Indian Journal of Chemical Technology
Vol.2, September 1995, pp. 271-275
Liquid-liquid extraction studies of some metal complexes using crown ethers
A G Gaikwada,
H Noguchib& M Yoshiob
"RegionalResearch Laboratory, CSIR,Trivandrum 695 019,India
bDepartmentof Applied Chemistry,Faculty of Scienceand Engineering,SagaUniversity, Saga 840, Japan
Received 10 August 1994;accepted 15February 1995
liquid-liquid extraction investigations of Pd(ll), V(V), Co(ID), Cu(ll), Ni(ll) and Fe(ll)-PAR[4-(2pyridyl azo )-resorcinol] complexes by using dicyclohexano-18-crown-6- have been carried out. Anioic
complexes of Pd(ll), V(V), Co(ID), Cu(ll), Ni(ll) and Fe(ID)-PAR extracted into organic phase as increase in order Pd > V> Co > Cu > Ni > Fe.-During thermodynamic studies, it was observed that the entropy effect is one of the important factors responsible for the selective extraction. However, the shape
of the complexes may be one of the causes for the selective change in entropy during the extraction of
chelate. In effect, planar palladium-PAR-SCN- ichelate was extracted selectively in comparison with
other chelates.
1
Liquid-liquid extraction is one of the powerful
techniques for identification, separation and preconcentration of metal ions. Crown ethers containing 0-, N -, S- have been successfully utilised
for the selective extraction purposes of ions 1. Enhanced extractabilities for some transition ·metal
ions were observed with use of macrocyclic polythia-coronandsz• The cationic complexes of
crown ethers are normally extracted into organic
phase as an ion-paif3-5• The methods for selecting
the best ligand-saIt-solvent combinations for the
effective and selective extraction have been
worked out and applied in ion pair extraction. In
general, the efficiency of cation extraction increases with the hydrophobic nature of the counter ion, e.g. picrate> SCN- =
> NOi >
Extraction applications of crown ethers are copper-zincon6, fhiorimetric detenilination of potassium, eosin and bromocresol extraction7, protonated amino acids8 and metanil yellow. When
crowned ion pairs of metal· cations are extracted
from aqueous phase to organic phase, water molecules which are strongly hydrated9 to cations cotransferred into organic phase. During complexation process, the release of water molecules from
cation and anion may play a significant role in extraction of ion pair. No attempt has been made on
investigation of extraction of Pd, ·V, Co, Cu, Ni
and Fe-PAR complexes with dicyclohexyl-18crown-6. The present work describes the extractiQl1Sof such complexes.
-
00;
0-.
Experimental Procedure
Picycloh~xyl-18-crown-6 lOCC) was obtained
from Nippon Soda and its solution was prepared
in 1,2-dichloroethane. The resulting solution was
washed with 0.01 M lithium hydroxide in order to
avoid pH change after extraction. The stock solutions of vanadium and the other metals used were
prepared by dissolving ammonium metavanadate
and metal chloride in 1 M hydrochloric acid, respectively. The concentration of stock solutions
was determined by the complexometry with disodium ethylenediamine tetra acetate10• The solution of 4-(2-pyridyl-azo)-resorcinol (PAR) (Dojin
Kagaku) was prepared in 0.01 M lithium hydroxide. The stock solution of lithium thiocyanate was
prepared and its concentration was determined by
precipitation titration with silver nitrate10• 1,2Dichloroethane and the othe'r.reagents were all of
analytical grade and used without further purification.
The proceudrl;used was as described earlier6·11•
Lithium thiocyanate solution was used in the case
of palladium in order to improve the efficiency of
extraction. The solution of cobalt(m)-PAR complex was prepared trom CoClz and PAR solutions,
and oxidation state of cobalt was ascertained12•
For pH adjustment dilute phosphoric acid and
lithium hydroxide (pH 5-8) or boric acid and lithium hydroxide (pH 7-11) were used. The resulting
aqueous solution (20 mL) was transferred into a
50 mL centrifuged tube equipped with well
grounded stopper. Then, 20 mL of'1,2-dichloroethane (10 mL for Fe(ll) and Ni(ll) containing the
crown ether (10-5-0.1 M) was added to the above
solution. The mixture of two phases was shaken in
1"111'1'111
IlIllIIIHI;.td·Mlit'llll",IOl~
••.• ,._, ••"· •• I~·I
272
INDIAN J. CHEM. TECHNOL.,
a thermostated water bath at different temperatures in the range 12-43°C for a day (20 min for
vanadium).
After separating both organic and aqueous
phases, the concentration of metal-PAR complexes
extracted into organic phase was determined spectrophotometrically at maximum absorption wave
length. These are 541, 552, 515, 513, 524 and
493 nrn for Pd(II), V(V), Co(III), Cu(n), Ni(n) and
Fe(II), respectively. The molar extinction coefficients determined of extracted ion pair complexes
into organic phase are 3.3 )(104, 3.3 x 104,
60 x 104, 5.4 x 104, 3.2 X 104 and 3.3 x 104 for
Pd(II), V(V), Co(III), Cu(II), Ni(II) and Fe(II), respectively. The concentration of metal ions in organic phase were confirmed after stripping and
determining with atomic absorption spectrophotometer. The concentration of metal-PAR
complex in the aqueous phase was calculated from the
concentration of that extracted in the organic
phase by using mass action's law.
SEPTEMBER
1995
... (3)
DCC-=DCCo
[DCC]o
[DCC]
Kd.DCC=
, j
l'llijll
... (4)
The 10gKd,Dcc was found to be 2.88 at 25°C13.
The complexation of potassium ion with DCC
in the aqueous phase is represented by Eqs (5)
and (6).
K+ + DCC-=KDCC+
[KDCC+]
KKDCC+
= [K+][DCC]
... (5)
... (6)
The association of the complexed potassiumDCC cation with anionic metal-PAR complexes in
aqueous phase is expressed by Eqs (7) and (8).
KDCC++ MC- -=KDCC+ MC[KDCC+MC-]
... (7)
... (8)
Results and Discussion
Ka,KDCCMC[KDCC+]rMC~]
Extraction equilibria-The overall extraction equilibrium between aqueous and organic phases for
the extraction of anionic metal-PAR complexes
with cationic potassium-crown
ether complex
would be represented as follows:·
The distribution of the ion pair complexes between aqueous and organic phases can be represented by Eqs (9) and (10).
DCC
... (9)
DCCK+MC[KDCC+MC-1,
Organic
Aqueous
1~
Kd,KDCCMC[KDCC+MC]
~
K" DeC
K,DCCK+MCDCC+K+ +MC- -=DCCK+MC-
The distribution ratio of metal-PAR complex,
D, between organic and aqueous phases can be
given in general by Eq. (11). However, the metalPAR complex is predominantly
present
as
KDCC+MCin organic phase and as MC- in
aqueous phase under the experimental conditions.
The distribution ratio of metal-PAR complex can
be represented by Eq. (12).
where MC- is an anionic metal-PAR complex and
KDCC+ - MC- is the ion-pair.
The expression for the extraction of anionic
metal-PAR complexes can be given by Eq. (1).
Kex
K++DCCo+MC-~
KDCC+MC;
... (1)
The overall extraction equilibrium constant
can be expressed by Eq. (2).
... (10)
(Kex)
total concentration of metal-PAR complex
[KDCC+MC-]o
Kex
D=
... (2)
= [K+] [DCC1,[MC-]
The subscript '0' denotes the organic phase.
The overall extraction equilibrium can be represented in terms of the. following equilibria. The
distribution of the uncomplexed DCC between
aqueous and organic phases can be given by Eqs
(3) and (4).
,
,
!
I
11'
'''1'1'1''
"I'
r
in the organic phase
total concentration of metal-PAR complex
in the aqueous phase
[MClo+[KDCC+ MC-]o
D= [MC]+[MC-]+[MC2-]
[KDCC+ MC-lo
D=----[MC-]
I
I'
I
f
II"
111'III
... (11)
... (12)
GAtKWAD et aL: liQUID-liQUID
EXTRACTION
where MC, MC- and MC2- denote neutral, single
and double negatively charged complexes. It is also reported 14 that the single negative charged
complexes predominantly present in the aqueous
phase. The following relation can be obtained
from Eqs (1) and (12).
logD;{K+] = logKex + log [DCC1
.,. (13)
Eqs (14) to (16) can be obtained from the mass
balances.
Cocc = [DCe] + [KDCC +J+ (Vo/V )[DCe]o
+ (Vo/V)[KDCC+MC-]o
CMC-
(14)
=[MC-]+(Vo/V)[KDCC+MC-1
(15)
CK+ = [K+][KDCC+] + (Vo/V)[KDCC+MC-Jo
... (16)
where Coco CMC + and CK+ are the initial concentrations of crown ehter, metal-PAR complex
and potassium ion in the aqueous phae. Vo and V
represent the volumes of organic and aqueous
phases. In the experimental studies, C~ + is much
larger than Cocc or CMC , hence CK = [K+J in
Eq. (16). Then, Eq. (13) is rewritten as follows:
logD'cK+ = logKe" + log [DCC1
... (17)
The concentrations of DCC and KDCC + in the
aqueous phase can be neglected in Eq. (14) because Kd,occ is much larger than unity. Finally,
the value of [DCClo can be calculated by using Eq.
(18).
DCCo=(V/Vo)Cocc-
[KDCC+MC-]o
'" (18)
The Eq. (17) shows that the plot of logD/CK+
versus log[DCC]o should give the straight line with
slope one and the extrapolation to [DCC]o = 0
gives 10gKex value.
The PAR forms negatively charged complexes
with several metal
different geometries
V(V)
Ni(Il)
\Co(IlI)
Fe(Il)
Cu(Il)
Pd(Il) ions with
and their complexation has been wellDCC
established14• During these studies different geometries
of metal-PAR complexes were selected such as
Pd(D}-PAR (square planar), V(V)-PAR (square
pyramid, semi-spherical), Co(m), Cu(II), Ni(II) and
Fe(II)-PAR (octahedral, spherical). The extraction
studies of negatively charged complexes of metalPAR with crown ethers are interesting and make
more attraction to new aspect of spectrophotometric studies in the field of crown ether chemistry.
Effect of thiocyanate, bromide, chloride, iodide
anions on extraction of palladium-The
effect of
chloride, bromide, iodide and thiocyanate anions
OF METAL COMPLEXES
273
on the extractability of Pd(ll)-PAR complex with
potassium-DCC cation have been studied at same
pH (8.25). It was observed that the extraction of
Pd(II)-PAR complex with potassium-DCC cation
complex depends on· the concentration and nature
of anion used along with them. During the investigation of effect of these ions on the extraction of
Pd(II)-PAR complex it has been observed that the
thiocyanate ion shows highest extractability among
the four anions. The constant absorbance was observed in the concentration range of thiocyanate·
ion from 2.5 x 10-4-4.0 x 1O-4M. Further, the extraction studies of Pd(II)-PAR-SCN- complex was
carried out in this concentration range.
Effect of pH on the extractability-The
extraction of metal-PAR complexes with potassium-DCC
cation complex into 1,2-dichloroethane was attempted at various pH. The constant absorbance
was observed in the pH range from 5 to 6.8 for
V(V), 5.8 to 8.5 for Pd(II), 6 to 7.5 for Co(I1I), 8
to 9.2 for Cu(II), 7.8 to 8.5 for Ni(II) and 7.8 to
9.9 for Fe(II), respectively. It can be inferred from
the maximum and constant absorbance in each
case of metals that the one type of complex is extracted into the organic phase and it exists predominantly in the aqueous phase. Therefore, further extraction studies of these metal-PAR complexes were carried out at these pH ranges.
Effect of Dee concentration variation-The investigation relation between logD/CK+
versus
10g[DCC]o for (Pd(II), V(V), Co(m), Cu(II), Ni(II)
and Fe(II)) metal-PAR complexes at 25°C shows a
linear relationship for the all metal ions with slope
one. It can be concluded that the overall extraction equilibrium described by Eq. (1) is valid for
these extraction equilibrium systems. Thus it
Table I-The log K ex values and the composition of extracted
metal-PAR complexes with potassium-DCC
and Zephiramine*
Metal ion
4.8
2.0
3.5
11.16***
6.37
8.59
6.1
3.3
compositon**
Ni(HL)LFe(HL)LPd(L)SCNV02LCu(HL)LlogKex
Zephiramine
CoLi
*obtained from Ref. 11.
**PAR is described by H2L and the ratio of PAR/metal
determined by using continuous variation method
***Divalent anion Ni~ - is extracted
ion is
j,lb,j·[j
-
;;
II
274
INDIAN 1. CHEM. TECHNOL.,
SEPTEMBER
1995
would indicate that the extraction of metal-PAR
cal complex. It may be possible that the contribucomplex is a negatively charged complex. The tion of entropy term to free energy would be one
composition of ion pair complex is 1: 1 with re- of the causes for the high extractability of planar
spect to anionic metal-PAR complex and pota- complex. It reflects the intramolecular interactions
within the complexes. These interactions seem to
ssium-DCC cation. The log Kex values extrapolated and the composition of the ion pairs extracted
be very different for the complexes fo Pd(IT), V(V)
are listed in Table 1. The 10gKex values are 6.1 and Cu(n). They are stronger in case of the planar
anion of Pd(ll)-PAR-SCN-.
However, symmetry
for planar Pd(II)-PAR-SCN-, 4.8 for semi-spheriplays an important role at the level of the entropy
cal V(V)-PAR and 3.5, 3.3,2 and 2 for spherical
contribution.
Co(III)-, Cu(II)-" Fe(II)- and Ni(II)-PAR complexes
at 25°C, respectively. It can be seen from the
There is no gradual change in ~Ho values for
above logKex values that the palladium-PAR com- the extraction of PAR complexes of palladium, vanadium and copper with potassium-DCe.
The
plex is extracted more easily than the complexes
of the other metal ions. The log K ex values of ex- thermal effect to the free energy is larger than the
entropy contribution in all three cases. But, the ortraction of metal-PAR complexs with potassiumDee complex are smaller than that of with ze- der of the decrease in T ~SJ values correspond
phiramine for the cobalt and iron cases. The ex- with that in K ex values. Therefore, it can be contraction sequence of metal ions can be given as cluded that the entropy effect may be one of the
follows Pd> V> Co> Cu > Fe > Ni. The log K ex causes for the high Kex value in planar palladiumvalue of Pd(II) complex is enough large to allow an PAR-SCN- complex.
The overall extraction constant can be rewritten
analytical application of this extraction method in
by using Eqs (1 )-(18) as follows:
comparison with the log K ex values of other metal
IOns.
Determination of thermodynamic quantitiesThe liquid-liquid extraction studies of palladium,
vanadium and copper-PAR complexes with potassium-DCC complex have been carried out at different temperatures such as 12, 16, 25, 34 and
43°e. The 10gKex values for palladium, vanadium
and copper-PAR complexes were determined at
different temperatures. It was observed that the
10gKex value decreases with increase in temperature for all elements. The determined 10gKex values were plotted against 1IT for these elements
to estimate the thermodynamic quantities. The determined ~ HO and T ~ So values are given in
Table 2. The extraction sequence of metal-PAR
complexes remains same as described at 2Ye.
The ~Go values for the metal-PAR complexes
show the negative sign. However, the entropy term
for these complexes shows the different signs. The
different sign of entropy contribution shows that
the contribution of entropy term to free energy is
decreased when the shape of metal-PAR complex
is changed from planar or semi-spherial to spheriTable
2-Thermodynamic
Dee
... (19)
Here, K d,DCC and K KDCC + are constant for all the
case8. The extraction of metal-PAR complex depends on the Ka,KDCCMC and Kd,KDCCMC values.
The high extractability of planar palladium-PAR
complex due to the entropy term during the association of ions in aqueous phase or the distribution of ion-pairs in the aqueous and organic
phases may be one of the causes. During the extraction process, the following model \yould be applicable for the entropy change of the association
of ions in aqueous phase. These are the combinations between planar-planar ions (a) and planarspherical ions (b) with the same sizes. On the basis
of the proposed models, it has been suggested that
the number of water molecules released during the
ion association of ion pairs and extraction of ion
pairs into organic phase would be large and more
in the case 'of (a) than that of in (b) can be represented by Eq. (20).
MC -. mH20
dHo,
TdSJ,
Kllmol
Kllmol
-34.8
-27.8
-25.0
-39.6
2.4
-27.4
-18.8
I
+. nH20
F MC -. DCCK
+
7.0
-20.8
n+x)H20
... (20)
where m, n and x denote the number of hydrated
water molecules to metal-PAR, potassium-DCC
and ion pair complexes, respectively. Since the
contact surface area in planar-planar complex (a)
is large in comparison with that of case (b), the
number of water molecules released, m + n - x,
III'
'I
+ DCCK
'xH20+(m+
complexes at 298K
d GO,
Pd(II)-PAR-SeNV(V)-PAR
eu(II)-PAR
K KDCC + • K a,KDCCM' K d,KDCCMC
quantities of metal-PAR-potassium-
Kllmol
Metal-PAR
K ex = K~,DCC'
1111111
GAIKWAD et al.: LIQUID-LIQUID
EXTRACTION
during the complexation process would also be
large in case of (a). The release of a large number
of water molecules would be one of the causes for
the enhancement of extraction. Therefore, it is expected that the entropy effect is higher for planarplanar ions, potassium-DCC-Pd(ll)-PAR-SCN"
complex than that of for planar-spherical ions, potassium-DCC-V(V)-PAR complex or potassiumDCC-Cu(II)-PAR complex.
Conclusion
It was observed that the planar-Pd(ll)-SCN-
complex shows the higher extraction in comparison with that of other metal ion-PAR complexes.
The entropy term plays an important role in the
ion pair formation process and its partition between aqueous and organic phases.
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275
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