Studies in monohydroxy lead chloride

J. inorg,nucl.Chem., 1968,Vol.30, pp. 2915to 2920. PergamonPress. Printedin GreatBritain

STUDIES

IN

MONOHYDROXY

K. L. Y A D A V A ,

LEAD

CHLORIDE

U. S. P A N D E Y * and K R I S H N A M U R A R I LAL

Chemical Laboratories, University of Allahabad, Allahabad, India

(Received 18January 1968) Abstract - The formation of Pb(OH)C! takes place in the-metathesis of PbCi~ and NaOH. Its existence has been confirmed with the help of i.r. absorption spectra and thermogravimetric analysis. The nature of ionisation of Pb(OH)Ci has been studied and it has been shown that it ionises mainly into CI- and Pb(OH) + ions. The value of[Pb(OH+)][Cl-I is approx. 7.6 x 10-~ at 35°C. INTRODUCTION

TI-I~ FOP.MATIONof hydroxy lead chlorides has received considerable attention since 1884, when Wood and Borden[l] reported the formation of Pb(OH)C1 as a basic lead chloride. Others[2-8] have concluded that monohydroxy lead chloride is formed as a basic salt, but some results also indicated the existence of higher hydroxy lead chlorides. In view of the conflicting results the hydrolysis of aqueous lead chloride has been investigated with a view to elucidating the species formed as basic salts. The studies included chemical analysis, conductometric and protentiometric titrations, infrared spectral studies and thermogravimetry. The behaviour of Pb(OH)CI as an electrolyte has also been studied and the solubility product determined. EXPERIMENTAL

Conductometric and analytical studies Lead chloride was obtained by precipitation from 0-5 M lead nitrate solution with hydrochloric acid (both B.D.H.A.R. chemicals) and the precipitate was thoroughly washed and dried and stored. From this sample a 0.01 M aqueous stock solution was prepared and standardised by determining lead and chloride[9, 10]. Sodium hydroxide (B.D.H.A.R.) was used to prepare a standard solution. To a fixed volume of the lead chloride solution, varying amounts of alkali were added and the total volumes were made up to 250 mi in every case. This was allowed to equilibrate for 6 hr with constant shaking at 30* ± 0-I°C. The lead and chloride contents of the supernatant liquid were determined and the pH and conductance were measured.

*Present address: H.D. Jain College, Magadh University, Arrah, India. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

J. L. Borden and J. Wood, J. Am. chem. Soc. 6, 218 (1884). A. Schulten, BMl. Soc; fr. Min~r. 20, 186 (1897). L. Cloutier, Annali Chim. 19, 5 (1933). E, Grillot, Thesis, Pads (1943). V. P. Schvedov,J. Gen. Chem. U.S.S.R. 16, 1561 (1946). H. Andre, Ann/s Chim. Phys. 44, 284 (1947). F. Calarco and A. H. Guerrero,A nnls,4s. Quin. Argent 41, 5 (1953). B. Charreton, Buil. Soc. chim. Fr. 323 (1956). F.J. Welcher, The Analytical Uses ofEDTA, p. 193. Van Nostrand, New York (1961). F. H. Welcher, Standard Method ofChemicalAnalysis, p. 265. Van Nostrand, New York (1963). 2915

2916

K . L . Y A D A V A , U. S. P A N D E Y and K. M. LAL

In another set of experiments, varying lead chloride solutions were added to a fixed volume of alkali and determinations were made as before. The observations were repeated at different concentrations of reactants. Typical results are presented in Figs. 1 and 2. Double distilled water (carbon dioxide free) was used for preparation of all the solutions and mixtures. The behaviour o f Pb(OH)CI as an electrolyte. A sample of Pb(OH)CI was prepared by the addition of 40 ml of 0.1 M NaOH to 1000 ml of 0-01 M PbCl2. The precipitated mass was washed with water and an aqueous suspension prepared and stored in a Jena bottle. The operations were performed under nitrogen in order to exclude atmospheric COs. The analysis of the precipitate indicated a ratio o f l : 1 for [Pb]: [CI]. Chloride ion activity in the suspension of Pb(OH)CI was determined potentiometrically. The Ag/AgCI electrode was prepared by the method of Carmody[l 1] and standardised using standard potassium chloride solution. Glass, and saturated calomel electrodes were also used as necessary. A specially designed cell, which avoided contamination of the solution with the chloride from the calomel electrode and allowed determinations to be made in an atmosphere of nitrogen was used. Measurements were made with the solutions exposed to air and in an atmosphere of nitrogen. For the determination of the solubility product, volumes of the filtrate from the Pb(OH)CI suspension were measured into several flasks and varying amounts of aqueous 0.1 M. NaCi solution were added. Volumes were made up to 250ml and the flasks shaken and maintained at 35_+0.1°C overnight for equilibrium to be reached. Aliquots were pipetted out using a G-4 Jena filter attached to Table 1. Determination of the solubility product of Pb(OH)CI filtrate from a Pb(OH)CI suspension taken in an atmosphere of N~ (T = 35°C) CI- ion activity mgm ions/l,

Pb(OH) + mg ions/l,

0.91 1.39 1.70 2.22 2.58 3.75 5.97

9.00 5.75 4.75 3.89 2.59 1.81 1.19

pH

K,p = [Pb(OH)÷][CI-] x l0 T

7.3 8.2* 7.3 8.0 7.3 8.0 7.3 8.6 7.3 6.7 7.3 6.8 7.3 7.1 Average = 7.6 × 10-r

*No added CITable 2. Effect of COs on a suspension of PB(OH)CI taken in a cell exposed to atmosphere ( T = 35"C)

Time 0 min 15 rain 30 min ~, 24 hr

Cl- ion activity mgm ions/l 1-01 1.15 1.27

2.45

pH 7-3 7.3 7.3 6.7

11. W. R. Carmody,J. Am. chem. Soc. 51,2901 (1929); 54, 3647 (1932).

Studies in monohydroxy lead chloride

2917

the pipette. Lead and chloride in the clear solution were determined volumetrically and potentiometrically respectively, and the pH was measured.

Thermogravimetric studies. The thermal decomposition of Pb(OH)CI, which had been dried in a vacuum dessiccator, was studied using a Stanton Thermobalance. Infrared absorption studies. Spectra were recorded using a Perkin-Elmer Grating IR spectrophotometer (Model 237) with samples mounted in KBr discs. RESULTS

AND DISCUSSION

The results of the conductometric measurements (Fig. 1) show two breaks, corresponding to [OH-]/[Pb ++] = 1 and 1-7 respectively. The first point corresponds to the formation of sparingly soluble Pb(OH)C1 and the second indicates the incomplete conversion of the monohydroxy chloride to the hydrous oxide (expected at the ratio 2). With more dilute reactants the second break tends to approach the ratio 2. However, with the dilute solutions the break at the ratio ! disappears due to the enhanced solubility of the monohydroxy chloride. These results are corroborated by the reverse titrations (Fig. 2). The pH curve (Fig. 1 .), however, indicates only one break in the neighbourhood of the ratio 1.3. A --

160

~

n o,

D

x, ._~

-

==.0

--

80

,I-I

2.5---

°ii

i

I

2

3

roH~ / ~

4.0

i 4

Fig. l. Analytical, Conductometric and electrometric results for the PbCI~-NaOH system, (NaOH varying). (PbCI= = 0.00680 M) Curve A = Conductance; Curve B = pH; Curve C = [Pb++]; Curve D = [CI-].

The results of the determinations of [Pb ++] and [C1-] in the supernatant liquids (Fig. 1) show that as the quantity of N a O H added to a fixed amount of Pb ++ increases a minimum is reached in the curves where [OH-]/[Pb ++] = 1. This is followed by an increase in C1- concentration till the value of [OH-]/[Pb ++] reaches 2. However, the chloride available in the supernatant liquid, always remains less than that present from PbClv This indicates that Pb(OH)2 is not

2918

K.L.

Y A D A V A , U. S. P A N D E Y and K. M. L A L

formed at any stage. In the reverse titration (Fig. 2.) similar observations are made. Here the C1- content in the supernatant liquid increases until [OH-]/ [Pb ÷ ÷] falls to 1, below which the increase is more rapid. The formation of Pb(OH) CI is thus indicated. The Pb ++ content inthe supernatant liquid shows a minimum at approximately [OH-]/[Pb ++] = 1.7 which again suggests the co-formation of sparingly soluble Pb(OH)CI and less soluble lead hydrous oxide from a highly alkaline medium capable of retaining lead partially in solution as plumbite.

75 X 15"0

-- 50

on

.=

r-m

¢) l-I

13.0

-- 25 I1.0

9'0

I

I

I

i

2

3

4

5

COH'-I /CPI~ ~]

Fig. 2. A n a l y t i c a l and c o n d u c t o m e t r i c results for the P b C i = - N a O H

s y s t e m , (PbCI=

varying). ( N a O H = 0-00687 M) C u r v e A = C o n d u c t a n c e ; C u r v e B = [Pb+÷]; Curve C = [C1-].

Charreton[8] obtained three inflexions in the pH titration curves and a break at [OH-]/[Pb +÷] = 1.5 in his conductance curve and suggested that a tribasic lead chloride is formed as an intermediate between Pb(OH)CI and lead hydrous oxide. We have observed only one inflexion in the pH curve at approximately [OH-]/[Pb ÷÷] = 1.3. Charreton, however, performed his experiments in the presence of added KCI. Our results do not support the formation of trihydroxy lead chloride. Thermogravimetric studies on Pb(OH)CI have shown a weight loss between 250 and 280 °, a plateau from 250 ° to 645 ° and a further weight loss above 645 °. It appears that the reaction proceeds as: -H=O

2Pb(OH)CI

250*-ZSO*C " ~

Pb2OCl~~

PbCl~+ PbO.

Studies in monohydroxy lead chloride

2919

The weight of the residue corresponded to that of PbO only (confirmed by analysis also). Anhydrous PbClz is volatile and is expected to sublime above 645°C. Duval[12] has recorded the pyrolysis curve of PbCl~ and he reported that there is a slight increase in weight from 55° to 525°C and volatilization commences at 526°C. Caven[13] has observed that Pb(OH)2 completely loses the water at 130°C to form the oxide. Thus the curve obtained in the present work does not correspond either to that of PbCI2 or Pb(OH)~ and suggests the existence of Pb(OH)C1 as a definite compound. The i.r. spectrum of Pb(OH)C1 is different from those of PbCI2 and Pb(OH)~. There is a sharp absorption at 3500 cm -1 for Pb(OH)CI, whereas in lead hydrous oxide it is weak and broad. This is the O H - stretching mode observed previously for lithium hydroxide [ 14, 15] calcium hydroxide [ 16, 17] and magnesium hydroxide [ 18, 19] Thus Pb(OH)CI is a definite hydroxy compound whereas lead hydrous oxide is essentially a hydrated oxide. Other absorption bands occur at 1600 and 1400cm -1 in the spectra of Pb(OH)CI and lead hydrous oxide. The former corresponds to the H - O H bending mode[20] and the latter is attributed to the P b - O H bending mode[21]. There is no perceptible absorption by PbCI2 in this region. The dissociation of Pb(OH)CI could occur via the following alternative schemes: Pb(OH)CI ~ P b O H + + C1-

(i)

Pb(OH)CI ~.~ PbCI + + O H -

(ii)

Both the equilibria are affected by changes in H ÷ ion concentration. Under our experiment of condition using a saturated solution of Pb(OH)CI at pH ca.7, ionisation of Pb(OH)C1 according to (i) is likely to be predominant. This view is substantiated by the value of [C1-]/[OH +] = 104 (Table 1) for solutions to which sodium chloride has not been added. The effect of C1- ion concentration on the Pb(OH)C1-NaC1 system (Table 1) shows that the H ÷ ion concentration remains unchanged on the addition of C1- ions. Thus the contribution of the reaction Pb(OH)C1 ~- PbC1 ÷ + O H C. Duval. Inorganic Thermogravimetric Analysis 2nd. Edn, p. 622. Elsevier, Amsterdam (1963). R. M. Caven, Text Book oflnorganic Chemistry, V, p. 400. Griffin London (1917). L. H. Jones,J. chem. Phys. 22,217 (1954). K. A. Wickersheim, J. chem. Phys. 31,863 (1959). W. R. Busing and H. W. Morgan,J. chem. Phys. 28,998 (1958). R. M. Hexter, J. opt. Soc. Am. 48,770 (1958). H . A . Benesi,J. chem. Phys. 30,852 (1959). R.T. Mara and G. B. B. M. Sutherland, J. opt. Soc. Am. 43, 1100 (1953). K. Nakamoto, Infra-red Spectra of Inorganic and Coordination Compounds, p. 156. Wiley, New York, (1963). 21. G. Duvai andJ. Lacomte, Bull. Soc. chim. Fr. 8,713 (1941). 12. 13. 14. 15. 16. 17. 18. 19. 20.

2920

K.L. YADAVA, U. S. PANDEY and K. M. LAL

to the equilibrium is negligibly small. On the other hand the reaction: Pb(OH)++ C1- ~ Pb(OH)CI appears more probable. The solubility product of the monohydroxy lead chloride, designated K,p = [Pb(OH)+][(C1-)], has been determined to be ca.7.6 × 10-7 at 35°C. Table 2 shows the effects of atmospheric COs on the monohydroxy lead chloride. The activity of chloride ion increases with time of contact of the suspension with the atmosphere. The pH of the suspension remains constant for 30 rain but definitely falls after 24 hr. These observations suggest the following mechanism for the action of COs on Pb(OH)CI: Pb(OH)CI ~--- P b O H + + CI-

(i)

Pb(OH) + + CO~ ~ PbCOa + H +

(iii)

Pb(OH) + + H + -~ Pb ++ + H 2 0

(iv)

Pb+++ CO~+ H~O ~ PbCOa + 2H +.

(v)

Equilibria (iii) and (iv) lie to the right and (v) contributes H + activity only to a very slight extent. The fall in pH after 24 hr is predominantly due to dissolution of COs in the system but the presence of carbonate in the precipitate has also been detected.