vanadate solution added to nickel chloride solution.