CHEM. RES. CHINESE UNIVERSITIES 2010, 26(1), 114—117
Solid-liquid Metastable Equilibria in Quaternary System
(NaCl+Na2CO3+Na2SO4+H2O) at 273.15 K
WANG Rui-lin and ZENG Ying*
Department of Chemical Engineering, Chengdu University of Technology, Chengdu 610059, P. R. China
Abstract The metastable phase equilibria of the quaternary system NaCl+Na2CO3+Na2SO4+H2O were studied at
273.15 K. The salts’ solubilities, densities and pH values of the equilibrated solution in this system were determined.
According to the experimental data, the metastable equilibrium phase diagram, the diagram of density vs. composition and pH vs. composition diagram were plotted. The phase diagram consists of five univariant curves, four crystallization fields and two invariant points. The four crystallization fields correspond to sodium carbonate decahydrate
(Na2CO3·10H2O), sodium sulfate decahydrate(Na 2SO4·10H2O), sodium chloride(NaCl) and burkeite(2Na2SO4·
Na2CO3), respectively. The crystallization field of sodium sulfate decahydrate(Na2SO4·10H2O) is the largest, which
indicates that sodium sulfate is easy to saturate and crystallize from solution at 273.15 K.
Keywords Quaternary system; Metastable phase equilibrium; Sulfate
Article ID 1005-9040(2010)-01-114-04
1
Introduction
The Zabuye Salt Lake, Tibet, located in the west
of China, is famous for its high concentrations of lithium, boron, and potassium in the world. The main
components of its brine are Li+, K+, Na+, B4O72–,
CO32–, Cl–, SO42– and H2O, including rare elements
such as Rb+ and Cs+[1,2]. Metastable phase equilibrium
and phase diagram play an important role in exploiting the brine resources. To economically exploit salt
lake brine resources, it is important to adopt the local
natural energy such as the sun and the wind, thus the
technique like the solar pond is widely used. The
comprehensive utilization of brines is strongly dependent on mutual metastable equilibrium solubilities of
salts, and therefore, the investigation of metastable
equilibrium is of important theoretical and practical
significance.
The metastable equilibrium phase diagram, also
called the “solar phase diagram”, was begun to be studied in the early 1920s. Metastable equilibrium studies aiming at the sea water system Na++K++Mg2++
Cl– +SO42–+H2O at 15, 25, and 35 °C[3―5] have been
reported, respectively. Metastable equilibria in the
quinary system Na++K++Cl– +CO32– +SO42– +H2O and
the quaternary system Li++Mg2++Cl–+SO42–+H2O at
25 °C have also been completed[6,7]. Although all the
mentioned researches have played an important role in
exploiting the salt lake brine resources, they are almost concentrated on the metastable equilibria of
temperature above 15 °C, while the climate conditions
in the region of Zabuye Salt Lake are generally windy,
arid, little rainfall and has great evaporating capacity,
its average temperature is about 273.15 K[8]. Aiming
at the characteristics of the clime and the composition
of Zabuye Salt Lake, the researches focused on the
metastable equilibrium at 273.15 K will have an important guiding significance[9].
The quaternary system NaCl+Na2CO3+Na2SO4+
H2O is a subsystem of the Zabuye Salt Lake brines. So
far, no report has been found about the metastable
phase equilibria of this quaternary system. The present
paper covers the metastable equilibria of the quaternary system(NaCl+Na2CO3+Na2SO4+H2O) at T=
273.15 K. The solubilities and the physicochemical
properties such as density and pH value of the equilibrated solution were measured.
2
2.1
Experiments
Reagents and Instruments
All the chemicals used were of analytical purity
———————————
*Corresponding author. E-mail: [email protected]
Received February 2, 2009; accepted April 8, 2009.
Supported by the National Natural Science Foundation of China(No.40673050), the Research Fund for the Doctoral Program
of Higher Education from the Ministry of Education of China(No.20070616008) and the Scholarship Leaders Training Fund from
Sichuan Province, China(No.2008-140).
No.1
WANG Rui-lin et al.
grade and obtained from Chengdu Kelong Chemical
Reagent Manufactory, China. They were sodium chloride(NaCl, 99.5%, mass fraction), sodium carbonate
(Na2CO3, 99.5%, mass fraction), sodium sulfate
(Na2SO4, 99.5%, mass fraction). Doubly deionized
water was obtained from a Millipore water system
with an electrical conductivity less than 1×10–4 S/m
and pH=6.6.
An SHH-250 type thermostatic evaporator made
by the Chongqing INBORN Instrument Corporation,
China, was used for the metastable phase equilibrium
experiments. There were temperature-controlling apparatus and blower accessory equipment in it to control the temperature and the evaporation quantity of
the evaporated system. The temperature controlling
precision was ±0.1 K. A PHS-25 precision pH meter
supplied by the Shanghai Leici Instrument Factory
was used to measure the pH value, with a precision of
±0.01. A SIMENS D500 X-ray diffractometer with
Ni-filtered Cu Kα radiation was used to analyze the
crystalloid form of the solid phase. An Optima 5300V
type ICP-OES made by PE Corporation, USA, was
used for the determination of the sodium ion concentration.
2.2
Experimental Methods
The isothermal evaporation method was employed in this study. According to the phase equilibrium composition, an appropriate quantity of salts
and distilled water were mixed together as a series of
artificial synthesized brines and loaded into clean polyethylene containers(15 cm long, 10 cm wide and 7
cm high) and then the containers were put into the
SHH-250 type thermostatic evaporator for the isothermal evaporation at (273±0.1) K. The crystal behaviors of the solid phases were observed periodically.
When enough new solids appeared, the solids were
separated from the solutions, dried at 273.15 K and
waited to identify. Meanwhile, 5.0 mL of the sample
of the clarified solution was taken from the liquid
phase and diluted to a final volume of 100 mL in a
volumetric flask filled with the deionized water to
analyze the liquid phase components. Another 5.0 mL
of sample was taken to measure the density. The remainder of the solution continued to be evaporated to
reach the next metastable equilibrium point.
The densities(ρ) of the equilibria solution were
measured by means of a density bottle method with a
115
3
precision of ±0.0002 g/cm . The pH value was
measured with a PHS-25 precision pH meter, with a
precision of ±0.01. The pH meter was calibrated with
standard buffer solutions prepared by either the
mixing agents of sodium dihydrogen phosphate and
dipotassium hydrogen phosphate(pH 6.86) or borax
(pH 9.18).
2.3
Identification of Solid Phases
When sufficient new solid phases appeared, the
solids were separated from the solutions. The obtained
wet crystals of the solid phase were separated from
each other according to the crystal shapes as much as
possible. The solids were then analyzed by chemical
methods to obtain the composition, and further identified by X-ray diffraction to ascertain the crystalloid
form.
2.4
Analytical Methods
According to the method reported in ref. [10], the
chlorine ion(Cl–) concentration was measured by
silver nitrate titration(uncertainty of 0.5%, mass fraction). The sulfate ion concentration(SO42–) was determined by a method of mixing barium chloride and
magnesium chloride-EDTA titration(uncertainty of
0.5%). The carbonate ion concentration(CO32–) was
determined by means of a method of acid-base titration. The sodium ion concentration(Na+) was evaluated on an ion balance, and assisted by ICP-OES
(uncertainty less than 0.5%, mass fraction).
3
Results and Discussion
The experimental results of solubilities and
physicochemical properties such as density and pH
value of the metastable equilibria of the quaternary
system NaCl+Na2CO3+Na2SO4+H2O at 273.15 K are
listed in Table 1. In Table 1, the salts’ solubilities of
the equilibrated solution are expressed as mass
fraction, and the composition of the solid phases are
calculated with 100 g of dry salt as benchmark, that is,
J(NaCl)+ J(Na2CO3)+J(Na2SO4)=100 g. According to
the composition of dry salt, the metastable phase diagram of this system at 273.15 K was plotted, as shown
in Fig.1(A). Fig.1(B) is the partial enlarged diagram of
Fig.1(A). In Fig.1, there are four crystallization fields
corresponding to sodium carbonate decahydrate
(Na2CO3·10H2O, G1EBG1), sodium sulfate decahydrate(Na 2 SO 4 ·10H 2 O, G 2 FAG 2 ), sodium chloride
116
CHEM. RES. CHINESE UNIVERSITIES
(NaCl, BEFAB) and burkeite(2Na 2 SO 4 ·Na 2 CO 3 ,
G1EFG2G1). The crystallization field of NaCl is the
smallest, while the crystallization field of Na2SO4·
10H2O is larger than the rest. The large crystallization
Table 1
regions indicate that Na2SO4 is of a low solubility;
therefore, most sodium sulfate salt can be easily crystallized from the solution.
Determined values of salts’ solubility, density and pH value of equilibrated solution in the quaternary
system NaCl +Na2CO3 +Na2SO4 +H2O at 273.15 K*
Composition of solution, w(B)(%)
No.
1,A
2
Vol.26
w(NaCl)
25.23
25.23
w(Na2CO3)
0.00
0.50
w(Na2SO4)
1.55
1.53
w(H2O)
73.23
72.74
Composition of solid phase Jb/(g·100 g–1 dry salt)
J(NaCl)
94.22
92.54
J(Na2CO3)
0.00
1.84
J(Na2SO4)
5.78
5.63
Density,
ρ/(g·cm–3)
pH
ncl+ns
ncl+ns
1.2109
1.2145
9.31
9.60
Solid phase
3
25.01
0.64
1.55
72.80
91.95
2.35
5.69
ncl+ns
1.2165
9.41
4
24.31
1.16
1.50
73.03
90.14
4.29
5.57
ncl+ns
1.2219
9.21
5
23.83
1.85
1.59
72.74
87.40
6.77
5.83
ncl+ns
1.2261
9.42
6
23.36
2.63
1.38
72.64
85.37
9.60
5.03
ncl+ns
1.2289
9.43
7,B
24.19
6.13
0.00
69.68
78.15
21.85
0.00
nc+ncl
1.2268
11.08
8
23.73
3.29
0.31
72.68
86.85
12.02
1.12
nc+ncl
1.2226
10.52
9
23.76
3.28
0.31
72.65
86.86
12.00
1.14
nc+ncl
1.2259
10.24
10
23.65
3.33
0.32
72.70
86.64
12.21
1.16
nc+ncl
1.2276
9.81
11
22.80
3.98
0.89
72.33
82.40
14.38
3.22
nc+ncl
1.2364
9.41
0.00
6.90
2.22
90.88
0.00
75.69
24.31
nc+nsc
1.0961
11.47
12,G1
13
0.47
6.03
1.91
91.60
5.58
71.72
22.69
nc+nsc
1.0921
11.26
14
5.38
8.77
2.35
83.50
32.61
53.13
14.27
nc+nsc
1.1628
10.61
15,E
21.85
5.69
1.01
71.45
76.52
19.93
3.55
nc+nsc+ncl
1.2440
10.41
16,G2
0.00
5.15
2.25
92.60
0.00
69.56
30.44
ns+nsc
1.0794
11.84
17
0.45
5.31
2.07
92.17
5.69
67.82
26.49
ns+nsc
1.0842
11.50
18
1.25
5.71
2.43
90.62
13.30
60.83
25.87
ns+nsc
1.1014
10.78
19
5.46
7.90
2.87
83.77
33.63
48.65
17.71
ns+nsc
1.1556
10.21
20,F
22.46
4.33
1.08
72.13
80.60
15.54
3.86
ns+nsc+ncl
1.2377
10.18
21
22.14
5.23
1.03
71.59
77.95
18.41
3.64
nsc+ncl
1.2426
10.36
22
22.35
4.73
1.06
71.85
79.40
16.82
3.78
nsc+ncl
1.2392
10.38
* ns: Na2SO4·10H2O; ncl: NaCl; nc: Na2CO3·10H2O; nsc: 2Na2SO4·Na2CO3.
Fig.1
Metastable phase diagram of quaternary system NaCl +Na2CO3 +Na2SO4 +H2O at 273.15 K(A) and
part enlargement diagram of Fig.1(A)(B)
The phase diagram has five univariant curves
(B→E, A→F, F→E, E→G1 and F→G2), and two isothermal invariant points E and F. The invariant point E
is saturated with three salts Na2CO3·10H2O, NaCl and
2Na2SO4·Na2CO3, and the invariant point F is saturated with three salts Na2SO4·10H2O, NaCl and
2Na2SO4·Na2CO3. Burkeite 2Na2SO4·Na2CO3 is found
in this quaternary system. The sodium sulfate(Na2SO4)
causes a weak salting-out effect on the salt sodium
carbonate(Na2CO3).
On the basis of data collected in Table 1, the relationship between the physicochemical properties of
solution(density and pH value) and the mass fraction
of sodium chloride w(NaCl) are shown in Figs.2 and
3, respectively. Fig.2 shows that in the process of water evaporation, the density of the equilibrated solution increases regularly with the increasing of sodium
chloride concentration in a w(NaCl) range of 0―22%,
and reaches the maximum values at the co-saturation
point E(1.2440 g/cm3) and point F (1.2377 g/cm3). In
No.1
WANG Rui-lin et al.
Fig.3, the pH value of the metastable equilibrium of
aqueous solution decreases gradually with the increasing of sodium chloride concentration.
Fig.2
Phase diagram of density-content of quaternary
system NaCl+Na2CO3+Na2SO4+H2O at 273.15 K
4
117
Conclusions
Metastable equilibria of the quaternary system
NaCl+Na2CO3+Na2SO4+H2O at 273.15 K were studied. Solubility of the salts and properties such as
density and pH value of the equilibrated solution were
determined experimentally. According to the experimental data, the phase diagram and corresponding
diagrams of properties vs. composition were plotted.
The experimental results show that there are two
invariant points, five univariant curves, and four crystallization fields in this system. Burkeite 2Na2SO4·
Na2CO3 is formed in this quaternary system. The
crystallization form of sodium sulfate is Na2SO4·
10H2O at 273.15 K. The salt sodium sulfate has the
smallest solubility and it can be easily crystallized
from the solution.
References
[1]
Zhang Y. S., Zheng M. P., Nie Z., et al., Sea-lake Salt and Chemical
Industry, 2005, 34, 1
[2]
Zheng X. Y., Tang Y., Xu Y., Salt Lakes of Tibet, Science and
Technology Press, Beijing, 1989, 5
Fig.3
Diagram of pH value vs. composition of quaternary
system NaCl +Na2CO3+Na2SO4+H2O at 273.15 K
As we known, the sodium sulfate has different
crystallization forms, its mainly forms are Na2SO4·
7H 2 O, Na 2 SO 4 ·10H 2 O and Na 2 SO 4 . The earlier
researches aiming at the metastable equilibria for the
systems including sodium sulfate, such as the quinary
system Na++K++Cl–+CO32–+SO42–+H2O at 298.15 K[7]
and the quaternary system Na++Li++SO42–+B4O72–
(CO32–)+H2O at 288.15 K[11,12], show that the crystallization form of sodium sulfate is anhydrous salt
Na2SO4 at 298 and 288 K, while in the ternary system
Na + +K + +SO 4 2– +H 2 O and the quaternary system
Na++B4O72–+SO42–+CO32–+H2O at 273.15 K[13,14], the
crystallization form of sodium sulfate is Na2SO4·
10H2O. In this paper, the results also show that the
crystallization form of sodium sulfate is Na2SO4·
10H2O at 273.15 K.
[3]
Jin Z. M., Zhou H. N., Wang L. S., Chem. J. Chinese Universities,
2002, 23(4), 690
[4]
Jin Z. M., Zhou H. N., Wang L. S., Chem. J. Chinese Universities,
2001, 22(4), 634
[5]
Su Y. G., Li J., Jiang C. F., Journal of Chemical Industry and Engineering, 1992, 43, 549
[6]
Guo Z. Z., Liu Z. Q., Chen J. Q., Acta Chimica Sinica, 1991, 49, 937
[7]
Fang C. H., Niu Z. D., Liu Z. Q., Acta Chimica Sinica, 1991, 49,
1032
[8]
Nie Z., Zheng M. P., Acta Geosicientia Sinica, 2001, 22, 271
[9]
Zheng M. P., Acta Geosicientia Sinica, 2001, 22, 97
[10]
Institute of Qinghai Salt-Lake, Chinese Academy of Science, Analytical Methods of Brines and Salts, Science Press, Beijing, 1984, 75
[11]
Sang S. H., Yin H. A., Zeng Y., Acta Chimica Sinica, 2006, 64,
2247
[12]
Sang S. H., Yu H. Y., Cai D. Z., Chinese Journal of Inorganic Chemistry, 2005, 21, 1316
[13]
Lin X. F., Zeng Y., Zheng Z. Y., Journal of Salt Lake Research,
2007, 15, 24
[14]
Zheng Z. Y., Zeng Y., Chen J., et al., Chem. J. Chinese Universities,
2008, 29(2), 336