Extraction of zinc and copper with acetylenic quaternary ammonium salts

Article abstract of DOI:10.1007/s11176-005-0296-9

Extraction of zinc(II) and copper(II) from HCl solutions with solutions of mono- and dialkynyl-substituted quaternary ammonium halides in chloroform and toluene is more efficient than the extraction with saturated quaternary ammonium salts and occurs by different mechanism. The dependences of the metal distribution factors on the HCl concentration have two maxima whose positions (absolute and relative) and relative intensities depend on the particular extractant, metal, and diluent. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0296-9

Russian Journal of General Chemistry, Vol. 75, No. 5, 2005, pp. 672 678. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005,
pp. 712 718.
Original Russian Text Copyright
2005 by Andreev, Tunina, Remizova.
Extraction of Zinc and Copper
with Acetylenic Quaternary Ammonium Salts
V. P. Andreev*, S. G. Tunina**, and L. A. Remizova***
* Petrozavodsk State University, Petrozavodsk, Karelia, Russia
** Karelian State Pedagogical University, Petrozavodsk, Karelia, Russia
*** St. Petersburg State University, St. Petersburg, Russia
Received December 26, 2003
Abstract Extraction of zinc(II) and copper(II) from HCl solutions with solutions of mono- and dialkynyl-
substituted quaternary ammonium halides in chloroform and toluene is more efficient than the extraction with
saturated quaternary ammonium salts and occurs by different mechanism. The dependences of the metal
distribution factors on the HCl concentration have two maxima whose positions (absolute and relative) and
relative intensities depend on the particular extractant, metal, and diluent.
There are only a few papers on recovery and puri-
fication of metal ions with unsaturated compounds;
nevertheless, their results allowed development of
very efficient methods for extractive concentration of
osmium with -olefins [1] and extractive chromatog-
raphy of actinides with allyltrialkylammonium nitrate
[2]. The effect exerted on the extraction by a triple
bond capable of complexation with soft Lewis acids
was not examined previously.
reactions at room temperature are complete within
1 2 h in acetonitrile and 1 2 days in pentane; the
yields of the quaternary ammonium salts are close to
quantitative.
A principal difference of the examined extractants
Ia Ie and II from those described in the literature
[5, 6] is the better steric accessibility of the nitrogen
atom due to the linear structure of the CH2 C C CH2
fragment. This is extremely important for the donor
acceptor bonding R4NX: MeXn [7] between the
anion X of the quaternary ammonium salt and the
metal cation Me in the extractable complex.
In [3] we described the extraction of Zn(II) from
solutions of alkali metal halides in HCl with hept-2-
ynyltrioctylammonium bromide and concluded that
further studies in this direction would be promising.
First we studied how the distribution factors of
Zn(II) and Cu(II) in extraction with quaternary ammo-
nium salts (QASs) Ia, Ib, and III depend on the HCl
concentration in the initial aqueous solution, varied
from 0.75 to 9.0 M (Figs. 1, 2). These dependences
appeared to be complex. In the extraction of the met-
als with solutions of Ia and Ib in chloroform and tolu-
ene, two local maxima are observed at HCl concentra-
tions of 2 4 and 5 7 M. On the contrary, in extraction
of Zn(II) with compound III containing no bonds,
the corresponding dependence is dome-shaped and has
only one broad maximum at an HCl concentration of
3 5 M, in full agreement with published data on the
extraction of Zn, Cu, Cd, and many other metals with
ammonium salts containing saturated hydrocarbon
substituents [8].
Here we continue a study of the effect exerted by
the structure of quaternary ammonium salts containing
a triple bond in the -position relative to the nitrogen
atom, and also by the diluent on extraction of copper
and zinc from HCl solutions. We chose the following
ammonium salts:
[R3NCH2C CR ]+X [(C8H17)2N(CH2C CC4H9)2]+Br
Ia Ie
II
[(C8H17)3NCH3]+I
III
R = C8H17, R = H, X = Br (a); R = C8H17, R = C4H9,
X = Br (b); R = C8H17, R = C4H9, X = I (c); R = C10H21,
R = C4H9, X = Br (d); R = C10H21, R = C6H13, X = Br (e).
The salts were prepared by quaternization of dioc-
tylamine, trioctylamine, and tridecylamine with ap-
propriate propargyl halides under very mild conditions
following the previously developed procedure [4]. The
The composition of the Zn and Cu complexes
extracted with Ib was determined by the equilibrium
shift method [9]. The composition of the aqueous
phase was kept constant (C0HCl 1, CM0 e 0.02 M at
1070-3632/05/7505-0672 2005 Pleiades Publishing, Inc.

EXTRACTION OF ZINC AND COPPER
673
DZn
1.4
DMe
6
1.0
0.6
5
1
4
2
0.2
3
1
3
5
7
9 CHCl, M
2
2
1
3
Fig. 1. Distribution factor of Zn(II) as a function of HCl
concentration in the aqueous phase in extraction with
1
4
(1) Ib and (2) III; diluent chloroform. C
0.05
QAS
and C
0.02 M; 25 C.
Zn
1
5
10
CHCl, M
25 C), and the concentration of the quaternary ammo-
nium salt in the organic phase was varied from 0.025
to 0.25 M. The dependences of the copper and zinc
distribution factors on the concentrations of Ib in tolu-
ene and chloroform, respectively, are monotonically
ascending (Fig. 3), and the corresponding log log
plots are straight lines with the slope close to unity
(except the case of extraction of Zn with 0.075
0.25 M solutions of Ib in chloroform; in this range,
the slope is close to 2, Fig. 3).
Fig. 2. Distribution factors of metals as functions of HCl
concentration in the aqueous phase. Metal: (1) Zn and
(2 4) Cu; extractant: (1, 2) Ia, (3) Ib, and (4) Id; diluent
toluene; C
0.05 and C
0.02 M; 25 C.
QAS
Me
Thus, two maxima appearing in the metal extrac-
tion curves may be caused by changes in the compo-
sition of the extractable species, associated to certain
extent with the presence of a triple bond in the ex-
tractant molecule.
These data indicate that zinc and copper are ex-
tracted from 1 M HCl in the form of 1 : 1 metal QAS
complexes. With zinc, at a 3 4-fold and greater ex-
cess of the extractant, 1 : 2 complexes are formed.
It should be emphasized that, according to published
data [5 7], 1 : 1 adducts are not formed in extraction
of copper and zinc with saturated ammonium salts.
On the one hand, it could be expected that, at HCl
concentration higher than 5 M, the addition of HCl
across the bond of the quaternary ammonium salt
would become more probable. However, in this case,
the metal distribution factors should decrease owing
DZn(Cu)
(a)
(b)
log DZn(Cu)
1.0
1
1
7
0.6
5
3
2
0.2
0.2
0.6
1.4
1
0.6
log CIb
2
1
0.05
0.15
0.25 CIb, M
Fig. 3. Distribution factors of (1) Zn(II) and (2) Cu(II) as functions of the concentration of hept-2-ynyltrioctylammonium
bromide Ib in (1) chloroform and (2) toluene at 25 C: (a) linear and (b) logarithmic coordinates. C
1 and C
0.02 M.
HCl
Me
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 5 2005

674
ANDREEV et al.
Table 1. Distribution factors D of Zn(II) and Cu(II) in
extraction from aqueous HCl with 0.05 M solutions of
quaternary ammonium salts in chloroform and toluene at
25 C
CuCl, which is a very sensitive qualitative reaction for
a terminal triple bond [14].
Thus, the second maximum (Figs. 1, 2) is apparent-
ly due to the unique structure of the quaternary ammo-
nium salt containing a triple bond in the -position
relative to the nitrogen atom. The extractants that we
studied are presumably capable to form under the
experimental conditions complexes of types IV and/or
V, rather than common double salts [R4N+X Me2+X
N+R4](X )2 (VI) formed by addition and/or anion ex-
change [5, 15] involving only the ammonium nitrogen
atom in saturated quaternary ammonium salts.
[HCl],
M
*
Extractant
Solvent
DZn DCu
[4, 12, 13]
Ia
3.5
3.5
5.0
3.5
3.5
Chloroform 0.69
Toluene 3.56 2.06
4.63 1.82
+0.37
+0.09
Ib
Chloroform 1.19
a
Toluene
2.5
5.0
3.5
3.5
5.0
3.5
3.5
5.0
3.5
3.5
5.0
3.5
3.5
4.85
H
H
C
Ic
Id
Chloroform 0.57
Toluene
+0.09
+0.09
a
1.85
1.90
R3N+
H
R3N+
C
C
C
R
C
Ie
II
Chloroform 1.62
+0.09
+0.7
0.39
H
C
X
a
X
X
C
Toluene
2+
2+
Me
2X
R
2X
R
Me
1.62
Chloroform 0.83
Toluene 10.47
N+R3
C
2.05
C
III
Chloroform 0.29
H
b
1-Octyne
H
b
b
3.5, 5.0
Toluene
IV
V
a
2+
The Me content in the aqueous phase after the extraction
Adducts IV and V, similarly to other chelate com-
pounds (e.g., those formed by Cu2+ with glycols or
-amino acids [16]), can show increased thermody-
namic stability due to intramolecular interaction of the
closely located fragments of the quaternary ammoni-
um salts with the metal ion. Apparently, the stability
of these adducts should largely depend on the HCl
concentration, and the presence of two maxima sug-
gests the realization of at least two major mechanisms
of extraction of zinc and copper from acid solutions
with acetylenic salts.
b
2+
was below the detection limit.
organic phase after the extraction was below the detection
limit.
The Me content in the
to an increase in the substituent volume upon transi-
tion from the sp to sp2 or sp3 state, and also to the
acceptor power of the chlorine atom appearing in the
substituent (negative inductive effect), rather than pass
through a second maximum.
Furthermore, it is indicated in the literature [5, 10]
that the extraction of metals with saturated quaternary
ammonium salts decreases at high HCl concentrations
because of the growing competition of the extraction
of the acid itself.
As expected, the metal distribution factors in ex-
traction with Ia Ie, as in extraction with common
quaternary ammonium salts, strongly depend on the
diluent. Replacement of chloroform by toluene sharp-
ly increases the extractability of Zn(II) and Cu(II),
especially in the range of the maximum at high HCl
concentrations (Figs. 1, 2). In particular, in toluene
salt Ib virtually quantitatively extracts zinc from
3.5 M HCl and extracts copper from 5.0 M HCl with
DCu 4.85, whereas in chloroform salt Ib does not
extract copper at all, and the maximal distribution
factor of zinc is as low as 1.3 (Table 1).
On the other hand, the triple bond capable of for-
mation of donor acceptor complexes with cations of
soft (concept of hard and soft acids and bases [11])
acids could enhance the extraction of Zn and Cu.
However, as shown by our experiments, 1-octyne
forms stable complexes with Zn and Cu ions neither
in aqueous nor in organic phase. In the organic phase,
we detected no metal ions (complexometric titration,
Table 1), and in the aqueous phase phase, after two-
fold washing with the diluent, 1-octyne was not de-
tected by the reaction with an ammonia solution of
This relationship can be accounted for as follows.
First, CHCl3, being somewhat more electrophilic than
toluene, better solvates the halide anion, reducing its
nucleophilicity toward the metal ion and thus decreas-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 5 2005

EXTRACTION OF ZINC AND COPPER
675
ing the extractive power of the quaternary ammonium
salt. Second, toluene, whose polarizability is much
higher than that of chloroform, should better dissolve
molecular complexes of acetylenic quaternary ammo-
nium bases with soft (readily polarizable [11]) Lewis
acids Zn2+ and Cu2+.
This trend is consistent with a decrease in the solu-
bility of the extractable metal complexes in the aque-
ous phase in going to more hydrophobic extractants,
and also with a decrease in the acceptor power of al-
kynyl substituents ( * +0.76 for HC CH2 [12, 18]
and +0.48 for C4H9C CCH2 and C6H13C CCH2 [4]).
In toluene, the most promising extractant is Ib
(Fig. 2, Table 1). For copper, a further increase in the
length of substituents at the nitrogen atom (Id) or
triple bond (Ie) leads to a decrease in the distribution
factor (CHCl 5.0 M):
It is interesting that the positions (absolute and
relative) and relative heights of the maxima in the
plots of the distribution factors vs. HCl concentration
depend on the structure of the compound and on the
kind of the metal and diluent (Figs. 1, 2). In particu-
lar, salt Ia in toluene approximately equally extracts
the metal from solutions with low and high HCl con-
centrations (in the points of maxima). Salt Ib in tolu-
ene extracts copper much better at high acid concen-
trations, and in chloroform it extracts zinc much
better at low acid concentrations, with the second
maxima being ill-defined. With Id, the curve becomes
dome-shaped, similarly to the curves observed with
saturated quaternary ammonium salts.
Ib > II > Id > Ia > Ie.
However, this order may depend on the HCl con-
centration. For example, at CHCl 3.5 M, salt Ia ex-
tracts Cu better than does Id. With zinc, no such fine
distinctions were found. In toluene, all the examined
acetylenic quaternary ammonium salts with the molec-
ular weight equal to or exceeding that of Ib virtually
quantitatively extract Zn from HCl solutions, i.e., this
diluent levels the extractive properties of these salts.
The concept considering quaternary ammonium
salts as nucleophilic agents forming donor acceptor
bonds with the metal cation through the anion in
extraction [5] allows the strength of the extractable
complexes to be correlated with the inductive and
steric effects of substituents at the N atom.
One of possible factors responsible for such a de-
pendence of the distribution factors on the length of
hydrocarbon substituents in quaternary ammonium
salts is association; its role increases with a decrease
in the diluent polarity [7, 19]: nitrobenzene < chloro-
form < benzene < toluene < xylene < CCl4 < alkanes.
We showed that the acetylenic salts extract Zn and
Cu better than does salt III, which is due, in particu-
lar, to the steric factors. It follows from construction
of molecular models [17] that the effective volume of
the RC CH2 group is somewhat smaller than that of
the CH3 group (if R is not too bulky). As alkynyl
groups, exhibiting negative inductive effect, should
decrease the nucleophilicity of the anion, the abnor-
mally high extractability of the acetylenic quaternary
ammonium salts also suggests that the triple bond
directly participates in the formation of molecular
complexes of types IV and/or V. At the same time,
the contribution of species VI (with no ,v interaction
of the triple bond with the metal ion) to extraction
with acetylenic quaternary ammonium salts is hardly
significant.
In more polar [20] chloroform ( 1.87 D, 4.70),
the association of quaternary ammonium salts is
somewhat weaker than in toluene ( 0.36 D, 2.38),
and, despite the hindering effect exerted by electro-
philic CHCl3, which tends to specifically interact with
halide anions, on the formation of molecular com-
plexes with metal cations, the distribution factors
grow with an increase in the length of substituents at
the N atom, owing to a decrease in the solubility of
the adducts in water.
In less polar toluene, an increase in the association
of quaternary ammonium salts with an increase in
their hydrophobicity results in a decrease in the active
active concentration of the free extractant and hence
in the worse extraction of copper. This effect becomes
more pronounced with a decrease in the HCl concen-
tration.
Our further experiments were aimed to reveal how
the degree of extraction depends on the extractant
structure.
In chloroform (Fig. 1, Table 1), the zinc distribu-
tion factors (CHCl 3.5 M) grow with an increase in the
length of substituents at the nitrogen atom and triple
bond, considerably exceeding the distribution factors
observed with the saturated analog III:
This trend is, apparently, responsible for the fact
that salt Ia in toluene, despite much stronger acceptor
properties of the alkynyl substituent, extracts copper
from 5 M HCl better than does Ie, and from 3.5 M
HCl, also better than does Id. As for zinc, all the
quaternary ammonium salts with long-chain alkynyl
groups in toluene are moch more efficient extractants
Ie > Ib > II > Ia > III.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 5 2005

676
logDMe
ANDREEV et al.
ity of the quaternary ammonium salt) compared to Ib,
Ib
Ib
3
0.6
in less polar toluene logD, on the contrary, decreases.
Ia
Ia
2
1
2
3
The abnormally high (from the viewpoint of elec-
tronic factors) extractive power of II (Fig. 4, Table 1)
should be particularly noted. Despite the presence of
two electron-withdrawing substituents, it extracts zinc
more efficiently than does Ia, and in extraction of
copper it is inferior only to Ib. This is apparently
caused also by the fact that in II, compared to the
other salts studied, the quaternary nitrogen atom is the
most accessible sterically. Furthermore, the second
triple bond creates prerequisites for formation of still
more stable (compared to IV and V) chelate com-
plexes, e.g., of type VII:
Id
II
0.2
0
Ie
Ib
III
1
0.2
0.4
0.2
0
0.2 0.4 0.6 0.8
*
Fig. 4. Distribution factors of Zn(II) and Cu(II) in extrac-
tion from aqueous HCl with quaternary ammonium salts
at 25 C, as functions of the Taft inductive constants
R
R
(
*). C
0.05 and C
0.02 M. (1) Zn(II), chloro-
QAS
Me
form, C
(3) Cu(II), toluene, C
quaternary ammonium salts is the same as in the text and
in Table 1. Dashed lines are drawn through the points for
the most structurally similar compounds Ia and Ib, with
the aim to emphasize the effect of growing hydropho-
3.5 M; (2) Cu(II), toluene, C
3.5 M; and
HCl
HCl
N+
X
5.0 M. The numbering of the
H
H
H
H
HCl
C
C
C
C
R
2X
C
C
R
Me2+
bicity of quaternary ammonium salts ( * and E
remain constant) at varied diluent polarity.
N
VII
than Ia (in chloroform, the trend is similar) and virtu-
ally completely extract zinc from the aqueous phase.
Thus, the efficiency of extraction with acetylenic
quaternary ammonium salts largely depends on their
structure.
Presumably, complexes of quaternary ammonium
salts with zinc ions are associated in the organic phase
more strongly than complexes with copper ions (espe-
cially in toluene). Since association of extractable
complexes enhances the transfer of metals to the or-
ganic phase, the distribution factor of zinc is higher.
A similar assumption was made to account for, e.g.,
higher distribution factors of Pt, compared to Pd, in
extraction with solutions of quaternary ammonium
salts in toluene [19].
The simultaneous influence of the inductive and
steric effects of substituents in amines on their nucleo-
philic reactivity can be quantitatively estimated with
the extended Taft equation [4, 17]
logk = logk0
+
*
* + EN.
As the constant EN characterizing the steric effects
in an amine R1R2R3N is used the Taft constant ES for
the isosteric hydrocarbon substituent R1R2R3C. This
equation can be applied to describing extraction proc-
esses [5] involving salts of primary, secondary, and
tertiary amines as nucleophilic agents.
Figure 4 shows the correlation of the distribution
factors of copper and zinc with the Taft inductive con-
stants ( *) of substituents at the ammonium nitrogen
atom. The fact that the extraction is not described by a
common straight line in the logDMe
* coordinates
suggests the significant role of some factors other than
electronic effects. Despite a limited data set (dashed
lines), we can speak of significant deviations of the
points for III (downward) and II (upward) from
straight lines 1 and 3, suggesting different mechan-
isms of formation of donor acceptor complexes of
metal halides with quaternary ammonium salts in the
series of saturated, monoacetylenic, and diacetylenic
derivatives. It is clearly seen from Fig. 4 that, whereas
in chloroform logD increases with increasing length
of substituents at the N atom (increasing hydrophobic-
As the steric effect is telescopic [13] (introduction
of the next radical exerts a greater steric effect than
introduction of the previous radical), its role in proc-
esses involving quaternary ammonium salts is particu-
larly significant. Unfortunately, steric constants for
quaternary ammonium salts are lacking. Therefore, to
compare the steric accessibility of the nitrogen atom
in quaternary ammonium salts, we took the EN values
for tertiary amines with linear substituents (Table 2).
Some of these values were calculated by us previously
from data on the quaternization kinetics.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 5 2005

EXTRACTION OF ZINC AND COPPER
677
Table 2 shows that, starting from tributylamine
VIIIc, an increase in the length of the alkyl chain does
not longer affect the steric accessibility of the nitrogen
atom in the saturated tertiary amine. At the same time,
comparison of EN for VIIIa, IXa, and IXb shows that
the steric constants of the CH3 and CH2C CR groups
are virtually equal, taking into account that the error
in their determination is 0.1 0.2 unit. However, a
shift of the triple bond away from the nitrogen atom
by even one or two carbon atoms (compounds IXa,
IXb, Xa, Xb) results in that the steric effect of - and
-acetylenic amines becomes the same as that of the
corresponding saturated analogs (VIIIb VIIId).
Table 2. Steric constants EN of tertiary -, -, and -ace-
tylenic and saturated amines [4]
Comp. no.
Amine
EN
VIIIa
VIIIb
VIIIc
VIIId
VIIIe
IXa
IXb
Xa
Xb
XIa
(CH3)2NCH3
(CH3)2NC5H11
(C4H9)2NC4H9
(C4H9)2NC7H15
1.54a
2.2
4.5a
4.4
4.5
2.6
4.5
2.6
4.5
1.8a
1.8
3.7
3.7
(C8H17)2NC8H17
(CH3)2N(CH2)3C CC4H9
(C4H9)2N(CH2)3C CC4H9
(CH3)2N(CH2)2C CC4H9
(C4H9)2N(CH2)2C CC3H7
(CH3)2NCH2C CCH=CH2
(CH3)2NCH2C CC2H5
(C4H9)2NCH2C CC4H9
(C8H17)2NCH2C CC5H11
These data confirm our assumption that the steric
accessibility of the nitrogen atom in quaternary am-
monium salts Ia Ie and III should be similar. There-
fore, the higher extractive power of the salts with
acetylenic substituents can be rationalized only by
assuming principally different complexation mechan-
isms (e.g., those involving chelation). Also, from this
standpoiont, a significant dependence of the distribu-
tion factors of zinc and copper in extraction with Ib,
Id, and Ie on the diluent can be rationalized only by
different effects of solvation of quaternary ammonium
salts and molecular complexes formed by them; such
an attempt was made in this paper.
XIb
XIc
XId
a
Data from [17].
copper and zinc ions with acetylenic quaternary am-
monium salts, as influenced by the extractant struc-
ture, kind of diluent, and concentration of HCl or
metal halide (salting-out agent) in the aqueous phase,
and to gain insight into the extraction mechanism.
EXPERIMENTAL
Thus, acetylenic quaternary ammonium salts that
we studied (especially Ib, Id, and Ie) under definite
conditions extract zinc from aqueous HCl much more
efficiently than do tributyl phosphate [21] (traditional-
ly used extractant) and ammonium salts with saturated
substituents at the nitrogen atom [5]. The acetylenic
quaternary ammonium salts also show promise for
extraction of copper from hydrochloric acid solutions,
taking into account the fact that the results of this
study were obtained at low (0.05 M) concentrations of
the extractants in the organic phase.
The procedure for preparing quaternary ammonium
salts and tertiary amines containing triple bonds in the
-position relative to the nitrogen atom was described
in [4, 22].
Extraction of zinc with solutions of quaternary
ammonium salts and 1-octyne in appropriate organic
diluent was performed at 25 C, with equal volumes
of the organic and aqueous phases. The initial concen-
tration of Zn(II) and Cu(II) was kept constant and
equal to 0.02 M. The extraction equilibrium with
quaternary ammonium salts is attained within 1 min.
The equilibrium concentrations of Zn(II) and Cu(II)
were found by complexometric titration with Xylenol
Orange indicator at pH 5. The acid concentration was
determined by titration with KOH (Methyl Orange
The relatively high extractive power of the salts
studied, and also the simplicity of procedures for the
recovery of Zn(II) and Cu(II) with these salts allow
these procedures to be recommended for analytical
practice.
Of considerable interest is also the fact that acetyl- indicator) [23].
enic quaternary ammonium salts efficiently extract
The concentration dependences and diluent effects
were studied at 25 C.
rare-earth elements, according to our preliminary data.
Doubtless advantages of these extractants are very fast
(within several minutes) attainment of the extraction
equilibrium and phase separation, and also considera-
bly lower concentrations of the extractant in the or-
ganic phase, as compared to the published systems.
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