Thermoanalytical study of coprecipitated lead and barium carbonates

Themrochimica Acta; 32 (1979) 310-316 @ Elsevier Scientific Publishing Company, Amsterdam-

Note --

_.I_.

Printed in The Netherlands

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fhermoanalytical

study of coprecipitated

M. E. GARCIA-CLAVEL,

M. T. CASAIS-ALVAREZ

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31 August

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lead and barium carbonates

i\ND

L. RAMOS-ALVARO

Secci611 de Termoamilisis p Reactiridad de .%lfdos, Dcpar-tamento de Quimica Alralirica Facrdrad de Qrrimicas, Cirrdad UMrersitar-ia, Macfrid 3 (Spain) (Received

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&I C.S.I.C.,

1978)

Following a study on coprecipitation of carbonates, which we began years ago with the alkaline-earth carbonates’ - 1, we present a study of the PbCO,-BaCO, system. Nitta et aI.‘, employing samples of various Pb/Ba ratios, obtained solid solutions whose decomposition proceeds in a manner similar to that of cerussite, with formation of the intermediate PbO - PbCO,. They carried out the precipitation at room temperature by adding aqueous solutions of FYJ(NO~)~ and Ba(N03)z to Na2C0, solution. They found adsorbed Na+ on the precipitates (0.08 rns 3-l) by atomic absorption measwements. We also used P’o(NO,)~ and Ba(NO,),, however, while they used Na2C03, we used (NH,),CO, because its easy voIatiIization ensures the non-existence of foreign ions in the precipitate. Starting from Pb(N03)I/Ba(N03)2 solutions in the relationship l/l, changes in the precipitation conditions produce a definite change in the composition and structure of the precipitates. The decomposition mechanism proposed by us is essentially different from that given by Nitta et al.‘. EXPERlhIENTAL

Reactants The reactants were Ba(N03j1 and Pb(NO,), supplied by Merck (A-R. quality) and (NH,)ZCO, supplied by Carlo Erba (A-R. quality). Preparatioil qf the samples We prepared 24 samples by precipitation from aqueous solutions of equimolecular mixtures of Ba(N0,)2 and Pb(NOJ) z with (NH,),CO, aqueous solution. We employed an amount of carbonate seven times larger than the theoretical one. We studied how the folIowing variable parameters affect precipitation conditions: (a) temperature (at room temperature and boiling point); (b) aggregation velocity of solutions (abruptly and by continuous drippins at an interval of 2 to 4 set between drops (-0.03 ml); (c) the way in which the said addition is carried out

B.p. B.p* B& B.p. B.p. B.p. B.p. B.p. Room Room Room Room

12

13 14 15 16 17 18 19 20 21 22 23 24

tcntpe temp. temp. temp.

temp.

I~oom

Room temp. Abruptly Abruptly 0.03 ml/2see 0.03m1/2 set 0.03ml/4see 0.03ml/4see 0.03ml/4WC 0.03ml/4see 0.03IllI/ see 0.03ml/4 see 0.03ml/4 set 0.03IllI/ set

C/N

N/C

C/N

N/C

C/N

N/C

C/N N/C C/N

C/N' N/C

N/C

N/C C/N 0.5 0.5 0 0 0 0 24 24 0 0 24 24

temp.

Room tcrnp. Room temp.

Room temp.

Room

B.p. B.p.

Room temp. Room temp.

l/l.00 I /0.9(14 I JO.967 I /0.968 I JO.990 110.975 l/I .Ol l/O,975 l/O.90 I /0.97 l/l,00 l/O.98

l/1.0(,8 l/1.071 l/l.071 l/1.072

0 0 24 24

N/C C/N

Abruptly Abruptly Abruptly Abruptly

Room temp. Room temp.

9 10 II

l/O.915 I JO.978

0 0

N/C C/N

Abruptly Abruptly

I /0*9x0 I to.982 l/O,253 1JO.256 l/O.137 l/O. 139

BP. Bq

0.p. B.p. Rap. B.p. B,p. B,p.

7 8

1.5 1.5 7.5 IS 12 12

Abruptly Abruptly Abruptly Abruptly Abruptly Abruptly

13-p. B.p. B.p. B.p. B.p. B.p.

1 2 3 4 5 6

N/C C/N N/C C/N N/C C/N

CONDITIONS 01’ I’RECII’I’I’ATION AND THE COMPOSITIDN AND STRUCTURE OF THE I’RECII’ITATES WTAlNI:D

TABLE1

!

1 Ccrilssite arid witheritc mixture

\

Solid solution

w

312 (nitrates

solution

on carbonate

solution

and vice versa);

(d) digesting

time of precipi-

tates in the mother liquors (0, 1.5 h, 7.5 h and 24 11); (e) digesting temperature (at room temperature and boiling point). All precipitates were filtered through a No. 4 coarse filter, washed with cold distilled water and dried at room temperature_ in Table I, we give conditions of precipitation, the composition and the structure of all precipitates obtained. Composition of the different precipitates has been determinated by quantitative chemical analysis. CO:-: elemental microanalysis of C, analyzer Perkin-Elmer 240; Pb” *: complexometric titration with EDTA usins as indicator Eriochrome Black T; Ba’ f : substitution titration with magnesium complexonate also using Eriochrome Black T. The study of precipitates has been carried out by thermogravimetry, DTA

and X-ray powder

diffraction.

Apparatus TG apparatus. A Chevenard thermobalance (model Adamel) was used with a -maximum operating temperature of 1250°C. Photographic recording was used. The heating rate was 300°C h-‘. The samples were packed into a Staatlich Berlin porcelain crucible

(A 4/O), and the temperature

crucible. DTA apparatzrs. This was constructed

measured

was the air surrounding

in the laboratory

using a vertical

the

furnace

and a temperature regulation system (both Adamel). A sintered alumina holder (19 X 12.5 :< 14 mm, shaped in the form of a rectangular parallelepiped divided into two equal cavities, with a wall thickness of 1.5 mm) was used. A differential chrome]/ alumel thermocouple was used and the data recorded using a Metrohm Labograph E47S. The temperature of the air surroundin g the specimen holder was measured. A heating rate of 300°C h-I was employed. X-Ray powder diflraction. A Philipps P.W. 1010 generator was used with a diffractometer P.W. 1050 Geiger counter and Cu Kr, radiation with a Ni filter. The data was recorded graphically (P-W. 1051). RESULTS

AND

DISCUSSION

According

to the obtained

fied into three groups. (I) Amorplzozts precipitates. addition

of the solutions

(nitrates

diffractograms, These solution

the different

precipitates on carbonate

are

samples

obtained solution

may be classiby the

abrupt

and vice-versa)

at

* Concerning this titration, literature about complexometry (by example Schwarzenbachfi, Bermejo Martinez;). suggests that the alkaline-earth metals are titrated together with the Pb”‘. Now, in the original papers, FlaschkaS and Flaschka and Huditz” definitely say that ions titrated together with the Pb”-, are Ca2+ and Mg”;. We have verified that the ion Ba”+ does not perturb the Pb”+ titration, since it does not form complex with the indicator, under the conditions of this titration.

(4)

50

60

Fig. 1. 0, B&t&; witherite

0, PbC03, (I)

coprecipitated

mixture;

I

I

I

40

30

20

2e’

Cerussite and .(3> solid solution;

witherite mechanical l.4) P~CQI.

mixture;

(2) ccrussite

and

314 high temperature and without digestion of precipitates in the mother liquors. If the precipitation is carried out at room temperature, with or without digestion of precipitates in the mother liquors, the start of crystallization is observed, but we cannot determine exactly if it is a cerussite and witherite mixture, or a solid solution. The composition of these precipitates is PbCOJBaCO, = l/l M, according to the quantitative chemical analysis. (2) Cerrtssite and witherite mixtzu-es, non-perfectly crystallized (Fis. 1). They have been obtained in most of the studied cases: (a) when the precipitate obtained at boiling point by abrupt addition of the solutions is heated in the mother liquors at this temperature for 30 min; (b) they are also obtained when solutions are added sIowly or very slowIy, with or without digestion of the precipitates in the mother liquors; operating in this manner it was found that no matter what the temperature of solutions, the structure of the precipitates was the same. The cerussite/witherite ratio is equimolecular in all samples. In Fig. 1, the diffractogram 1 corresponds to an equimolecular mixture obtained by mechanical agitation of PbCO, and BaCO,. both obtained by us. Diffractogram 2 corresponds to the diagram of sample number 22, one of the I2 giving identica1 diagrams. In both diagrams all the characteristic lines of the PbCO, and BaCO, appear. We can also see the worse crystallization of the coprecipitated mixture respecting the mechanical mixture. (3) So/k1 solution. Solid solutions are obtained when the precipitates, obtained with heat and by means of mixin g solutions abruptly, remain in the mother liquors at the boiling point for I.5 h or longer. Composition of the solid solution obtained boiling for 1.5 h is Pb/Ba = l/l M. By boiling it for a longer period, BaCO, dissolves partially, so the composition for 7.5 h and 12 h is Pb/Ba = l/O.25 and l/O.1, respectively. The diffractogram of sample 5 is shown in Fig. 1, diagram 3, (which is identical with the diffractogram shown by sample number 6). The diffractogram of PbCO, prepared by us* is shown in diagram 4. The diagram of the solid solution is similar to the diffractogram of the PbCO, with the lattice slightly defo.rmed. Both the chemical analysis and the thermal studies (TG and DTA) s h ow the existence of barium in samples 5 and 6. We also made the diffractogram of a mechanical mixture with a relationship in similar order to the relationship of the formerly mentioned samples, that is PbCOJBaCO, = l/0.1, In this diagram appear all the characteristic lines of both compounds, so we can deduct that in the coprecipitated samples the barium is forming part of the lattice of the PbCO,. * We have prepared the PbCOa by mixing hot solutions of Pb(NOa)e and (NH&C03 and keeping the precipitate obtained in the mother liquors at boiling point for a long time. In the literaturelO, we had found that solutions must be kept cold in order to avoid formation of basic carbonate, but the diagram of the precipitate obtained by us confirms the formation of PbCOa, in spite of the prolonged boiling.

315 (11 3000.

loo0

YOOO

250°

3500

-

qnno I”“-

2750

16ii0°C

-a ’ 900”

looo”c

\ I

I41

100

9oo”

so-

(3)

loo0

I

550°

(21

6000

500”

2000\

4000

T

CCfiO 22”

I

700° 7nnO ,“”

9oo”

lllo”c

1

9600

11ooo

115ooc

mg

Fig. 2. TG curves. (1) Amorphous precipitate sample weight 290.6 mg; (2) ccrussitc and withcrite coprecipitated mixture, 360.2 mg; (3) solid solution sample weight 413.0 mg: (4) cerrusitc and withcrite mechanical mixture sample weight 347.7 mg.

Fig. 3. DT’A curves. (I) Cerussite and witherite mechanical mixture; cerrusite and witherite coprecipitated mixture; (4) solid solution.

(7) amorphous

precipitate;

(3)

On making

the

thermal

study

of the

different

precipitates and examining samples by X-ray diffraction taken at predeterminated points of the TG (Fig. 2) and ‘0 3), it has been found that in the calcination of all of them the same DTA curves (F13.

316 reactions are produced. These reactions and the approximate for which the reactions take place, are as follows PbCO,

- BaCO,

Pb30,

4- BaCO,

PbO t

(PbC03

intervals

+ BaCO,

(4

-520-620°C _ _.~ __ _ -+ PbO + BaCO, -

BaCO,

(b)

650-880 “C _ BaPb03 .____

w 1050-I 150°C _ ___

BaPbO,

- 250-520 “C _-_--, Pb,O,

-+ BaCO,)

temperature

(c)

BazPbOs

(d)

The presence of Pb,O, (sometimes with BaCO, alone, and sometimes with red PbO too) is detected by TG in the curves of solid solutions and cerussite and witherite mixtures, but it is not verified in the case of amorphous samples. The transformation carbonates

a-PbO 4 mixtures.

Reaction

/J-PbO is d etected

(c) correspondin,

only by DTA

* to the formation

curves

of BaPbO,

of soiid solutions is registered

and

in DTA

curves as two consecutive endothermic peaks. Reaction (d) due, on one hand, to the high temperatures in which this reaction takes place, and on the other, to the fact that the residue, a BazPbOS and PbO mixture, is melted, has been established by TG only. The thermal behaviour is completely different from that of the mechanical mixture, in which the following reactions are produced; they are the same as that of cerussite decomposition’ 1 PbCO,

t

PbC03

-

BaCO, 2Pb0

Later

--40-3*o”c

4 BaCO,

the formation

l

PbC03

-310-400”c of BaPbO,

. 2Pb(-j

+

BaCO,

(a>

___ PbO + BaCO, and Ba,PbO,

takes

place

accordins

(b)

to the

reactions (c) and (d) aforementioned. REFERENCES I

2 3 4 5 6 7 S 9 10 11

F. Burriel-Marti, M. E. Garcia-Clavel and M. Rodriguez de la Peiia, C.R. XXXVI Cmgr. Put. Chits. Id., Bsusscls, 1967, Vol. I, p. 205. F. Burriel-Marti, M. E. Garcia-Clavel and M. Rodriguez de la Peiia, Memaria XXX VI Cotlgr. Irrt. Qztinr. it&., Mud-id, 1967, Vol. 1, p. 237. M. E. Garcia-Clavel, F. Burriel-Marti and M. Rodriguez de la Pefia, II&. Quittz. Atlal., 24 (1970) 31. F. Burriel-Marti, M. E. Garcia-Clavei and M. Rodriguez de la Peiia, Quitn. Znd. (Madriri), 18 (2) (1972) 4. Y. Nitta. T. Yamaguchi and H. Kuno, Nippotr Kagaku Zasshi, 92 (1971) 219. G. Schwarzenbach, Cottqtle_~ottwiric Titrations, Interscience, New York, 1957. F. Bermejo Martinez and A. Prieto Bouzas, Aplicaciones Atraliticas &I AEDT y andlogos, Universidad Compostelana, Santiago de Compostela, 1975. H. Flaschka, Mikrocl~!rr~. Ada, 39 (1952) 313. H. Flaschka and F. Huditz, 2. Atzal. Cizenz., 137 (1952) 172. Cotnprehettsive Ittorgatric Chemisrry, Vol. 2, Pergamon, Oxford, 1973. F. Burriel-Marti y M. E. Garcia-Clavel, An. R. Sot. Esp. Fis. Qubn., Ser. B, LIII (1967) 317.