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ION-EXCHANGE SEPARATIONS STEPHEN S. WINTER Northeastern University, Boston, Massachusetts
The physical chemical principles that are involved in reactions of ion-exchange resins (1, 2) can be applied in a number of different ways in order to effect analytical separations. It is possible to group these techniques into a set of distinct processes: removal of ions from solution by absorption,1 separation of ions by absorption, separation by selective desorption of ions from the resin, removal of ions by ion-exclusion, and electrochemical separations by ion-exchange membranes. The present paper discusses the principles that are involved in each of these processes. The chief physical chemical considerations that govern ion-exchange reactions are that: (1) ions tend to concentrate in the resin phase, and (2) exchanges between ions are equilibrium processes, with equilibrium constants that do not differ greatly from unity. The first of these implies that an ion within the resin phase will not readily leave the resin unless it is displaced by other ions. Ion-exchange columns can therefore be rinsed with pure solvents without leaching the ions. The second factor limits the extent to which ions can be displaced from the resin by contact with an electrolyte solution. Batchwise contact displaces only To obtain complete exchange, a fraction of the ions. the solution must flow through beds of resin in which contact between fresh portions of the resin and the solution is continuously established. The absorption processes are thus similar to those occurring in chromatographic columns; hence the frequently used term, ion-exchange chromatography.
pletely removed, leaving water with a smaller content of ionic impurities than is normally obtained by distillation (3, 4)- The complete removal of ions from solution has found many analytical uses as well: determination of total salt concentration (5a), removal of interfering anions in cation analysis (5b), concentration of trace constituents (5c).
QUANTITATIVE ABSORPTION OF IONS FROM
SOLUTION
The first extensive use of ion exchangers was the treatment of hard water by zeolites which replace the calcium and magnesium ions by sodium ions. As synthetic organic resins became available, it became possible to replace all cations by hydrogen ions, and all anions by hydroxyl. Thus, the electrolytes are com1 Because ion-exchange processes occur within the resin domain, the term absorption seems preferable to the frequently used adsorption.
The principles upon which the absorption of an ionic species from solution rest are illustrated by the following hypothetical experiment. A solution containing a sodium salt flows over a bed of cationic resin in the hydrogen form. At the beginning of the experiment the resin particles are surrounded by distilled water. The salt solution displaces the distilled water in the downward direction. To keep track of the changes that take place as the solution flows through the column, the resin bed is divided into a large number of small zones (Figure 1) which are lettered consecutively from top to bottom. The solution is divided into small-volume portions, each of which exactly fills the interstitial volume between the resin particles contained in one zone. The volume portions are numbered in the order in which they enter the resin bed. For simplicity, the concentration of the solution is adjusted so that the number of sodium ions in each volume portion exactly equals the number of hydrogen ions bound to the resin in each zone. It is assumed that both sodium ions and hydrogen ions
473
JOURNAL OF CHEMICAL
474
have equal affinity for the resin,
so that the absorption will favor neither ion but will be determined equilibrium solely by their respective concentrations. As the solution flows downward through the column, first enters Zone A, then B, volume portion Zone C, etc. As Volume 1 leaves Zone A, it is followed by Volume 2. This is in turn replaced by Volume 3. The exchanges that occur during this downward flow are the following (see the table): Volume 1 in Zone A. The resin contains no sodium ions, the solution no hydrogen ions. Exchange occurs until the two are distributed equally between the two phases. The solution that leaves Zone A, therefore, has exchanged half of its sodium ions for protons. Volume 1 in Zone B. This fresh portion of resin contains no sodium ions. Exchange occurs until both ions are equally distributed between the resin and the solution, so that when Volume 1 of the solution leaves Zone B it contains only one-fourth of the original 1
sodium ions. Volume 1 in Zone C. The solution again flows into fresh resin which contains no sodium ions. Exchange therefore further decreases the sodium ion concentration of the solution. This process recurs as the solution flows downward through the resin until the first volume portion becomes essentially depleted of sodium ions. Volume 2 in Zone A. The resin already contains one-half sodium ions from the previous exchange, but the solution that enters contains no hydrogen ions. Exchange, therefore, decreases the sodium ion concentration of the solution. Only three-fourths of the sodium ions remain in the solution. Volume 2 in Zone B. The resin contains one-fourth sodium ions, the solution three-fourths sodium ions. After exchange, each phase contains equal amounts. Although Volume 2 encounters resin that already contains some sodium ions, as it flows into the lower zones its sodium ion concentration is always greater than that of the resin it encounters so that the exchange removes It too becomes gradually some of the sodium ions. depleted of its sodium ions. The exchanges that occur with other volume portions are similar, and have been calculated for the first few volume portions in the table. In each case, the volume portions flow into resin zones having a smaller sodium ion content than the solution. Exchange, therefore, reduces the sodium ion concentration of the solution during each contact. The complete absorption of cations can thus be thought of as the result of a large number of stepwise exchanges (6).
EDUCATION
The only limitation to this depletion through exchange is the capacity of the resin. As the upper zones become saturated with sodium ions, only the lower zones exchange until the entire ion-exchange column is converted to the Na+ form. The assumptions made about the concentration of the solution and the affinity of the resin in no way restrict. this general pattern of behavior during exchange. As a matter of fact, because of the tendency of ions to concentrate within the resin phase, the fraction of the ions within the resin will generally be greater than onehalf, thus speeding up the absorption. From the trend of the hypothetical concentrations as shown in the table it can be seen that after a considerable amount of the solution has passed through the resin, the upper zones have completely exchanged and contain only sodium ions, the lowest zones remain unexchanged, and there is an intermediate region, in which the ion content of the resin varies between these two extremes. As additional solution enters the resin, this intermediate region gradually moves downward through the column, until its capacity is nearly exhausted. At this point the ion concentration of the effluent changes and begins to contain small amounts of sodium ions which passed through the resin without exchanging. As more solution passes through the resin, the sodium ion content of the effluent gradually increases, until, after saturation, the solution passes through unchanged. This behavior is shown in Figure 2b, in which the relative concentrations of hydrogen ions and sodium ions in the effluent are plotted against the volume of solution that has passed through the resin. Such curves are known as break-through curves. They reflect the concentrations of the two ionic species in the various zones of the resin (Figure 2a) as given in the table. Break-through curves give the capacity of the resin for the ion absorbed by showing the amount of the solution that must pass through the column before the ion leaves the column unexchanged. Furthermore, the steepness of the curve in the intermediate region is related to the exchange equilibrium. The larger the distribution ratio of the ion between the resin and the solution, the sharper is the “S” in the curve. The shape of the break-through curves is governed not only by the equilibrium distribution of the ion between the solution and the resin phase, but also by the rate at which the equilibria are attained during exchange. Sharper separations are generally obtained by slow flow rates, long and narrow columns, and small resin particles (5d).
TABLE
1
Fraction of Ions in External Solution After Exchange Exchange
Volume
1
Volume 8
zone
Na+
H+
Na+
H+
Na+
Zone A Zone B Zone C
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Zone# Zone E
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Vs
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Vs
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Volume 3
H+
Na+
Vs
‘7:6 ’Vis
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Volume 4
H+
Nar
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VOLUME 33, NO. 9, SEPTEMBER, 1956
475
DIRECT SEPARATION OF IONS
to
In the separation of several ions from one another by ion-exchange processes, the affinities of the various ions for the resin must be considered in addition to the equilibria described in the previous section. A detailed examination of the various steps is much more difficult in this case, because of the several concurrent equilibria. But one can obtain a satisfactory picture from qualitative considerations. The solution that flows into Zone A of Figure 1 may contain two or more ions that absorb to different extents, for instance, equinormal amounts of calcium and ammonium ions (5d). Both of these ions replace some of the protons from the resin, but the more strongly absorbed calcium ions displace a greater number than do the ammonium ions. As before, the primary process is the exchange of the ions from the solution for those of the resin. Superimposed upon this, however, is the unequal absorption of the two ions.
When the second volume portion enters Zone A further exchange takes place. Protons are again displaced unequally. In addition, the more strongly absorbed calcium ions may also displace some of the ammonium ions from the column. Thus there is absorption of both ions from the solution with the displacement of hydrogen ions, and a concurrent partial displacement of ammonium ions. Both of these processes continue as additional volume portions enter Zone A, and similar exchanges occur in the lower zones, with the result that the solution becomes depleted of calcium ions in the upper part of the column. Conversely, the upper zones of the exchanger contain mostly calcium ions. Because all of the calcium ions are absorbed in the upper sections of the column, only hydrogen ions and ammonium ions reach lower zones. There, the displacements are those described in the previous section. Below the nearly pure calcium ion part of the exchanger there is a region containing only ammonium ions, and below it, the unexchanged resin (Figure 3a). Separating these are regions in which the concentration gradually changes from one ion to the other. Because separation through absorption is only partially effective, the region of transition between the calcium and ammonium ions reaches far into the calcium ion
Figure (a) Ion content
2.
Ion Concentration
of resin.
(£>)
During Replacement
Break-through
curve.
another by altering the technique as follows. The inflow of solution is stopped before the resin capacity is fully used—often long before that point is reached—and the column is rinsed with a solution that contains another ion. The purpose of the rinse solution is to replace the ions already on the column gradually and selectively. The ions of the rinse solution therefore pass from the exchanger together with the ions to be separated, and must not interfere with their later identification. Acids of various strengths are thus suitable for the desorption of cations and are generally used. The action of the ions of the rinse solution is simply displacement. They replace in part the ions already on the exchanger. If an acid solution is added to the resin described in the previous section, the hydrogen ions displace the calcium ions to a small extent, the ammonium ions nearly completely. Thus in the upper portions of the column, both the hydrogen ions from one
zone.
As additional solution flows through the ion exchanger, the five regions spread downward. A breakthrough curve as shown in Figure 36, which reflects the ion content of the resin, is obtained. The steepness of the breaks in that curve-again gives information about the relative affinities of the ions for the resin. If the resin is properly chosen so that the affinities differ sufficiently, nearly complete separation can be effected although some overlap of the ions to be separated cannot be avoided altogether. SELECTIVE DESORPTION
It is possible to improve the separation of ions from
Figure 3.
Separation of Ions
in
an
Exchange Column (After Samuelson
(5c/))
(a) Ion content
of resin.
(6) Break-through
curve.
JOURNAL OF CHEMICAL EDUCATION
476
Figure
4.
Electrochemical
Separation by
cations; filled circles Open circles attached to membrane walls. =
=
art
Ion-exchange Membrane
anions.
Bound ions
are
shown
the rinse solution and the calcium ions that were displaced by them tend to push the ammonium ions ahead of them. This results in the concentration of nearly all of the ammonium ions in the lower part of the column. It also results in a more complete separation of the ammonium ions from the calcium ions. The ammonium ions therefore pass from the ionexchange column as a distinct zone free of calcium ions. The latter follow in the effluent after additional rinsing. By the proper choice of acid concentration in the rinse solution, it is sometimes possible to displace only the less strongly absorbed ions to any considerable extent. This is the case when the absorbed ions differ greatly in their affinity for the resin, and the affinity of the rinse ion is near that of the less strongly absorbed In this case one can use several concentrations one. of the rinse ion to selectively replace each kind of ions absorbed on the column. The displacement of the ions from the ion-exchange column by a rinse solution has been used in many of the separations that can be found in the literature (7, 8). The scope of this technique, particularly when displacing solutions of various concentrations are used, is illustrated by the successful separations of such similar substances as amino acids (9) and the borate complexes of several sugars (10). ION EXCLUSION
The methods described so far are useful in separating ionic species for analytical purposes. They make use of the different absorbing powers of ions, and, in the ease of the concentration of trace amounts, of the fact that ions are absorbed whereas neutral molecules are not. In all these separations, the resin is in the acid form at the beginning and must be returned to that state by an appropriate treatment, before another separation can be carried out. The amount of material that can be separated is limited by the capacity of the resin. For large-scale industrial use of ion-exchange resins, the limited capacity and the expense of regeneration of the resin are serious drawbacks. Another technique has therefore been devised to separate ions from molecules (11). Resin particles absorb solvent molecules as well as
ions (1). The former diffuse into the resin structure and solvate the resin granules, whereas the latter are bound by electrostatic forces to the ionic groups of the resin. If the solvent surrounding the resin particles is changed, exchange occurs between the molecules of the different solvents in a manner similar to the exchange of ions. Thus, if a glycerine solution flows through an ion-exchange column that is solvated with water molecules, the glycerine replaces part of the water within the resin particles. This factor is used in the technique called ion exclusion. The salt-containing glycerine solution is passed through a column which is saturated with the salt impurity and is solvated by water. Since the resin is already saturated with the ions of the glycerine solution, no exchange takes place between ions. The salt remains in the external solution, and thus flows through the ion-exchange column. But the glycerine diffuses into the resin and replaces the water solvating the resin particles. The exchange that takes place thus causes the retention of the molecular glycerine, whereas the ionic impurities pass from the column. After the column becomes saturated with glycerine, the column is rinsed with pure water. This reverses the solvent exchange so that the water replaces the glycerine within the resin particles. The glycerine flows from the column free of ionic impurities. As soon as the wash cycle is completed, the column is ready for another separation. This method thus avoids the use of expensive reagents as regenerants, and is not limited by the concentration of ionic groups in the resin for its exchange capacity. Large amounts of ionic impurities can be displaced as easily as small amounts. Because of these features, the method seems commercially feasible where separations involving the absorption and desorption of ions are too expensive. ION-EXCHANGE MEMBRANES
Ion-exchange resins cast into sheets or membranes offer still another method for carrying out separations, which, however, is distinct from the processes described before in that it does not use beds of granular resins. The technique depends upon the mobility of ions in the resin sheet under the influence of an externally imposed electric field. Transference of current through ion-exchange membranes occurs almost exclusively by the flow of counterions (12), since the ions of the opposite polarity are chemically bound to the resin2 and cannot move. In the process illustrated in Figure 4, in which a salt solution is contained between anionic- and cationicexchange membranes under the influence of an electric field, only the positive ions travel through the cationic resin, and only the anions through the anionic resin. As a result, if the salt contained in the central compartment is sodium chloride, it can be converted Ion exchangers also contain small amounts of free ions of the polarity as that of the bound ions which may carry a limited fraction of the current, especially when the concentration of electrolyte in the surrounding solution is large. 2
same
VOLUME 33, NO. 9, SEPTEMBER, 1956
into sodium hydroxide and hydrochloric acid by such
477
734(1951). (5) Samuelson, O., “Ion Exchangers in Analytical Chemistry,” John Wiley & Sons, Inc., New York, 1953, (a) Chap. VIII, (b) Chap. IX, (c) Chap. XI, (d) Chap. V. (6) Martin, A. J. P., and R. L. M. Synge, Biochem. J., 35, 1358(1941). (7) Rieman, W., Ill, J. Chem. Educ., 31, 212 (1954). (8) Reviews appear frequently in Ann. Rev. Phys, Chem. For instance, Boyd, G. K, 2, 309 (1951); W. C. Bauman, R. E. Anderson, and R. M. Wheaton, 3, 109 (1952); W. Juda, J. A. Marinsky, and N. W. Rosenberg, 4, 373 (1953); J. Schubert, 5, 413 (1954). (9) Moore, S., and W. H. Stein, J. Biol. Chem., 192, 663 (1951). (10) Khym, J. X., and E. P. Zill, J. Am. Chem. Soc., 73, 2399 (1951). (11) Wheaton, R. M., and W. C. Bauman, Ind. Eng. Chem., 45,228(1953). (12) Jenny, H., T. R. Nielsen, N. T. Coleman, and D. E. Williams, Science, 112, 164 (1950). (13) Winger, A. G., G. W. Bodamer, R. Kunin, C. J. Prizer, and G. W. Harmon, Ind. Eng. Chem., 47, 50 (1955).
a
cell.
number of such cells in of electrodes with a set series, using only single in the of current amount saving required. The number of cells so arranged is limited chiefly by the high resistance of the membranes to electrical conductance. The problem is to prepare membranes thin enough for good conductance, and yet mechanically strong. Such membranes are a fairly recent development, so that they are not yet widely used. Their successful application in the desalting of sea water has been reported (18).
It is possible to connect
a
a
LITERATURE CITED (1) Winter, S. 8., J. Chem. Educ., 33, 246 (1956). (2) Tompkins, E. It., J. Chem. Educ., 26, 32, 92 (1949). (3) Reents, A. C., and F. H. Kahler, Ind. Eng. Chem., 43, 730(1951). (4) Kunin, R., and F. X. McGarvey, Ind. Eng. Chem., 43,
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