January, 19581 HALE 3 Recent Advances in the Preparation and Uses of Ion-exchange Resins A Review* BY D. K. HALE (High Polymers Group, Chemical Research Laboratory, Teddington, Middlesex) SUMMARY OF CONTENTS Introduction New ion-exchange materials Chelating resins Ion-exchange papers Ion-exchange membranes Other new materials Gradient elution Ion-exclusion and partition chromatography Other new techniques New techniques Future developments MANY of the most important recent developments in pure and applied chemistry have taken place largely as a result of improvements in separation techniques. In many of these develop- ments, ion-exchange resins and ion-exchange chromatography have played an important part. Some of the more spectacular successes of ion-exchange techniques have been in the separation of the rare earths1 and in the isolation of promethium2 and the transuranium element~,3~~J0~7 but, in many laboratories, ion-exchange resins are now also used as established analytical tools.They are widely employed not only in simple analytical problems, such as the deter- mination of salt concentrations, but also for the separation of complex mixtures of closely related substances, such as amino acidss and nucleic acid degradation prod~cts.~ Since the publication of the American work on the separation of fission products, a wide variety of procedures for the separation of inorganic ions by ion-exchange chromatography have been described, and BoydlO has said- “That it is now possible to separate every element in the periodic table which forms an ion in solution from every other such element by proper employment of the techniques of ion-exchange chromatography appears to be a reasonably safe conclusion.” The principles and methods employed in the established ion-exchange techniques are well described in the book by Samuelson,ll whose pioneer work on the application of ion- exchange resins in analytical chemistry has been largely responsible for the widespread adoption of ion-exchange methods in the laboratory.The methods used for the separation of complex mixtures by ion-exchange chromatography are also described in two recent books on ion-exchange technology12 and on the use of ion-exchangers in organic chemistry and biochemistry.13 Since the field of application of ion-exchange resins is now so wide, in this review it will be possible to describe only some of the more recent advances that are likely to be of interest to the analytical chemist.The more important developments may be classified somewhat arbitrarily into two groups: (a) new ion-exchange materials and (b) new techniques. NEW ION-EXCHANGE MATERIALS CHELATING RESINS- Although different ions have different affinities for ion-exchange resins of the con- ventional type containing either acidic groups, such as the -SO,H group, or basic groups, such as the quaternary ammonium group, these differences in affinity are frequently too small to form the basis of a readily effected separation, especially when the ions to be separated are very similar in their properties. Most of the separations carried out by ion-exchange chrom- atography have therefore been based on differences in the degree of dissociation of the sub- stances being separated or on the stability of their complexes, rather than on differences in the * Based on a lecture delivered a t the meeting of the Midlands Section on Thursday, October 25th, 1956.HALE: RECENT ADVANCES IN THE PREPARATION AND [Vol.83 4 affinities of the ions for the resin. For example, in the separation of the rare earths, use is made of the differences in the stability of their complexes with citric acid or ethylenediamine- tetra-acetic acid. This lack of highly selective properties has, however, meant that the con- ventional ion-exchange resins have been used extensively in chromatographic procedures, since, by appropriate selection of eluting agents, they can be used for a wide variety of applica- tions.In the past few years there has, however, been increasing interest in the development of resins containing special functional groups that would be expected to result in highly selective behaviour. Interest has very largely centred on the preparation of resins containing chelating groups that might be expected to !;how highly selective absorption behaviour towards metal cations. These resins have been described as chelating resins or chelating ion-exchange resins. Their preparation and properties have been described in recent reviews14J6 and only a short account of them will be given in this review. The first attempt to prepare a resin showing highly selective behaviour was made in 1940 by Skogseid,l6 who synthesised a polystyrene resin containing functional groups similar in structure to dipicrylamine.Dipicrylamine forrns a sparingly soluble potassium salt, which may be a chelate compound, and Skogseid was able to show that the resin containing similar groups had a higher affinity for potassium than had other ion-exchange resins. This resin has recently been studied in more detail, and Skogseid’s results have been confirmed.17 Most of the chelating resins so far described have been obtained by condensing compounds such as m-phenylenediglycinel* with formaldehyde, or anthranilic acid with resorcinol and f o m - aldehyde.l81l9 920 Chelating resins with polymer networks similar to those in the more recently developed ion-exchange resins should, however, have greater physical and chemical stability.Cornaz and Deuel21 have described a chelating resin containing hydroxamic groups that was based on a commercial carboxylic resin ; this absorbed ferric ions selectively. Chelating resins prepared from cross-linked polystyrene and containing 8-hydroxyquinoline22 and amino acid groups14 have also been prepared. The former resin strongly absorbed copper, nickel and cobalt ions in the pH range 2 to 3, whereas resins of the latter type showed very similar behaviour, the order of affinity for bivalent ions of the first transition series corresponding to the Irving - Williams order of stability observed with soluble complexes. Until recently, little has been published on the characterisation or possible applications of chelating resins, but it appears likely that they may eventually prove useful for the deter- mination of trace metals and in some inorganic separations.Blasius and O l b r i ~ h ~ ~ have described the application of a chelating resin prepared from m-phenylenediaminetetra-acetic acid to the separation of alkali-metal ions from copper or nickel, to the separation of alkaline earths from copper and to the separation of cobalt and nickel. In the first example, the alkali metal was eluted from the chelating resin with 0.01 N hydrochloric acid, the effluent was passed through a column of a strongly basic resin in the hydroxyl form, and the solution of the alkali-metal hydroxide was titrated with 0.1 N hydrochloric acid. The copper or nickel was then eluted from the chelating resin with 2 N hydrochloric acid and determined by direct titration.A major disadvantage of the chelating resins containing weakly acidic groups is the slow rate of exchange, so that long columns or very slow flow-rates have to be employed. Resins containing stronger acidic groups may prove superior in this respect, and Kennedy and D a v i e ~ ~ ~ have described the application of phosphonic acid chelating resins to the separa- tion of uranium from heavy metals. It may be noted that some of the commercially available ion-exchange resins show highly selective behaviour in special instances. For example, some of the phenolic resins containing methylenesulphonic acid groups have an exceptionally high affinity for caesium2s and have been used foir the determination of caesium in sea water26 and for the separation of milligram amounts of caesium from large amounts of other alkali- metal ION-EXCHANGE PAPERS- In conventional ion-exchange chromatography a column of resin is employed, and the effluent is usually collected in fractions, which are subsequently analysed.To obtain a satisfactory separation, local equilibrium should. be closely approached a t all points in the column; if the rates of exchange are slow, this means that an extremely finely divided resin and a very slow flow-rate have to be employed. In qualitative analysis a procedure similar to that in paper partition chromatography should possess advantages. Ion-exchange properties can be conferred on filter-paper by the introduction of acidic or basic groups into the celluloseJanuary, 19581 USES OF ION-EXCHANGE RESINS.A REVIEW 5 molecule, and the application of ion-exchange papers of this type containing carboxylic, sulphonic and quaternary ammonium groups has been des~ribed.~81~~ More recently, Kember and Wells30 have described chromatographic separations on papers prepared from aminated and phosphorylated cellulose. Cation- and anion-exchange papers can also be made by incorporating finely divided ion-exchange resins in the cellulose pulp used for the preparation of the paper31; the chromatographic behaviour of ions on the resultant ion-exchange paper has been found to be similar to that observed in column experiments with the same resin. A similar technique has been described by led ere^,^^ who incorporated particles of ion- exchange resin in filter-paper by passing a strip of the paper through a suspension of colloidal aggregates of Dowex 50 or an anion-exchange resin of a fine particle size.Paper prepared in this way has been used for the separation of selenium and tellurium.= A spot of a solution containing a mixture of sodium selenite and sodium tellurite was applied to a strip of paper impregnated with Dowex 50. The paper was then eluted with acid and sprayed with a solution of stannous chloride in 5 N hydrochloric acid. It was found that the tellurium, unlike the selenium, exhibited a marked cationic character in acid solution, and good separa- tions of the two elements were achieved. The use of filter-paper containing finely divided ion-exchange resins for the quantitative determination of cations and anions has also been described.% ,35 ION-EXCHANGE MEMBRANES- There is at present considerable interest in the development of ion-exchange materials in sheet form for use as membranes in electrodialytic and electrochemical processes.Cation- exchange membranes are permeable to cations, but are relatively impermeable to anions; similarly, anion-exchange membranes allow the passage of anions, but are relatively imperme- able to cations. Ion-exchange membranes of both types are commercially available ; the cation-exchange membranes, e.g., Permaplex C-10 (The Permutit Co. Ltd.)? contain sulphonic acid groups and the anion-exchange membranes, e.g., Permaplex A-10, contain strongly basic quaternary ammonium groups. Both types of membranes have a low electrical resistance and show highly selective behaviour towards cations and anions, respectively.If these ion- exchange membranes are used instead of the comparatively non-selective membranes pre- viously employed in electrodialysis, the process becomes very much more efficient and salts can readily be removed from solutions that are too concentrated to be treated by the usual ion-exchange procedures. Although the commercially available ion-exchange membranes have been developed primarily for large-scale applications, they are also suitable for laboratory use. In the analysis of mixtures of amino acids and other substances by paper chromatography, it is often necessary to remove inorganic salts before the mixture is analysed.The usual methods may result in the loss of 10 or 20 per cent. of many of the amino acids and, moreover, arginine may be largely converted to ornithine. Small laboratory electrodialysers have been d e ~ c r i b e d ~ ~ * ~ ~ in which an electrolytic cell fitted with platinum or carbon electrodes is divided into three compartments by a cation- and an anion-exchange membrane. The solution to be desalted is placed in the central compartment and tap water or dilute electrolyte solution in the electrode compartments. When a potential is applied to the cell, cations and anions in the central compartment pass into the electrode compartments through the ion-exchange mem- branes, which prevent the entry of anions and cations moving in the opposite directions.A similar method has been employed for the de-ionisation of aqueous solutions of plant extracts.38 The use of ion-exchange membranes in the electrodialysis cell permitted de-ionisation of the solutions without loss of sugars. An electrolytic cell divided into two compartments by a cation-exchange membrane has been used in the determination of boron in a sodium hydroxide solution.39 The anode compartment was filled with the hydroxide solution and the cathode compartment with 0.5 N sodium hydroxide. On passage of an electric current, sodium ions passed through the membrane into the catholyte and the boron remained in the anolyte. The concentration of sodium hydroxide in the anolyte was reduced to 0.007 N , and the boron was determined colorimetrically after removal of interfering elements such as aluminium, chromium and manganese on a column of a cation-exchange resin. The method could be used for the determination of boron in sodium metal and has proved valuable in the preparation of boron-free sodium hydroxide.Although membranes based on ion-exchange resins are a relatively new development, the properties of biological membranes that show ion-selective behaviour had been the subject6 HALE: RECENT ADVANCES IN THE PREPARATION AND [Vol. 83 of extensive investigation many years before the discovery of ion-exchange resins, and highly selective membranes based on collodion had been prepared. The more recent membranes of this type that have been developed by Sollne~“~ have proved to be suitable for the deter- mination of ion activities in solutions of electrolytes. The most effective cation-exchange membranes of this type were prepared bmy incorporating sulphonated polystyrene in a collodion membrane.Highly selective anion-exchange membranes were prepared by treating collodion membranes with solutions of protamine sulphate. Membranes prepared in this way may be used for the determination of the activities of many ions for which specific reversible electrodes are not available, e.g., fluoride, nitrate and acetate, or when it is difficult to set up a suitable electrode, e.g., with alkali-metal and alkaline-earth cations. These membranes can therefore be used for the determination of ion activities in single electrolyte solutions in much the same way that a glass electrode is used for the determination of hydrogen- ion activities. The membranes developed by Sollner appear to be particularly suitable for this purpose and have been used successfully in studies on the binding of alkali-metal and alkaline-earth cations in protein solutions41742 and in potentiometric titrati0ns.4~ Membranes prepared in the laboratory by moulding a finely divided ion-exchange resin with an inert binder such as polystyrene or polyethylene have also been used successfully for the determina- tion of ionic activities and in potentiometric tit ration^.^,^^,^^ When commercial ion-exchange resin membranes are being applied in the determination of ionic activities, the use of a strip or ribbon of the ion-exchange material has been found to be ad~antageous.4~ OTHER NEW MATERIALS- In the separation of large molecules, such as proteins, by ion-exchange chromatography, the exchange process is confined to the surface of the resin particles.In most of the earlier work on protein separations the weakly acidic cation-exchange resin Amberlite IRC-50 was employed. Recently, special ion-exchange materials for protein separations have been developed. These include chemically modified c~~lluloses48 yg9 9 5 0 9 5 1 and ion-exchange materials prepared by coating the surface of kieselguhr with sulphonated cross-linked p~lystyrene.~~ y53 The recent introduction of a new type of highly porous polystyrene ion-exchange resinM may lead to further developments in the ion-exchange separation of large molecules. The use of ion-exchange chromatography for the separation of peptides, proteins and nucleic acids has been described in two recent review article~.~~~~G NEW TECHNIQUES The use of ion-exchange resins in the study of complexes has led in turn to the develop- ment of a wide variety of procedures for the separation of inorganic ions by ion-exchange chromatography.The methods developed by Kraus, Moore and Nelson57 for the separation of metals in the form of their chloro complexes on anion-exchange resins are of very wide application and some remarkable separations have been achieved by this simple yet elegant technique. The general principles involved in the application of these methods in analytical chemistry have been described by Jent~sch,~* and a summary in tabular form of the work of Kraus and his colleagues has recently been publi~~hed.~~ Many other methods involving the use of selective eluting agents, e g ., phosphoric acid6076l and organic J~~ have been described. And, as such a wide selection of procedures is now available for the separation of inorganic ions, improvements in experimental techniques such as the use of gradient elution appear to be most likely t o lead to important developments. In the organic field some interesting separations have been carried out by the processes known as “ion exclusion” and by partition ~hromatography,~~ GRADIENT ELUTION- The technique of gradient elution is being employed to an increasing extent in ion- exchange chromatography. In elution procedures in which it is necessary, for example, t o increase the acidity of the eluting agent to remove the more strongly absorbed components a gradient-elution technique can be used, in which the acid concentration of the eluting agent is increased continuously.When this procedure is employed, “tailing” is reduced or eliminated and the discontinuities or duplication of peaks, which may arise on changing an eluting agent, are avoided. In a typical gradient-elution procedwre the eluting agents are passed successively into a mixing chamber, which is mounted over a magnetic stirrer. If the mixing chamber initially contains a solution of one concentration and a solution of a higher concentration isJanuary, 19581 USES OF ION-EXCHANGE RESINS. A REVIEW 7 added continuously, the concentration of the eluting agent will at first increase rapidly and then slowly approach that of the added solution.The composition of the eluting agent may be varied in different ways by suitable adjustment of the rates at which the solutions flow into and out of the mixing vessel.65 Gradient elution is now employed in the procedure developed by Moore and Stein for the separation of amino acids,8 and its application to the separation of the rare-earth elemenW and to the separation of ortho-, pyro-, tri-, trimeta- and tetrameta-phosphates67 has recently been described. The theory of gradient elution has been discussed by Drake,68 F r e i l i ~ ~ g ~ ~ 770 and Piez.71 ION-EXCLUSION AND PARTITION CHROMATOGRAPHY- If a strongly acidic cation-exchange resin in the sodium form is allowed to come in contact with a solution containing sodium chloride and a non-electrolyte such as glycerol, the equilibrium concentration of sodium chloride inside the re+n particles will be very much less than in the external solution, owing to the Donnan equilibrium effect.The uncharged glycerol molecules will, however, be absorbed by the resin, and in this way a partial separation of sodium chloride and glycerol could be obtained. By using a column of the resin and a chromatographic procedure with water as eluting agent, a complete separation of charged molecules from uncharged molecules can be achieved. The use of this process, which is known as “ion exclusion,” for the purification of sugar72 and glycerol73 has been described. Although water is used as an eluting agent and acidic or basic regenerants are not required, the solutions to be treated are diluted in the process unless special procedures are adopted and, moreover, slow flow-rates and resin of fine particle size have to be employed.It is, however, likely that ion-exclusion techniques may eventually prove of value in the laboratory for the separation of strong electrolytes from uncharged molecules, especially as uncharged molecules are sometimes strongly absorbed by ion-exchange re~ins.7~ ~7~ Since the extent to which it is absorbed depends on the nature of the uncharged molecule, ion-exchange resins can be used for the separation of mixtures of non-electrolytes or weak electrolytes by partition chromatography.76 With alcohols the separation is greatly improved by the use of a salt solution as the eluting agent instead of water, and this method has been described as “salting- out ~hromatography.”~~ 3 7 8 OTHER NEW TECHNIQUES- A number of analytical procedures involving ion-exchange resins have recently been developed in which chemical reactions take place either at the surface of, or within, the resin particles.For example, the use of anion-exchange resins for the adsorption of trace amounts of cations by the formation of insoluble salts has recently been de~cribed.7~ In experiments with a column of Amberlite IRA-400 and solutions containing trace amounts of caesium, strontium and a zirconium - niobium mixture, it was found that a column in the hydroxide form did not retain the caesium and strontium, but collected quantitatively the zirconium - niobium mixture; a column in the oxalate form retained most of the strontium, but not caesium; and a column in the carbonate form retained strontium.This method is reported to be more efficient and more selective than the conventional co-precipitation techniques and has been found to be suitable for the rapid separation and determination of radiostrontium and radiocaesium in fission-product mixtures.80 Y81 A similar procedure has been described for the separation of barium-140 from lanthanum-140.82 This technique has also been used in a method for the rapid determination of the major anions in fresh water,83 in which a column of a strongly acidic cation-exchange resin in the silver form was used for the selective retention of chloride ions, a process similar in principle to the technique developed during the war for the removal of salt from sea water with silver zeolites.It has also been found that the sensitivity of precipitate-forming spot-tests can be improved by the use of ion- exchange resins.84 As an example, a strongly basic resin in the chloride form was employed in the detection of silver. The use of ion-exchange resins for increasing the sensitivity and selectivity of a number of other spot-tests has also been reported re~ently.8~98~ In a typical example, an anion- exchange resin that has been treated with haematoxylin solution is used for the detectionof germaniumIv. An anion-exchange resin with a low degree of cross-linking is added to a haematoxylin solution; after a few hours the resin particles become bright yellow.A few of the dried resin particles are then added to a drop of the sample solution. If germanium is present, the resin particles become brown to red-violet in colour. The limit of identification8 HALE: RECENT ADVANCES I N THE PREPARATION AND [Vol. 83 is said to be remarkably low (0.005 pg) and the limiting concentration 1 in 6 x 108. Inter- fering ions such as Fe3+ and Bi3+ can be removed by preliminary treatment of the sample solution with a cation-exchange resin. 1; UTURE DEVELOPMENTS Many of the special ion-exchange resins that have been developed recently are only of ephemeral interest, but some may become of importance to the analytical chemist. Further research on the preparation and properties of ion-exchange membranes may lead to new techniques for the separation and determination of ions by electrochemical methods.In recent years, rapid progress has been made in the use of ion-exchange resins in analytical procedures and it may be expected that they will find even wider application in the future, particularly in problems involving the separation of complex mixtures. 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