Study of the system Pr2O3-SeO2-H2O at 100°C and the properties of the compounds obtained

Article abstract of DOI:10.1023/B:JTAN.0000017332.63963.6b

The solubility of the system Pr₂O₃-SeO₂-H₂O at 100°C was studied. The fields of crystallization of Pr₂(SeO₃)₃·H₂O, PrH(SeO₃)₂, Pr(HSeO₃

Full text of DOI:10.1023/B:JTAN.0000017332.63963.6b

Journal of Thermal Analysis and Calorimetry, Vol. 75 (2004) 95–100
STUDY OF THE SYSTEM Pr2O3–SeO2–H2O AT 100°C
AND THE PROPERTIES OF THE COMPOUNDS
OBTAINED
G. G. Gospodinov1* and M. G. Stancheva2
1Department of Inorganic Chemistry, University ‘Prof. Assen Zlatarov’, Bourgas 8010, Bulgaria
2Technological College, Razgrad, 7200, Bulgaria
(Received June 7, 2003; in revised form September 9, 2003)
Abstract
The solubility of the system Pr2O3–SeO2–H2O at 100°C was studied. The fields of crystallization of
Pr2(SeO3)3⋅H2O, PrH(SeO3)2, Pr(HSeO3)3 were established. The compounds obtained were identi-
fied by means of chemical, X-ray and thermal analysis. The thermal decomposition of
Pr2(SeO3)3⋅H2O, PrH(SeO3)2, and Pr(HSeO3)3 was determined.
Keywords: praseodymium selenites, reaction mechanisms, solubility isotherm, X-ray pattern
Introduction
The selenites of the rare earths (praseodymium in particular) are used to synthesize
selenides, which find application as semiconducting materials. They are also used as
initial compounds in the production of luminophors, pigments, etc. The method of
obtaining selenides by reduction of selenites of the rare-earth elements has the advan-
tage of taking place quickly and at relatively low temperatures. The composition of
the reduction products may vary depending on the composition of the reaction gas
medium [1–3].The data concerning the selenites of praseodymium are rather scarce
[4–11]. Authors reported about the synthesis of normal and acid selenites of praseo-
dymium at fixed concentrations of the initial solutions. No studies were carried out
for all concentration intervals. No answer was given to the question whether these
compounds are the only possible ones in the three-component system Pr–Se–O.
Experimental
High-purity Pr2O3 (99.99 mass%) and SeO2, obtained by the authors by oxidation of
high-purity Se with nitric acid, were used to prepare the system Pr2O3–SeO2–H2O
at 100°C. SeO2 was subjected to additional triple sublimation to achieve better purifi-
*
Author for correspondence: E-mail: ggospodinov@btu.bg
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

96
GOSPODINOV, STANCHEVA: Pr2O3–SeO2–H2O
cation. According to spectral analysis data the product obtained has the purity
of 99.999% (the calculation was made on the concentration of 10 impurities).
In order to study the system Pr2O3–SeO2–H2O at 100°C, 25 samples were pre-
pared each containing 2 g of Pr2O3 and varying concentrations of selenious acid (from 0
to 85 mass%). The ampoules were sealed and thermostated in an air thermostat at
100 0.5°C, and were periodically shaken to ensure further precrystallization. To deter-
mine the necessary time for reaching chemical equilibrium, kinetic curves were ob-
tained. For that purpose, more ampoules with the same composition were prepared and
opened periodically. Equilibrium was reached when chemical analysis showed that the
liquid and the solid phases did not change their composition. It was considered that
crystallographic equilibrium was established when the peak intensities and the
interplanar distances in the X-ray patterns no longer changed. After chemical and X-ray
equilibrium was reached, the liquid and the solid phases were separated at the experi-
mental temperature and subjected to chemical, thermal and X-ray phase analyses.
Chemical analysis was made complexonometrically [12] for Pr3+ as Pr2O3 using
xylenol orange as an indicator, and iodometrically and gravimetrically [13] for sele-
nite ions. The concentration of Pr3+ ions in the liquid phase was determined spectro-
photometrically on a Specol-11 apparatus (Karl Zeiss, Germany) [14].
X-ray analysis was carried out on a URD-6 apparatus (Germany) at Cu anode for
Kα-emission and a nickel filter for β-emission. An OD-102 derivatograph (MOM,
Hungary) was used for thermal analysis by heating the phases from 25 to 1000°C at a
rate of 5°C min–1 in metal ceramic crucibles; α-Al2O3 was used as a standard substance.
Results and discussion
The results from studying the system Pr2O3–SeO2–H2O are presented in Fig. 1 and
Table 1. Three salts were formed in the system – one normal and two acid ones. In the
concentration interval 0.52–3.24 mass% SeO2 in the liquid phase, a normal salt is ob-
Fig. 1 Solubility isotherm of the system Pr2O3–SeO2–H2O at 100°C
J. Therm. Anal. Cal., 75, 2004

GOSPODINOV, STANCHEVA: Pr2O3–SeO2–H2O
97
tained with composition Pr2(SeO3)3⋅H2O. In the interval 3.24–50.48 mass% SeO2 in
the liquid phase, the acid salt PrH(SeO3)2, [Pr(SeO3)(HSeO3)] is formed, and
at 50.48–68.74 mass% SeO2, the salt Pr(HSeO3)3 is formed. The phases obtained
were identified by the Schreinemakers’ method and also by chemical and X-ray
phase analyses after isolating them in the pure state.
Table 1 Solubility isotherm of the system Pr2O3–SeO2–H2O at 100°C
Liquid phase, mass%
Solid phase, mass%
Formula
composition
No.
Pr2O3
SeO2
Pr2O3
SeO2
1.
2.
5.2⋅10–3
6.0⋅10–3
6.4⋅10–3
7.0⋅10–3
8.1⋅10–3
8.7⋅10–3
8.9⋅10–3
9.3⋅10–3
9.4⋅10–3
9.8⋅10–3
1.2⋅10–2
1.8⋅10–2
2.5⋅10–2
3.0⋅10–2
4.6⋅10–2
4.8⋅10–2
5.2⋅10–2
5.8⋅10–2
6.4⋅10–2
0.52
1.08
46.20
44.05
46.35
45.06
38.10
33.89
35.90
38.47
35.55
34.60
37.55
32.60
38.25
32.05
29.10
27.15
28.62
25.48
27.63
46.50
45.48
45.00
46.40
50.52
47.60
50.20
54.06
52.20
53.70
55.08
53.00
55.56
55.06
60.14
62.51
63.05
63.54
64.08
Pr2(SeO3)3⋅H2O
Pr2(SeO3)3⋅H2O
Pr2(SeO3)3⋅H2O
Pr2(SeO3)3⋅H2O
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
PrH(SeO3)2
Pr(HSeO3)3
Pr(HSeO3)3
Pr(HSeO3)3
Pr(HSeO3)3
Pr(HSeO3)3
3.
2.50
4.
3.24
5.
3.24
6.
10.34
16.10
22.26
26.50
33.70
39.60
41.42
46.20
50.48
50.48
55.00
57.71
63.48
68.74
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
Well-formed crystals of PrH(SeO3)2 were identified by determining their crys-
tallographic parameters: a=7.049, b=8.354, c=19.012 Å, Dx=4.697 g cm–3. The
compound crystallizes in an orthorhombic system with a space group Pbc21.
Figure 2 shows the derivatogram of Pr2(SeO3)3⋅H2O. The compound is thermally
stable at 250°C. At 390°C crystallization water is liberated, and anhydrous praseo-
dymium selenite is formed. The mass loss indicated by the TG curve is 2.50 mass%
(theoretical calculation for this transition is 2.64 mass%). Chemical analysis gave the
composition of Pr2O3 as 49.66 mass%, and that of SeO2 as 50.23 mass% (theoretical
calculations for Pr2O3 and SeO2 are 49.47 and 50.33 mass%, respectively). In the
temperature interval 600–640°C, Pr2(SeO3)3 decomposes, 1 mole of SeO2 is liberated
J. Therm. Anal. Cal., 75, 2004

98
GOSPODINOV, STANCHEVA: Pr2O3–SeO2–H2O
Fig. 2 Thermoanalytical curves of Pr2(SeO3)3⋅H2O
and oxoselenite Pr2O3⋅2SeO2 is obtained. The mass loss resulting from this transition
is 17.99 mass% (theoretical calculation is 18.94 mass%). Chemical analysis gave the
composition of Pr2O3 as 59.82 mass% and that of SeO2 as 40.33 mass% (theoretical
calculations for Pr2O3 is 59.78 mass% and that for SeO2 is 40.22 mass%). The
endothermal peak at 700°C corresponds to liberation of another mole of SeO2 and
transformation of the compound into oxoselenite with composition Pr2O3⋅SeO2.
However, at 940°C, which is the temperature at which the sample was heated for
derivatograph analysis, the mass loss due to thermal decomposition is 25.00 mass%,
while the theoretically calculated mass loss is 35.25 mass%, which shows that the
process is incompleted. SeO2 was completely liberated and a pure phase of Pr2O3 was
obtained in the temperature interval 1000–1200°C.
Figure 3 shows the TG, DTA, T curves of PrH(SeO3)2. This compound is ther-
mally stable at 150°C. In the temperature interval 180–250°C adsorbed and constitu-
tionally bounb water is liberated and PrH(SeO3)2 turns into tetraselenite Pr2Se4O11. The
mass loss due to this transition is 3.33 mass% (theoretical calculation is 2.27 mass%).
Chemical analysis gave the composition of Pr2O3 as 42.59 mass%, and that of SeO2
as 57.40 mass% (theoretical calculation for Pr2O3 is 42.63 mass%, and that for SeO2
is 57.37 mass%). In the temperature interval 395–440°C, 1 mole of SeO2 is liberated,
and tetraselenite is transformed into a normal salt Pr2(SeO3)3. The total mass loss due to
these transitions is 22.33 mass% (theoretical calculation is 16.29 mass%). The differ-
ence between the experimental mass loss and the theoretical one is due to presence of
small amount of adsorption water. At 450°C another mole of SeO2 is liberated and the
basic salt Pr2O3⋅2SeO2 is formed. The mass loss according to the data from TG curve
is 33.33 mass% (theoretical calculation is 33.47 mass%). Chemical analysis gave the
composition of Pr2O3 as 59.82 mass% and that of SeO2 as 40.28 mass% (theoretical cal-
J. Therm. Anal. Cal., 75, 2004

GOSPODINOV, STANCHEVA: Pr2O3–SeO2–H2O
99
Fig. 3 TG, DTA, T curves of PrH(SeO3)2
culation for Pr2O3 is 59.78 mass% and that for SeO2 is 40.22 mass%). In the tempera-
ture interval 610–700°C yet another mole of SeO2 is liberated, and a second basic salt
Pr2O3⋅SeO2 is obtained. The total mass loss due to all decomposition reactions so far
is 42.00 mass% (theoretical calculation is 44.33 mass%). X-ray phase analysis data
show that the main phase obtained in this transformation is Pr2O3⋅SeO2. Insignificant
amount of Pr2O3⋅2SeO2 is also present. Complete liberation of SeO2 takes place at
1100–1200°C, and Pr2O3 and insignificant quantities of Pr6O11 are obtained.
The compound Pr(HSeO3)3 is thermally stable at 300°C. Dehydration with liber-
ation of constitutional water takes place at higher temperatures. The mass loss due to
this transformation is 5.17 mass% (theoretical calculation is 5.14 mass%). Chemical
analysis gave the composition of Pr2O3 as 33.20 mass% and that of SeO2
as 66.73 mass% (theoretical calculation is 33.13 mass% for Pr2O3, and 66.87 mass%
for SeO2). At 460°C two moles of SeO2 are liberated and the initial selenite turns into
Pr2(SeO3)3⋅SeO2. The mass loss due to this transformation is 26.34 mass% (theoreti-
cal calculation is 26.28 mass%). At 600°C yet another mole of SeO2 is liberated and
the compound is transformed into a normal salt Pr2(SeO3)3. The mass loss in this
transformation is 36.90 mass% (theoretical calculation is 36.86 mass%). Chemical
analysis gave the composition of Pr2O3 as 49.83 mass%, and that of SeO2 as
50.25 mass% (theoretical calculation for Pr2O3 is 49.77 mass% and that for SeO2
is 50.23 mass%). The selenite obtained has stoichiometric composition.
Further decomposition of Pr2(SeO3)3 takes place by the scheme of the normal salt
described by us. The suggested mechanism of thermal decomposition of Pr(HSeO3)3 is
in good agreement with the one described in [15, 16].
J. Therm. Anal. Cal., 75, 2004

100
GOSPODINOV, STANCHEVA: Pr2O3–SeO2–H2O
Identification of the intermediate phases obtained in the course of thermal de-
composition of the three salts was confirmed by chemical and X-ray phase analyses
of these products by modeling the conditions of thermal analysis. This paper repre-
sents continuation of our investigation on the three-component systems of the type
Ln2O3–SeO2–H2O [17–21].
Conclusions
• The solubility isotherm of the system Pr2O3–SeO2–H2O at 100°C was studied. It
was proved that three salts are formed in the system, namely: Pr2(SeO3)3⋅H2O,
PrH(SeO3)2 and Pr(HSeO3)3.
• The parameters of the crystal lattice of PrH(SeO3)2 were determined.
• Complex thermal analysis by means of the derivatograph of the compounds ob-
tained was made. Furthermore, the intermediate phases and the mechanism of the
decomposition were studied.
References
1 L. Ja. Markovskij, M. C. Soboleva and Ya. A. Sapognikov, J. Prikl. Chem., 38 (1965) 717.
2 R. A. Safina, Avtoref. Dis. Kand. Chim. Nauk, L. GIJH, 1969, 17.
3 E. Immonen, M. Koskenlinna, L. Ninissto and T. Pakanen, Finn. Chem. Lett., 3 (1976) 67.
4 E. Giesbrecht and I. Giolito, An. Da Acad. Brasileira de Ciencias, 39 (1967) 233.
5 K. I. Petrov, Ya. M. Golovin, M. B. Varfolomeev and E. M. Remennik, J. Neorg. Chem.,
18 (1973) 385.
6 A. I. Majer, Ya. L. Syponickij and M. H. Karapetyanc, Izv. AN SSSR. Neorg. Mater., 2 (1966) 778.
7 A. I. Majer, T. A. Soldatova and M. H. Karapetyanc, Tr. MHTI ‘D. I. Mendeleeva’, 62 (1969) 39.
8 A. I. Majer, T. A. Soldatova and M. H. Karapetyanc, Izv. AN SSSR. Neorg. Mater., 5 (1969) 1712.
9 Ya. L. Syponickij, A. I. Majer and M. H. Karapetyanc, Tr. MHTI ‘D. I. Mendeleeva’, 54 (1967) 27.
10 A. I. Majer, Ya. L. Syponickij and M. H. Karapetyanc, Izv. Vuz. Chem. i Chem.Tech., 14 (1971) 3.
11 M. Pedro, J. C. Trombe and A. Castro, J. Materials Science Letters, 14 (1995) 994.
12 V. Umlang, A. Ianssen, D. Tierg and C. Winsch, Theorie und Praktische Anwendung von
Complexbildnern, Dechema, Frankfurt am Main 1971.
13 I. I. Nazarenko and E. I. Ermakov, Analiticheskaia Himia Selena i Telura, Nauka, Moscow
1974, pp. 5., 59
14 D. I. Ryabchikov and V. A. Ryabuhin, Analiticheskaia Himia Redkozemelnih elementov,
Nauka, Moscow 1966, p. 187.
15 G. S. Savchenko, I. V. Tananaev and A. N. Volodina, Izv. AN SSSR. Neorg. Mater., 4 (1968) 1097.
16 M. H. Karapetyanc, A. I. Majer and T. A. Soldatova, Izv. AN SSSR. Neorg. Mater., 6 (1970) 1118.
17 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 55 (1999) 221.
18 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 65 (2001) 275.
19 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 67 (2002) 463.
20 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 73 (2003) 835.
21 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 73 (2003) 859.
J. Therm. Anal. Cal., 75, 2004

100
GOSPODINOV, STANCHEVA: Pr2O3–SeO2–H2O
Identification of the intermediate phases obtained in the course of thermal de-
composition of the three salts was confirmed by chemical and X-ray phase analyses
of these products by modeling the conditions of thermal analysis. This paper repre-
sents continuation of our investigation on the three-component systems of the type
Ln2O3–SeO2–H2O [17–21].
Conclusions
• The solubility isotherm of the system Pr2O3–SeO2–H2O at 100°C was studied. It
was proved that three salts are formed in the system, namely: Pr2(SeO3)3⋅H2O,
PrH(SeO3)2 and Pr(HSeO3)3.
• The parameters of the crystal lattice of PrH(SeO3)2 were determined.
• Complex thermal analysis by means of the derivatograph of the compounds ob-
tained was made. Furthermore, the intermediate phases and the mechanism of the
decomposition were studied.
References
1 L. Ja. Markovskij, M. C. Soboleva and Ya. A. Sapognikov, J. Prikl. Chem., 38 (1965) 717.
2 R. A. Safina, Avtoref. Dis. Kand. Chim. Nauk, L. GIJH, 1969, 17.
3 E. Immonen, M. Koskenlinna, L. Ninissto and T. Pakanen, Finn. Chem. Lett., 3 (1976) 67.
4 E. Giesbrecht and I. Giolito, An. Da Acad. Brasileira de Ciencias, 39 (1967) 233.
5 K. I. Petrov, Ya. M. Golovin, M. B. Varfolomeev and E. M. Remennik, J. Neorg. Chem.,
18 (1973) 385.
6 A. I. Majer, Ya. L. Syponickij and M. H. Karapetyanc, Izv. AN SSSR. Neorg. Mater., 2 (1966) 778.
7 A. I. Majer, T. A. Soldatova and M. H. Karapetyanc, Tr. MHTI ‘D. I. Mendeleeva’, 62 (1969) 39.
8 A. I. Majer, T. A. Soldatova and M. H. Karapetyanc, Izv. AN SSSR. Neorg. Mater., 5 (1969) 1712.
9 Ya. L. Syponickij, A. I. Majer and M. H. Karapetyanc, Tr. MHTI ‘D. I. Mendeleeva’, 54 (1967) 27.
10 A. I. Majer, Ya. L. Syponickij and M. H. Karapetyanc, Izv. Vuz. Chem. i Chem.Tech., 14 (1971) 3.
11 M. Pedro, J. C. Trombe and A. Castro, J. Materials Science Letters, 14 (1995) 994.
12 V. Umlang, A. Ianssen, D. Tierg and C. Winsch, Theorie und Praktische Anwendung von
Complexbildnern, Dechema, Frankfurt am Main 1971.
13 I. I. Nazarenko and E. I. Ermakov, Analiticheskaia Himia Selena i Telura, Nauka, Moscow
1974, pp. 5., 59
14 D. I. Ryabchikov and V. A. Ryabuhin, Analiticheskaia Himia Redkozemelnih elementov,
Nauka, Moscow 1966, p. 187.
15 G. S. Savchenko, I. V. Tananaev and A. N. Volodina, Izv. AN SSSR. Neorg. Mater., 4 (1968) 1097.
16 M. H. Karapetyanc, A. I. Majer and T. A. Soldatova, Izv. AN SSSR. Neorg. Mater., 6 (1970) 1118.
17 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 55 (1999) 221.
18 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 65 (2001) 275.
19 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 67 (2002) 463.
20 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 73 (2003) 835.
21 G. G. Gospodinov and M. G. Stancheva, J. Therm. Anal. Cal., 73 (2003) 859.
J. Therm. Anal. Cal., 75, 2004