Selective Liquid Membrane Transport of Lead(II) by an Acyclic Polyether Dicarboxylic Acid Ionophore

Article abstract of DOI:10.1021/ac9701593

The effects of the chain structure and substitutents in six acyclic polyether dicarboxylic acids and one acyclic polyether carboxylic acid upon the efficiency and selectivity of pH-driven Pb²⁺ transport in a bulk chloroform membrane system have

Full text of DOI:10.1021/ac9701593

Anal. Chem. 1997, 69, 3002-3007
Selective Liquid Membrane Transport of Lead(II)
by an Acyclic Polyether Dicarboxylic Acid
Ionophore
Kazuhisa Hiratani,*,† Toshikazu Takahashi,† Hideki Sugihara,† Kazuyuki Kasuga,† Kyoko Fujiwara,†
Takashi Hayashita,‡ and Richard A. Bartsch§
National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan, Department of Chemistry,
Faculty of Science and Engineering, Saga University, 1 Honjo, Saga 840, Japan, and Department of Chemistry and
Biochemistry, Texas Tech University, Lubbock, Texas 79409
chloroform membrane and into the aqueous receiving phase, the
rate of Pb2+ transport is anion-dependent.
The effects of the chain structure and substitutents in six
acyclic polyether dicarboxylic acids and one acyclic poly-
ether carboxylic acid upon the efficiency and selectivity
of pH-driven P b2 + transport in a bulk chloroform mem-
brane system have been assessed. Among the carriers,
1 ,2 -bis[2 -(o-carboxyphenoxy)ethoxy]-4 -t er t -bu tylben-
zene (1 ) is found to exhibit high selectivity for transport
of P b2 + compared with alkali metal cations and a variety
of other divalent metal ion species. Ionophore 1 also
extracts P b2 + from aqueous solution into chloroform with
the loss of two protons. A 1 :1 complex of P b2 + with di-
ionized 1 was isolated.
In earlier work, we observed that the acyclic polyether
dicarboxylic acids 1 and 2 exhibit selectivities for Ba2+ and Mg2+
respectively, in competitive proton-coupled transport of alkali metal
and alkaline earth metal cations from a basic aqueous solution
(source phase) through a chloroform membrane into an acidic
aqueous solution (receiving phase).8 For these di-ionizable
carriers, transport of the divalent metal ions does not require
concomitant transport of anions from the aqueous phase. Also,
the pH gradient may be used to drive metal ion transport against
its concentration gradient.
,
Recently we discovered that 1 and 2 also exhibit selectivity
for Pb2+ transport from weakly acidic aqueous solutions through
a chloroform membrane and into a strongly acidic aqueous
solution.9 To probe the influence of chain structure and substit-
uents within the carrier upon the efficiency and selectivity of Pb2+
transport, the study has now been expanded to include four
additional acyclic polyether dicarboxylic acids 3-6 and the acyclic
polyether carboxylic acid 7 . In addition to Pb2+, divalent metal
ions of Ba2+, Cd2+, Cu2+, Hg2+, and Zn2+ were chosen for the
following reasons: (i) Earlier, 1 was found to selectively transport
Ba2+ across a bulk chloroform membrane;8 (ii) Cd2+ is a different,
toxic, divalent, heavy metal ion; (iii) Cu2+ is at the top of the
Irving-Williams order;10 (iv) Zn2+ is an important metal ion in
biological systems; and (v) Hg2+ is a toxic, divalent heavy metal
ion with approximately the same ionic radius as Pb2+. In addition
to the liquid membrane transport studies, the stoichiometry of
the solvent extraction complex formed from Pb2+ and 1 has been
determined and spectroscopic properties of the isolated complex
have been examined.
Selective removal of Pb2+ for environmental remediation and
in the treatment of acute and chronic lead poisoning remains an
important objective.1-3 Attempts to remove toxic heavy metal ions
(e.g., Cd2+, Hg2+, and Pb2+) from the environment and from
biological systems have utilized a variety of separation methods,
such as adsorption, precipitation, and solvent extraction.
Compared with solvent extraction, liquid membrane transport
systems have the advantage that the amounts of organic liquid
and metal ion complexing agent are markedly reduced.4-6 Selec-
tive transport of lead salts across bulk liquid membranes with
dicyclohexano-18-crown-6 (DC18C6) has been reported by Lamb
et al.7 Since in this system both Pb2+ and one or more anions
must be transported from the aqueous source phase through the
† National Institute of Materials and Chemical Research.
‡ Saga University.
§ Texas Tech University.
While this research was underway, good selectivity for Pb2+
over Cu2+ in competitive transport across plasticized cellulose
triacetate membranes containing acyclic polyether dicarboxylic
acid 8 was reported.11
(1) Izatt, R. M..; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J.;
Sen, D. Chem. Rev. 1 9 8 5 , 85, 271.
(2) Hancock, R. D. Pure Appl. Chem. 1 9 8 6 , 58, 1445.
(3) Damu, K. V.; Shaikjee, M. S.; Michael, J. P.; Howard, A. S.; Hancock, R. D.
Inorg. Chem. 1 9 8 6 , 25, 3879.
(4) Liquid Membranes; ACS Symposium Series 347; Noble, R. D., Way, J. D.,
Eds.; American Chemical Society: Washington, DC, 1987.
(5) Araki, T.; Tsukube, H. Liquid Membranes: Chemical Applications; CRC
Press: Boca Raton, FL, 1990.
(6) Chemical Separations with Liquid Membranes; ACS Symposium Series 642;
Bartsch, R. A., Way, J. D., Eds.; American Chemical Society: Washington,
DC, 1996.
(8) Hiratani, K.; Taguchi, K.; Sugihara, H. J. Membr. Sci. 1 9 9 1 , 56, 960.
(9) Hiratani, K.; Sugihara, H.; Kasuga, K.; Fujiwara, K.; Hayashita, R.; Bartsch,
R. A. J. Chem. Soc., Chem. Commun. 1 9 9 4 , 319.
(10) Irving, H.; Williams, R. J. P. J. Chem. Soc. 1 9 5 3 , 3192; Nature 1 9 4 8 , 162,
746.
(7) Lamb, J. D.; Izatt, R. M.; Robertson, P. A.; Christensen, J. J. J. Am. Chem.
Soc. 1 9 8 0 , 102, 2452.
(11) Hayashita, T.; Fujimoto, T.; Moriata, Y.; Bartsch, R. A. Chem. Lett. 1 9 9 4 ,
2385
3002 Analytical Chemistry, Vol. 69, No. 15, August 1, 1997
S0003-2700(97)00159-5 CCC: $14.00 © 1997 American Chemical Society

of 3 with mp 113-114 °C: 1H NMR (CDCl3) δ 3.76 (4H, s,
OCH2CH2O), 3.92 (4H, t, J ) 4 Hz, OCH2CH2O-Ar), 4.36 (4H, t, J
) 4 Hz, OCH2CH2O-Ar), 7.02 (2H, dd, J ) 8 and 8 Hz), 7.09 (2H,
dd, J ) 8 and 8 Hz), 8.10 (2H, d, J ) 8 Hz) ppm; IR (KBr) 3264
(CO2H), 1714 (CdO), 1244 (CO) cm-1; UV (CHCl3) λmax 292 nm,
ꢀ 6180. Anal. Calcd for C20H22O8: C, 61.53; H 5,68. Found: C,
61.63; H, 5.85.
P reparation of 3 ,3 -Bis[o-(carboxyphenoxy)methyl]oxe-
tane (5 ). In a manner similar to that described above for the
synthesis of 3 , reaction of 3,3-bis(chloromethyl)oxetane with the
sodium salt of ethyl salicylate in DMF followed by hydrolysis gave
oxetane derivative 5 with mp 162-162.5 °C in 65% yield: 1H NMR
(CDCl3) δ 4.57 (4H, s, cyclic-OCH2), 4.75 (4H, s, Ar-OCH2), 7.13
(2H, m), 7.55 (2H, m), 8.07 (2H, dd, J ) 2 and 8 Hz) ppm; IR
(KBr) 2665, 2558 (CO2H), 1690 (CdO), 1250 (CO) cm-1; UV
(CHCl3) λmax 292 nm, ꢀ 6650. Anal. Calcd for C19H18O7: C, 63.68;
H, 5.06. Found: C, 63.39; H, 5.05.
P reparation of 1 ,2 -Bis[o-(carboxyphenyl)aminocarbon-
ylmethoxy]-4 -ter t-butylbenzene (6 ). To 1.60 g (5.0 mmol) of
1,2-bis[(chlorocarbonyl)methoxy]-4-tert-butylbenzene, which was
obtained from the reaction of 1,2-bis[(hydroxycarbonyl)methoxy]-
4-tert-butylbenzene with thionyl chloride, in 50 mL of benzene were
added 1.70 g (10 mmol) of ethyl 2-aminobenzoate and 1.0 g (10
mmol) of triethylamine. The mixture was stirred at 70 °C for 24
h, cooled to room temperature, and poured into water (300 mL).
The mixture was extracted with benzene (200 mL), and the
benzene layer was washed with water (3 × 100 mL), dried over
MgSO4, and evaporated in vacuo. The residue was chromato-
graphed on silica gel with chloroform as eluent to give 2.30 g
(79%) of 1,2-bis[(o-ethoxycarbonylphenyl)aminocarbonylmethoxy]-
4-tert-butyl-benzene as a colorless liquid. A solution of this di-
(ethyl ester) (1.40 g) and 1.0 g of KOH in 30 mL of EtOH was
refluxed overnight and evaporated in vacuo. Water was added
to the residue, and the solution was acidified with concentrated
HCl and extracted twice with chloroform. The chloroform layer
was washed twice with water, dried over MgSO4, and evaporated
in vacuo. The residue was recrystallized from benzene/ cyclo-
hexane (2:1) to give 0.80 g (67%) of 6 with mp 157-158 °C: 1H
NMR (CDCl3) δ 1.21 (9H, s), 4.75 (2H, s), 4.78 (2H, s), 6.81 (1H,
EXPERIMENTAL SECTION
Reagents. Inorganic and organic compounds were reagent-
grade commercial products and were used as received. 1,2-Bis-
[2-(o-carboxyphenoxy)ethoxy]-4-tert-butylbenzene (1 ),8 1,2-bis[3-
(o-carboxyphenoxy)propoxy]-4-tert-butylbenzene (2 ),8 1,2-bis[2-
(o-carboxyphenoxy)ethoxy-2-ethoxy]-4-tert-butylbenzene (4),12 and
1-[(2-quinolyl-8-oxy)ethoxy]-2-[(o-carboxyphenoxy)ethoxy]-4(5)-
tert-butylbenzene (7 )13 were prepared as previously reported.
P reparation of 1 ,8 -Bis(o-carboxyphenoxy)-3 ,6 -dioxaoc-
tane (3 ). A mixture of 3.30 g (20 mmol) of ethyl salicylate, 0.50
g (20 mmol) of oil-free NaH, and 50 mL of DMF was stirred until
it became homogeneous and 1.90 g (10 mmol) of 1,2-bis(2-
chloroethoxy)ethane was added. The mixture was stirred at 70
°C for 24 h, cooled to room temperature, and poured into 300 mL
of water. The mixture was extracted with benzene (200 mL), and
the benzene layer was washed with water (3 × 100 mL), dried
over MgSO4, and evaporated in vacuo. The residue was chro-
matographed on silica gel with chloroform as eluent to give 3.20
g (72%) of 1,8-bis[(o-ethoxycarbonyl)phenoxy]-3,6-dioxaoctane as
a colorless liquid. A solution of this di(ethyl ester) (2.20 g) and
1.0 g of KOH in 30 mL of EtOH was refluxed overnight and
evaporated in vacuo. Water was added to the residue, and the
solution was acidified with concentrated HCl and extracted twice
with chloroform. The combined chloroform layers were washed
twice with water, dried over MgSO4, and evaporated in vacuo.
Recrystallization of the residue from benzene gave 1.20 g (62%)
(12) Hiratani, K.; Sugihara, H.; Taguchi, K.; Iio, K. Chem. Lett. 1 9 8 3 , 1657.
(13) Hiratani, K.; Taguchi,; Sugihara, H.; Iio, K. Bull. Chem. Soc. Jpn. 1 9 8 4 , 57,
1976.
Analytical Chemistry, Vol. 69, No. 15, August 1, 1997 3003

Figure 2. Pb2+ concentrations in the source phase (b) and receiving
phase (O) vs time for transport across a chloroform membrane by
carrier 1. Initial conditions: source phase, 15 mL of 0.10 M Pb2+ in
acetate buffer at pH 6.18; membrane phase, 30 mL of 5.0 mM 1 in
chloroform; receiving phase, 15 mL of 0.10 M nitric acid.
Figure 1. Cell for measuring metal ion transport across a chloroform
membrane: (a) aqueous source phase, (b) chloroform membrane,
(c) aqueous receiving phase, (d) glass stirrer, and (e) water bath at
25 °C.
time both phases were clear in all cases. The concentration of
Pb2+ in the aqueous solution was determined by atomic absorption
spectroscopy, and the amount of extracted Pb2+ was calculated
by difference from the concentration of Pb2+ in the original
aqueous solution.
d, J ) 8 Hz), 6.92 (1H, d, J ) 8 Hz), 6.95 (1H, m,), 7.18 (2H, m),
7.64 (2H, m), 8.05 (2H, dd, J ) 4 and 8 Hz), 8.85 (2H, dd, J ) 4
and 8 Hz), 11.64 (2H, s, NH) ppm; IR (KBr) 3194, 2616 (CO2H),
1691 (CdO), 1235 (CO) cm-1; UV (CHCl3) λmax 309 nm, ꢀ, 9700.
Anal. Calcd for C28H28N2O8: C, 63.51; H, 5.52; N, 5.29. Found:
C, 64.43; H, 5.77; N, 5.25.
RESULTS AND DISCUSSION
P reparation of the P b2 + Complex of 1 ,2 -Bis[2 -(o-carbox-
yphenoxy)ethoxy]-4 -ter t-butylbenzene (1 ). Lead(II) acetate
trihydrate (0.38 g, 1.0 mmol) was added to a solution of 1 (0.49
g, 1.0 mmol) in 2 mL of DMSO. The mixture was heated to obtain
a solution which was allowed to cool to room temperature and
stand gradually forming colorless crystals. The crystals were
filtered and dried in vacuo at 100 °C overnight to give 0.51 g (71%)
of the complex with mp 132-135 °C: 1H NMR (CDCl3) δ 1.20
(9H, s), 4.23 (4H, br m, OCH2CH2O), 4.50 (4H, br m, OCH2CH2O),
6.67 (2H, br, m), 6.90 (4H, br m), 7.04(1H, br s), 7.30 (2H, br m),
and 7.70 (2H, br m) ppm; IR (KBr) 1588, 1556, 1520 (CO2-), 1219
(CO) cm-1; UV (CHCl3) λmax 283 nm, ꢀ, 7300. Anal. Calcd C28-
H28O8Pb‚H2O: C, 46.86; H, 4.21. Found: C, 46.51; H, 4.14.
Metal Ion Transport across Bulk Chloroform Membranes.
Single species and competitive metal ion transport experiments
were conducted in a U tube-type glass cell (Figure 1) thermostated
at 25.0 ( 0.2 °C. Each phase was mechanically stirred at 200
rpm. The acetate buffer for the source phase was prepared from
195 mL of 1.00 M AcONa, 25 mL of 0.20 M AcOH, and 780 mL of
deionized, distilled water. Compositions, concentrations, and
volumes of the weakly acidic (pH 5.2-6.2) aqueous source phase,
the chloroform membrane phase, and the more strongly acidic
aqueous receiving phase are given in the table footnotes. The
amounts of metal ions transported into the receiving phase and
metal ions remaining in the source phase were determined by
atomic absorption spectrophotometry.
Acyclic polyether dicarboxylic acids 1 -6 each contain two
ortho-substituted benzoic acid units with variation in the length
of the spacer that joins the two carboxylic acid-containing end
groups, as well as the number and type(s) of potential coordinating
atoms. Compounds 1 ,8 2 ,8 and 4 12 form a series in which the
length of the spacer and number of polyether oxygen atoms are
systematically varied. 3 is an analog of 1 in which the tert-
butylbenzo group of the latter has been replaced by an ethylene
unit. Since the oxetane ring oxygen is directed away from the
dicarboxylic acid groups, which should limit its role in metal ion
coordination, 5 is a close analog of 3 . In 6 , two of the ether
oxygens in 1 have been replaced by two amide groups. 7 was
included to provide an example of an acyclic polyether with only
one carboxylic acid group.
Transport of P b2 + across Bulk Chloroform Membranes
by Acyclic P olyether Dicarboxylic Acids 1 -6 and Carboxylic
Acid 7 . The metal ion transport experiments were conducted in
the U tube-type glass cell shown in Figure 1. The source phase
“a” was a 0.10 M aqueous solution of Pb2+ in acetate buffer at pH
6.18. The membrane phase “b” was a 5.0 mM solution of the
acyclic polyether dicarboxylic or carboxylic acid carrier in
chloroform. In nearly all cases, the aqueous receiving phase “c”
was 0.10 M nitric acid. All phases were stirred mechanically at
200 rpm. Samples of the aqueous receiving and source phases
were removed periodically, and the Pb2+ concentrations were
determined by atomic absorption spectrophotometry. In the
absence of carrier, no Pb2+ transport was detected.
Solvent Extraction of P b2 + by 1 . Into a 20-mL sample tube
with a screw cap were added 8.0 mL of an aqueous 10 mM Pb2+
solution and 8.0 mL of a 1.0 mM chloroform solution of 1 . The
pH of the aqueous solutions ranged from 4.57 to 6.28 as achieved
by appropriate combinations of 1.0 M NaOAc and 1.0 M HNO3.
The mixture was shaken vigorously for 24 h at 25 °C, after which
A plot of the Pb2+ concentrations in the aqueous source and
receiving phases vs time for single-species transport by acyclic
polyether dicarboxylic acid 1 is shown in Figure 2. After one
day, the Pb2+ concentration in the receiving phase exceeds that
in the source. Thus, proton-driven transport of Pb2+ against its
3004 Analytical Chemistry, Vol. 69, No. 15, August 1, 1997

Table 2. Competitive Transport of Pb2+ and Cu2+
across Bulk Chloroform Membranes by Acyclic
Polyether Carboxylic and Dicarboxylic Acids and by
Dicyclohexano-18-crown-6 at 25 °Ca
initial
transport rate ×106
b
Pb2+/ Cu2+
(µmol h-1 cm-2
)
pH of the
source phase
transport
entry
carrier
Pb2+
Cu2+
selectivity
1
2
3
4
5
6
7
1
2
3
4
5.27
5.27
5.27
5.27
6.18
6.18
6.18
46
20
26
13
2
0.48
1.3
0.66
4.0
2
12
<0.1
96
15
40
3.3
1
6 c
7
3.0
0.1
0.25
DC18C6d
Initial conditions: source phase, 15 mL of 0.10 M Pb2+ and 0.10
M Cu2+ in acetate buffer; membrane phase, 30 mL of 5.0 mM carrier
in chloroform; receiving phase, 0.10 M nitric acid. b Reproducibility
was (10% or less of the stated value. c The chloroform membrane
became pale blue and turbid. d Dicyclohexano-18-crown-6.
a
Figure 3. Pb2+ concentrations in the source phase (O) and receiving
phase (b) vs time for Pb2+ transport across a chloroform membrane
by carrier 1. Initial conditions: source phase, 15 mL of 1 mM Pb2+ in
acetate buffer at pH 6.18; membrane phase, 30 mL of 5.0 mM 1 in
chloroform; receiving phase, 15 mL of 0.10 M nitric acid
polyether dicarboxylic acid 5 , a white precipitate formed in the
chloroform membrane, suggesting that the Pb2+ complex of the
dicarboxylate form of 5 is insoluble in chloroform. Therefore,
no further attempts were made to study divalent metal ion
transport by potential carrier 5 .
The acyclic polyether carboxylic acid 7 (Table 1, entry 8) is
less effective in Pb2+ transport than the acyclic polyether dicar-
boxylic acids 1 -4 and 6 . For 7 , transport of Pb2+ requires either
two molecules of ionized carrier or one molecule of ionized carrier
and a monovalent anion from the aqueous source phase for
electroneutrality.
Competitive Transport of P b2 + and Cu2 + across Bulk
Chloroform Membranes. To explore the influence of structural
variation within the carrier upon selectivity, competitive transport
of Pb2+ and Cu2+ across chloroform membranes by 1 -4 and 6 ,
7 , and the crown ether DC18C6 was examined. The initial Pb2+
and Cu2+ transport rates for these carriers are presented in Table
2.
Table 1. Transport of Pb2+ across Bulk Chloroform
Membranes by Acyclic Polyether Carboxylic and
Dicarboxylic Acids Carriers at 25 °Ca
initial Pb2+
transport rate × 106
b
entry
carrier
(µmol h-1 cm-2
)
1
2c
3
4
5
6
7
8
1
1
2
3
4
5
6
7
53
54
20
34
24
<1d
29
9.2
a
Initial conditions: source phase, 15 mL of 0.10 M Pb2+ in acetate
buffer at pH 6.18; membrane phase, 30 mL of 5.0 mM carrier in
chloroform; receiving phase, 15 mL of 0.10 M nitric acid. b Reproduc-
ibility was (10% or less of the stated value. c Receiving phase was 0.20
M nitric acid. d A white precipitate formed in the chloroform membrane.
As can be seen, the nature of the spacer that joins the two
ortho-substituted benzoic acid groups in the acyclic polyether
dicarboxylic acid carrier has a profound influence upon the relative
rates of Pb2+ and Cu2+ transport. The Pb2+/ Cu2+ selectivity ratios
vary from 96 for carrier 1 to 1 for carrier 6 . The very high
selectivity obtained with 1 is reduced somewhat when the tert-
butylbenzo group in the spacer is replaced with a more flexible
ethylene unit in 3 . Replacing two of the ethylene units in the
spacer of carrier 1 with propylene units in 2 diminishes the
selectivity to an even lower level than was observed for 3 .
Replacing two of the ether linkages in carrier 1 with amide groups
in 6 produces a profound diminution in the selectivity.
concentration gradient is achieved. The transport rate decreases
gradually with time, presumably due to a lowering of the source
phase pH by the antiport of protons. Figure 3 shows a plot of
the Pb2+ concentrations in the aqueous source and receiving
phases vs time for single species transport by carrier 1 from a
solution that was initially 1.0 mM in Pb2+. As can be seen, Pb2+
transport from the source phase into the receiving phase is nearly
quantitative after 12 h. The Pb2+ concentration in the source
phase after 24 h is reduced to less than 2 ppm.
Initial rates of Pb2+ transport across the chloroform membrane
by carriers 1 -7 are recorded in Table 1. Identical Pb2+ transport
rates by 1 into receiving phases of 0.10 and 0.20 M nitric acid
(Table 1, entries 1 and 2, respectively) are consistent with
transport through the chloroform membrane as the rate-limiting
step. For the acyclic polyether dicarboxylic carriers, the rate of
Pb2+ transport decreases in the order 1 > 3 g 6 > 4 g 2 with
a variation of (5.3-2.0) × 10-5 µmol h-1 cm-2 for the series. Thus,
the nature of the spacer that joins the two ortho-substituted
benzoic acid units in the carrier is found to have a modest
influence upon the rate of Pb2+ transport. For the oxetane
7 is found to transport Cu2+ more effectively than Pb2+
.
Presumably this results from preferred interaction of Cu2+ with
the quinolinyl unit in the carrier.
Compared with the nonionizable, cyclic polyether carrier
DC18C6, pH-driven Pb2+ transport by carriers 1-4 and 6 is much
more rapid (Table 2).
Single-Species and Competitive Metal Ion Transport
across Chloroform Membranes by 1 . Since 1 was found to
provide the most efficient Pb2+ transport in single-species transport
experiments and the greatest Pb2+/ Cu2+ selectivity in competitive
Analytical Chemistry, Vol. 69, No. 15, August 1, 1997 3005

Table 3. Single-Species and Competitive Divalent Metal Ion Transport across Bulk Chloroform Membranes by
Acyclic Polyether Dicarboxylic Acid 1a
b
initial transport rate × 106 (µmol h-1 cm-2
)
initial pH of
source phase
entry
Pb2+
Cu2+
Zn2+
Cd2+
Ba2+
Hg2+
selectivity
1
2
3
4
5
5.80
6.26
6.24
6.47
6.13
9.2
53
44
35
15
<0.07
Pb2+/ Zn2+ g 760
Pb2+/ Cd2+ ) 150
Pb2+/ Ba2+ ) 87
Pb2+/ Hg2+ ) 3.3
0.30
0.46
4.5
a
Initial conditions: source phase, 15 mL of 0.10 M metal ion (each) in acetate buffer; membrane phase, 30 mL of 5.0 mM 1 in chloroform;
receiving phase, 15 mL of 0.10 M nitric acid. b Reproducibility was (10% or less of the stated value.
transport experiments, the transport behavior of this carrier for
alkali metal cations and several divalent metal cations was
examined.
In single-species transport experiments conducted with carrier
1 , no transport of alkali metal cations was detectable.
The initial rate for the single-species transport of Cu2+ across
the chloroform membrane by carrier 1 is presented in Table 3
(entry 1). The flux for Cu2+ is 9.2 × 10-6 µmol h-1 cm-2 compared
with a single species Pb2+ flux of 53 × 10-6 µmol h-1 cm-2 (Table
1, entry 1). Based upon these single species transport behaviors,
a competitive Pb2+/ Cu2+ transport ratio of ∼5.5 is estimated.
Although the single-species fluxes for Pb2+ and Cu2+ transport
are not directly comparable due to somewhat differing source
phase pH values, it appears certain that this estimated Pb2+/ Cu2+
transport selectivity is considerable less than the measured
competitive selectivity ratio of nearly 100 (Table 2, entry 2). The
Figure 4. Concentrations of Pb2+ (b) and Zn2+ (O) in the aqueous
primary causative factor for the considerable higher competitive
Pb2+/ Cu2+ selectivity in competitive transport is a marked diminu-
tion of Cu2+ transport compared to that observed in the single-
species transport experiment.
receiving phase vs time for competitive transport across a chloroform
membrane by carrier 1. Initial conditions: source phase, 15 mL of
10 mM Pb2+ and 1.0 M Zn2+ in acetate buffer at pH 6.18); membrane
phase, 30 mL of 5.0 mM 1 in chloroform; receiving phase, 15 mL of
0.10 M nitric acid.
For competitive transport of Pb2+ and other divalent metal ion
species (Table 3), selectivity ratios are Pb2+/ Zn2+ g 760, Pb2+
/
Cd2+ ) 150, Pb2+/ Ba2+ ) 87, and Pb2+/ Hg2+ ) 3.3. Thus, with
the exception of Hg2+, the Pb2+ selectivity in competitive transport
is very high for carrier 1 .
Results for competitive transport by carrier 1 from a source
phase solution 10 mM in Pb2+ and 1.0 M in Zn2+ are presented in
Figure 4. Even though there was an initial 100-fold excess of Zn2+
in the source phase, 75% of the Pb2+, but only 3% of the Zn2+, was
transported into the receiving phase after two days.
Solvent Extraction of P b2 + by 1 . To gain insight into the
nature of the complex between carrier 1 and Pb2+ in membrane
transport, solvent extraction experiments were conducted.
If extraction of Pb2+ with 1 (LH2) occurs as follows
where CM,o and CM,w denote the Pb2+ concentrations in the organic
and aqueous phases, respectively. When the volumes of the
organic and aqueous phases are the same and the concentration
of Pb2+‚L2- in the aqueous phase is negligible, the following
relationships can be derived from eqs 2 and 3
D ) Kex[LH2]o/ [H+]2
(4)
(5)
w
log D ) log Kex + log[LH2]o - 2 log[H+]w
Thus if the extraction occurs according to eq 1, plots of log D vs
-log[H+]w () pH) at a constant [LH2]o and of log D vs log[LH2]o
at a constant pH would be straight lines with slopes of 2 and 1,
respectively.
(Pb2+)w + (LH2)o S (Pb2+‚L2-)o + (2H+)w
(1)
Data for the solvent extraction of Pb2+ from aqueous solutions
into chloroform by 1 are shown in Figure 5. A plot of log D vs
pH (Figure 6) shows a linear relationship with a slope of 1.9. Thus
the formation of the extraction complex in solvent extraction of
Pb2+ with 1 is accompanied by the loss of two protons. For
extractions conducted at pH 6.28, a plot of log D vs log[LH2] gives
a straight line with slope 1.05 (Figure 7). These results verify
the extraction constant may be written as
Kex ) [Pb2+‚L2-]o[H+]2w/ [Pb2+]w[LH2]o
(2)
where the subscripts “o” and “w” denote the organic and aqueous
phases, respectively. The distribution coefficient may be written
as
the stoichiometry of the extraction complex as Pb2+‚L2-
.
Further Investigation of the Complexation of P b2 + by 1 .
D ) CM,o/ CM,w ) [Pb2+‚L2-]o/ [Pb2+
]
(3)
w
Although lead(II) acetate is insoluble in chloroform, rapid dis-
3006 Analytical Chemistry, Vol. 69, No. 15, August 1, 1997

Figure 5. pH dependence for solvent extraction of 10 mM Pb2+ in
acetate buffer with 1.0 mM 1 in chloroform.
Figure 7. Plot of log D vs log [1] for solvent extraction of Pb2+ from
aqueous solution at pH 6.28 into chloroform by 1.
Figure 6. Plot of log D vs pH for solvent extraction of Pb2+ from
aqueous solution into chloroform by 1.
Figure 8. Aromatic region of the 1H NMR spectrum of 1 in the
solution of lead acetate was observed in a solution of 1 in
chloroform. To examine this process in a more quantitative
fashion, 1H NMR spectroscopy was utilized. The aromatic region
absence and presence of lead(II) acetate.
selectivity for Pb2+ transport over Ba2+, Cd2+, Cu2+, and Zn2+, as
well as over alkali metal cations. Solvent extraction of Pb2+ from
aqueous solutions into chloroform occurs with formation of a 1:1
complex of Pb2+ and the di-ionized form of 1 .
1
of the H NMR spectrum of 1 in deuteriochloroform is shown in
Figure 8. Addition of a slight excess of lead(II) acetate and
shaking for 10 min produced marked changes in these absorp-
tions. Only limited further changes were noted after one day.
Thus formation of the complex between Pb2+ and 1 is shown to
be rapid.
From an equimolar solution of lead acetate trihydrate and 1
in dimethyl sulfoxide, a 1:1 complex of Pb2+ and the di-ionized
form of 1 crystallized as a monohydrate. This complex was
characterized by 1H NMR and IR spectra and its composition
verified by combustion analysis.
ACKNOWLEDGMENT
R.A.B. gratefully acknowledges support by the Division of
Chemical Sciences of the Office of Basic Energy Sciences of the
U.S. Department of Energy (Grant DE-FG03-94ER14416).
Received for review February 10, 1997. Accepted May 21,
1997.X
CONCLUSIONS
AC9701593
1 is shown to provide efficient and selective transport of Pb2+
across bulk chloroform membranes. Carrier 1 exhibits high
X Abstract published in Advance ACS Abstracts, July 1, 1997.
Analytical Chemistry, Vol. 69, No. 15, August 1, 1997 3007