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Ion-exchange foam chromatography
Arttrl_~~kcr Chirtticct Acta 68 ( 1974) 1 19-I 30 Q Elscvicr Scicntilic Publishing Company. Amsterdam
ION-EXCHANGE
FOAM
Ncthcrlands
ON HETEROGENEOUS
CATION-
and A. B. FARAG
Itisrirfrte 01’ Iww~gtiuic mtl (Nitrigcrry) (Rcccivcd
in The
CHROMATOGRAPHY
PART II. RAPID SEPARATIONS EXCHANGE FOAMS*
T. BRAUN
119 - Printcd
6th July
Atltrlyrictrl
Chwislry,
L. l5iiri~ii.s Uuirer.vity.
P.O. Bos
123.
I443
i3lldtlpcsr
1973)
During the last few years, there has been considerable interestz.3 in preparing ion-exchange materials with reduced solid-phase mass transport in order to increase the rate of exchange processes. It has been realized’ that the application of very small resin beads, which have a low resistance .to mass transport, allows a rapid equilibrium with the mobile phase. However, it is necessary to employ forced flow to attain reasonable flow-rates in column operations. The speed and efficiency of ion-exchange separations have been grcntly increased by the development2.5 of packings which consist of a thin layer of ion-exchange resin coated on the surface of a solid-core support (pellicular ion exchangers). This type of resin has been shown to be extremely useful in reducing solid-phase mass transport resistances and thus accelerating the exchange processes. However, the capacity of these pellicular resins is extremely low, and chromatographic systems based on these materials are limited to very small amounts of solute. The preparation of a superficially sulphonated, highly cross-linked exchange resin on styrege-divinylbenzene base has also been described 3*6 . Again, the application of these resins allowed the exchange equilibrium to be attained rapidly, but their capacities are several orders of magnitude smaller than that of common exchange resins. In a recent paper’ various methods have been tested for the preparation of homogeneous and heterogeneous ion-exchange foams. Homogeneous ion-exchange foams were prepared by introducing ion-exchange groups on previously prepared phenol-formaldehyde, polyurethane and polyethylene foams. Heterogeneous ion-exchange foams were prepared by foaming a fine powder of a commercially available cation exchanger with the precursors of open-cell polyether-type polyurethane foam. Preliminary investigations’ on heterogeneous ion-exchange foams showed that the kinetics of the exchange processes is fast and the capacity is quite suitable. In previous work’-lo on “foam chromatography”, i.e. the application of foams as chromatographic column fillings, the hydrodynamic properties and the kinetics of partition on foam-packed chromatographic columns proved to be extremely favourable. It was decided to investigate the applicability of the recently * For Part I xc ref. 1.
120
T. BRAUN.
described flexible ion-exchange foams in quick column separations. of the exchange processes and the selectivity of foam ion exchanger in comparison with the normal ion-exchange beads.
A. B. FARAG
The kinetics were studied
EXPERIMENTAL
CFFlCl r11aterici1.s All reagents used were of analytical grade unless otherwise specified. Copper, zinc and cadmium solutions were prepared by dissolving the sulphate salts in water. Iron and calcium chloride solutions were prepared by dissolving the chloride salts in 0.03 M hydrochloric acid and distilicd water, respectively. Polyurethane-Varion KS heterogeneous ion-exchange foam was prepared as previously described I. The foam material contained 40% (w/w) cation exchanger. The ion-exchange resin used was the powdered Varion KS sulphonated polystyrene cation exchanger of 0.3-W mm particle original diameter (Nitrokemia, Fi.izfdgyhrtelep, Ftizf??, Hungary). Rrage?rts
Apparatus Culum~~s. Glass columns of 15 mm diameter and 15 cm long were used. A separating funnel was fitted at the top of the column as a reservoir of the eluting solution. Counter. For activity measurement, a Nai(T1) detector and an energyselective counting device (type NK-107/B, Gamma, Budapest, Hungary) were employed. The polyurethane-Varion KS heterogeneous ion-exchange foam (cubes of about 5 mm edge) was washed several times with water and allowed to stand with it overnight. The water was poured off and the foam material was shaken 3 times with 3 .M hydrochloric acid solution. The foam material was washed with a large portion of water, and after decantation, it was shaken with 1 M sodium hydroxide solution for 3 h and again washed several times with water. Finally, the foam material was shaken with 2 M hydrochloric acid solution to change it to the H+- form, and then washed with distilled water. The waterwetted foam exchanger was packed in the column by the procedure previously developeds. Experiments at controlled temperatures were done with water-jacketed columns. Distribution ratios The distribution ratios were determined for 5 mg of the elements in 50 ml of solution and 0.2 g of resin beads or foam in the *,H-form which had been dried at 80” in a drying oven. After equilibration for 6 h at room temperature in a mechanical shaker, the ion-exchange beads or foams were separated from the aqueous phase by filtration and the amount of the elements on the beads or foam were determined by suitable analytical methods. From the results, the distribution ratios
ION-EXCHANGE
D =
FOAM
CHROMATOGRAPHY
amount of element in resin amount of element in solution
PART II.
121
ml of solution * g of dry resin
were calculated. Determinution of the rate of exchange of copper by ion-exchange &am In 15 stoppered flasks of 25-ml capacity, S-ml portions of water were added to 0.1 g of washed, dry foam. The flasks were shaken at room temperature in a mechanical shaker for 4 h, and then 5 ml of 64Cu-labelled copper sulphate solution (1 mg Cu2+ ml- ‘) were added to each flask. The flasks were then shaken for different times (I-90 min) and copper(H) was determined in the aqueous phase by measuring the radioactivity of 2 ml of each solution. Elrrtiorl of copper( II) by hydroxyanlmorlirrnt chloride Copper(I1) sulphate solution (I ml of a 5 mg Cuzf ml -l solution) was used as the feed. Sorption took place at a flow-rate of 2 ml min- l and then the column was washed with 20 ml of distilled water. A 0.5 M hydroxyammonium chloride solution was used to elute copper at flow-rates of 2-10 ml min- ‘. Separation of ctrdmiuni, zinc, irori arul cctlciwn The feed solution contained 1 ml of cadmium sulphate (5 mg Cd2+ ml- ‘), 0.5 ml of zinc sulphate (10 mg Zn2+ ml-‘), 1 ml of iron(II1) chloride (5 mg Fe’+ ml-‘) and 1 ml of calcium chloride (5 mg Ca2+ ml-‘) solutions. The mixture was allowed to pass through the foam ion-exchange column (bed height ca. 12 cm, 5 g of dry foam) at a flow-rate of 2 ml min-’ and the column was then washed with 20 ml of distilled water. The elements were then eluted in the following sequence: 200 ml of 0.2 M hydrochloric acid in 40% ethanol for cadmium(II), 150 ml of 0.2 M hydrochloric acid in 8O”/0ethanol for zinc(II), 150 ml of 1.0 M hydrochloric acid in 80% ethanol for iron(III), and 150 ml of 2.0 M hydrochloric acid in 20”/, ethanol for calcium. All metal of an appropriate radiometrically. RESULTS
AND
ions were titrated with 0.01 M EDTA solution in the presence indicator. Copper when present in trace amounts was measured
DISCUSSION
Although the results obtained by applying columns packed with pellicular resins2”, in which a solid core is surrounded by a thin film of cross-linked ionexchange material, and with superficially sulphonated resins3*6 are very promising, yet the low capacity of these resins remains a serious limitation. Many investigators, e.g. Pinfold and Karger 4, have shown that the exchange processes on finely pulverized ion exchangers proceed substantially faster thn the processes on bigger beads of the same composition. As previously mentioned’, the use of fine grains causes difficulties even in the “batch technique*‘, since they form colloidal suspensions. An acceptable solution for this problem was found by the
I-. BRAUN.
122
A. B. FARAG
successful preparation of heterogeneous ion-exchange foams in which finely ground commercial ion-exchange resin was built into a polyurethane foam matrix of an open-cell polyether type. The mechanical properties of this ion-exchange foam were found’ to be the same as those of loam containing no ion-exchange resin. Furthermore, the distribution of the exchanger grains was proved’ to be uniform. Several questions have to be answered before this ion-exchange foam can be used for analytical purposes: (1) should the ion-exchange foam be homogeneously packed to produce sharp break-through curves‘?: (2) will the ionexchange process be fast enough to permit the application of high flow-rates?; (3) will the selectivity of the ion exchanger be affected?; and (4) does the swelling of the foam support limit the application of foam ion exchanger in water-miscible organic solvents? In order to answer these and -other questions, two separation models were investigated in detail. The first model was the separation of copper(H) by aqueous hydroxyammonium chloride solution I ‘. Previous studies8-r0 on foam chromatography showed that the vacuum technique of packing was the most suitable for foam column packing. One of the advantages of this method is that it allows the application of a high flow-rate simply by gravity flow. This method of column packing was therefore used again.
Fig. 1. Brmk-through capacity curve copper sulphatc. I mg Cu” ml-l; of the exchange foam. I.8 mcq. g- *.
of coppcr(lI) flow-rate,
on cation-exchange foam. Follm weight, I g; i&d: I ml mine’ ;tt room tcmpcraturc. Total capcity
The break-through capacity of a column packed with 1 g of cationexchange foam was determined by passing copper(M) sulphate solution (1 mg Cu2+ ml-‘) through the column at a 1 ml mine-’ flow-rate. A plot of percent copper in the effluent US. effluent volume is shown in Fig. 1. The sharp slope of the curve indicates that the column is homogeneously packed and also that . the exchange equilibrium of copper(H) on the ion-exchange fdam is attained rapidly.
ION-EXCNANGE
FOAM CH ROMATOGRAPI-IY
PART Il.
123
The rate of sorption of copper(Il)‘by ion-exchange foam The rate of sorption of copper by the ion-exchange foam is fast. The half-life of equilibrium sorption, calculated from the curve of Fig. 2, is 0.6 mini It is clear from the curves that sorption of copper(I1) takes place in ‘one .step, i.e. gel diffusion is not the rate-controlling step as in the case of common ionexchange beads. The two-step sorption previously reported on ion-exchange foams’ was probably due to the use of coarser ion-exchange powder in foam preparation.
Fig. 2, Rate of sorption of coppcr(I1) on ion-exchange foam at room temperature, [Cu2*]. S mg.
It is interesting to note that the half-life of equilibrium sorption on the present ion-exchange foam is exactly the same as that obtained’ in the case of the first rapid step on surface-sulphonated pearl polymer. However, the capacity of the proposed ion-exchange foam is much higher than that of the superficially sulphonated resin. These results suggest the application of ion-exchange foam for rapid separations in column operation, The effect of how-rate on the elution of copper~I~~ by 0.5 M ~lytlroxy~~lmo~liurn chloride solution Flow-rates ranging between 2 and 10 ml min” were applied at SO”. The etution curves are shown in Fig. 3. A slight increase in ,peak width witli increased llow-rate occurs but the elution of copper is quantitative even at flowrates as high as 10 ml min -I. These results agree well with the previously discussed .kinetic results obtained in batch experiments.
124
T. BRAUN.
A. B. FARAG
3 s
MO-
, 15.0 -
loo-
so-
0
Fig. 3. Effect of flow-rate on the clution of coppcr(il) with 0.5 M hydroxyammoniurn chloride solution from a foam column .of 12-cm bed height at 50”. (a) At 2 ml mine’: (b) ;lt 6 ml min-‘: (c) at 10 ml min-‘. Fig. 4. Effccl of flow-rate on solution at SO”. (a) Ion-cxchangc
the clution of copper(I1) with 0.5 M hydroxyammonium foam column; (b) ion-cxchunge bcod column.
chloride
A comparison between the elution of copper(H) on ion-exchange resin beads (Varion KS) and on ion-exchange foam columns was also carried out. Figure 4 shows the elution curves obtained at 2, 4 and 6 ml min -I for columns packed with resin beads and foam under otherwise identical experimental conditions. The elution of copper(H) from the bead columns is greatly affected by flow-rate and quantitative elution is obtained only at a flow-rate of 2 ml min- ‘. Worth mentioning is that complete retention and elution of microgram quantities of copper(H) is possible on foam-filled columns. Figure 5 shows the elution curve obtained for 0.6”peCu spiked copper at 50” with a flow-rate of 2 ml min-‘. Comparative selectivity of cation-exchange foums and beds in HCl-ethanol mixtures A systematic investigation was next undertaken to determine the selectivity of the cation-exchange foam in comparison with the ion-exchange beads (Varion KS). In order to carry out this study, initial experiments were conducted to
ION-EXCHANGE
FOAM
CHROMATOGRAPHY
PART
II.
125
(320.1
s
151
1O.f
51
0.c Effluent , ml Fig. 5. Elution of microgram amount of coppcr(II) (0.6 pg) with 0.5 M hydroxyammonium on a foam column of 12-cm bed height at 50” with a 2 ml min-’ flow-rate.
chloride
determine the distribution ratios of a set of elements with the conventional cation-exchange beads and the cation-exchange foam. The cation-exchange distribution ratios of cadmium(H), zinc(II), iron(II1) and calcium(H) were measured in a wide range of hydrochloric acid-ethanol mixtures. The curves of Fig. 6 represent plots of the distribution ratios as a function of hydrochloric acid concentration in the absence and presence of ethanol at different concentrations. The set of curves in Fig. 7 show plots of the distribution ratios us. ethanol concentration at various hydrochloric acid strengths. As may be seen from these figures, hydrochloric acid and ethanol have more or less the same general effect on the distribution ratios of the elements tested. However, the results obtained for the distribution ratios on the foam are generally shifted to slightly lower values. As is evident from the curves, the selectivity seems to be about the same as that of the original cation-exchange beads. This was further confirmed by the results obtained for the separation of a synthetic mixture of cadmium, zinc, iron and calcium on foam columns. It was found that these metal ions are eluted by hydrochloric acid-ethanol mixtures in the same order as that reported by Strelow et al . I2 for AG SOW-X8 sulphonqted polystyrene cation-exchange resin (100-200 or 200400 mesh particle size). Cadmium(H) was eluted with 0.2 M hydrochloric
4 MO
EmH
I
20%
EIOH
I to %
FOAM ElOH
60%
t3OH
D n /.
EIOH
90%
Eloli
L-Jb
I1
1.0
05
N.
HCI
l3EADS
Fig. 6. Cadmium. zinc. iron and copper distribution dimerent ethanol concentrations. for foam and bead cu* +.
ratios columns.
acid concentration PS. hydrochloric (a) Cd’+; (b) Zn*+; (c) Fe’+;
at (d)
10
(b)
EtOH%
,.
10
5
10’
10
5
10’
10
5
10’
10
5
2
I&H%
Fig. 7. Cadmium, zinc, iron and copper distribution ratios vs. ethyl alcohol hydrochloric acid strengths for foam and bead columns. (a) Cd2+; (b) Zn2+:
concentration at various (c) Fe’* ; (d) Ca’+.
T. BRAUN.
A. B. FARAG
a ,b
Fig. 8. Elution curve for Cd(II)-Zn(lI)-Fc(III)-Cn(II) of 12-cm bed height at room tcmpcraturc. Fig. 9. Elution curves for Ion-exchange foam column
u flow-rate
mixture
calcium(ll) with 3 M hydrochloric at a flow-rate of IO-12 ml min-‘:
on a cation-cxchangc
foam
column
acid in 20% ethanol solution. (b) ion-exchange bcud column
(a)
nt
of 2 ml rnin- *.
acid in 4o’i/, ethanol, zinc(I1) ‘with 0.2 M hydrochloric acid in 80’;/, ethanol, iron(II1) with 1.0 M hydrochloric acid in 80% ethanol, and calcium with 2.0 M hydrochloric acid in 202, ethanol (Fig. 8). The elution of these elements was quantitative at room temperature with flow-rates of 3-7 ml min- ‘. It is worth mentioning that calcium(H) was quantitatively eluted from foam columns with 3 M hydrochloric acid in 20”/, ethanol at a flow-rate of lo-12 ml min-’ (Fig. 9). ‘It was found that the elution curve of calcium at this relatively high flow-rate is sharper than the elution curve obtained with the conventional cation-exchange bead column (Varion KS, 0.3-0.5 mm diameter) at a flow-rate of 2 ml min- l, under otherwise the same experimental conditions (Fig. 9). As is obvious from these results, cation-exchange foam columns seem to work more efliciently than the common cation-exchange bead columns in watermiscible organic solvents also.
ION-EXCHANGE
FOAM
CHROMATOGRAPHY
PART II.
129
CONCLUSIONS
The results of this investigation indicate that cation-exchange foam can be homogeneously packed in columns by the previously developed” vacuum method of packing. The kinetic processes on the linely grained cation exchanger supported on the polyurethane foam are extremely fast and are not affected by the foaming process. Gel-diffusion has no part in the exchange equilibrium. The selectivity of the cation-exchange foam is similar to that of the original ion-exchange beads. The application of cation-exchange foam columns for separations in aqueous solutions and water-miscible organic solvents seems to have advantages over both conventional ion-exchange and superficially coatedte5 or sulphonated3 exchange resins. One of the most obvious advantages of the cation-exchange foam is its reasonable capacity and high efficiency. There are no special requirements for sample size; amounts of sample which are commonly used in conventional ionexchange bead columns can be successfully separated on foam columns. Work is in progress on the preparation of anion-exchange foams and their application for quick separation. Grateful acknowledgement is made to Dr. K. Khdar, Mr. 0. BCkeffy and the North Hungarian Chemical Works, Sajobabony, Hungary for the preparation of the heterogeneous ion-exchange foams used in this work. SUMMARY
The possibility of using polyurethane-Varion KS heterogeneous cationexchange foam for rapid separations in aqueous and alcoholic solutions was investigated. The exchange equilibria on the ion-exchange foam were attained rapidly and the break-through capacity of columns packed with the foam material was quite acceptable. Relatively high flow-rates could be applied without any appreciable loss in column performance. With foam columns, microgram and milligram amounts of copper could be retained and eluted quantitatively with hydroxyammonium chloride solution. The overall capacity of the examined ionexchange foams was about 40”/0 of the original capacity of beads. Cationexchange distribution coefficients of cadmium(H), zinc(H), iron(II1) and calcium(H) were determined for foam material and conventional bead exchanger; the selectivity of both was about the same.
On examine les possibilites d’utilisation de l’echangeur de cations, heterogene mousse polyurCthane-Varion KS, pour des separations rapides dans des solutions aqueuses et alcooliques. L’equilibre d’echange est atteint tres vite. A l’aide des colonnes de mousse, des quantites de cuivre, de l’ordre du micro- et du milligramme peuvent Qtre retenues et Cluees quantitativement au moyen dune solution de chlorhydrate d’hydroxylammonium. On examine Cgalement la capacite de ces mousses, de m&me que leur selectivite.
130
T. BRAUN.
A. B. FARAG
ZUSAMMENFASSUNG
Die Anwendbarkeit von heterogenem Polyurethan-KationenaustauschcrSchaum (Varion KS) fiir schnelle Trennungen in w%srigen und alkoholischen Lijsungen wurde untersucht. Die Austauschgleichgewichte am IonenaustauscherSchaum stellten sich schnell ein, und die Durchbruch-KapazitPt der mit dem Schaummaterial gefiillten SPulen war annehmbar. Relativ hohe Fliessgeschwindigkeiten konnten ohne Verminderung der Stiulenleistung angewendet werden. Mit Schaumtiulen konnten Mikrogrammund Milligramm-Mengen von Kupfer zuriickgehalten und mit Hydroxylammoniumchlorid-L6sung quantitativ eluiert werden. Die Gesamt-Kapazitgt der untersuchten Ionenaustauscher-Schiiume war etwa 40% der urspriinglichen Kapazitiit von Perlen. Die Kationenaustausch-Verteilungskoefflzienten von Cadmium( II), Zink( II), Eisen( III) und Calcium( II) wurden fiir Schaum-Material und fiir k’onventionellen Perlen-Austauscher bestimmt; die Selektivitgt von beiden war etwa dieselbe. REFERENCES 1 Part
I. T. Braun.
0.
BCkcmy.
1. Haklits.
K.
Khdhr
and
G.
Majoros.
An&
C/Jim. Acta,
( 1973) 45.
2 3 4 5 6 7 8 9 10 11 12
J. J. Kirkland, J. Chronurogr. Sci., 7 (1969) 361. M. Skafi and K. H. Licser. Z. Ar~ol. Chetn., 249 (1970) 182. T. A. Pinfold and B. L. Karger. Separ. Sci.. 5 ( 1970) 183. G. G. Horvath, B. A. Preiss and S. R. Lipsky. Anal. CYw~tt., 39 (1967) 1422. M. Skali and K. H.-Licser, Z. natal. Chefn.. 250 ( 1970) 306; 251 (1970) 177. T. Braun and A. B. Farag, Tahro. 19 (1972) 828. T. Braun a,nd A. B. Farag, Anal. C/Jim Acre, 61 ( 1972) 265. T. Braun. E. Husi?iir and L. Bakos. Amd. Chim. Acra. 64 (1973) 77. T. Braun, A. B. Farag and A. Klimcs-Szmik, Anal. Chim. Acra. 64 (1973) 71. Y. Shigctomi, R. Arimoto and T. Nagahota, Talanrrr, 19 (1972) 1210. F. W. E. Strclow. C. R. Van Zyl and C. J. C. Bothma, Atd. C/rim Acra. 45 (1969)
81.
64
ION-EXCHANGE
FOAM
Ncthcrlands
ON HETEROGENEOUS
CATION-
and A. B. FARAG
Itisrirfrte 01’ Iww~gtiuic mtl (Nitrigcrry) (Rcccivcd
in The
CHROMATOGRAPHY
PART II. RAPID SEPARATIONS EXCHANGE FOAMS*
T. BRAUN
119 - Printcd
6th July
Atltrlyrictrl
Chwislry,
L. l5iiri~ii.s Uuirer.vity.
P.O. Bos
123.
I443
i3lldtlpcsr
1973)
During the last few years, there has been considerable interestz.3 in preparing ion-exchange materials with reduced solid-phase mass transport in order to increase the rate of exchange processes. It has been realized’ that the application of very small resin beads, which have a low resistance .to mass transport, allows a rapid equilibrium with the mobile phase. However, it is necessary to employ forced flow to attain reasonable flow-rates in column operations. The speed and efficiency of ion-exchange separations have been grcntly increased by the development2.5 of packings which consist of a thin layer of ion-exchange resin coated on the surface of a solid-core support (pellicular ion exchangers). This type of resin has been shown to be extremely useful in reducing solid-phase mass transport resistances and thus accelerating the exchange processes. However, the capacity of these pellicular resins is extremely low, and chromatographic systems based on these materials are limited to very small amounts of solute. The preparation of a superficially sulphonated, highly cross-linked exchange resin on styrege-divinylbenzene base has also been described 3*6 . Again, the application of these resins allowed the exchange equilibrium to be attained rapidly, but their capacities are several orders of magnitude smaller than that of common exchange resins. In a recent paper’ various methods have been tested for the preparation of homogeneous and heterogeneous ion-exchange foams. Homogeneous ion-exchange foams were prepared by introducing ion-exchange groups on previously prepared phenol-formaldehyde, polyurethane and polyethylene foams. Heterogeneous ion-exchange foams were prepared by foaming a fine powder of a commercially available cation exchanger with the precursors of open-cell polyether-type polyurethane foam. Preliminary investigations’ on heterogeneous ion-exchange foams showed that the kinetics of the exchange processes is fast and the capacity is quite suitable. In previous work’-lo on “foam chromatography”, i.e. the application of foams as chromatographic column fillings, the hydrodynamic properties and the kinetics of partition on foam-packed chromatographic columns proved to be extremely favourable. It was decided to investigate the applicability of the recently * For Part I xc ref. 1.
120
T. BRAUN.
described flexible ion-exchange foams in quick column separations. of the exchange processes and the selectivity of foam ion exchanger in comparison with the normal ion-exchange beads.
A. B. FARAG
The kinetics were studied
EXPERIMENTAL
CFFlCl r11aterici1.s All reagents used were of analytical grade unless otherwise specified. Copper, zinc and cadmium solutions were prepared by dissolving the sulphate salts in water. Iron and calcium chloride solutions were prepared by dissolving the chloride salts in 0.03 M hydrochloric acid and distilicd water, respectively. Polyurethane-Varion KS heterogeneous ion-exchange foam was prepared as previously described I. The foam material contained 40% (w/w) cation exchanger. The ion-exchange resin used was the powdered Varion KS sulphonated polystyrene cation exchanger of 0.3-W mm particle original diameter (Nitrokemia, Fi.izfdgyhrtelep, Ftizf??, Hungary). Rrage?rts
Apparatus Culum~~s. Glass columns of 15 mm diameter and 15 cm long were used. A separating funnel was fitted at the top of the column as a reservoir of the eluting solution. Counter. For activity measurement, a Nai(T1) detector and an energyselective counting device (type NK-107/B, Gamma, Budapest, Hungary) were employed. The polyurethane-Varion KS heterogeneous ion-exchange foam (cubes of about 5 mm edge) was washed several times with water and allowed to stand with it overnight. The water was poured off and the foam material was shaken 3 times with 3 .M hydrochloric acid solution. The foam material was washed with a large portion of water, and after decantation, it was shaken with 1 M sodium hydroxide solution for 3 h and again washed several times with water. Finally, the foam material was shaken with 2 M hydrochloric acid solution to change it to the H+- form, and then washed with distilled water. The waterwetted foam exchanger was packed in the column by the procedure previously developeds. Experiments at controlled temperatures were done with water-jacketed columns. Distribution ratios The distribution ratios were determined for 5 mg of the elements in 50 ml of solution and 0.2 g of resin beads or foam in the *,H-form which had been dried at 80” in a drying oven. After equilibration for 6 h at room temperature in a mechanical shaker, the ion-exchange beads or foams were separated from the aqueous phase by filtration and the amount of the elements on the beads or foam were determined by suitable analytical methods. From the results, the distribution ratios
ION-EXCHANGE
D =
FOAM
CHROMATOGRAPHY
amount of element in resin amount of element in solution
PART II.
121
ml of solution * g of dry resin
were calculated. Determinution of the rate of exchange of copper by ion-exchange &am In 15 stoppered flasks of 25-ml capacity, S-ml portions of water were added to 0.1 g of washed, dry foam. The flasks were shaken at room temperature in a mechanical shaker for 4 h, and then 5 ml of 64Cu-labelled copper sulphate solution (1 mg Cu2+ ml- ‘) were added to each flask. The flasks were then shaken for different times (I-90 min) and copper(H) was determined in the aqueous phase by measuring the radioactivity of 2 ml of each solution. Elrrtiorl of copper( II) by hydroxyanlmorlirrnt chloride Copper(I1) sulphate solution (I ml of a 5 mg Cuzf ml -l solution) was used as the feed. Sorption took place at a flow-rate of 2 ml min- l and then the column was washed with 20 ml of distilled water. A 0.5 M hydroxyammonium chloride solution was used to elute copper at flow-rates of 2-10 ml min- ‘. Separation of ctrdmiuni, zinc, irori arul cctlciwn The feed solution contained 1 ml of cadmium sulphate (5 mg Cd2+ ml- ‘), 0.5 ml of zinc sulphate (10 mg Zn2+ ml-‘), 1 ml of iron(II1) chloride (5 mg Fe’+ ml-‘) and 1 ml of calcium chloride (5 mg Ca2+ ml-‘) solutions. The mixture was allowed to pass through the foam ion-exchange column (bed height ca. 12 cm, 5 g of dry foam) at a flow-rate of 2 ml min-’ and the column was then washed with 20 ml of distilled water. The elements were then eluted in the following sequence: 200 ml of 0.2 M hydrochloric acid in 40% ethanol for cadmium(II), 150 ml of 0.2 M hydrochloric acid in 8O”/0ethanol for zinc(II), 150 ml of 1.0 M hydrochloric acid in 80% ethanol for iron(III), and 150 ml of 2.0 M hydrochloric acid in 20”/, ethanol for calcium. All metal of an appropriate radiometrically. RESULTS
AND
ions were titrated with 0.01 M EDTA solution in the presence indicator. Copper when present in trace amounts was measured
DISCUSSION
Although the results obtained by applying columns packed with pellicular resins2”, in which a solid core is surrounded by a thin film of cross-linked ionexchange material, and with superficially sulphonated resins3*6 are very promising, yet the low capacity of these resins remains a serious limitation. Many investigators, e.g. Pinfold and Karger 4, have shown that the exchange processes on finely pulverized ion exchangers proceed substantially faster thn the processes on bigger beads of the same composition. As previously mentioned’, the use of fine grains causes difficulties even in the “batch technique*‘, since they form colloidal suspensions. An acceptable solution for this problem was found by the
I-. BRAUN.
122
A. B. FARAG
successful preparation of heterogeneous ion-exchange foams in which finely ground commercial ion-exchange resin was built into a polyurethane foam matrix of an open-cell polyether type. The mechanical properties of this ion-exchange foam were found’ to be the same as those of loam containing no ion-exchange resin. Furthermore, the distribution of the exchanger grains was proved’ to be uniform. Several questions have to be answered before this ion-exchange foam can be used for analytical purposes: (1) should the ion-exchange foam be homogeneously packed to produce sharp break-through curves‘?: (2) will the ionexchange process be fast enough to permit the application of high flow-rates?; (3) will the selectivity of the ion exchanger be affected?; and (4) does the swelling of the foam support limit the application of foam ion exchanger in water-miscible organic solvents? In order to answer these and -other questions, two separation models were investigated in detail. The first model was the separation of copper(H) by aqueous hydroxyammonium chloride solution I ‘. Previous studies8-r0 on foam chromatography showed that the vacuum technique of packing was the most suitable for foam column packing. One of the advantages of this method is that it allows the application of a high flow-rate simply by gravity flow. This method of column packing was therefore used again.
Fig. 1. Brmk-through capacity curve copper sulphatc. I mg Cu” ml-l; of the exchange foam. I.8 mcq. g- *.
of coppcr(lI) flow-rate,
on cation-exchange foam. Follm weight, I g; i&d: I ml mine’ ;tt room tcmpcraturc. Total capcity
The break-through capacity of a column packed with 1 g of cationexchange foam was determined by passing copper(M) sulphate solution (1 mg Cu2+ ml-‘) through the column at a 1 ml mine-’ flow-rate. A plot of percent copper in the effluent US. effluent volume is shown in Fig. 1. The sharp slope of the curve indicates that the column is homogeneously packed and also that . the exchange equilibrium of copper(H) on the ion-exchange fdam is attained rapidly.
ION-EXCNANGE
FOAM CH ROMATOGRAPI-IY
PART Il.
123
The rate of sorption of copper(Il)‘by ion-exchange foam The rate of sorption of copper by the ion-exchange foam is fast. The half-life of equilibrium sorption, calculated from the curve of Fig. 2, is 0.6 mini It is clear from the curves that sorption of copper(I1) takes place in ‘one .step, i.e. gel diffusion is not the rate-controlling step as in the case of common ionexchange beads. The two-step sorption previously reported on ion-exchange foams’ was probably due to the use of coarser ion-exchange powder in foam preparation.
Fig. 2, Rate of sorption of coppcr(I1) on ion-exchange foam at room temperature, [Cu2*]. S mg.
It is interesting to note that the half-life of equilibrium sorption on the present ion-exchange foam is exactly the same as that obtained’ in the case of the first rapid step on surface-sulphonated pearl polymer. However, the capacity of the proposed ion-exchange foam is much higher than that of the superficially sulphonated resin. These results suggest the application of ion-exchange foam for rapid separations in column operation, The effect of how-rate on the elution of copper~I~~ by 0.5 M ~lytlroxy~~lmo~liurn chloride solution Flow-rates ranging between 2 and 10 ml min” were applied at SO”. The etution curves are shown in Fig. 3. A slight increase in ,peak width witli increased llow-rate occurs but the elution of copper is quantitative even at flowrates as high as 10 ml min -I. These results agree well with the previously discussed .kinetic results obtained in batch experiments.
124
T. BRAUN.
A. B. FARAG
3 s
MO-
, 15.0 -
loo-
so-
0
Fig. 3. Effect of flow-rate on the clution of coppcr(il) with 0.5 M hydroxyammoniurn chloride solution from a foam column .of 12-cm bed height at 50”. (a) At 2 ml mine’: (b) ;lt 6 ml min-‘: (c) at 10 ml min-‘. Fig. 4. Effccl of flow-rate on solution at SO”. (a) Ion-cxchangc
the clution of copper(I1) with 0.5 M hydroxyammonium foam column; (b) ion-cxchunge bcod column.
chloride
A comparison between the elution of copper(H) on ion-exchange resin beads (Varion KS) and on ion-exchange foam columns was also carried out. Figure 4 shows the elution curves obtained at 2, 4 and 6 ml min -I for columns packed with resin beads and foam under otherwise identical experimental conditions. The elution of copper(H) from the bead columns is greatly affected by flow-rate and quantitative elution is obtained only at a flow-rate of 2 ml min- ‘. Worth mentioning is that complete retention and elution of microgram quantities of copper(H) is possible on foam-filled columns. Figure 5 shows the elution curve obtained for 0.6”peCu spiked copper at 50” with a flow-rate of 2 ml min-‘. Comparative selectivity of cation-exchange foums and beds in HCl-ethanol mixtures A systematic investigation was next undertaken to determine the selectivity of the cation-exchange foam in comparison with the ion-exchange beads (Varion KS). In order to carry out this study, initial experiments were conducted to
ION-EXCHANGE
FOAM
CHROMATOGRAPHY
PART
II.
125
(320.1
s
151
1O.f
51
0.c Effluent , ml Fig. 5. Elution of microgram amount of coppcr(II) (0.6 pg) with 0.5 M hydroxyammonium on a foam column of 12-cm bed height at 50” with a 2 ml min-’ flow-rate.
chloride
determine the distribution ratios of a set of elements with the conventional cation-exchange beads and the cation-exchange foam. The cation-exchange distribution ratios of cadmium(H), zinc(II), iron(II1) and calcium(H) were measured in a wide range of hydrochloric acid-ethanol mixtures. The curves of Fig. 6 represent plots of the distribution ratios as a function of hydrochloric acid concentration in the absence and presence of ethanol at different concentrations. The set of curves in Fig. 7 show plots of the distribution ratios us. ethanol concentration at various hydrochloric acid strengths. As may be seen from these figures, hydrochloric acid and ethanol have more or less the same general effect on the distribution ratios of the elements tested. However, the results obtained for the distribution ratios on the foam are generally shifted to slightly lower values. As is evident from the curves, the selectivity seems to be about the same as that of the original cation-exchange beads. This was further confirmed by the results obtained for the separation of a synthetic mixture of cadmium, zinc, iron and calcium on foam columns. It was found that these metal ions are eluted by hydrochloric acid-ethanol mixtures in the same order as that reported by Strelow et al . I2 for AG SOW-X8 sulphonqted polystyrene cation-exchange resin (100-200 or 200400 mesh particle size). Cadmium(H) was eluted with 0.2 M hydrochloric
4 MO
EmH
I
20%
EIOH
I to %
FOAM ElOH
60%
t3OH
D n /.
EIOH
90%
Eloli
L-Jb
I1
1.0
05
N.
HCI
l3EADS
Fig. 6. Cadmium. zinc. iron and copper distribution dimerent ethanol concentrations. for foam and bead cu* +.
ratios columns.
acid concentration PS. hydrochloric (a) Cd’+; (b) Zn*+; (c) Fe’+;
at (d)
10
(b)
EtOH%
,.
10
5
10’
10
5
10’
10
5
10’
10
5
2
I&H%
Fig. 7. Cadmium, zinc, iron and copper distribution ratios vs. ethyl alcohol hydrochloric acid strengths for foam and bead columns. (a) Cd2+; (b) Zn2+:
concentration at various (c) Fe’* ; (d) Ca’+.
T. BRAUN.
A. B. FARAG
a ,b
Fig. 8. Elution curve for Cd(II)-Zn(lI)-Fc(III)-Cn(II) of 12-cm bed height at room tcmpcraturc. Fig. 9. Elution curves for Ion-exchange foam column
u flow-rate
mixture
calcium(ll) with 3 M hydrochloric at a flow-rate of IO-12 ml min-‘:
on a cation-cxchangc
foam
column
acid in 20% ethanol solution. (b) ion-exchange bcud column
(a)
nt
of 2 ml rnin- *.
acid in 4o’i/, ethanol, zinc(I1) ‘with 0.2 M hydrochloric acid in 80’;/, ethanol, iron(II1) with 1.0 M hydrochloric acid in 80% ethanol, and calcium with 2.0 M hydrochloric acid in 202, ethanol (Fig. 8). The elution of these elements was quantitative at room temperature with flow-rates of 3-7 ml min- ‘. It is worth mentioning that calcium(H) was quantitatively eluted from foam columns with 3 M hydrochloric acid in 20”/, ethanol at a flow-rate of lo-12 ml min-’ (Fig. 9). ‘It was found that the elution curve of calcium at this relatively high flow-rate is sharper than the elution curve obtained with the conventional cation-exchange bead column (Varion KS, 0.3-0.5 mm diameter) at a flow-rate of 2 ml min- l, under otherwise the same experimental conditions (Fig. 9). As is obvious from these results, cation-exchange foam columns seem to work more efliciently than the common cation-exchange bead columns in watermiscible organic solvents also.
ION-EXCHANGE
FOAM
CHROMATOGRAPHY
PART II.
129
CONCLUSIONS
The results of this investigation indicate that cation-exchange foam can be homogeneously packed in columns by the previously developed” vacuum method of packing. The kinetic processes on the linely grained cation exchanger supported on the polyurethane foam are extremely fast and are not affected by the foaming process. Gel-diffusion has no part in the exchange equilibrium. The selectivity of the cation-exchange foam is similar to that of the original ion-exchange beads. The application of cation-exchange foam columns for separations in aqueous solutions and water-miscible organic solvents seems to have advantages over both conventional ion-exchange and superficially coatedte5 or sulphonated3 exchange resins. One of the most obvious advantages of the cation-exchange foam is its reasonable capacity and high efficiency. There are no special requirements for sample size; amounts of sample which are commonly used in conventional ionexchange bead columns can be successfully separated on foam columns. Work is in progress on the preparation of anion-exchange foams and their application for quick separation. Grateful acknowledgement is made to Dr. K. Khdar, Mr. 0. BCkeffy and the North Hungarian Chemical Works, Sajobabony, Hungary for the preparation of the heterogeneous ion-exchange foams used in this work. SUMMARY
The possibility of using polyurethane-Varion KS heterogeneous cationexchange foam for rapid separations in aqueous and alcoholic solutions was investigated. The exchange equilibria on the ion-exchange foam were attained rapidly and the break-through capacity of columns packed with the foam material was quite acceptable. Relatively high flow-rates could be applied without any appreciable loss in column performance. With foam columns, microgram and milligram amounts of copper could be retained and eluted quantitatively with hydroxyammonium chloride solution. The overall capacity of the examined ionexchange foams was about 40”/0 of the original capacity of beads. Cationexchange distribution coefficients of cadmium(H), zinc(H), iron(II1) and calcium(H) were determined for foam material and conventional bead exchanger; the selectivity of both was about the same.
On examine les possibilites d’utilisation de l’echangeur de cations, heterogene mousse polyurCthane-Varion KS, pour des separations rapides dans des solutions aqueuses et alcooliques. L’equilibre d’echange est atteint tres vite. A l’aide des colonnes de mousse, des quantites de cuivre, de l’ordre du micro- et du milligramme peuvent Qtre retenues et Cluees quantitativement au moyen dune solution de chlorhydrate d’hydroxylammonium. On examine Cgalement la capacite de ces mousses, de m&me que leur selectivite.
130
T. BRAUN.
A. B. FARAG
ZUSAMMENFASSUNG
Die Anwendbarkeit von heterogenem Polyurethan-KationenaustauschcrSchaum (Varion KS) fiir schnelle Trennungen in w%srigen und alkoholischen Lijsungen wurde untersucht. Die Austauschgleichgewichte am IonenaustauscherSchaum stellten sich schnell ein, und die Durchbruch-KapazitPt der mit dem Schaummaterial gefiillten SPulen war annehmbar. Relativ hohe Fliessgeschwindigkeiten konnten ohne Verminderung der Stiulenleistung angewendet werden. Mit Schaumtiulen konnten Mikrogrammund Milligramm-Mengen von Kupfer zuriickgehalten und mit Hydroxylammoniumchlorid-L6sung quantitativ eluiert werden. Die Gesamt-Kapazitgt der untersuchten Ionenaustauscher-Schiiume war etwa 40% der urspriinglichen Kapazitiit von Perlen. Die Kationenaustausch-Verteilungskoefflzienten von Cadmium( II), Zink( II), Eisen( III) und Calcium( II) wurden fiir Schaum-Material und fiir k’onventionellen Perlen-Austauscher bestimmt; die Selektivitgt von beiden war etwa dieselbe. REFERENCES 1 Part
I. T. Braun.
0.
BCkcmy.
1. Haklits.
K.
Khdhr
and
G.
Majoros.
An&
C/Jim. Acta,
( 1973) 45.
2 3 4 5 6 7 8 9 10 11 12
J. J. Kirkland, J. Chronurogr. Sci., 7 (1969) 361. M. Skafi and K. H. Licser. Z. Ar~ol. Chetn., 249 (1970) 182. T. A. Pinfold and B. L. Karger. Separ. Sci.. 5 ( 1970) 183. G. G. Horvath, B. A. Preiss and S. R. Lipsky. Anal. CYw~tt., 39 (1967) 1422. M. Skali and K. H.-Licser, Z. natal. Chefn.. 250 ( 1970) 306; 251 (1970) 177. T. Braun and A. B. Farag, Tahro. 19 (1972) 828. T. Braun a,nd A. B. Farag, Anal. C/Jim Acre, 61 ( 1972) 265. T. Braun. E. Husi?iir and L. Bakos. Amd. Chim. Acra. 64 (1973) 77. T. Braun, A. B. Farag and A. Klimcs-Szmik, Anal. Chim. Acra. 64 (1973) 71. Y. Shigctomi, R. Arimoto and T. Nagahota, Talanrrr, 19 (1972) 1210. F. W. E. Strclow. C. R. Van Zyl and C. J. C. Bothma, Atd. C/rim Acra. 45 (1969)
81.
64