A study of the selenites of cerium phase states in aqueous medium and thermal properties

Article abstract of DOI:10.1023/A:1025846901134

The solubilities of the systems CeO₂-SeO₂-H ₂O and Ce₂O₃-SeO₂-H₂O were studied at 100°C. The field of crystallization of Ce(SeO ₃)₂ was established in the system CeO₂-SeO ₂-H₂O, and fields of crystallization of Ce ₂(SeO₃)₃ and Ce₂(SeO ₃)₃ H₂SeO₃ were established in the system Ce₂O₃-SeO₂-H₂O. The compound obtained were identified by means of chemical, X-ray and derivatograph analysis. The mechanism of thermal dissociation of Ce(SeO₃)₂, Ce₂(SeO₃)₃ and Ce₂(SeO ₃)₃H₂SeO₃ was studied.

Full text of DOI:10.1023/A:1025846901134

Journal of Thermal Analysis and Calorimetry, Vol. 73 (2003) 859–865
A STUDY OF THE SELENITES OF CERIUM
Phase states in aqueous medium and thermal properties
G. G. Gospodinov1* and M. G. Stancheva 2
1Prof. Assen Zlatarov University, Bourgas 8010, Bulgaria
2Technological College, Razgrad 7200, Bulgaria
(Received October 31, 2002; in revised form February 16, 2003)
Abstract
The solubilities of the systems CeO2–SeO2–H2O and Ce2O3–SeO2–H2O were studied at 100°C. The
field of crystallization of Ce(SeO3)2 was established in the system CeO2–SeO2–H2O, and fields of crys-
tallization of Ce2(SeO3)3 and Ce2(SeO3)3 H2SeO3 were established in the system Ce2O3–SeO2–H2O.
The compound obtained were identified by means of chemical, X–ray and derivatograph analysis. The
mechanism of thermal dissociation of Ce(SeO3)2, Ce2(SeO3)3 and Ce2(SeO3)3·H2SeO3 was studied.
Keywords: cerium selenites, equilibrium, phase diagrams, solubility, X–ray
Introduction
Bercelius [1] and Nilson [2] were the first to publish data about the selenites of cerium.
Jolin [3] obtained amorphous cerium selenite, and Espil [4] obtained yellow powder of
anhydrous cerium selenite. Nilson [2] and Jolin [3] obtained cerium hydrogenselenites
having a different number of moles of crystallization water by treating a normal salt with
selenious acid. Barbieri and Calzolari [5] obtained orange-yellow powder of cerium sele-
nite by heating a mixture of cerium nitrate, selenious acid and nitric acid.
Markovski and Safina [6] described syntheses of the selenites of tri- and four-valent
cerium at different ratios of cerium and selenious acid in the temperature range from 25
to 40°C. Depending on the conditions of precipitation, several salts were obtained:
Ce(SeO3)2, Ce(HSeO3)3 H2O, Ce2(SeO3)3 H2SeO3, Ce2(SeO3)3 H2O, Ce2(SeO3)3 3H2O
and Ce2O3 2SeO2 15H2O. The authors were not able to obtain acid selenite of Ce(IV).
In [6], standard heats of formation of cerium selenites were calculated by deter-
mining the heats of dissolution of the selenites of cerium in nitric acid.
The structure of Ce(IV) was determined and reported in [7]. Ce(SeO3)2 crystal-
lizes in an orthorhombic system. The authors [8] determined the parameters of the
crystal lattice of Ce2SeO5.
The aim of the present work is to study the solubility of the systems
CeO2–SeO2–H2O and Ce2O3–SeO2–H2O at 100°C and to determine the thermal dis-
sociation of the compound obtained.
*
Author for correspondence: E-mail: ggospodinov@btu.bg
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

860
GOSPODINOV, STANCHEVA: CERIUM SELENITES
Experimental
25 samples were prepared to study each of the systems. For that purpose, Ce(SeO3)2 was
obtained from high-purity cerium(IV) chloride and sodium selenite, and Ce2(SeO3)3 –
from high-purity cerium(III) chloride and sodium selenite. Razotherm glass ampoules
were used and each was filled with 2 g of Ce(SeO3)2, respectively Ce2(SeO3)3 and with
H2SeO3 in various concentrations (from 0.1 to 80 mass%). The ampoules were sealed
and placed in an air thermostat at 100±0.5°C. They were periodically shaken. After
chemical and X–ray phase equilibrium was reached, the system was studied. For that
purpose, the ampoules were opened, the liquid and solid phases were separated at the ex-
perimental temperature and were subjected to chemical analysis.
The procedure of preparing the systems and studying the kinetics of reaching an
equilibrium were described in [9].
Chemical analysis for Ce3+ ions was made by direct complexometric titration us-
ing xylenol orange as an indicator [10]. Analysis for Ce4+ ions was made by reverse
complexometric titration with eriochrome black T as an indicator [10], and that for
selenite ions was made iodometrically and gravimetrically [11].
X–ray analyses were made on a URD–6 apparatus (Germany) at Cu anode and
Kα emission, and a Ni filter for β emission.
An OD–102 derivatograph (MOM, Hungary) was used for derivatograph analy-
sis. The experimental temperature interval was 25–1000°C. The experiments were
made in inert medium using ceramic crucibles at a heating rate of 5°C min–1 and sam-
ple mass 250 mg. α–Al2O3 was used as a standard substance.
Results and discussion
Figure 1 and Table 1 show the results from studying the system CeO2–SeO2–H2O at
100°C. The only compound obtained in the whole concentration range of the experi-
ments was anhydrous cerium(IV) selenite Ce(SeO3)2. The composition of the com-
pound was determined by the Schreinemakers’ method after analyzing the washed
Fig. 1 Solubility isotherm of the system CeO2–SeO2–H2O at 100°C
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GOSPODINOV, STANCHEVA: CERIUM SELENITES
861
and dried solid phases. Chemical analysis gave the composition of the solid phase as
43.62 mass% CeO2 and 56.33 mass% SeO2. This composition was also confirmed by
decomposition of the salt and gravimetric analysis of the residue of CeO2 and volu-
metric determination of the sublimate of SeO2.
Table 1 Solubility isotherm of the system CeO3–SeO2–H2O at 100°C
Liquid phase/mass%
Solid phase/mass%
Formula
composition
No.
CeO2
SeO2
CeO2
SeO2
1
2
5.20 10–3
5.00 10–3
4.80 10–3
5.50 10–3
4.60 10–3
4.20 10–3
4.20 10–3
4.10 10–3
3.90 10–3
3.70 10–3
3.70 10–3
3.80 10–3
3.70 10–3
3.40 10–3
3.10 10–3
3.00 10–3
1.52
6.13
37.00
38.11
30.36
32.00
32.01
31.37
32.84
33.09
35.25
27.40
28.34
28.78
29.37
23.60
24.01
23.87
47.00
49.62
41.50
47.01
48.23
48.50
51.02
51.60
53.07
51.63
52.75
55.51
56.30
59.03
63.93
63.75
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
Ce(SeO3)2
3
10.10
22.80
25.48
31.37
35.65
37.37
42.78
44.08
46.94
54.45
58.18
62.20
69.03
72.84
4
5
6
7
8
9
10
11
12
13
14
15
16
The presence of only one compound in the system CeO2–SeO2–H2O at 100°C
was confirmed by the results from X–ray phase analysis. It should be noted that acid
salts were not obtained at 250°C by the authors [6] in 1910 and by Markovski and
Safina [7] in 1968. In this respect, Ce(IV) is similar to Sn(IV), for which no hydrogen
selenites were obtained at 100 and 250°C, either [12].
Figure 2 gives the simultaneous T, TG and DTA curves of Ce(SO3)2. There are a
number of endothermal peaks on it due to liberation of adsorption water and decomposi-
tion of the selenite. The slight endothermal peaks at 120, 180, 260 and 360°C are associ-
ated with liberation of adsorption water. As we have observed it many times, liberation of
this water is accompanied by partial high-temperature hydrolysis and liberation of CeO2
in a gaseous form. This can explain the loss of 7.24 mass% of the initial sample. Further
on, thermal decomposition takes place in two phases: at 550, 640 and 700°C, passing
through a stage at which oxoselenite CeO2 SeO2 is formed. The highest temperature at
which the sample was heated was 900°C but complete decomposition did not take place,
J. Therm. Anal. Cal., 73, 2003

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GOSPODINOV, STANCHEVA: CERIUM SELENITES
Fig. 2 Simultaneous T, TG and DTA curves of Ce(SeO3)2
and CeO2 was not formed. This is characteristic of other selenites and hydrogen selenites
of the rare earth’s and it was described by other authors, too [13]. Complete liberation of
SeO2 is achieved after continued heating of the sample at 1100–1200°C. These conclu-
sions were made both on the basis of thermogravimetric analysis data and by chemical
and X-ray phase analyses.
Table 2 Solubility isotherm of the system Ce2O3–SeO2–H2O at 100°C
Liquid phase/mass%
Solid phase/mass%
Formula
composition
No.
Ce2O3
SeO2
Ce2O3
SeO2
1
1.10 10–2
1.20 10–2
2.10 10–2
2.10 10–2
3.10 10–2
3.50 10–2
3.60 10–2
4.00 10–2
4.10 10–2
4.40 10–2
4.70 10–2
4.85 10–2
4.90 10–2
5.30 10–2
1.52
6.42
45.60
44.58
43.08
40.21
40.20
40.23
37.93
38.23
41.00
30.82
31.30
30.17
29.19
32.04
48.80
46.90
46.12
44.87
54.50
55.13
53.08
53.20
55.25
52.50
54.60
56.08
57.14
56.95
Ce2(SeO3)3
Ce2(SeO3)3
Ce2(SeO3)3
Ce2(SeO3)3
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
CeH(SeO3)2
2
3
9.24
4
15.25
15.25
16.17
21.13
29.39
33.05
44.91
50.30
57.45
59.95
61.35
5
6
7
8
9
10
11
12
13
14
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GOSPODINOV, STANCHEVA: CERIUM SELENITES
863
Figure 3 and Table 2 show the results from studying the system
Ce2O3–SeO2–H2O at 100°C. Two salts were formed in the system. In the concentra-
tion range 1.52–15.25 mass% of SeO2 in the liquid phase, Ce2(SeO3)3 was formed.
Hydrogen salt CeH(SeO3)2 was formed in the interval 15.25–61.35 mass% of SeO2 in
the liquid phase. The phase obtained were identified by the Schreinemakers’ method
and also by other methods of physicochemical analysis.
Fig. 3 Solubility isotherm of the system Ce2O3–SeO2–H2O at 100°C
Figure 4 presents the simultaneous T, TG and DTA curves of Ce2(SeO3)3. The
compound starts liberating adsorption water at 230°C, but visible decomposition be-
gins at 605°C. The endothermal peak in the temperature interval 605–810°C consists
of three superposed endothermal peaks, which form the total endothermal peak. In
Fig. 4 Simultaneous T, TG and DTA curves of Ce2(SeO3)3
J. Therm. Anal. Cal., 73, 2003

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GOSPODINOV, STANCHEVA: CERIUM SELENITES
the temperature range 605–690°C, the selenite loses 1 mol of SeO2 and turns into
Ce2O3 2SeO2. This transformation is confirmed by the mass change of the selenite
from 16.90 mass% against 16.78 mass% calculated from the TG curve. Ce2O3·2SeO2
loses one mole of SeO2 in the temperature range 690–765°C and turns into
(CeO)2SeO3 or Ce2O3 SeO2. The mass loss due to that transformation is 33.85 mass%
(theoretical calculation is 33.56 mass%). The selenite was heated at 940°C but com-
plete decomposition transforming it into Ce2O3 did not take place.
Figure 5 shows the simultaneous T, TG and DTA curves of CeH(SeO3)2
(Ce2(SeO3)3 H2SeO3). The compound is thermally stable at 400°C. In the temperature
range 400–740°C, the selenite loses 1 mole of constitutional water and turns into ce-
rium tetraselenite Ce2Se4O11 or Ce2(SeO3)3 SeO2. The mass loss is 3.6 mass% (theo-
retical calculation is 2.8 mass%). At 740–820°C, tetraselenite loses 1 mol of SeO2,
and Ce2(SeO3)3 is obtained. The mass loss is 19.40 mass% against 18.32 mass% cal-
culated theoretically. In the temperature interval 820–860°C, another mole of SeO2 is
liberated, and a base salt with composition Ce2O3 2SeO2 is formed. The mass loss is
30.40 mass% (theoretical calculation for the corresponding transition is
30.36 mass%). In the temperature range 860–920°C, decomposition takes place ac-
companied by liberation of another mole of SeO2 and the selenite turns into
Ce2O3 SeO2. The mass loss is 43.21 mass% (theoretical calculation is 44.40 mass%).
Decomposition of Ce2O3 SeO2 to Ce2O3 is not complete at 1000°C, which is the high-
est temperature at which the selenite was heated. The mass loss at 1000°C is
54.8 mass% (theoretical calculation for complete decomposition and transition of the
selenite into an oxide is 58.45 mass%). The results from chemical analysis of samples
heated at the corresponding temperature and losing equivalent mass during heating
are a confirmation of the above schemes of thermal decomposition. This paper repre-
sents continuation of our investigation on the three-component systems of the type
Ln2O3–SeO2–H2O [14–16]
Fig. 5 Simultaneous T, TG and DTA curves of CeH(SeO3)2
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GOSPODINOV, STANCHEVA: CERIUM SELENITES
865
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