XIX-th ARS SEPARATORIA – Złoty Potok, Poland 2004
ISOTOPE EFFECTS FOR 34S/32S DURING DISSOLVING
PROCESS OF SO2 IN POLAR SOLVENTS
Agnieszka MIKOŁAJCZUK1), R. WIERZCHNICKI
and Andrzej G. CHMIELEWSKI
Instytut Chemii i Techniki Jądrowej,
ul. Dorodna 16, 03-195 Warszawa,
1)
[email protected]
In this study sulfur fractionation factor between sulfur dioxide in gas
phase and sulfur dioxide dissolved in three polar solvents has been
investigated experimentally. Water, methanol and nitrobenzene were used in
the experiment as solvents. Such measurements were carried out at the
temperature range 20-70oC. The isotope composition was measured using
mass spectrometry.
The first investigation on sulfur isotope effects was experimentally
performed in 1945 by Thode et al. [1]. The authors investigated the sulfur
isotope effect in SO2(gas)-HSO3-(aq) system and found that the fractionation
factor for the sulfur isotope pairs (34S/ 32S) and (36S/32S), was 1.019±0.02 and
1.043±0.02 at 25oC respectively. Such system is applied to the separation
technology of sulfur isotopes till now [2].
Recently, sulfur isotope fractionation factors (34S/32S) were
determined experimentally for the systems SO2 in the gas phase and
dissolved in water [3] and methanol [4]. The isotope effects were
investigated when these systems reached equilibrium states. and the solvent
phases were saturated with sulfur dioxide. The influence of temperature on
investigated sulfur isotopes separation between phases was studied as well.
With increasing temperature, both the solubility of sulfur dioxide and the
fractionation factor decrease, at 22oC α=1.00147, and at 70oC
α=1.00086±0.000054 for SO2 saturated solutions. In the methanol isotope
the fractionation factor is lower than in the water, and changes from
α=1.00030 at 20oC to α =1.00041±0.00002 at 58oC.
The solubility of sulfur dioxide in nitrobenzene is higher than in water
and lower than in methanol. Sulfur dioxide dissolved in water reacts with the
molecules of solvent. The precisely is not known which kind of structures is
formed in methanol and nitrobenzene after SO2 dissolution.
According to the force constant of the Lennard-Jones potential of
gases and liquids the maximum solubility can be predicted. The force
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XIX-th ARS SEPARATORIA – Złoty Potok, Poland 2004
constant of the Lennard-Jones potential of the liquids are 809.2K, 609.0K
and 481.8K for methanol, nitrobenzene and water [6]. The solubility of gases
in liquids depends on the force constant of the Lennard –Jones potential of
the liquid. Methanol has the lowest force constant of the Lennard –Jones
potential, than higher values have nitrobenzene and water, this is a reason
that solubility of SO2 in methanol is the highest [6].
The nitrobenzene has the higher dipole moment than water and
methanol. The values are 4.23D, 1.84D and 1.69D respectively. Sulfur
dioxide has the dipole moment equal to 1.6D. Methanol is a polar solvent
and creates hydrogen–bonds like water. Oxygen in -NO2 group creates
hydrogen-bonding with donors of proton. Dielectric constants of dipolar
liquids are large because dipolar molecules have a high free motion. In water
the dielectric constant depends on a temperature, for example the dielectric
constant at 25oC is equal to 78, at 100oC 55.33, for nitrobenzene 34.1 at 25oC
and for methanol only 31.2 at 25oC. Water and methanol have the similar
moment of dipole, both creates hydrogen-bonding, but solubility of SO2 in
methanol is higher than in water; 35g of sulfur dioxide is dissolved at 20oC
in methanol [4], 15.3 g in nitrobenzene and 10.64 g in water [5].
Figure 1 shows comparison sulfur isotope effects between sulfur
dioxide in the gas and dissolved in water, methanol and nitrobenzene.
1.002
1.00175
1.0015
water [3]
1.00125
α
1.001
1.00075
1.0005
1.00025
methanol [4]
1
nitrobenzene
0.99975
0.9995
0.99925
10
20
30
40
50
60
70
80
temperature [oC]
Fig. 1. Sulfur separation factor (α) between sulfur dioxide in the gas phase
and liquid phase (water, methanol, nitrobenzene), solution saturated with sulfur
dioxide, p=1013 hPa, precision of the measurement of δ34S is 0.052; 0.020; 0.013
respectively, for water, methanol and nitrobenzene.
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XIX-th ARS SEPARATORIA – Złoty Potok, Poland 2004
In water and methanol the normal isotope effects were observed, while in
nitrobenzene the reverse isotope effect was observed. The spectroscopic data
to theoretical calculation are not available nowadays.
The internal vibrational frequencies of SO2 molecule, when it is
transferred from the gaseous into the “solvents” phase should be determined.
Therefore, the fractionation factors were determined only experimentally in
this work. When water was SO2 saturated, the isotope fractionation factor
was equal to 1.00147 at 22oC, when the concentration SO2 in water was
0.292 mol/l (water partly saturated) the isotope fractionation factor was
equal 1.00256 at 18oC [3].
In SO2 saturated water solution both, the solubility of sulfur dioxide
and the fractionation factor decrease with increasing temperature, at 22oC
α=1.00147, and at 70oC α=1.00086 ± 0.000052. In methanol the solubility of
SO2 decrease with increasing temperature, however, the isotope fractionation
factor changes from α = 1.00030 at 20oC to α=1.00041±0.00002 at 58oC. In
this case, the changes of isotope fractionation factors were small and equal to
about 0.15 ‰ [4].
In the case of nitrobenzene solution, the solubility of sulfur dioxide
decrease when the temperature increase. In this case, the reverse isotope
effect is observed, the heavier sulfur isotope is enriched in the gas phase in
comparison with the solvent phase. The isotope fractionation factor changes
from α=0.99961 at 20oC to α=0.99948± 0.000013 at 71oC. The changes of
isotope effects are quite small, within the temperature range 20÷71oC, the
changes are equal to about 0.18± 0.013‰.
ACKNOWLEDGMENT
This work was supported by a grant of Polish Committee for Scientific
Research No. 4T09A 039 24.
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