Indian Journal of Chemistry
Vol. 44A, August 2005, p ~. 1594-1596
Notes
The effect of 18-crown-6 on the solubility
and thermodynamic parameters of Li 2C0 3,
Na2C03, NaCl, CH 3COONa and KCI in
methanol and ethanol
F I EI-Dossoki
a
Chemi stry Department, Faculty of Ed ucation,
Suez - Canal University, Port - Said, Egypt
Received 19 September 2004; revised 8 lillie 2005
The effect of diffe rent concentrations of a cyclic polyether
IS-crown-6 ( I RC6) on the solvati on parameters of so me alkali
metal salts has been studied. The mol al so lubi lities of Li cCO),
Na2C03, NaC I, CH)COONa and KCI have becn determined
expe rimentll y (using vo lumetric and precipitation titrations) in the
absence and in presence of different concentrations of ISC6 in
methanol and ethano l at 20, 25 , 30 and 35"C. From the
experim ental res ults , the so lvation thermodynamic parameters
(the free energy, th e enthalpy and the entropy) have been
ca lculated and di scussed. Also the effects of different
co ncentrati ons of ISC6, temperature, anion, ioni c radiu s and
solvent on th e parameters have been discussed.
IPC Code: Int. CI. 7 C07C59100
Among crown ethers, 18-crown-6 has a high degree
of flexibility and a number of donating oxygen atoms
in the macrocyclic ring. Because of thi s, 18C6 can
wrap itself around the metal ion of proper size to form
"wrap around " complex both in so lution I and in so lid
state.
Solvation process is considered to be one of an
important processes affecting the nature of many
interactions in solutions. Hence, this process was
studied by many authors 2-4. The influence of the
complexation power of 18C6 on the so lvation of some
alkali earth metal chlorides in methan o l and ethanol at
different temperatures was studied earlier5 . We report
in this note the solubility of Li 2CO" Na:>CO" NaCI,
CH 3COONa and KCl in methanol and ethanol at 20,
25 , 30 and 35°C.
Experimental
Lithium carbonate, sodium carbonate, sodium
ch loride, sodium acetate trihydrate, po tassium
"Prescn t address: Rep ubli c of Yemen - AI - Hod ieda h UniversityFaculty of Education - Chemistry Dep. P.O 3114
chloride, potassium chromate, hydrochloric acid (all
from G . P. R), 18C6(C2oH2406) (Fluka), silver nitrate
(Windsor laboratory limited) were all of highest
commerical purity available and were used as sLl ch .
Methanol and ethanol of G . P. R type were used after
bidi stillation and drying over a dehydrating agent
(molecular sieve (0.8-] .2 mm diameter).
Saturated solutions of Li 2C0 3 , Na 2C0 3 , NaCI ,
CH 3COONa and KCI , in the absence and in presence
of different concentrations of 18C6 , were prepared by
shaking 10 ml solution in a tube containing excess of
the so lid substances in a thermostated water bath of
the type (Clifton) at time interval of 1·2 weeks at 20,
25,30 and 35°C. The molal solubilities of substances
were analysed using volumetric and precipitation
(Mohr's) titration s with standard so lution of HCI and
AgNO, respectively.
Results and discussion
The molal solubilities, (C), of Li 2CO J , Na 2 C0 3 ,
NaCl , CH3COONa, and KCl in methanol and ethan o l
at 20, 25 , 30 and 35°C in the absence and presence of
different concentrations of 18C6 (10.4 , 10.3 , 10.2 and
5xlO·2 molar) were determined by volumetric and
precipitation titrations as reported in the experimental
section .The results which are average of a'minimum
two e xpe riments calculated using Eq . (I) in molal
scale with standard diviation ± 0.0011 are shown in
Tables I and 2.
wxlOOO
..
Molal solubility (c)=
g moll 1000 g so lve!1!
dox M
... ( I )
where w is the weight of the salts in one millimeter
saturated solution, M is the molecular weight of salt
and do is the den sity of so lvent (methanol a nd
ethanol).
The results show that the molal solubilities of
Li 2C O " Na 2C0 3 , NaC!, CH }COO la, and KCI in
methanol increase as the concentration of 18C6
increased indicating a higher solvation of the used
sa lts. Th e mo lal so lubility of CH,COONa in creases as
the concentration of 18C6 increases till 0.01 M and
then decreases up to 0 .05 M. Thi s may be due to the
formation of a solid complex in presence of a high
NOTES
1595
Table I-The molal solubilities (g moll 1000 g solvelll ) of Li 2CO, . Na2CO,. CH,COONa. NaCI and KCI in methanol in absence and in
presence of different concentrations of ISC6 at 20. 25 . 30 and 35°C .
Na ~CO,
[18C6]
20 uC
Li 2CO,
25°C
30"C
35°C
20 "C
25"C
30"e
35"C
20 u C
CH, COONa
30 °C
25 "C
35 °C
0.0000
0.0001
0.0010
0.0100
0.0500
0.0 16
0.021
0.023
0.026
0.028
0.019
0.023
0.026
0.028
0.031
0.024
0.026
0.028
0.031
0.033
0.028
0.021
0.024
0.026
0.028
0.052
0.061
0.064
0.069
0.08 1
0.057
0.066
0.071
0.076
0.088
0.066
0.069
0.074
0.078
0.093
0.072
0.096
0.110
0.120
0.124
3.752
3.812
3.872
3.967
2.501
3.822
3.881
3.941
4.039
2.627
3.946
4.006
4.125
4.185
3.826
4.806
5.047
5.167
5.407
3.965
20 °C
0.266
0.336
0.363
0.366
0.375
NaCI
25 "C
30"C
0.273
0.284
0.324
0.305
0.355
0.325
0.355
0.330
0. 350
0.355
35"C
0.299
0.300
0.314
0.323
0.338
20"C
0.122
0.126
0.132
0.136
0.163
KCI
25"C
0.125
0.138
0.144
0.150
0.188
30°C
0. 130
0.143
0.155
0.166
0.2 14
35°C
0.130
0.136
0.140
0.147
0. 170
[18C6]
0.0000
0.0001
0.0010
0.0100
0.0500
Table 2-The molal solubilities (g mol 11000 g solvent) of Li 2C0 3• Na~CO , . CH ,COONa. NaCI and KCI in ethanol in absence and in
presence of different concentrations of 18C6 at 2025. 30 and 35"C
[ISC6]
0.0000
0.0001
0.0010
0.0100
0.0500
[1 8C6]
0.0000
0.0001
0.0010
0.0100
0.0500
20 °C
0.014
0.014
0.014
0.0 14
0.0 14
20nC
0.025
0.059
0.064
0.077
0.089
Li 2CO,
25 "C
30 u C
0.016 0.0 19
0.016 0.019
0.016 0.019
0.016 0.019
0.016 0.019
NaCI
25 °C
30 n C
0.037
0.050
0.058
0.053
0.063
0.059
0.076
0.063
0.089
0.076
35"C
0.021
0..()J2
0.012
0.012
0.012
20 "C
0.021
0.021
0.021
0.021
0.021
35 "C
0.051
0.052
0.053
0.056
0.060
20nC
0.012
0.025
0.038
0.050
0.063
Na2CO,
25"C
30 nC
0.023
0.026
0.023
0.026
0.023
0.026
0.023
0.026
0.023
0.026
KCI
25"C
30"C
0.025
0.037
0.037
0.049
0.049
0.063
0.063
0.077
0.076
0.089
concentration of l8C6 . In ethanol the mo lal
so lubilities of NaCI and KCI increased as the
concentration of l8C6 increased whi le the molal
solubilities of Li 2C O}, Na2C03 and CH 3COONa were
unaffected by the presence of l8C6. This may be due
to the high solvophobic character of Li 2C03 • Na2COJ
and CH 3COONa and the solvophilic character of
NaC! and KCl in ethanol. Also it was noted that the
molal solubilities increase as the concentration of
18C6 increases ti II concentration of 18C6 is 10.2 M
and then decreases up to 5x I 0. 2 M.
In studying the effect of solvent, it was noted that
the molal solubilities of all alkali metal salts used in
the absence and presence of 18C6 are lower in ethanol
than in metha'lol. This may be due to the lower
dielectric constant of ethanol and the lower
solvophilic character of the used salts in ethanol.
The effect of temperature on the solvation of the
alkali metal salts has also been studi ed. The molal
35°C
0.028
0.028
0.028
0.028
0.028
20"C
1.075
1.075
1.075
1.075
1.075
CH , COONa
30 nC
25 nC
1.1 37
1.198
1.1 37
1.1 98
1.1 37
1.198
1.1 37
1.1 98
1.198
1.1 37
35 "C
1.322
1.322
1.322
1.322
1.322
35"C
0.043
0.056
0.069
0.084
0.094
solubilities of al l sa lts used increased as the
temperature increased from 20nC to 35"C except in
case of NaC! (in methanol and ethanol), KCl (in
methanol) and Li 2CO) (in methanol). In presence of
D
l8C6 solubilities increased till 30 C then dec reased at
35"C, probably due to the io ,ver formation constant of
the complex of l8C6 with the metal ion at hi gh
temperature which may be due to the small e r radiu s of
Li+ and Na+ with respect to the cavity diameter of
l8C6 and the dielectric constant of the so lvent. As the
temperature increased the percentage of the effect of
l8C6 on the solubility of all used salts in methano l
and for ['-laC! and KCI in ethanol was decreased .
Comparing the effect of the ionic radius, it was
observed that the molal so lubiliti es of NaCI are hi g her
than those of KCl in methanol and ethanol in the
absence and in presence of 18C6.
From the solubility measurement, the so lvation free
enegies (L'.G s ) of Li 2C0 3 , Na2C03, NaC! , CH)COONa
INDIAN J CHEM, SEC A, AUGUST 2005
1596
and KCI in methanol and ethanol at 20, 25, 30 and
35°C were calculated applying Eg. (2).
6.G,
= -2. 303 RT log Ksp
(2)
The value of log K,p depends mainly on the nature of
he sol ute in the solvents under investigation. The
value of log K..p can be determined using Eg. (3).
log
K,p =
2
log c + 2 log y±
(3)
where c is the molal solubility , y± is the mean activity
coefficient of ions which is a function of ion-ion
interacti on and can be estimated from the Debye
Huckel limiting law as modified by Robinson and
Stoke~ Eg. (4)6'9.
... (4)
calculated by adding the crystal io nic rad ius of the salt
to the radii of solvent at diffe rent temperatures
applied .
From the relation of 10gKsp versus liT, the changes
in the enthalpy, the free energy and the entropy at
different temperatures were calculated. As the molal
solubility of the used salts increase the solvation free
energy change has a lower positive va lue (increase in
the negative direction) and vice-versa. This indi cate a
higher solvation process in the same direction .
The positi ve value of the enthalpy change indicates
the endothermic character of the solvation process of
the used salts in methanol and ethanol in absence and
in presence of 18C6 except NaCI. The results sho w
that the entropy change often has a negative valu e
indicating a higher solvation process.
References
I
where Z+ and Z_ are the charges of ions in solutions,
.n-
n-
3/2
1/2
O
A= 1.823x IQ6(E
, B = 50.29 (E .
, r is the
solvated radius, E is the dielectric constant for the
solvents . The values of E for methanol and ethanol at
d ifferent temperatures are taken from ref. (4) ; the
solvated radii of methanol and ethanol at different
temperature are calculated using Eg. (5)10 by
cons idering spherical form of the sol vated molecules.
2
3
4
5
6
7
... (5)
where V) is the partial molar volume of the solvent
calculated by dividing their molecular weights by
their densities. The solvated radii (rO) of the salts are
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