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Elimination of acid-base generation (‘water-splitting’) in electrodialysis
Desalination, 51 (1984)
55-60
Eisevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ELIMINATION OF ACID-BASE GENERATION (‘WATER-SPLITTING’) IN ELECTRODIALYSIS*
I. RUBINSTEIN,
A. WARSHAWSKY,
L. SCHECHTMAN AND 0. KEDEM
The Weizmann Institute of Science, Rehovot (Israel) (Received October 21,1983;
in revised form December 18,1983)
SUMMARY
Membranes containing crown ether were prepared. The membranes become positively charged by complexing alkali-metal ions. In these anionexchange membranes, not containing amino groups, pH changes caused by above-limiting currents were very small in contrast to the substantial acidbase generation by conventional A membranes. This is consistent with Simons’ model. It is suggested that both suppression of acidification and the dynamic nature of the positive charges may help to avoid fouling.
Polarization in ED at low current densities has been described in detail and analyzed successfully on the basis of the concept of the Nernst unstirred layer [ 1, 21. It is however well known that the curves of current versus voltage do not approach a limiting value assymptoticahy, as expected from theory. The ‘limiting’ current appears only as a narrow flat region or as an inflexion point [ 31. At higher voltage the current rises again, and this abovelimiting current has been attributed to ‘water-splitting’, i.e. transfer of H’ and m through cation- and anion-exchange membranes respectively, as a consequence of total salt-depletion in the unstirred layers. During the last years it was shown by a number of authors [3-6] that, in the cation exchange (C) membranes studied, even under conditions of severe polarization nearly all of the current was carried-by the salt cation. The significant pH changes observed so far appeared only at anion exchange (A) membranes. At high, above-limiting, current densities a decrease in the coulomb efficiency of desalting, r), is generally observed. Forgacs et al. [2] have shown *Presented at the Fourth Symposium on Synthetic Membranes in Science and Industry, sponsored by the European Society of Membrane Science and Technology, Tiibingen, September 6-9, 1983.
I. RUBINSTEIN et al.
56
that this loss in q could be quantitatively correlated to m transport, in an A-membrane. The different phenomena observed at A- and C-membranes strongly suggest that the generation of H’ and m ions does not take place in the unstirred layers, common to both, but in the A-membrane itself. R. Simons [6] suggested that the dissociation of water, and the removal of H+ and m in opposite directions, occur in a thin surface layer of the A-membrane, adjacent to the dialysate. It is caused by reversible protonation of weakly basic groups, namely tertiary amines. --B
+
Hz0 --+-BH++m
&BH+ + HZ0 -Q+H;O Based on the known kinetic constants, Simons showed that this process should be substantially faster than direct dissociation of water. To explainwater-splitting at fully iquatemized membranes Simons further assumed that in the strong electric field established in the surface layer, tertiary groups can be created by decomposition of the quaternary groups; his experiments indicated that this process was likely taking place. Without entering into the quantitative arguments, the most direct test of the basic concept, reversible protonation, would be the performance of an Amembrane carrying only strongly basic groups. In addition, no chemical transformation of these to weakly basic ones should be possible. From a general chemical point of view, alkali metals are excellent candidates for the required positive charges. An A-membrane carrying alkali-ions as ‘fixed’ charges can be prepared by the attachment of uncharged crown ether sites to a neutral matrix. In contact with alkali salt solution, cations are bound and the anions enter the membrane as mobile counter-ions. The alkali ions are of course not irreversibly immobilized. They are exchanged between membrane and solution, and among sites, according to the rate of each reaction. It is however readily seen that only very fast association-dissociation will contribute significantly to a diffusion process.* The average mobility of the bound alkali ions may be assumed to be negligible compared to that of the anion in the membrane. Invasion of free salt is governed by the same distribution equilibria as in conventional ion exchange membranes, and hence permeselectivity may be expected whenever the concentration of the binding sites is larger than the concentration of the external salt solution. For the preliminary study reported here a benzo-crown ether bound to a hydrophobic chain [8] was incorporated into dense cellulose acetate (CA) *According to the Eyring theory of transport phenomena [ 7 ] D = x2 KO. Here D is the diffusion constant ofthe considered diffusion process, x is the distance between the adjacent planes of sites of the order of lo-’ cm, and ZCOir the appropriate reaction rate. A significant contribution of the bound ions to the electrodiffusion process would arise for D of the order of lo-’ - lo@ cm2/swhich would in turn require a very fast diisociation with KO greater in order than 10” 8’ .
ELIMINATION OF ACID-BASE GENERATION
IN ED
57
membranes. When the mixture of CA and complex (ligand + CsBr) was solvent cast and evaporated to dryness, clear fiis were obtained. These proved to be non-ideal anionexchange membranes, as expected. Transport and polarization tests were carried out in a four-compartment cell (Fig. l), with the tested membrane in the middle. Carbon suspension electrodes [9] were used to avoid pH changes by the electrode reactions. The area of the membranes confining the electrode compartments (C-membrane at the cathode and normally, except where stated otherwise, A-membrane at the anode) was 28.3 cm* while the area of the tested membrane varied between 0.8 and 3.1 cm*. No stirring was applied in compartments I and II, so that polarization was limited by natural convection only, and due to the different membrane areas was much more pronounced at the examined membrane. Current was determined as a function of the potential across the tested membrane, measured between a pair of Ag/AgCl electrodes inserted on both sides at a distance of 3.5 cm. The potential thus contains some of the IR drop in the solutions. The plot of I versus V shows a linear region at low voltage, a plateau of varying breadth, and then a further rise of current at higher voltage, reaching a slope similar to the initial one. The current in the flat region is the limiting current. Following this characterization, current was passed through the membrane for l-2 hours, at two current densities: one in the low linear region, and the other above the limiting value. Salt concentrations and pH were measured at start and end of this period. Table I gives the results for different membranes. The coulombic efficiency, 7, refers to desalting of compartment I for the tested A-membranes, and to desalting in II for the C-membrane which was
Fig. 1. Scheme of the cell A-anode, C-cathode, AM and CM-membranes confining the respective anode and cathode compartments, TM - tested membrane.
KCl O.OlN
KCI 0.ol.N
KC1 O.OlN
KCI O.OlN
KCl
Dllysis
Dialysis
Neglnst C
Neginet C
Neginst A fresh
KC1 0.02N
KCI O.OlN
KCI OhlN
NeginstA after paesege of current
Heterogeneous A
Heterogeneow
A
KCI O.OlN
Neginet A after passage of current
O.OlN
KCl O.OlN
KC2 O.OlN
Solution
Dialysis
Tested membrane
2.00
1.50
-
-
0.92 0.92
0.94
0.90
0.90 0.90
0.90
0.95
0.95
8.00
-
2.00 0.60
2.00 0.50
12.00 3.00
2.00 0.60
2.00 0.50
2.00 0.50
2.00 0.60
2.00
fnu9)
7
2.56 0.75
2.55 0.63
3.82 0.95
2.56 0.75
2.55 0.63
2.55 0.63
2.56 0.75
-
-
-
-
;mA/cm2)
1.2
1.0
2.4
1.2
1.2
1.0
1.0
-
-
-
P (mA/cm’)
120 180
120 180
60 120
120 120
120 120
120 180
120 180
120
120
120
120
At(min)
7.20 6.50
7.40 6.75
6.60 6.60 6.65 6.45
6.50 6.45
9.45 6.56
6.46 6.30
7.10 6.80
6.55 6.35
6.15
6.20
6.20
4.75
pH;
6.15 6.46
6.50 6.40
6.25 6.25
6.40 6.60
6.40 6.40
6.80
6.80
6.50
6.80
pHo
3.40 6.08
3.35 6.20
4.40 6.50
3.20 6.20
4.15 6.10
5.60 6.45
5.20 6.18
6.50
6.40
6.30
6.65
pH;
44 93
52 97
63 92
81 94
91 94
-
89 96
-
-
-
-
0 96
Neginet A
Neginet A
Heterogeneous A
NeghtA
Neginet A
Neginst C
Neginrt A
Neginet A
Heterogeneous A
Heterogeneous A
Heterogeneous A
Protective A
WATER SPLITTING AND COULOMBIC EFFICIENCY FOR DIFFERENT CONVENTIONAL MEMBRANES The coulombic efficiency, Q, refers to desalting of compartment I for the tested A-membranes and to that of compartment II for C-membranes, Z stands for the total electric current in the cell, 7 for the counterion transport number of the tested membrane, calculated from the membrane potential i-current density at the tested membrane, 1e&m- limiting current density, determined from the “plateau” of the voltage/current curve, pHe refers to the initial state of solution, pHZrJr - to the final state in the appropriate compartments after time At of passage of current. Protective A refers to the type of membrane confining the anode compartment.
TABLE I
kz
ELIMINATION OF ACID-BASE GENERATION
59
IN ED
checked for comparison. 7 is the transport number of the counter-ion, calculated from the membrane potential measured between 0.01 and 0.1 solution. pHo is the initial value and pH,*+I1,the final ones. The first lines in Table I give results for a dialysis membrane, showing slight pH effects caused by the A-membrane confining the anode-space. (In the electrode-compartments themselves no pH-changes were observed.) The C-membrane results given next were obtained with a C-membrane confining both anode and cathode compartments for pH changes, and with the usual A-membrane at the anode for 7. Finally, water-splitting by the A-membranes is shown in the lower part of Table I. As already pointed out by Simons, the polyethylene-based Neginst membranes give less water-splitting than polystyrene-based ones. Table II shows the corresponding results with the cellulose acetate complex (CAC) membranes. These results show clearly that water-splitting is indeed determined by the character of the charged groups in the anion-exchange membrane. pH-changes obtained at strong polarization near the complexing membranes were very small. The decrease in coulombic efficiency for removal of Cl- ion is small, TABLE II WATER-SPLITTING AND COULOMBIC EFFICIENCY IN CAC MEMBRANES Electrolyte solution was O.OlN CsCl (initial concentration). The rest of notations as in Table I.
Tested membrane
7
I
II
z (mA)
*
ih
At
;mAlcma)
(mA/cm2)
(min)
PHO PH;
PH;
7) (96)
Protective A
0.89
2.00 0.70
2.55 0.89
1.5
125 180
5.67 5.95
6.25 5.67
5.30 6.06
83 89
Heterogeneous A
0.83
2.00 0.60
2.55 0.76
2.2
120 121
6.40 6.45
6.22 6.20
6.10 6.31
90 98
Hetero-
geneous A
III
0.89
8.00 1.50
2.55 0.48
1.5
120 120
6.50 7.25
6.15 6.10
6.45 6.20
64 68
Heterogeneous A
IV
0.92
8.00 1.50
2.55 0.48
1.5
60 180
6.50 6.50
6.35 6.30
6.20 6.88
77 98
Heterogeneous A
V
0.92
8.00 2.00
2.55 0.63
1.6
60 150
6.62 6.62
6.10 6.25
6.22 6.65
90 92
Hetero-
60
I. RUBINSTEIN et al.
but still evident, and is not necessarily accounted for by m transfer. It should be remembered that we are dealing with fixed charges which are not covalently bound, and it is possible that permselectivity decreases at high current density. This and other points will be studied in future with a series of covalently bound ligands, different bound ions, etc. The results shown here demonstrate in principle that water splitting can be avoided by suitable chemistry of the A-membrane. The direct practical advantage with respect to membrane fouling is evident, since local acidity near the membrane enhances precipitation of colloids. A further property of these complexes may help to fight fouling: reversible decomposition takes place at mildly elevated temperature (N60”C) [lo]. The membrane can thus be ‘discharged’, removing the anchoring points for eleo trostatic adsorption.
ACKNOWLEDGEMENT
This work was supported in part by the National Council for Research and Development.
REFERENCES 1. K.S. Spiegler, Desalination, 9 (1971) 367. 2. C. forgacs, N. Ishibashi, J. Leibovitn, J. Sinkovic and K.S. Spiegler, Desalination, 10 (1972) 181-214. 3. B.A. Cook, Electrochim. Acta, 3 (1961) 307. 4. A.J. Mackai and J.C.R. Turner, JCS Faraday Transactions I, 74 (1978) 2860. 5. I. Rubinstein and L. Shtihnan, JCS Faraday Transactions II, 75 (1979) 281. 6. R. Simons, Nature, 280 (30) (1979) 824-826; Desalination, 28 (1979) 41-20. 7. S. Glasstone, K.J. Laidler and H. Byring, The Theory of Rate Processes, McGraw-Hill, New York, NY, 1941, p. 519. 8. A. Warshawsky and N. Kahana, Polymeric crown ethers for the extracting of alkali cations, Report to Israel Chemicals Ldt., November 1981, Rehovot. 9. 0. Kedem, J. Cohen, A. Warshawsky and N. Kahana, Desalination, 46 (1983) 291299. 10. A. Warshawsky and N. Kahana, J. Amer. Chem. Sot., 104 (1982) 2663-2664.
55-60
Eisevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ELIMINATION OF ACID-BASE GENERATION (‘WATER-SPLITTING’) IN ELECTRODIALYSIS*
I. RUBINSTEIN,
A. WARSHAWSKY,
L. SCHECHTMAN AND 0. KEDEM
The Weizmann Institute of Science, Rehovot (Israel) (Received October 21,1983;
in revised form December 18,1983)
SUMMARY
Membranes containing crown ether were prepared. The membranes become positively charged by complexing alkali-metal ions. In these anionexchange membranes, not containing amino groups, pH changes caused by above-limiting currents were very small in contrast to the substantial acidbase generation by conventional A membranes. This is consistent with Simons’ model. It is suggested that both suppression of acidification and the dynamic nature of the positive charges may help to avoid fouling.
Polarization in ED at low current densities has been described in detail and analyzed successfully on the basis of the concept of the Nernst unstirred layer [ 1, 21. It is however well known that the curves of current versus voltage do not approach a limiting value assymptoticahy, as expected from theory. The ‘limiting’ current appears only as a narrow flat region or as an inflexion point [ 31. At higher voltage the current rises again, and this abovelimiting current has been attributed to ‘water-splitting’, i.e. transfer of H’ and m through cation- and anion-exchange membranes respectively, as a consequence of total salt-depletion in the unstirred layers. During the last years it was shown by a number of authors [3-6] that, in the cation exchange (C) membranes studied, even under conditions of severe polarization nearly all of the current was carried-by the salt cation. The significant pH changes observed so far appeared only at anion exchange (A) membranes. At high, above-limiting, current densities a decrease in the coulomb efficiency of desalting, r), is generally observed. Forgacs et al. [2] have shown *Presented at the Fourth Symposium on Synthetic Membranes in Science and Industry, sponsored by the European Society of Membrane Science and Technology, Tiibingen, September 6-9, 1983.
I. RUBINSTEIN et al.
56
that this loss in q could be quantitatively correlated to m transport, in an A-membrane. The different phenomena observed at A- and C-membranes strongly suggest that the generation of H’ and m ions does not take place in the unstirred layers, common to both, but in the A-membrane itself. R. Simons [6] suggested that the dissociation of water, and the removal of H+ and m in opposite directions, occur in a thin surface layer of the A-membrane, adjacent to the dialysate. It is caused by reversible protonation of weakly basic groups, namely tertiary amines. --B
+
Hz0 --+-BH++m
&BH+ + HZ0 -Q+H;O Based on the known kinetic constants, Simons showed that this process should be substantially faster than direct dissociation of water. To explainwater-splitting at fully iquatemized membranes Simons further assumed that in the strong electric field established in the surface layer, tertiary groups can be created by decomposition of the quaternary groups; his experiments indicated that this process was likely taking place. Without entering into the quantitative arguments, the most direct test of the basic concept, reversible protonation, would be the performance of an Amembrane carrying only strongly basic groups. In addition, no chemical transformation of these to weakly basic ones should be possible. From a general chemical point of view, alkali metals are excellent candidates for the required positive charges. An A-membrane carrying alkali-ions as ‘fixed’ charges can be prepared by the attachment of uncharged crown ether sites to a neutral matrix. In contact with alkali salt solution, cations are bound and the anions enter the membrane as mobile counter-ions. The alkali ions are of course not irreversibly immobilized. They are exchanged between membrane and solution, and among sites, according to the rate of each reaction. It is however readily seen that only very fast association-dissociation will contribute significantly to a diffusion process.* The average mobility of the bound alkali ions may be assumed to be negligible compared to that of the anion in the membrane. Invasion of free salt is governed by the same distribution equilibria as in conventional ion exchange membranes, and hence permeselectivity may be expected whenever the concentration of the binding sites is larger than the concentration of the external salt solution. For the preliminary study reported here a benzo-crown ether bound to a hydrophobic chain [8] was incorporated into dense cellulose acetate (CA) *According to the Eyring theory of transport phenomena [ 7 ] D = x2 KO. Here D is the diffusion constant ofthe considered diffusion process, x is the distance between the adjacent planes of sites of the order of lo-’ cm, and ZCOir the appropriate reaction rate. A significant contribution of the bound ions to the electrodiffusion process would arise for D of the order of lo-’ - lo@ cm2/swhich would in turn require a very fast diisociation with KO greater in order than 10” 8’ .
ELIMINATION OF ACID-BASE GENERATION
IN ED
57
membranes. When the mixture of CA and complex (ligand + CsBr) was solvent cast and evaporated to dryness, clear fiis were obtained. These proved to be non-ideal anionexchange membranes, as expected. Transport and polarization tests were carried out in a four-compartment cell (Fig. l), with the tested membrane in the middle. Carbon suspension electrodes [9] were used to avoid pH changes by the electrode reactions. The area of the membranes confining the electrode compartments (C-membrane at the cathode and normally, except where stated otherwise, A-membrane at the anode) was 28.3 cm* while the area of the tested membrane varied between 0.8 and 3.1 cm*. No stirring was applied in compartments I and II, so that polarization was limited by natural convection only, and due to the different membrane areas was much more pronounced at the examined membrane. Current was determined as a function of the potential across the tested membrane, measured between a pair of Ag/AgCl electrodes inserted on both sides at a distance of 3.5 cm. The potential thus contains some of the IR drop in the solutions. The plot of I versus V shows a linear region at low voltage, a plateau of varying breadth, and then a further rise of current at higher voltage, reaching a slope similar to the initial one. The current in the flat region is the limiting current. Following this characterization, current was passed through the membrane for l-2 hours, at two current densities: one in the low linear region, and the other above the limiting value. Salt concentrations and pH were measured at start and end of this period. Table I gives the results for different membranes. The coulombic efficiency, 7, refers to desalting of compartment I for the tested A-membranes, and to desalting in II for the C-membrane which was
Fig. 1. Scheme of the cell A-anode, C-cathode, AM and CM-membranes confining the respective anode and cathode compartments, TM - tested membrane.
KCl O.OlN
KCI 0.ol.N
KC1 O.OlN
KCI O.OlN
KCl
Dllysis
Dialysis
Neglnst C
Neginet C
Neginst A fresh
KC1 0.02N
KCI O.OlN
KCI OhlN
NeginstA after paesege of current
Heterogeneous A
Heterogeneow
A
KCI O.OlN
Neginet A after passage of current
O.OlN
KCl O.OlN
KC2 O.OlN
Solution
Dialysis
Tested membrane
2.00
1.50
-
-
0.92 0.92
0.94
0.90
0.90 0.90
0.90
0.95
0.95
8.00
-
2.00 0.60
2.00 0.50
12.00 3.00
2.00 0.60
2.00 0.50
2.00 0.50
2.00 0.60
2.00
fnu9)
7
2.56 0.75
2.55 0.63
3.82 0.95
2.56 0.75
2.55 0.63
2.55 0.63
2.56 0.75
-
-
-
-
;mA/cm2)
1.2
1.0
2.4
1.2
1.2
1.0
1.0
-
-
-
P (mA/cm’)
120 180
120 180
60 120
120 120
120 120
120 180
120 180
120
120
120
120
At(min)
7.20 6.50
7.40 6.75
6.60 6.60 6.65 6.45
6.50 6.45
9.45 6.56
6.46 6.30
7.10 6.80
6.55 6.35
6.15
6.20
6.20
4.75
pH;
6.15 6.46
6.50 6.40
6.25 6.25
6.40 6.60
6.40 6.40
6.80
6.80
6.50
6.80
pHo
3.40 6.08
3.35 6.20
4.40 6.50
3.20 6.20
4.15 6.10
5.60 6.45
5.20 6.18
6.50
6.40
6.30
6.65
pH;
44 93
52 97
63 92
81 94
91 94
-
89 96
-
-
-
-
0 96
Neginet A
Neginet A
Heterogeneous A
NeghtA
Neginet A
Neginst C
Neginrt A
Neginet A
Heterogeneous A
Heterogeneous A
Heterogeneous A
Protective A
WATER SPLITTING AND COULOMBIC EFFICIENCY FOR DIFFERENT CONVENTIONAL MEMBRANES The coulombic efficiency, Q, refers to desalting of compartment I for the tested A-membranes and to that of compartment II for C-membranes, Z stands for the total electric current in the cell, 7 for the counterion transport number of the tested membrane, calculated from the membrane potential i-current density at the tested membrane, 1e&m- limiting current density, determined from the “plateau” of the voltage/current curve, pHe refers to the initial state of solution, pHZrJr - to the final state in the appropriate compartments after time At of passage of current. Protective A refers to the type of membrane confining the anode compartment.
TABLE I
kz
ELIMINATION OF ACID-BASE GENERATION
59
IN ED
checked for comparison. 7 is the transport number of the counter-ion, calculated from the membrane potential measured between 0.01 and 0.1 solution. pHo is the initial value and pH,*+I1,the final ones. The first lines in Table I give results for a dialysis membrane, showing slight pH effects caused by the A-membrane confining the anode-space. (In the electrode-compartments themselves no pH-changes were observed.) The C-membrane results given next were obtained with a C-membrane confining both anode and cathode compartments for pH changes, and with the usual A-membrane at the anode for 7. Finally, water-splitting by the A-membranes is shown in the lower part of Table I. As already pointed out by Simons, the polyethylene-based Neginst membranes give less water-splitting than polystyrene-based ones. Table II shows the corresponding results with the cellulose acetate complex (CAC) membranes. These results show clearly that water-splitting is indeed determined by the character of the charged groups in the anion-exchange membrane. pH-changes obtained at strong polarization near the complexing membranes were very small. The decrease in coulombic efficiency for removal of Cl- ion is small, TABLE II WATER-SPLITTING AND COULOMBIC EFFICIENCY IN CAC MEMBRANES Electrolyte solution was O.OlN CsCl (initial concentration). The rest of notations as in Table I.
Tested membrane
7
I
II
z (mA)
*
ih
At
;mAlcma)
(mA/cm2)
(min)
PHO PH;
PH;
7) (96)
Protective A
0.89
2.00 0.70
2.55 0.89
1.5
125 180
5.67 5.95
6.25 5.67
5.30 6.06
83 89
Heterogeneous A
0.83
2.00 0.60
2.55 0.76
2.2
120 121
6.40 6.45
6.22 6.20
6.10 6.31
90 98
Hetero-
geneous A
III
0.89
8.00 1.50
2.55 0.48
1.5
120 120
6.50 7.25
6.15 6.10
6.45 6.20
64 68
Heterogeneous A
IV
0.92
8.00 1.50
2.55 0.48
1.5
60 180
6.50 6.50
6.35 6.30
6.20 6.88
77 98
Heterogeneous A
V
0.92
8.00 2.00
2.55 0.63
1.6
60 150
6.62 6.62
6.10 6.25
6.22 6.65
90 92
Hetero-
60
I. RUBINSTEIN et al.
but still evident, and is not necessarily accounted for by m transfer. It should be remembered that we are dealing with fixed charges which are not covalently bound, and it is possible that permselectivity decreases at high current density. This and other points will be studied in future with a series of covalently bound ligands, different bound ions, etc. The results shown here demonstrate in principle that water splitting can be avoided by suitable chemistry of the A-membrane. The direct practical advantage with respect to membrane fouling is evident, since local acidity near the membrane enhances precipitation of colloids. A further property of these complexes may help to fight fouling: reversible decomposition takes place at mildly elevated temperature (N60”C) [lo]. The membrane can thus be ‘discharged’, removing the anchoring points for eleo trostatic adsorption.
ACKNOWLEDGEMENT
This work was supported in part by the National Council for Research and Development.
REFERENCES 1. K.S. Spiegler, Desalination, 9 (1971) 367. 2. C. forgacs, N. Ishibashi, J. Leibovitn, J. Sinkovic and K.S. Spiegler, Desalination, 10 (1972) 181-214. 3. B.A. Cook, Electrochim. Acta, 3 (1961) 307. 4. A.J. Mackai and J.C.R. Turner, JCS Faraday Transactions I, 74 (1978) 2860. 5. I. Rubinstein and L. Shtihnan, JCS Faraday Transactions II, 75 (1979) 281. 6. R. Simons, Nature, 280 (30) (1979) 824-826; Desalination, 28 (1979) 41-20. 7. S. Glasstone, K.J. Laidler and H. Byring, The Theory of Rate Processes, McGraw-Hill, New York, NY, 1941, p. 519. 8. A. Warshawsky and N. Kahana, Polymeric crown ethers for the extracting of alkali cations, Report to Israel Chemicals Ldt., November 1981, Rehovot. 9. 0. Kedem, J. Cohen, A. Warshawsky and N. Kahana, Desalination, 46 (1983) 291299. 10. A. Warshawsky and N. Kahana, J. Amer. Chem. Sot., 104 (1982) 2663-2664.