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VANADATES OF LEAD (II) 1 7 3

Vanadates of Lead (II): An Electrometric StudyR. S. S a x e n a and M. C. J a in

Department of Chemistry, Malaviya Regional Engineering College, Jaipur (India)

(Z. Naturforschg. 24 b, 173— 176 [1969] ; eingegangen am 22. Juli 1968)

The formation and compositions of lead vanadates formed by the interaction of lead nitrate and different alkali vanadates (ortho, pyro, meta and poly) have been investigated by means of pH, e.m.f. and conductometric titrations between the reactants at several concentrations in the presence of 20% alcoholic media. The well defined breaks and inflections in titration curves provide definite evidence for the formation and precipitation of ortho-3PbO • V20 5 , pyro-2PbO • V20 5 and meta- P b 0 -V 20 5 vanadates of lead at pH ranges 7.5 —9.0, 5.75 — 7.25 and 4.25 — 5.25 respectively.

The literature on the study of vanadates of lead(II) is scanty and m ost of the earlier workers had confined themselves to the preparative and analytical study of these com pounds. R o s c o e 1 prepared sparingly soluble lead or^Ao-vanadate P b3(V0 4)2 by adding lead acetate to sodium orthovanadate solu­tion. E p h r a im and B e c k 2 reported the form ation of lead orfAo-hexavanadate 3 P b O '2 V20 5-2 H 20 by treating m anganese hexavanadate with lead nitrate. B e r z e l i u s 3 obtained the yellow precipitate of lead due-tero-tetravanadate PbV 40 11 . D lT T E 4 prepared lead pyrovanadate P b 2V20 7 by pouring am m onium meta vanadate solution into an acetic acid solution of lead n itrate. R o s c o e (Loc-cit) on mixing sodium pyro-vanadate and lead acetate solutions, obtained lead oxy-pyro-vanadate P b O '2 P b 2V20 7; possibly a m ixture of P b 3(V 0 4) 2 and P b 2V20 7 . C a r n o t 5, Z o l o t a v i n 6 and G u i t e r 7 reported the form ation of lead meta vanadate under different conditions. In view of the insufficient and conflicting details of the reports of earlier w orkers on the composition of lead vanadates, and in the absence of any electro- metrie data on the subject, it was considered w orth­while to study the form ation and com positions of lead vanadates, obtained by the interaction of lead n itra te and alkali vanadates at different pH levels, by means of electrom etric techniques which have

1 H. E . R o s c o e , Phil. Trans. Roy. Soc. [London] 138, 1 [1868] ; Proc. Roy. Soc. [London] 16, 220 [1868] ; 18, 37, 313 [1870] ; J. diem. Soc. [London] 21, 322 [1868].

2 F. E p h r a i m and G . B e c k , Helv. chim. Acta 9, 38 [1926].3 J. J. B e r z e l i u s , Acad. Handl. Stockholm 1831. 1; Schweig-

ger’s J. 62, 323 [1831] ; Pogg. Ann. 22, 1 [1831],4 A. D i t t e , C. R. hebd. Seances Acad. Sei. 96, 1050, 121

[1883] ; 104, 1707 [1887].5 A. C a r n o t , C . R . hebd. Seances Acad. Sei. 103,121 [1887].6 V. L. Z o l o t a v i n , Zhur. Anal. Khim. 2, 364 [1947].7 H. G u i t e r , Ann. Chim. 15, 5 [1941],

provided m ore conclusive evidences on the com po­sition of metal v an ad a tes8-12. Hence the present investigation has been undertaken.

Experimental

Anal. R. (B.D.H.) reagents NaOH, V20 5 , P b (N 0 3) 2 and reagent grade H N 03 were used and their solutions prepared in air-free conductivity water.

A standard solution of sodium ortho-\anadate was prepared by digesting one mole of V20 5 in boiling solution of NaOH containing six moles of it. The alkali pyro and meta vanadate solutions were prepared by ad­ding two and four moles of H N 03 to a solution con­taining one mole of ortho-vanadate at 100 °C.

pH and e.m.f. measurements were carried out on Cambridge null-deflection type pH-meter using a wide range glass electrode and a bright platinum foil as an indicator electrode in conjunction with S.C.E. The e.m.f. and pH were noted after each addition of the titrant in small increments which were further de­creased in the region of the end point and plotted against the volume of the titrant added. The end points were determined from the sharp inflection in titration curves and the maximum values of dpH /dV and dE/dV.

The conductance measurements were carried out on an electronic eye indicator type conductometer. Twenty ml of the solution was taken in the conductivity cell each time. The corrected conductances were plotted as a function of the ml of the titrant added and the end points were judged from the breaks in titration curves.

8 R. S . S a x e n a and O. P. S h a r m a , J. inorg. nuclear Chem. 28, 195 [1966].

9 R. S. S a x e n a and M . L. M it t a l , J. inorg. nuclear Chem. 27,2553 [1965].

10 R. S . S a x e n a and O. P. S h a r m a , Indian. J. Chem. 5, 9[1967].

11 R. S . S a x e n a and M . L. M t t a l , J. Indian. Chem. S o c . 41, 125 [1964].

12 R. S . S a x e n a and O. P. S h a r m a , J. Indian. Chem. S o c . 43, 209 [1966].

1 7 4 R. S. SAXENA AND M. C. JAIN

Molarity o f solutions Equivalance points [ml]Pb(NC>3)2 3 N a2 0 -V 205 Calculated Observed

E.M.F. p n Cond.

Direct ortho vanadate titrations Figs. 1, 2 and 3 Curve I

m /5 m / 1 0 0 3 .0 3 .0 3 .0 2 .8m / 3 0 m / 5 0 0 3 .6 3 .6 3 .6 3 .5 0m / 1 0 0 m / 1 5 0 0 4 .0 3 .9 5 3 .9 5 3 .8 0

Reverse ortho-vanadate titrations Figs. 1 , 2 and 3 Curve I I

m /1 0 m / 5 3 .3 3 3 .3 5 3 .3 5 3 .3 0m / 2 0 0 m / 8 0 2 .6 6 2 .6 0 2 .6 5 2 .6 0m / 1 0 0 0 m / 6 0 0 4 .0 3 .9 5 3 .9 5 3 .9 0

Pb(N03)2 2 N a20 • V2O5 Direct p y ro - \anadate titrations Figs. 1, 2 and 3 Curve I I I

m /1 0 m /1 2 5 3 .2 0 3 .2 0 3 .2 0 3 .2 0m /6 0 m / 6 0 0 4 .0 3 .9 5 4 .0 3 .9 5m / 1 0 0 m / 1 5 0 0 2 .6 6 2 .6 0 2 .6 0 2 .5 5

Reverse p y ro -\anadate titrations Figs. 1 , 2 and 3 Curve IVm / 2 0 m /5 2 .5 2 .5 2 .5 2 .5m / 4 0 0 m /1 2 5 3 .1 2 3 .1 5 3 .1 5 3 .1 0m / 1 0 0 0 m / 4 0 0 4 .0 3 .9 5 3 .9 5 3 .9 0

Pb(N03)2 N a20 • \ 2O5 Direct meta-vanadate titrations Figs. 1, 2 and 3 Curve V

m / 1 0 m / 6 0 3 .3 3 3 .2 5 3 .2 5 3 .2 0m /2 5 m / 1 2 0 4 .1 6 4 .0 4 .0 5 4 .0m / 1 0 0 m / 5 0 0 4 .0 3 .9 3 .9 0 3 .8 5

Reverse meta-vanadate titrations Figs. 1 , 2 and 3 Curve V I

m / 5 0 m / 1 0 4 .0 4 .0 4 .0 5 4 .1 0m / 2 0 0 m /2 5 2 .5 2 .5 5 2 .5 5 2 .5 0m / 5 0 0 m /8 0 3 .2 3 .2 5 3 .3 0 3 .3 0

Table I. Summary of the results of E.M.F., pH and Conductometric titrations.

Using different concentration of reactants, a series of glass electrode, e.m.f. and conductometric titrations were carried out both by the direct and reverse methods [i.e. when P b (N 0 3) 2 solution from the microburette was added to different alkali vanadate solutions and /ice-versa]. The same strengths of reagents were em­ployed in all measurements for the sake of comparison of results which have been summarised in table I. Three figures illustrating direct and reverse pH (fig. 1), e.m.f. (fig. 2) and conductometric (fig. 3) titrations curves have been given.

Results and Discussion

S a x e n a and C o-w orkers13-15 have shown that the addition of acid to alkali ortho-vanadate and NaOH to V20 5 solutions at room tem perature causes the form ation of various p o ly -anions of uncertain com position, but when the solutions were heated after each addition of the titran t, three different iso-

13 R. S . S a x e n a a n d O . P. S h a r m a , J. i n o r g . n u c l e a r C h e m .2 8 , 1 8 8 1 [ 1 9 6 6 ] .

14 R. S . S a x e n a a n d M . L. M i t t a l , A e t a c h i m . A c a d . S e i .h u n g . 40, 1 0 9 [ 1 9 6 4 ] .

p o ly -anions viz. (V2O7)40, (VO3) 10 and (V10O27)4£ are formed in the vicinity of pH 10, 7 and 4.5 re ­spectively. Hence it was considered of interest to ascertain whether sim ilar salts of heavy metals may be precipitated as a result of double decom position. The reaction between P b ( I I ) and alkali vanadates has therefore, been studied by means of pH, e.m.f. and conductom etric titra tions with regard to the changes occurring in H 0 ion concentrations and the com positions of precipitates form ed.

Ortho-vanadate titrations: The pH of lead n itrate and alkali ortho-vanadate solutions were m easured by a wide range glass electrode and found to be in the vicinity of 4.5 and 12.0 respectively. Fig. 1 illustrattes the changes occurring in the pH values during the reaction between P b 2i and VO430 ions. In the case of direct titra tions (fig. 1, Curve I ) , when lead-nitrate solution from the m icroburette is

15 R. S. S a x e n a and M . L. M i t t a l , Naturwissenschaften 51.1 [ 1 9 6 4 ] ,

VANADATES OF LEAD (II) 1 7 5

Fig. 1. Direct and Reverse pH titrations between P b (N 03) 2 and alkali vanadates. Curve I. ml of m /5 P b (N 0 3) 2 added to 2 0 ml of m /1 0 0 3 N a ,0 • V20 5 . Curve II. ml of m / 5 3N a20 -V 20 5 added to 2 0 ml of m / 1 0 P b (N 0 3) 2 . Curve III. ml of m /1 0 P b (N 0 3) 2 added to 2 0 ml of m /1 2 5 2 Na20 ’V20 3 . Curve IV. ml of m /5 2 Na20*V 20 5 added to 2 0 ml of m /2 0 P b (N 0 3) 2 . Curve V. ml of m / 1 0 P b (N 0 3) 2 added to 2 0 ml of m /6 0 Na20 -V 20 5 . Curve VI. ml of m /2 5 N a ,0 -V 20 5 added to 2 0 ml

of m /2 0 0 P b (N 0 3) 2 .

Fig. 3. Direct and Reverse Conductometric titrations between lead nitrate and alkali vanadates. Curve I. ml of m /5 P b (N 0 3) 2 added to 2 0 ml of m /1 0 0 3 Na20 'V 20 5 . Curve II. ml of m /5 3 N a o0 -V o 0 5 added to 2 0 ml of m /1 0 P b (N 0 3) , . Curve III. ml of m /1 0 F-b (N 0 3) 2 added to 2 0 ml of m / 1 2 5 , 2 Na20 -V 20 5 . Curve IV. ml of m /5 2 N a ,0 , V20 r> added to 2 0 ml of m /2 0 P b (N 0 3) 2 . Curve V. ml of m / 1 0 P b (N 0 3) 2 added to 2 0 ml of m /6 0 Na20 -V 20 3 . Curve VI. ml of m /2 5 Na20 - V 20 5 added

to 2 0 ml of m / 2 0 0 Pb (N 0 3) 2 .

added to ortho-vanadate solution a gradual decrease in pH was observed till the stoichiom etric end-point is reached and after which the smallest addition of the titran t causes a sharp fall in pH, indicating the com pletion of reaction and suggesting the form ation of lead or^Ao-vanadate at the pH range (7 .5 —9 .0 ). In the case of reverse titra tions (fig. 1, Curve I I ) , when ortho-vanadate solution was used as the titrant, the pH first increases slowly and at the end point a m arked jum p in pH was observed, corresponding to the m olar ratio of PbO : V20 5 as 3 : 1, confirm ing the form ation of the same com pound (3 PbO 'V 20 5) according to the equation.

3 Na20 • V20 3 + 3 Pb (N 03) 2 = 3 PbO • V20 5 + 6 N aN 03 .

Em ploying sim ilar concentrations of reactants, both the direct and reverse e.m .f. (Fig. 2, Curves I and

Fig. 2. Direct and Reverse e.m.f. titrations between lead nitrate and alkali vanadates. Curve I. ml of m / 5 P b (N 0 3) 2 added to 2 0 ml of m / 1 0 0 3 Na20 -V 20 5 . Curve II. ml of m /5 3 N a 20 -V 20 5 added to 2 0 ml of m /1 0 P b (N 0 3) 2 . Curve III. ml of m /1 0 P b (N 0 3) 2 added to 2 0 ml of m / 1 2 5 , 2 Na20 -V 20 5 . Curve IV. ml of m / 5 2 N a 20 - V 20 5 added to 2 0 ml of m / 2 0 P b (N 0 3) 2 . Curve V. ml of m / 1 0 P b (N 0 3) 2 added to 2 0 ml of m / 6 0 Na20 , V20 5 . Curve VI. ml of m / 2 5 Na20 -V 20 5 added

to 2 0 ml of m / 2 0 0 Pb (N 0 3) 2 .

1 7 6 VANADATES OF LEAD (II)

II) and conductom etric (fig. 3, Curves I and II) titrations were also carried out. The titra tion curves yield well defined breaks and inflections at the sto i­chiometric end point corresponding to the reacting ratio of PbO : V20 5 as 3 : 1 and confirm the fo rm a­tion of the identical com pound, lead ortho-vanadate, 3 P b O V20 5 .

Pyro-vanadate ti tra tions: Alkali pyro-vanadate solutions were prepared as described earlier. The d irect titra tion curves (fig. 1, Curve III) are sim ilar to those of d irect ortho-v anadate titra tion curves, w ith the inflection corresponding to the m olecular ratio of PbO : V 20 5 as 2 : 1 , suggesting the fo r­m ation of lead pyro-vanadate, 2 P b 0 -V20 5 in the pH range (5.75 — 7 .2 5 ). In the case of inverse titra ­tions (fig. 1, Curve IV ), the first addition of alkali pyro-vanadate (pH äs 10.0) to lead nitrate solution (pH ä j 4.5) causes an initial decrease in pH till about half the volume of titran t required for the precipitation of lead pyro-vanadate, is added. This initial lowering in pH value is due to the presence of hydrolysed acid from lead salt. Later on, with the progress of the reaction, pH begins to rise and a pronounced upw ard inflection is obtained a t the stoichiom etric end point, corresponding to the fo r­m ation of lead pyro-vanadate, having the m olecular com position 2 P b 0 -V20 5 . The form ation of lead pyro-vanadate may be represented as follows:

3 Na20 ■ V20 5 + 2 H N 03= 2 Na20 • V20 5 + 2 N aN 03 + H.,0 .

2 Na20 • V20 5 + 2 Pb (N 0 3) 2= 2 PbO • V20 5 + 4 N aN 03 .

The course of this reaction was also followed by m eans of potentiom etric and conductom etric titra ­tions. A sharp break in titra tion curves (figs. 2 — 3, Curves III — IV) was obtained at a point correspon­ding to the form ation of lead pyro-vanadate.

Meta-vanadate t i tra tions: The solution of sodium m eta-vanadate was prepared as described earlier and its reaction with lead salt has been investigated by perform ing pH, e.m .f. and conductom etric titra ­

tions. The end-points obtained from the titra tion curves (figs. 1 — 3, Curves V and VI) indicate that P b2° and VO310 ions com bine in the ratio of 1 : 2 and suggest the form ation of lead m eta-vanadate P b 0 -V20 5 at the pH range 4.25 — 5.25. The reac­tion can be represented as follows:

3 Na20 • V20 5 + 4 H N 03- Na20 • V20 5 + 4 N aN 03 + 2 H ,0 .

Na.,0 • V20 5 + Pb (N 03) 2 = PbO • V20 5 + 2 N aN 03 .

The feeble breaks obtained in reverse conducto­metric titrations curves (fig. 3, Curve V I) as a con­sequence of the appreciable increase in conductance values in initial stages of titra tion indicate clearly the liberation of highly mobile H ions and provide strong support to the observations noted in reverse pH and e.m.f. titrations.

The reaction between lead nitrate and alkali poly- vanadate has also been studied, but the curves do not exhibit any appreciable break or inflection at the stoichiom etric end point. This may be ascribed to (I) small difference in the pH values of reactants(II) the presence of N aN 0 3 in appreciable am ount in po/y-vanadate solution, preventing the occurence of breaks in the conductom etric titra tion curves.

It is observed that after each addition of the titrant, it takes a little time for the pH, e.m .f. and conductance values to become steady. T horough stirring in the vicinity of the end point has a favourable effect. Each titra tion takes half an hour for completion.

It is apparent from the present study that the lead salt with different alkali vanadates yields co r­responding salts by meta thesis, namely lead ortho 3 P b 0 'V 20 5, pyro-2PbO • V20 5 and meta-VbO' V20 5 vanadate at pH ranges 7.5 —9.0, 5.75 — 7.25, 4.25 — 5.25 respectively.

The authors wish to express their sincere thanks to C.S.I.R., New Delhi, for the award of a fellowship to one of us (M.C.J.) and to Professor R. M. A d v a n i ,

Principal, Malaviva Regional Engineering College, Jaipur, for providing research facilities.