California Association of Chemistry Teachers
H. F. Walton University of Colorado Boulder
Ion Exchange in Analytical Chemistry
Analytical chemistry begins wheu a chemist asks, "What's in it?" If the "it" is at all cornplicated, like a rock or part of an animal or plant, the analytical chemist must he ready to use a wide variety of physical and chemical techniques to answer his question. Sooner or later he will have to perform chemical separations. To take only one example, let us consider the deternrinat,iouof nlinor elements in a rock. Table 1 shows the average concerrtration of certain elements in the rocks of the earth's crust, along with the estimated h i t s of spectrographic detect,ion. Evidently, nlany of these elelneuts could not he detected, let alone quantitatively det,ermined,without preliminary separation.
of opposite charge are mobile and can be exchanged for other ions of similar charge by contact with solutio~is cont,ainingthese ions. For exchange t,o take place at a reasonable speed the ion exchanger must have an open stxucl.ure, one which is porous on the molecular scale. Most analytical work is done with ion-exchange resins based on polystyrene. Styrene is copolymerized with divinylbenzene to give spherical heads that are then treated chen~icallyto introduce ionic groups. To make a cation exchanger the polymer is sulfonated to give a s h c t u r e like this:
Table 1 ( 1 )
Element
Spectrographic detection limit (PP~)
Ag B Bi Co
0. 5
10 20 10 10 5 5 5
La Ma Ni Pb Sn Zn
10 (100)
Crustal abundance (PP~) 0.08 3 0.2 20 18
1 35 15 2 40
To achieve this separation Ahrens, Edge and Brooks (1) used a combination of solvent extraction, cation ex-
change, and anion exchange. Their separation schenie is unusually comprehensive, hut it is typical of many that have been worked out for complex mixtures, both inorganic and organic, over the past ten years. "Ion exchange" is the reversible exchange of iorrs of like sigu het,ween solutions and granular solids called i a exchangers. Because these mat,erials can be used in colunrns, even small differences in selectivity can he made t,o yield useful separations. Ion Exchanging Materials
To be an iou exchanger, a material must have ionic groups attached to an insoluble framework. The ions Based on leetnres given at the Sixth Annual Summer Conference on Recent Advmres in Chemistry, s~onsoredbv the CACT.
This material swells in water and is freely permeable; ot,her positive ions can diffuse in and replace the hydrogen ions. To make an anion exchanger the group -CHI-I\'(CH&+Clor -CH2-X(CH&C2H40H +Clis introduced. These types of exchanger are used in the great majority of analytical applications. Other resins contain the chelating group, -CH2N(CHr COOH)%, and the phosphoric group, -OPO(OH)l. Resins containing the -COOH group are made by hydrolyzing crosslinked polymethyl methacrylate. These resins are available wit,h different degrees of crossliuking. The crosslinking is usually expressed as the proportion of divinylbenzene in t,he monomer mix. Most comn~oulya crosslinking of 8% is used for the sulfonic acid cation exchangers and quaternary a~nine anion exchangers based on polystyrene. Higher crosslinkiug gives more ions per unit volunie; lower crosslinking allows more space for large ions to diffuse in and out. The interior of t,he resin bead resenibles a drop of concentrated electrolyte solution, whose concentration is 6-8 molal in an 8% crosslinked resin. A different type of exchanger is made from cellulose fibers by introducing ionic groups. These are on the outside of the fibers and, therefore, accessible to very large ions such as those of polypeptides and proteins. Even bacteria have been separat,ed on columns of ulodified cellulose (8). Yet another type of exchanger is inorganic in nature, based on the hydrous oxides of elenleuts of the fourth, Volume 42, Number 2, February 1965
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fifth and sixth groups of the periodic table. The commonest of these is zirconium phosphate, a cation exchanger. These materials are precipitated from aqueous solutions and are gritty, granular materials whose particles are aggregates of microcrystals. They exchange ions extremely rapidly with aqueous solutions and show very marked selectivity among the alkali and alkaline earth metal ions (3). lon-exchange Selectivities
For the exchange of an ion A in solution with an ion B attached to an exchanger R, we can write an equation and define a "selectivity quotient"
where the bars indicate the exchanger phase. Ideally, Q is independent of the proportion of A to B, but in practice this is not so, because the activity coefficients of AR and BR vary in a complex manner with exchanger composition. If the ions A and B are of unequal charge, the distribution depends on solution concentration. Increasing dilution forces the ion of higher charge into the exchanger, as one expects from the mass action law. By choosing appropriate standard states, usually the pure homoionic resins AR and BR, one can evaluate thermodynamic equilibrium constants. A selection of these is plotted in Figures 1 and 2. From Figure 1 we note the increase of selectivity with crosslinking, and the fact that the greater the hydration of the cation, the more weakly it is bound by the exchanger. This is not the case with carboxylic cation-exchange resins. One theory to account for this is that the electrostatic field of the -C02- ionis more intense than that of the S O s - ion and that it removes part of the water of hydration from Na+ and K+, making Na+ now the smaller ion (normally, in aqueous solution, hydrated Na+ is larger than hydrated K+) and, therefore, more strongly held by the exchanger.
Flgure 1. Selectivity constants for rulfonated polystyrene cation-exchange resins, referred to U+; see Bonner and Smith, J. Phyr Chem., 61, 326 (1 957). Cradlnkings as shown.
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Comparing Figures 1and 2, we note the much greater selectivity found in anion exchange. We shall later see very great selectivity effects with the anion exchange of metal chloride complexes. Many factors are involved in ion exchange selectivity. One factor that must be borne in mind is that the polystyrene resin matrix has the character of an "organic solvent." This becomes especially clear when dealing with ions containing aromatic rings. The benzylammonium cation, CsHsCH2NH3+,is far more strongly held by a polystyrene sulfonic acid cation exchanger than the cations of aliphatic amines of similar molecular weight. In general, however, ion exchange is not a particularly selective process in itself. To achieve maximum selectivity, ion exchange is combined with complex ion formation. This is done in the cation-exchange separation of the rare earths, where buffered citrate of EDTA solutions are used as eluting agents. lon-exchange Columns: Practical Details
Ion exchangers are nearly always used in columns. As a solution passes down an exchanger column it continually meets fresh unreacted exchanger, so that the equilibrium is displaced in the direction one wants. By this multi-stage effect, even an unfavorable equilibrium can give exchange which is complete' within the limits of analytical detection. The column must, of course, be long enough and it must be carefully packed to avoid "channelling" or irregular flow. A good design for analytical work is shown in Figure 3. The funnel a t the top serves not only for pouring in solutions but also to receive the resin when it is "backwashed" before use. A stream of water is driven upward through the column, driving the resin into the upper part of the column. This releases air bubbles and classifies the resin beads so that they settle uniformly, the larger beads a t the bottom and the smaller a t the top, with spheres of roughly uniform size in any segment of the tube. For exacting chromatographic separations, one must start with particles carefully screened to narrow size limits.
Figure 2. Selectivity conrtanb for quaternary bare anion-exchange resins referred to CI-i see Gmgor, Belle, and Mars-, I . Am. Chem. Soc.,
77, 2731 (1 9551.
For general analytical use, particles of 50-100 mesh are best. For difficult chromatographic separations a smaller size, say 200400 mesh, should be chosen. The finer the particles, of course, the greater is the resistance to flow. A common mistake is to use a column that is too big. For most analytical work a resin bed 1 cm in diameter and 15 cm deep is suitable. This volume of polystyrene sulfonic acid resin contains about 20 milliequivalents of ions. Longer columns are needed in difKcult separations. The column must never be allowed to drain; if it does, it must be backwashed to remove the air. A "goose-neck" outlet tube is shown in Figure 3; this ensures that the solution level will not fall below the top of the bed. Commercial-grade resins are seldom pure enough to be directly used inanalytical work. They containmetallic impurities and,even more important, organic impurities, including incompletely polymerized material, that leak slowly in use. This is especially true of quaternary amine anion exchangers. These must be washed r e peatedly with acid, alkali, and methanol before use. The best solution for the analyst is to purchaseprepurified resins from supply houses such as the Bio-Rad Corn.. Richmond. CaliforFigure 3. Ion-exchange column. nia, or ~ ~ l l i n kChemi~ ~ & The "gwre-nect" mn b e omitted calWorks, St. Louis. These i f surface tension will prevent the resins need only brief washcolumn from draining. ing to be ready for use. &
,
Elution Chromatogmphy: Basic Theory
In elution chromatography a small amount of substance (for example, sodium ions) is adsorbed a t the top of a column and is gradually displaced down the column by passing an appropriate "eluent" or displacing solution (for example, dilute hydrochloric acid). If the concentration of adsorbed substance in the "effluent," the solution emerging from the column, is plotted against the volume passed, a curve like Figure 4 is obtained. If the quantity of adsorbate is very small compared to the total capacity of the column, the curve approaches the shape of the standard curve of error. If two adsorbed substances are present which travel down the column at different rates, two distinct concentration peaks are obtained, and the two substances are thereby separated. There is a very simple relation between the volume, V,,,',.,, needed to elute the peak of the band and the distribution ratio of adsorbate between the exchanger and the solution. I t can be stated a8
sorbate in the resin) to (adsorbate in the free solution) in any given resin segment. The derivation assumes that D is independent of the proportion of adsorbate on the resin, and this will be strictly true only if the proportion is infinitesimally small. But the relation is accurate enough for practical use. The peak volume, V,., depends only on the distribution coefficient and not on the flow rate. The width of the band is seen from Figure 4. The ratio (6v/Vm.3 depends inversely on the square root of the number of theoretical plates, N. The "theoretical plate" is a concept borrowed from fractional distillation. Its thickness is an experimental parameter, a number which expresses the closeness of approach to equilibrium as the solution flows past the adsorbent. True equilibrium is never reached; changes in resin composition always lag behind changes in solution composition. To handle this delay mathematically we divide the column into imaginary segments called "theoretical plates" and suppose that the solution flowing into any given plate mixes completely with the resin in this "plate," and comes to equilibrium with it, before discharging into the next "plate," where it comes to equilibrium all over again. The narrower the "theoretical plates," the closer the approach to equilibrium. The thickness of the theoretical plates has been calculated by Glueckauf (4) and others from diffusion coefficients, bead size, distribution coefficient, and flow rate. The relations are complex and cannot be discussed in detail here. Under the conditions most often used in ion exchange chromatography, however, where diffusion within the beads is the rate-controlling step, the plate thickness is proportional inversely to the flow rate and directly to the square of the particle radius. I t cannot, however, be smaller than the particle diameter, and under typical operating conditions it is about 100 times the particle diameter. The number N in Figure 4 can be increased by decreasing the flow rate and the particle size and by increasing the length of the column. The resolution between bands, however, increases as the square root of N. To improve separations by a factor of two we must make the column four times as long, with other conditions, including flow rate, remaining the same. The flow rates normally employed range from 0.2 to 2 ml per square centimeter of column area per minute.
Conc.
,
,.
I I
"
I
t
I ,,nil(
where Vloid = the void volume of the column (that is, the volume of free solution in between the resin beads), and D = a distribution coefficient = ratio of (ad-
Effluent Volume, v
Flgure 4. Shape of ideal elution band with inflniterimal quantity of adsorbed material. The quontlty e is the bore of natural logarithmr.
Volume 42, Number 2, February 1965
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Simple Ion-exchange Separations: Exchanging Ionic Partners
ions from boiler water, traces of heavy metals from milk, and traces of copper from lubricating oils.
The simplest analytical separations made by ion exchange are separations of ions of opposite charge. One cannot, of course, pull posit,ive ions away from negative ions, but one can give them different partners. For example, one may wish to know the total electrolyte concentration of an aqueous solution, such as a sample of natural water or a salt brine. One simply passes the solution through a short column of strongacid cation-exchange resin in the hydrogen form. The cations of the solution are exchanged for their equivalent of hydrogen ions, which can be determined by titration with standard base. There are a number of sensitive analytical met,hods for anions in which many cations interfere. An example is the determination of traces of dissolved fluoride ions by the color of the complex formed with cerium(111) ions and "alizarin complexone." Many metal ions affect the test because they associate with the reagent. For most natural watersit suffices to pass the sample through a column of strong-acid cation-exchange resin before making the test. Another example is the titration of sulfate with barium perchlorate solution in aqueous isopropanol, using an adsorption indicator. Again, t,he solution should be passed through a column of hydrogen-ion resin before titrating.
Anion Exchange of Metal Chloride Complexes
The Concentration of Traces
Short columns of ion exchangers are sometimes used to collect trace elements from large volumes of solutions. To do this the exchanger must have considerable selectivity for the element or ion sought, or else the solution must contain few competing ions. As examples of high selectivity one may cite the use of a chelatiog resin (5) to concentrate traces of iron and manganese from salt brines or caustic soda (which is neutralized with hydrochloric acid before passing through the exchanger) and the use of a carboxylic resin to concentrate traces of copper from sea water. Anion-exchange resins are used to collect traces of gold from cyanide or chloride solutions. Many examples could be cited to show the use of ion exchange for concentrating ions from dilute solutions, such as silicate
N HCI Figure 5.
Anion ox&ange of meld-&bride mmplexer. ldistribution coefficient); abxiua, concentration of HCI.
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Journal o f Chemical Education
Ordinate, log
One of the most powerful ways to use ion exchange in inorganic analysis is as anion exchange in hydrochloric acid solutions (6). Most metals form chloride complexes, and these differ greatly in stability and in their ability to be adsorbed by polystyrene-type anionexchange resins. For some metals, such as gold, the distribution coefficient is as high as lo6. I t depends on the hydrochloric acid concentration, generally rising with the concentration as the anionic con~plexesare stabilized, then falling again a t high hydrochloric acid concentrations. For each metal there is a characteristic curve; examples are given in Figure 5. The only metal ions that are not held by quaternary anmepolystyrene exchangers are those of the alkali and alkaline earths, Ala+, La3+, Th4+, and (strangely enough) NiZ+. It is thus possible to devise a large number of separation schemes in which a group of metals is adsorbed on a resin from concentrated hydrochloric acid, then each metal is eluted in turn by progressively lowering the acid concentration. The separation of iron(III), cobalt, and nickel is a well-known example which makes a good lecture demonstration because of the colors. Iron and cobalt are strongly absorbed from 9M hydrochloric acid while nickel passes through the column; cobalt is driven out by 4-GM acid, then iron by 1M acid. (We mention again the need for using carefully washed, pre-purified anion-exchange resin; see above.) The method has found use in the systen~aticanalysis of rocks (with concentration of trace metals) and of complex alloys. We emphasize again that the stability of the anionic complexes in aqueous solution is not the only factor determining the distribution coefficients. The "nonaqueous solvent" character of the resin is a t least as important. The uptake of iron(III), for example, is far stronger than the very low stability of I'eCI4in aqueous solution would suggest, and closely parallels the extraction of iron by ether and other polar solvents. Binding of Orgonic Complex-Formers; Ligand Exchange
One of the earlier analytical applications of ion exchange was the separation of sugars and other polyhydroxy compounds by adsorption on columns of anion-exchange resins loaded with borate ions. The sugars formed neetively-charged borate complexes and could be separated by fractional elution. Another application was the separation of aldehydes by adsorption on anion exchangers loaded with hisulfite ions. A new development that uses the same principle is the separation of amines by selective adsorption on cation-exchange resins carrying ions such as NiZ+or Cut+. These ions are held on the resin in the form of their ammonia complexes. When an anline is introduced a t the top of the colun~n,its molecules displace ammonia molecules and coordinate with the metal ions. If an aqueous ammonia solution is now passed down the column, it displaces the amine dowuward while the metal ions stay bound to the resin. By this technique of "ligand exchange," mixtures of anlines can be separated (7). The method is specially promising with diamines. The selectivity order depends only to a
nrinor extent on the stabilities of the niet,al-amine con~plexes in aqueous solutions; the %on-aqueous solvent" character of the resin is very evident. Entirely different selectivity orders are obtained with a polystyrene sulfonic acid resin, a polyacrylic acid resin, and a zirconium phosphate cation exchanger, even though all three exchangers are loaded with the same metal ions. Figure 6 shows the elution sequence of four amines on a nickel-loaded polystyrene sulfonic acid exchanger; the least polar and most hydrocarbon-like, n-butylamine, is eluted last. On zirconiun~phosphate,
which has polar hydrophilic character, the order is exactly the reverse. Summary
These examples show some of t,he ways in which ion exchange can be used in chemical analysis. There are many other; elution chromatography of amino-acids is one of the most important. Details may be found in these reference books: SAMUELSON. 0.. "Ion Exchanee Seosratians in Andvtical Chem" istry," ~ o h ~ n i l e& y Sons, i?ew kork, 1963. This is the standard work on the subject, but is restricted almost entirely to inorganic applications. INCZEDY, J., "Analytische Anwendungen von Ionenaustauschern," AkadCmiai Kiado, Budapest, 1964. Stronger on theory than Samuelson, and somewhat more comprehensive. HEFTMANN, E., Editor, "Chromatography," Reinhold Publishing Corp., New York, 1961. Includes several chapters on ion exchange. MEITES, L., Editor, "Handbook of Analytical Chemistry," McGraw-Hill Book Co., New York, 1963, pp. 137-71. HELFFERICH, F., "Ion Exchange," MeGraw-Hill Book Co., New York, 1962. The standard work on the principles of ion exchanse, hut hae little suecific information a n analvtical auplica.. tionsy AHPHLETT,C. B., "Inorgmie Ion Exchangers," Elsevier, New York, 1964. ~~~
~
Literature Cited (1) AHRENS, L. H., EDGE,R. A., AND BROOKS, R. R., Anal. Chim. Ada., 28, 551 (1963). (21 PENN~NGTON. L. D.. AND WILLIAMS. M. B.. Ind. Eno. Chem.. 51, 759 (1959). ' Kmus, K. A,, ET AL.,P ~ cS. e d United Nations Cmf. on Peaceful Uses of AtomicEnergy, 28,3 (1959). GLUECKAUF, Trans. Faraday Soe., 51, 34 (1955); "Ion Ex-
Figure 6. Separation of aminer by ligand exchange. Resin, sulfonated polystyrene. 2% crosslinked. Ni'+ loaded; bulk column volume. 1 0 ml. Eluent, 0.94 M NHs to 5 0 mi, 1.8 M beyond. (From the author's laboratory.)
change and its Applications," Society for Chemical Industry, London, 1955, p. 34. IMOTO, H., Bunseki Kagaku, 10, 124 (1961). KRAUS,K. A., AND NELSON,F., PTOC.Fir81 United Nations Conf. m Peaceful Uses of Atomic Energy, 7 , 113 (1955). LATTERELL, J. J., AND WALTON, H. F., A m l . Chim. A&, in press.
Volume 42, Number 2, February 1965
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