International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:04
43
The Role of Dielectric Constant in Fractional
Separation of Alkali Metals Salts from Aqueous
Solutions
Mutasim I. Khalil, Reem A.H.Al-Yami, Amani H.Al-Khabbas
The effect of dielectric constant (ε) on the
precipitation of Lithium, Sodium and Potassium chlorides has
been investigated. At ε≈35 , 100%KCl, 58% NaCl , and 0.0%LiCl
is separated from an aqueous solution mixture . Repeated
procedures at selected dielectric constants achieved complete
fractional separation of the three salts. Results are correlated to
dielectric constant, cation size and hydration energies.
Abstract--
Index Term-- Alkali metals chlorides, Fractional separation,
Dielectric constant, Desalination, Cation size, Hydration energy.
INTRODUCTION
I.
The spontaneous crystallization of sodium chloride from
aqueous ethanol solutions was reported by Linnikov [1] who
employed ethanol for inducing super saturation in solutions.
Doki et al.[2] crystallized sodium chloride from aqueous
solution by adding pure ethanol or an ethanol –water (75vol%)mixture as diluents. Khalil and Al-Resayes[3]argued that
the solvent dielectric constant , which is a measure of
orientational correlation between molecules , is one
characteristic of great importance in causing ion formation by
a solute. For systems containing electrolytes , one should take
into account the effect of ions and ion pairs that produce the
dependence of the dielectric constant on electrolyte
concentration. Hence, the degree of solute ionization,
precipitation and coagulation could be varied by varying the
dielectric constant of the medium [3].
Mutasim I. Khalil is serving as
Department of Chemistry, Science Faculty, King Saud
University,Riyadh,11451,Saudi Arabia;
Tel:+496614675973;
fax;+496614675992;
email:[email protected]
Reem A.H.Al-Yami is serving as
Department of Chemistry,Girls College, Hail University, Saudi Arabia.
University,Riyadh,11451,Saudi Arabia;
Tel:+496614675973;
fax;+496614675992;
Amani H.Al-Khabbas is serving as
Department of Chemistry,Girls College, Hail University, Saudi Arabia.
University,Riyadh,11451,Saudi Arabia;
Tel:+496614675973;
fax;+496614675992
This has a great impact on experimental applications to
chemical and industrial problems, least to mention sea water
desalination.
Since solubility of alkali metals salts is a function of cation
size and dielectric constant ,then , fractional separation of
alkali metals salts from a solution mixture is a feasible
phenomenon which could be manipulated ,attained and
controlled by varying the dielectric constant of the
medium.Luckly enough , for most solvent mixtures ,
the dielectric constants could be easily predicted using pure
solvents properties[3,4].
In this paper we investigated the precipitation of LiCl, NaCl
and KCl at different values of dielectric constant of the liquid
medium. The optimum value of dielectric constant that
achieves maximum separation of the three salts is reported
.The results are interpreted in terms of three variables, that is,
dielectric constant, cation size and hydration energies, ∆Hhydr.
II.
EXPERIMENTAL
MATERIALS
LiCl, NaCl And KCL (BDH) are all analytical reagents (AR).
96% ethanol (BDH) was used as received. De-ionized water
was used .
Procedure:
An aqueous 23.6% solution of each salt was prepared and kept
in a 1.0 L volumetric flask. To six 10.0ml portions of each salt
solution different volumes of 96% ethanol were added and the
mixtures were left to reach equilibrium until a clear
supernatant layer was obtained above the precipitate formed.
The concentration of salt left in solution was then determined
for each experiment by drying a known volume of the
supernatant liquid. The amount of precipitated salt was then
calculated by difference.The dielectric constant was estimated
by Khalil,s proposed equation[3].
III.
RESULT AND DISCUSSIONS
Correlations of dielectric constant for commonly used solvent,
water , have been extensively studied[5-8].Models for the
dielectric constant of liquid mixtures have also been
reported[9]. Although the concentration dependence of the
dielectric constant of ionic systems has been recognized from
experimental evidence , yet no attempts were made to employ
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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:04
those evidences in separating metal salts from aqueous
solution mixtures into their individual components.
In this paper we report a procedure for separating alkali metal
salts by varying the dielectric constant of the medium. The
optimum value of dielectric constant (ε) that achieves the
maximum separation of the individual alkali metal chlorides is
reported.
Table 1 and Figure 1 show the percent of precipitation of each
salt at different dielectric constants.
TABLE I % precipitation of Li,Na,K chlorides at different dielectric constants. ε
80
57
55
52
35
%Precipitation
LiCl
0
0
0
0
0
NaCl
0
0.014
5.5
13.1
58.5
44
results show a linear correlation between cation size and and
percent precipitation of salt at a fixed dielectric constant as
shown by Table II and figure 2.
TABLE II %Precipitation of Li,Na,K chlorides and cation size at ε=35 KCl
0
29
42
47.18
100
% Precipitation
Cation size ˚A
0
0.5
54.5
1
100
1.5
Fig. 2. Plot of %precipitation of Li,Na,K chlorides versus cation size (Å) at
ε=35.
Fig. 1. Plot of %precipitation of Li,Na,K chlorides versus dielectric constants. It is evident that the solubility of such salts are dielectric
constant (ε) dependent. While LiCL is completely soluble in
all values of dielectric constant, the solubility of NaCL and
KCL decreases dramatically with decrease of ε value reaching
a 0.0% and 42% solubility for KCL and NaCL respectively
at ε = 35.Such a trend of behavior is correlated to the size of
the three cations revealing a linear relationship as indicated
by Table II and figure 2.The effect of ions on the dielectric
constant for the chlorides of alkali metals has been reported
[4]. The cation contribution to the reduction of dielectric
constant is primarily dependent on the charge and the effect
of ion size on the static dielectric properties of aqueous alkali
solutions has also been reported. However, a weak correlation
with radius was observed by Wang and Anderko [4].Our
In fact the dielectric behavior of electrolyte solutions is more
complicated in comparison to that of solvent mixtures because
of ion solvation and dissociation. It has been reported that the
addition of alkali and alkaline earth halides to water and
methanol results in a decrease in the dielectric constant of the
solution [10].This is due to ion-solvation effects. An ultimate
consequence is the lowering of solubility. A plot of hydration
energies of these chlorides shows an increase with increasing
dielectric constant, Table II and figure 3, and a decrease with
increasing cation radius, Table IV and figure 4.
TABLE III +
+
‐
Heat of hydration (∆Hhydr) of Li ,Na , K in kjmol at some dielectric constant +
∆Hhydr
ε
Li
Na
K
16
1301.21
652.7
435.13
32
1347.24
673.61
447.68
64
1363.98
681.98
456.05
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could be completely separated from LiCL but mixed with
≈58.5% NaCL. The components of this mixture was then
concentrated by heating and dissolved in a water-ethanol
mixture having a dielectric constant of 57 where upon 29% of
pure KCL was separated.The procedure could then be repeated
several times until complete separation is achieved. The
solution containing 100% LiCl and 42% NaCL was
concentrated by heating and dissolved in a water-ethanol
mixture having a dielectric constant of <35 where upon NaCL
was completely precipitated.
V.
CONCLUSIONS
The method of precipitating alkali metal salts described could
be of industrial application in increasing the rate of sodium
chloride precipitation from partially evaporated sea water.
‐
+
+
VI.
+ Fig. 3. Plot of ∆Hhydr (kj mol ) of Li , Na , K versus dielectric constant . One could interpret such a trend in terms of the fact that for
aqueous solutions the formation of hydration shells around
ions prevents the coordinated water molecules from being
oriented in the external field, thus causing a decrease in the
dielectric constant that results in precipitation of salt.
TABLE IV
Variation of ∆Hhydr with cations size at different dielectric constants.
Cation
size ˚A
0.5
1.0
1.5
This project was supported by King Saud University,
Deanship of Scientific Research,College of Science Research
Center.
VII.
REFERENCES
[1] Linnikov, O.D. 2006( a) . Spontaneous crystallization of potassium
chloride from aqueous and aqueous ethanol solutions . Cryst. Res.
∆Ηhydr
ε=16
1301.21
652.7
435.13
ACKNOLEDGEMENT
Technol. 41. No. 1, 10-17 41,; 2006( b) .Spontaneous
ε=32
1347.24
673.61
447.68
ε=64
1363.98
681.98
456.05
crystallization of
sodium chloride from aqueous and aqueous –
ethanol solutions Part 2 .
ibid, No. 2, 138-144);
2007.
Spontaneous crystallization of sodium chloride from aqueous and
aqueous –ethanol solutions Part 3 . ibid, 42, No. 8, 758-765. 2004
. Spontaneous crystallization of potassium chloride from aqueous
and aqueous-ethanol solutions Part 2 :Mechanism of aggregation
and coalesce of crystals. Ibid ,39,6,529-539.
[2] Doki, N. , Kubota, N., Yokota ,M .,Kimura, S., Sasaki , S . 2002 .
Production of sodium chloride crystals of
uni-modal size
distribution by Batch Dilution crystallization . J. Chem. Eng. Jpn.,
35, 1099.
[3] Khalil, M .I. ,Al-Resayes, S. I. 2012. The Role of Dielectric Constant
in Sodium Chloride Solution Chemistry
Part 1: Magnitude of
Supersaturation. Int.J.Phys.Sci., 7(4),578-583.
[4] Wang ,P., Anderko, A . (2001).Computation of Dielectric Constants
of Solvent Mixtures and Electrolyte
[5] Solutions. Fluid Phase Equilibrium, 186, 103-122.
Fig. 4. Plot of ∆Hhydr (kj mol-) versus cations size(Å).
[6] Bradley, D.J., Pitzer, K.S . 1979. Thermodynamics of Electrolytes.
12. Dielectric properties of water and Debye-Huckel parameters to
IV.
350 degree .C. and 1 kbar. J.Phys.Chem., 53,1599-1603.
APPLICATION
[7] Uematsu ,M., Frank ,E.U . 1980. Static Dielectric Constant of Water
The above data obtained was applied to separate Li,Na,K
chlorides from an aqueous solution mixture .At ε=35 , KCl
123704-8989-IJBAS-IJENS © August 2012 IJENS
and Steam.J.Phys.Chem.Ref.Data ,9,1 291-1306.
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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:04
46
[8] Archer, D.G., Wang, P . 1990. The Dielectric Constant of Water and
Debye-Huckel
Limiting
Law
Slopes.
J.Phys.Chem.Ref.Data
19,371411.
[9] Fernandez, D.P., Goodwin, A.R.H., Lemon, E.W., Sengers, J.M.H.L.,
Williams, R.C. 1997. A formulation for theStatic Permittivity of
Water and Steam at Temperatures from 238 K to 837 K at Pressure
up to 1200 MPa including Derivatives and Debye-Huckel
Coefficients. J.Phys.Chem.Ref.Data, 26, 1125-1166.
[10] Harvey, A.H., Prausnitz , J.M . 1987. Dielectric Constants of Fluid
Mixtures over a wide range of
Temperature and Density. J.Solut.Chem.,16,857-869.
[11] Wei ,Y.Z., Chiang, P., Sridhar ,S.
1992. Ion Size Effects on the
Dynamic and Static Dielectric Properties of Aqueous Alkali Halide
Solutions. J.Phys.Chem.,96,4569-4573.
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