Synthesis, characterization and thermal decomposition kinetics of barium(II)bis(oxalato)barium(II)dihydrate and lead(II)bis(oxalato)lead(II)monohydrate

Article abstract of DOI:10.1016/S0040-6031(99)00042-8

Barium(II)bis(oxalato)barium(II)dihydrate (BOD), Ba[Ba(C₂O₄)₂]·2H₂O and lead(II)bis(oxalato)lead(II)monohydrate (LOM), Pb[Pb(C₂O₄)₂]·H₂O have been synthesized and characterized by elemental analysis, conductance measurements, IR spectral, reflectance and X-ray powder diffraction studies. Thermal decomposition studies (TG, DTG and DTA) in air showed that at ca. 1000°C, a mixture of BaO₂ and BaCO₃ is generated from the compound, BOD, through the formation of BaO₂ and BaC₂O₄ at around 514°C as intermediates. LOM gave Pb₂O₃ as final product at ca. 390°C. DSC study in nitrogen showed a different decomposition pattern from that in air for both compounds. Using seven mechanistic equations, the rate controlling process of the dehydration and decomposition mechanism of BOD is inferred to be one- and three-dimensional diffusion, respectively. The decomposition mechanism of LOM is a phase boundary reaction having cylindrical symmetry. Some of the decomposition products were identified by analytical, IR spectral and X-ray powder diffraction studies. A tentative reaction mechanism for the thermal decomposition of both the complexes is proposed.

Full text of DOI:10.1016/S0040-6031(99)00042-8

Thermochimica Acta 329 (1999) 129±139
Synthesis, characterization and thermal decomposition kinetics of barium(II)-
bis(oxalato)barium(II)dihydrate and lead(II)bis(oxalato)lead(II)monohydrate
a,*
N. Deb , S.D. Baruah , N. Sen Sarma , N.N. Dass
b
c
c
a
Department of Chemistry, North Eastern Regional Institute of Science and Technology, Nirjuli 791 109, Arunachal Pradesh, India
b
Regional Research Laboratory, Jorhat 785 006, Assam, India
c
Department of Chemistry, Dibrugarh University, Dibrugarh 786 004, Assam, India
Received 19 June 1997; received in revised form 26 September 1998; accepted 15 January 1999
Abstract
Barium(II)bis(oxalato)barium(II)dihydrate (BOD), Ba[Ba(C O ) ]Á2H O and lead(II)bis(oxalato)lead(II)monohydrate
2
4 2
2
(
IR spectral, re¯ectance and X-ray powder diffraction studies. Thermal decomposition studies (TG, DTG and DTA) in air
LOM), Pb[Pb(C
2
O
4
)
2
]ÁH
2
O have been synthesized and characterized by elemental analysis, conductance measurements,
showed that at ca. 10008C, a mixture of BaO and BaCO is generated from the compound, BOD, through the formation of
2
3
BaO
2
and BaC
2
O
4
2 3
at around 5148C as intermediates. LOM gave Pb O as ®nal product at ca. 3908C. DSC study in nitrogen
showed a different decomposition pattern from that in air for both compounds. Using seven mechanistic equations, the rate
controlling process of the dehydration and decomposition mechanism of BOD is inferred to be one- and three-dimensional
diffusion, respectively. The decomposition mechanism of LOM is a phase boundary reaction having cylindrical symmetry.
Some of the decomposition products were identi®ed by analytical, IR spectral and X-ray powder diffraction studies. A
tentative reaction mechanism for the thermal decomposition of both the complexes is proposed. # 1999 Elsevier Science B.V.
All rights reserved.
Keywords: Synthesis; Characterization; Oxalato; Thermal decomposition; Kinetics
1
. Introduction
[8] reported the formation of BaCO at >3808C in
3
vacuum as well as lead oxide as ®nal product from
PbC O . Further they showed [9] that both the com-
The chemistry of metal oxalato complexes of both
2
4
simple and complex types are classi®ed and reviewed
by Krishnamurty and Harris [1]. Transition and non-
transition metal oxalates of different types have been
extensively studied by many workers [2±6]. The
structure of BaC O ÁH O was con®rmed [7] by X-
pounds decomposed to oxide or oxide and metal. A
gas chromatography study showed [10] PbO formed
as end product from PbC O via the formation of
2
4
carbonate as intermediate. Usha et al. [11] suggested
that the compound, NH Ba[Co(C O ) ]ÁnH O decom-
2
4
2
4
2
4 3
2
ray diffraction which led to BaCO and CO at around
poses in a complex manner and the ®nal product is
in¯uenced by the surrounding atmosphere. The oxa-
late of the mixed metal of barium and titanium of the
3
3
958C in N atmosphere, whereas Fabbri and Baraldi
2
type BaTiO(C O ) was reported by Gopalakhrishna-
2 4 2
*
Corresponding author. Tel.: +91-360-257659; fax: +91-360-
2
0
44307/+91-360-257696; e-mail: chem@nerist.ernet.in
Murthy et al. [12]. The same workers [13] prepared the
040-6031/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 9 9 ) 0 0 0 4 2 - 8

1
30
N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
compound PbTiO(C O ) Á4H O, and reported PbTiO
study. The furnace atmosphere was static air. The
2
4 2
2
3
�
1
as the ®nal product in air or O . Compounds of the
2
heating rate and sample mass were 108C min and
28.25 mg, respectively. DSC curves were recorded up
to 6708C using Perkin Elmer DSC-7 at a heating rate
type M U(C O ) ÁnH O (where MˆBa, Pb, Ca, Cd,
2
2
4 4
2
Sr) have been reported [14]. A mossbauer study [15]
of the thermal decomposition of Ba [Fe(C O ) ]Á
�
1
of 108C min in nitrogen atmosphere. The kinetic
parameters of the phase transformation process were
determined.
3
2 4 3
III
nH O led to BaFe O at higher temperatures.
2
4
Recently, Dollimore [16] reviewed oxalato com-
pounds and emphasized [17] the dependence of the
thermal stability of oxalate on the environmental
atmosphere, sample preparation and prehistory.
Mechanistic equations are used [18±20] to study the
kinetics and mechanism of the thermal decomposition.
In continuation of our work [21±31] on the compounds
of the type M[M(C O ) ]ÁxH O (where Mˆsame
The electrical conductivity of the compounds were
measured at 1 kHz with the help of Aplab (India)
LCR-Q meter (model no. 4910). The powder samples
9
� 2
were pressed at 3.73Â10 kg m to make pellets and
the graphite paint was used as electrode. The X-ray
powder diffraction pattern of the compounds and the
decomposition products were taken using a Geiger¯ex
Microprocessor controlled Automated Rigaku (Japan)
X-ray diffractometer system D/Max IIIC. Sometimes
the model JDX-11P3A JEOL diffractometer using Ni
2
4 2
2
metal), we are reporting here the synthesis, character-
ization and thermal decomposition of Ba[Ba-
(
(
C O ) ]Á2H O (BOD) and Pb[Pb(C O ) ]Á2H O
2
4 2
2
2
4 2
2
LOM) in air and nitrogen media. Mechanism of
®lter with Cu K radiation at 35 kV and 10 mA in the
ꢀ
the decomposition have been investigated on the basis
of different mechanistic equations. The tentative
mechanism for the decomposition in air has also been
proposed.
wide angle 28<2ꢁ<608 was also used to take the XRD
pattern of the powder sample.
3
. Theoretical
2
. Experimental
Evaluation of the mechanism of reaction using
mechanistic equations has been discussed by several
authors [18±20]. We have used the following kinetic
expression [20]:
The compounds were prepared by the method
described [22±31] earlier from the respective metal
chlorides. The compounds were puri®ed by standard
procedures and were air dried at ambient temperature
and stored in a desiccator. Water and metal contents
were determined thermogravimetrically and gravime-
trically by known methods. Carbon and hydrogen
were analyzed using Carlo Erba 1108 elemental ana-
lyzer.
0
Ã
Á ln ꢀ
Á lnꢀ1� ꢀ†
E
Áꢀ1=T†
R Á lnꢀ1 � ꢀ† Á lnꢀ1 � ꢀ†
1)
Á ln fꢀꢀ† ;
ˆ�
‡
(
where the terms have their usual meaning and f(ꢀ) is a
function depending on the actual mechanism of the
process. A series of seven forms of f(ꢀ) are proposed
in [20] out of the possible mechanisms [19] of thermal
The IR spectra were measured from 200 to
�
1
000 cm in nujol mulls with a Perkin-Elmer 883
4
0
decomposition. Thus a plot of Á ln ꢀ � Á ln f(ꢀ)/
spectrophotometer. IR spectra of the gases evolved
during decomposition were recorded by the reported
[21] method. Diffuse re¯ectance spectra were
recorded with a Shimadzu UV-240 spectrophotometer
Á ln(1� ꢀ) against Á(1/T)/Á ln(1� ꢀ) is a straight
*
line whose slope is � E /R, irrespective of the form
of f(ꢀ) employed. One can select the f(ꢀ) that best ®ts
the actual mechanism of reaction corresponding to the
intercept value close to zero.
using BaSO as the reference material.
4
TG, DTG and DTA of lead compounds were
�
Further, to test the correctness and validity of the
above conclusion regarding the identi®cation of the
actual mechanism of the process, the Arrhenius equa-
tion of the following type [20] is used.
1
recorded in static air at a heating rate of 108C min
using a Shimadzu DT 30B thermal analyzer. The
amount of sample taken was 15.48 mg. For barium
compound CHAN (USA), DTA and TGA units
0
Ã
(
model 131) were used to carry out the thermal
ln ꢀ � ln fꢀꢀ† ˆ lnꢀA=ꢂ† � E =RT;
(2)

N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
131
where the terms have their usual meaning. The slope,
*
E /R, and intercept, ln(A/ꢂ), can be obtained from
cule of water, respectively. The electrical conductivity
� 4
�
were
measured
� 7 � 1
to
cm for barium and lead com-
be
2.67Â10
and
0
� 1
the plot of ln ꢀ � ln f(ꢀ) against 1/Twhich is a straight
line. It now follows that the mechanism proposed on
the basis of the kinetic Eq. (1) is correct provided the
1.02Â10
ꢀ
pounds, respectively. No appreciable change has
occurred in the value of conductivity of lead com-
pound in relatively more humid atmosphere which is
*
E value obtained from the above plot of (2) turns out
to be the same.
�
7
� 1 � 1
measured to be 4.86Â10
ꢀ
cm . Selected bands
*
Thus, we have evaluated the E from TG curves
(Table 1) in the IR spectra of the compounds sug-
gested [29±32] the chelating character of the oxalato
group. The electronic spectrum of the solid sample of
BOD showed that the bands around 20 000, 23 800
employing the kinetic Eqs. (1) and (2) for seven forms
of f(ꢀ) and thus inferred the rate controlling process
[
20] for the dehydration and decomposition steps of
�
1
BOD and the only decomposition step of LOM as
described above. The details of the kinetic parameters
calculated can be seen from Table 3.
(small), 26 730 and 38 900 cm are due to L!M or
*
M!L charge-transfer and intraligand ꢃ!ꢃ transi-
tions. Similar bands observed at 20 000 and
2
�
1
*
7 390 cm are due to ꢃ!ꢃ transitions for LOM,
�
1
whereas intense band at 33 112 cm could be possi-
bly due to transition involving the Pb±Pb bond
4
. Results and discussion
[
22,33]. X-ray powder diffraction (XRD) pattern
4
.1. Characterization of complexes
(Table 2) of BOD and LOM are, however, different
from each other which suggests that they are not
isomorphous.
The complexes Ba[Ba(C O ) ]Á2H O (BOD) and
2
4 2
2
Pb[Pb(C O ) ]ÁH O (LOM) were isolated as white
2
4 2
2
powders and were insoluble in water and common
organic solvents, but decomposed in the presence of
strong acid or alkali.
4.2. Decomposition of the complexes
The TG curve (Fig. 1) of Ba[Ba(C O ) ]Á2H O
2
4 2
2
Water analysis, microanalytical and thermogravi-
metric results as well as estimated metal contents
suggested the proposed formula of the compound of
barium and lead, which contains two and one mole-
(BOD) showed that the mass loss was started around
1108C and an inclined slope up to 1708C with mass
loss of 5.5% (calculated, 5.55%) indicated the removal
of one and a half molecules of water. DTG change
Table 1
Selected bands in the IR spectra of Ba[Ba(C
2
O
4
)
2
]Á2H
2
O (BOD) and Pb[Pb(C
2
O
4
)
2
]ÁH
2
O (LOM)
a
IR bands (cm
� 1
)
Assignments
BOD
LOM
2
1
1
400±3840b (split into 2960, 3280, 3340, 3560)
640s
±
1600sb
ꢄ
sy(O±H)‡ꢄasy(O±H) or hydrogen bonding
ꢅsy(O±H±O)
480±1760b (split into 1660s, 1600s, 1560s)
1400±1700b
1370 small
ꢄ
ꢄ
ꢄ
ꢄ
asy(C=O)
ꢁ
1395 small, 1475s
320s
sy(C±O) and/or ꢄ(C±C)
sy(C±O)and/or ꢅ(O±C=O)
sy(C±O) and/or ꢅ(O±C=O)
1
8
ꢁ1320m, ꢁ1295sm
50s, 880sh
±
ꢁ
750, ꢁ770m, ꢁ790m
ꢁ790 (split into 780)
ꢁ600 small
±
ꢄ(M±O) and/or ꢅ(O±C=O) or co-ordinated water
Water of crystallization
ꢄ(M±O) and/or ꢄ(C±C)
6
5
4
3
3
±
00m
40m
80m, 450sh
95m
485, 470 (split)
380m
ꢅ(O±C=O) and/or ring deformation
ꢅ(O±C=O)
50m
360m
294 small
ꢅ(O±C=O) and/or ꢄ(C±C)
ꢃ (out-of-plane bending)
a
bˆbroad; mˆmedium; sˆstrong and shˆshoulder.

1
32
N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
Table 2
Prominent lines in the X-ray powder diffraction pattern of
place with lower activation energy in most of the
*
mechanisms. In the ®rst two processes the E are
Ba[Ba(C
2
O
4
)
2
]Á2H
]Á2H
2
O (BOD) and Pb[Pb(C
2
O
4
)
2
]ÁH
2
O (LOM)
more than double of other mechanisms. In DSC in
nitrogen a strong endothermal change is observed for
dehydration between 137.38C and 1728C and the
kinetic parameters are recorded in Table 4. The
XRD data of the calcined residue in furnace at
Ba[Ba(C
2
O
4
)
2
2
O
Pb[Pb(C
2
O
4
)
2
]ÁH
2
O
Ê
d (A)
Ê
d (A)
I/I
0
(Rel)
I (Rel)
6
6
5
5
5
5
5
4
4
4
4
3
3
3
3
2
2
2
2
2
2
2
2
2
2
2
2
2
1
1
1
1
.538
.197
.719
.542
.375
.218
.056
.877
.685
.420
.079
.529
.403
.177
.055
.942
.855
.790
.748
.688
.523
.486
.456
.434
.384
.363
.079
.011
.962
.950
.860
.628
85
83
100
74
69
68
66
92
69
66
52
56
58
41
47
73
36
44
66
39
36
31
31
29
42
50
59
44
27
33
23
23
7.369
6.321
6.188
5.981
5.604
5.434
5.273
4.951
4.741
4.480
4.288
4.149
4.001
3.783
3.720
3.573
3.490
3.411
3.229
3.140
3.097
2.996
2.486
2.220
2.174
2.102
2.004
4.72
3.60
2.34
3.64
3.28
3.48
1.31
22.16
17.59
10.71
20.40
7.16
100.00
3.33
1.50
5.29
1.44
6.54
2.10
2.57
1.96
1.21
1.69
1.21
1.67
1.70
1.30
3008C are included in Table 5. Most of the d values
are similar to the parent compound. The mass loss
started in case of BaC O ÁH O [7] was 978C.
2
4
2
Decomposition of the anhydrous species started
from 4508C and a steep slope in TG up to 5148C
apparently indicates that the product might be a
mixture of BaO2 and BaC O4 (mass loss, found,
2
18.5%; calculated, 18.94%). This type of species is
anticipated earlier [27,28,31]. Pyrolysed compound at
3
308C (mass loss, 17.5%) in furnace is grayish white
in color. The chemical and IR spectral studies of the
separated compounds suggest that the species might
be BaC O and barium oxide. The trace amount of
2
4
CO(g) and CO (g) evolved were detected by their IR
2
study and the band position were in conformity with
the works reported elsewhere [31,32]. Interestingly,
the mass loss occurs very slowly after 5148C in TG
with two small breaks at 8008C (21.4% mass loss) and
8
the conversion of BaO to BaO. The theoretical mass
868C (22.5% mass loss) which might be ascribed to
2
loss to get BaO and BaC O from the parent com-
4
2
pound is 22.23%. The reduction of BaO to BaO by
2
CO(g) evolved during rupture of C2O2 group cannot
�
4
be neglected. This type of conversion by heating in air
is also possible. Hence, the two small breaks in TG
curve might be responsible for the above changes. Two
small peaks appear at 8008C and 8508C in DTA pro®le
after a strong exothermic peak between 4508C and
5308C (ÁTmaxˆ5158C) which is due to main decom-
position phase are in good agreement with the TG
changes and the presumed phenomena. DTG change
between 4508C and 5308C for the main decomposition
followed by two small changes at 8008C and 8908C
are accounted for by the subsequent changes of the
product. The decomposition of BaC O starts at about
(
112±1808C) and an endotherm between 1408C and
2
108C (ÁTminˆ1868C) in DTA correspond to the
dehydration step. The inclined nature of the TG curve
has changed nearly to ¯at with two minor breaks at
2
058C and 2388C which may be accounted for the
elimination of remaining traces of water. Two small
peaks at 2058C and 2358C are distinct in DTG curve.
The complete deaquated species is assumed to be
around 3258C in TG (mass loss, found, 7.5%; calcu-
lated, 7.39%) and is stable upto 4508C. The activation
2
4
3958C as reported by Nagase et al. [7], whereas in this
study the stability of the intermediate BaC O4 is
enhanced due to the in¯uence of BaO, like CaO on
2
*
energy (E ) of this step was calculated (Table 3) using
mechanistic equations. The rate controlling process of
deaquation is found to be one-dimensional diffusion.
The study suggested that the dehydration had taken
CaC O [31]. After the small in¯exion in TG at 8868C,
the mass loss continued slowly and ®nally 24.5% mass
loss at 10008C indicates that the product may be a
2
4

N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
133
�
1
Fig. 1. TG, DTG and DTA curves of Ba[Ba(C
2
O
4
)
2
]Á2H
2
O at 108C min in air.
Table 3
Kinetic data evaluated by the seven mechanistic equations
*
Compound
Step
Mechanism
(A)
(B)
Difference of E
between (A)
and (B) (%)
*
*
Correlation
E
E
A
�
1
)
� 1
� 1
)
coefficient (r) (kJ mol
(kJ mol
)
(s
2
7
BOD
Dehydration
D1
D2
D3
D4
F1
0.9989
0.9865
0.9953
0.9899
0.9847
0.9868
0.9976
237.53
244.51
90.92
95.23
83.14
93.85
93.51
237.53
301.37
99.75
1.06Â10
2.33Â10
4.78Â10
1.25Â10
5.19Â10
1.12Â10
1.66Â10
00.00
18.86
8.85
3
1
1
1
1
1
4
2
3
2
3
0
110.83
109.12
114.31
92.34
14.08
23.81
17.89
1.25
R2
R3
2
2
7
1
2
2
1
6
Decomposition
D1
D2
D3
D4
F1
0.9166
0.9793
0.9972
0.8784
0.9276
0.9372
0.9012
346.39
389.69
113.36
191.84
311.74
283.38
249.41
415.68
445.39
113.37
249.41
415.68
415.68
311.75
2.95Â10
1.69Â10
3.01Â10
6.46Â10
1.02Â10
4.89Â10
3.09Â10
16.67
12.50
00.00
23.08
25.00
31.83
19.99
8
5
7
6
9
R2
R3
4
5
1
2
2
2
2
9
2
6
5
5
3
1
LOM
Decomposition
D1
D2
D3
D4
F1
0.9987
0.9928
0.8963
0.9982
0.9620
0.9989
0.9212
508.06
531.16
115.44
207.86
254.01
300.20
207.86
623.54
669.69
170.87
323.29
323.29
300.20
277.11
1.01Â10
2.82Â10
1.64Â10
2.69Â10
2.24Â10
1.41Â10
1.32Â10
18.52
20.69
32.44
35.71
21.43
00.00
24.98
R2
R3
0
(
Arrhenius plot of ln ꢀ /f(ꢀ) vs. 1/T, for Ba[Ba(C
A) Results obtained by using the seven mechanisms for the plot of (Á ln ꢀ � Á ln f(ꢀ)/Á ln(1� ꢀ)) vs. Á(1/T)/Á ln(1� ꢀ), and (B) for the
0
2
O
4
)
2
]Á2H
2
O (BOD) and Pb[Pb(C
2
O
4
)
2
]ÁH
2
O (LOM).

Table 4
�
1
DSC data of Ba[Ba(C
2
O
4
)
2
]Á2H
2
O (BOD) and Pb[Pb(C
2
O
4
)
2
]ÁH
2
O (LOM) in nitrogen at a heating rate of 108C min
*
*
Compound
Step Temperature
range (8C)
Peak
temperature (8C) (s
ln k
�
0
E
(kJ mol
ÁH
(kJ mol
ÁS
(J K mol
Heat of fusion Order of
� 1
Reaction
1
)
� 1
� 1
� 1
� 1
)
)
)
(kJ mol
)
reaction
BOD
1
2
3
1
137.3±172 (endo)
157.4
108.8Æ2.4
102.0Æ2.2
427.3Æ9.2
88.68Æ1.91
400.3Æ8.6 101.8
697.6Æ15.1 21.7
236.6
27.5
138.9
31.1
1.38Æ0.02 Dehydration
1.17Æ0.02 Decomposition
1.12Æ0.02
490.3±537.8 (endo) 515.7
538.5±547
358.0±424.0
542.7
394.0
2916.4Æ63.0
0.56
0.69
LOM
514.2Æ11.1 203.1
304.5
282.3
1.34Æ0.02 Decomposition

N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
135
Table 5
Prominent lines with d-spacing and intensity of the pyrolysed product at 3008C and 7008C of Ba[Ba(C
Pb[Pb(C O (LOM)
]ÁH
2
O
4
)
2
2
]Á2H O (BOD) and at 6508C of
2
O
4
)
2
2
Product at 3008C of BOD
Product at 7008C of BOD
Product at 6508C of LOM
Ê
d (A)
Ê
d (A)
Ê
d (A)
I/I
0
(Rel)
I/I
0
(Rel)
0
I/I (Rel)
4
4
3
3
2
2
2
2
1
.850
.625
.748
.679
.985
.369
.291
.088
.950
100
77
62
66
46
56
37
28
32
8.350
7.800
7.170
4.566
3.725
3.665
3.228
3.034
2.946
35
34
34
24
60
35
18
19
15
23
19
13
22
13
14
17
14
10
9
8.984
5.988
4.051
3.075
2.956
2.752
2.509
2.381
2.205
2.011
1.853
1.800
1.726
1.643
34
25
16
100
35
36
12
22
12
21
22
19
25
24
2
2
2
2
2
2
2
1
1
1
.634
.590
.294
.152
.100
.048
.020
.948
.917
.677
mixture of BaO and BaCO (calculated, 24.69%).
3
position is higher than dehydration, although it is
endothermic. The mass loss of 12.93% after DSC
scan suggests that the end product is a mixture of
BaO and BaC O . The derived mixture might be
2
The oxidation of BaO again to BaO could therefore
2
be assumed. An endotherm at 8508C in DTA gives
credence to this view. The TG study reveals that
instead of completing the decomposition of BaC O
2
4
formed through the formation of intermediate BaO2
and BaC O at 537.88C and the small overlapping
2
4
at around 4908C, it is generated at around 5148C
2
4
which decomposes slowly to BaCO in the presence
of oxide. The formation of a trace of BaC cannot be
peak is responsible for the reduction of BaO to BaO
2
3
by evolved CO(g) as well as the interaction of BaO2/
BaO with carbon. The low value of enthalpy (H) and
entropy (S) changes of the smaller peak suggest that
the reduction and/or interaction phenomena rather
than the decomposition may have occurred. The
2
excluded from the ®nal residue due to the interaction
of BaO or molten BaO with carbon formed during
2
disproportionation of CO (g) to CO (g). From the
2
present study it can be concluded that the gray white
compound at 10008C may be a mixture of BaO/BaO2
and BaCO along with a trace of BaC . XRD pattern
*
higher value of E corresponding to the smaller peak
indicates that the possible reduction process is slow.
For the main decomposition step the slope between
4508C and 5148C in TG pro®le is chosen for kinetic
study using mechanistic equations [20]. The para-
meters are given in Table 2. The rate controlling
process is found to be three-dimensional diffusion.
The compound, Pb[Pb(C O ) ]ÁH O (LOM),
3
2
(Table 5) of non-separable products exhibit the peaks
with d-values and intensities which are in close proxi-
mity with the oxide, carbonate and carbide of barium
[
35]. The BaCO is found to be stable up to 10008C in
3
TG (indeed it decomposes to oxide at 13608C [34]).
The DSC study (Fig. 3(a)) in nitrogen shows that two
overlapping endotherms from 490.38C to 5478C is
split into two peaks and their kinetic parameters are
given in Table 4. Both the dehydration and decom-
2
4 2
2
started to lose mass around 3008C and a steep slope
in TG (Fig. 2) up to 3908C (mass loss, found, 24.3%)
was observed. The product might be Pb O (calcu-
2
3
position steps in DSC are nearly to three-halves and
®
lated, 24.03%). Corresponding to the decomposition
stage in DTA exotherm between 3208C and 3708C
*
rst-order in kinetics, respectively. The E of decom-

1
36
N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
�
1
Fig. 2. TG, DTG and DTA curves of Pb[Pb(C
2
O
4
)
2
]ÁH
2
O at 108C min in air.
followed by a very small concomitant peak between
decomposes [34] at 3008C and PbCO at 3408C to
3
3
3
708C and 4028C is recorded. DTG change between
108C and 4008C is accounted for the corresponding
Pb O which behaved chemically as a mixture of PbO/
PbO ; the formation of Pb O might also be assumed
3
4
2
3 4
changes in TG and DTA. The exothermic nature of the
DTA curve from room temperature to 3208C is indi-
cative of the formation of an intermediate species;
some phase changes (or changes of crystallinity) and
intramolecular rearrangement might also take place.
Independent pyrolysis of the compound at 2008C in air
found a species with a mass loss of 7.47%. It is
assumed that the compound is a mixture of PbCO3
and PbC O (calculated, 7.59%). The compound in
as transient intermediate in our study which immedi-
ately changes to Pb O . The XRD data (Table 5) of the
calcined product around 6508C with mass loss of
2
3
24.96% con®rmed [35] the residue to be Pb O . A
few d-values are similar with the d-values of PbO and
2
3
2
Pb O , which suggest [35] their presence in trace
3
4
along with the main product Pb O . In TG after
3
2
3908C, a slight gain of mass (0.3%) is due to partial
oxidation of residue in air which is stable up to
10008C. DTG curve shows no changes up to
10008C. The DSC study in nitrogen (Fig. 3(b))
showed an endotherm between 3588C and 4248C.
The kinetic parameters are included in Table 4. The
mass loss at the end of the scan is 21.83%, which
indicates the formation of 2PbO2 (calculated,
21.39%).
2
4
humid atmosphere absorbs water and might be aqua-
ted to lead oxalate. There is no indication in TG and
DTG for the intermediate. However, the exothermic
nature of the DTA pro®le from starting temperature
gives credence to this view. Similar intermediates
were generated earlier [30]. The removal of single
water molecule at high temperature during decom-
position of the complex is thought to be H-bonded
A trace of C may also be assumed in the residue due
to disproportionation of CO(g) in inert medium. The
decomposition follows three-halves kinetic order from
with C O2 group and co-ordinated to Pb atom. The
�
2
4
IR spectrum showed the same nature. As PbC O
2
4

N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
137
�
1
Fig. 3. DSC curves of (a) Ba[Ba(C
2
O
4
)
2
]Á2H
2
O and (b) Pb[Pb(C
2
O
4
)
2
]ÁH
2
O at 108C min in nitrogen.

1
38
N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
DSC. From the kinetic study using mechanistic equa-
tions (Table 3), the rate controlling process of the
decomposition is inferred to be a phase boundary
reaction (cylindrical symmetry). Noticeable disagree-
10008C through the formation of a mixture of barium
peroxide and barium oxalate at 5148C, whereas the
compound LOM produces Pb O as the end product at
ca. 3908C. The rate controlling process of the dehy-
dration and decomposition step of BOD is inferred to
be one- and three-dimensional diffusion, respectively,
but in the case of LOM the decomposition mechanism
is identi®ed to be a phase boundary reaction.
2
3
*
ments in E values (Table 3) were found between all
the mechanistic equations and also from their Arrhe-
nius plots. Differences are observed in the work of
other authors [20].
The water vapor, carbon monoxide and carbon
dioxide evolved during decomposition were identi®ed
by IR spectroscopy [31,32].
References
The above results suggests the following tentative
scheme of the thermal decomposition in air.
In air,
[1] K.V. Krishnamurty, G.M. Harris, Chem. Rev. 61 (1961) 213.
[
2] W.W. Wendlandt, T.D. George, K.V. Krishnamurty, J. Inorg.
Nucl. Chem. 21 (1961) 69.
[3] D. Dollimore, D. Nicholson, J. Chem. Soc. (1962) 960.
Ba‰BaꢀC O † Š Á 2H O
2
4 2
2
[
[
[
[
4] D. Dollimore, D.L. Griffiths, D. Nicholson, J. Chem. Soc.
(1963) 2617.
1
10� 1708C
!
Ba‰BaꢀC2O4† ŠÁ0:5H2O‡1:5H2Oꢀv†
2
5] W.W. Wendlandt, E.L. Simmons, J. Inorg. Nucl. Chem. 28
1966) 2420.
up to 3258C
(
!
Ba‰BaꢀC2O4† Šꢀs† ‡ 0:5H2Oꢀv†
2
6] K. Nagase, K. Sato, N. Tanaka, Bull. Chem. Soc. Jpn. 48
ꢂ
4
50� 514 C
(1975) 868.
!
BaO ꢀs† ‡ BaC O ꢀs† ‡ x COꢀg†
2
2
4
7] K. Nagase, K. Sato, N. Tanaka, Bull. Chem. Soc. Jpn. 48
(1975) 439.
ꢂ
8
00� 886 C
‡
yCO ꢀg†
!
BaOꢀs† ‡ BaC O ꢀs†
[8] G. Fabbri, P. Baraldi, Atti. Soc. Nat. Mat., Modena 106
(1975) 57.
2
2
continues slow decomposition
4
ca10008C
!
[9] G. Fabbri, P. Baraldi, Atti. Soc. Nat. Mat., Modena 106
1975) 81.
BaO2ꢀs† ‡ BaCO3ꢀs† ‡ mCOꢀg†
finally
(
[
10] T.S. Rao, B.R. Gandhi, J. Chromatogr. 88 (1974) 407.
‡
nCO ꢀg† ! BaO ꢀs† ‡ BaC ꢀs†
[11] M.G. Usha, M. Subba Rao, T.R. Narayanan Kutty, J. Therm.
Anal. 31 (1986) 7.
2
2
2
‡
BaCO3ꢀs†
[12] H.S. Gopalakhrishna-Murthy, M. Subba Rao, T.R. Narayanan
Kutty, J. Inorg. Nucl. Chem. 37 (1975) 1875.
In air,
[13] H.S. Gopalakhrishna-Murthy, M. Subba Rao, T.R. Narayanan
Kutty, J. Inorg. Nucl. Chem. 38 (1976) 417.
Pb‰PbꢀC O † Š Á H O
2
4 2
2
[14] S.K. Awasthi, K.L. Chawla, D.M. Chakraburty, J. Inorg. Nucl.
Chem. 36 (1974) 2521.
ca:2008Cꢀpyrolysis†
!
PbCO ꢀs† ‡ PbC O ꢀs†
[15] A.S. Brar, S. Brar, S.S. Sandhu, J. Therm. Anal. 31 (1986)
3
2
4
1083.
‡
H2Oꢀv† ‡ xCOꢀg† ‡ yCO2ꢀg†
[
16] D. Dollimore, Anal. Chem. 66 (1994) 17R.
3
00� 3908C
[17] D. Dollimore, T.A. Evans, Y.F. Lee, Thermochim. Acta 194
(1992) 215.
!
Pb O ꢀs† ‡mCOꢀg† ‡ yCO ꢀg†
3
4
2
ꢀ
transient†
[
18] K.N. Ninan, C.G.R. Nair, Thermochim. Acta 30 (1979) 25.
finally
!
[19] W.W. Wendlandt, Thermal Methods of Analysis, Wiley, New
York, 1974, p. 45.
Pb O ꢀs†
2
3
ꢀ
stable†
[
20] R. Lozano, J. Roman, J.C. Aviles, A. Moragues, A. Jerez, E.
Ramos, Trans. Met. Chem. 12 (1987) 289.
[
[
21] T.K. Sanyal, N.N. Dass, J. Inorg. Nucl. Chem. 42 (1980) 811.
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(
1988) 4485.
23] N. Deb, P.K. Gogoi, N.N. Dass, J. Ind. Council Chemists 3
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1989) 145.
[25] N. Deb, P.K. Gogoi, N.N. Dass, J. Therm. Anal. 35 (1989) 27.
5
. Conclusions
The thermal studies (TG, DTG and DTA) in air
[
[
(
suggests that the compound BOD decomposes to
peroxide, carbide and carbonate of barium around
(

N. Deb et al. / Thermochimica Acta 329 (1999) 129±139
139
[
[
[
[
[
[
26] N. Deb, P.K. Gogoi, N.N. Dass, J. Inst. Chemists (India) 61
1989) 185.
27] N. Deb, P.K. Gogoi, N.N. Dass, Thermochim. Acta 145
1989) 77.
28] N. Deb, P.K. Gogoi, N.N. Dass, J. Therm. Anal. 36 (1990)
65.
29] N. Deb, P.K. Gogoi, N.N. Dass, Thermochim. Acta 198
1992) 395.
30] N. Deb, S.D. Baruah, N.N. Dass, J. Therm. Anal. 45 (1995)
57.
31] N. Deb, S.D. Baruah, N.N. Dass, Thermochim. Acta 285
1996) 301.
[32] K. Nakamoto, Infrared Spectra of Inorganic and Co-
ordination Compounds, 2nd ed., Wiley/Interscience, New
York, 1969, p. 83, 89, 219, 245.
(
(
[33] M.G.B. Drew, A. Lavery, V. Mckee, S.M. Nelson, J. Chem.
Soc. (1985) 1771.
4
[34] J.A. Dean, Lange's Handbook of Chemistry, 13th ed. (Int.
ed.), Mc-Graw Hill, New York, 1987, pp. 4±25, 4±27,
4±66.
(
[35] Inorganic Index to the Powder Diffraction File, published by
Joint Committee on Powder Diffraction Standards, 1601,
Parklane, Pennsylvania, 1971, p. 797, 800, 991.
4
(