Sulfonation of Polystyrene Preparation and Characterization of an Ion Exchange Resin in the Organic Laboratory Andrea E. Holboke and Robert P. Plnnelll Scripps, Pitzer and Claremont McKenna Colleges, Claremont, CA 91711 Experiments dealing with polymers, suitable for use in the undergraduate laboratory, have usually involved the prepaor condensation nolvmers bv relaration of homonolvmers " . . tively simple free radical addition or polar elimination reactions ( I , 2). Characterization studies have been left to laboratory courses in physical chemistry although a number of recent articles in this Journal have attempted to redress this deficiency. Very few experiments have been described in which polvmers are derivatized and functionality demonto describe the preparation and characterstrated ~ ' wish e ization of sulfonated polystyrene, a derivative of which many students are aware, or have actually used, i.e., a n ion exchange "resin". This material has been nrenared . commerciallv- bv.treatment of halocarbon solutions or slurries of polystyrene with sulfur trioxide (3). concentrated sulfuric acid (4).or chloro, and sulfonate substitusulfonic acid (5).~ o r o s i t yswelling, tion levels can be varied, depending upon the physical and chemical properties desired. The resins are marketed under a number of trade names and have become a valued tool in the laboratory. In the undergraduate laboratory, however, these rearents and halocarbon solvents present a significant hazard in inexperienced hands. An alternative p;ocedure has been described in the literature, i.e., the reaction of nolvstvrene with concentrated sulfuric acid in the nresence bf s:lv& sulfate catalyst (6, 7). We have modified the literature conditions to allow completion of the sulfonation and titration of active sites within a 4-h laboratory period; although concentrated sulfuric acid is still required, its manipulation has been minimized. T h e reaction involved is as follows: &
where @ represents the polystyrene substrate. The exchange capacity of the resulting resin is determined by a simple titration with sodium hydroxide; sodium ion displaces hydronium ion, which is then neutralized hy the hydroxide. Assuming the resin has been washed free of excess sulfuric acid, the quantity of hydrogen ion titrated should be equivalent to the number of sulfonic acid groups present in the sample of resin. Experimental Sulfonation of Polystyrene Place 15 mL of concentrated sulfuricacid in a 50-mL Erlenmeyer flask. Add 0.02 e of silver sulfate followed bv careful stirrine to d i s s h the precipitate. Warm the mixturr to9O0Con a afenm hath, andadd 1.0gofpoly~tyrenrbeads(4%crms.linkedwlrhdivmyllwn~ r n e )in small portions. I.oosely propper [he flask with a grouved
cork or one-hole stopper. Place the mixture on a steam bath, and heat for two hours with occasional stirring. When the reaction period is complete, carefully add the mixture, with stirring, to 100 mL of cold 6 M sulfuricacid. Filter the slurry on a Buchner funnel, and wash with five 10-mL portions of distilled water; after the fifth washing a portion of the filtrate should be checked with pH paper to ensure the residual sulfuric acid has been removed from the resin. The polymer is then rinsed with two 10-mL portions of anhydrous methanol. Spread the sulfonated resin on a watch glass, and dry to a constant weight by placing in a drying oven held at 105 'C for 10-15 min. The solid should be stirred occasionally to prevent charring and facilitate drying. Yield 1.6-2.1 g. Determination of Sulfonate Substitution Place a 0.2-g sample, weighed to 1-mg precision, in a 125-mL flask, and add 20 mL of distilled water. Add a drop of phenolphthalein indicator. and titrate with 0.100 M sodium hvdroxide solution t o the oink ekdooint. Slow diffusion of bvdronlrum ion from the interior of the beads may necessitate addithmal small aliquors of base to achieve a stable endpoint. Exchange rapacity in the range of :i5-4.4mmol sulfonatelg of resin should he olwrved. ~
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Separation of a Mixture of Copper(ll) and Chromium(l1l) Nitrates A column bed of resin can be prepared by transferring %6 g of resin, slurried in water, into a 50-mL burette using standard ehromatographicteehniques. A 1.0 mL aliquot of solution, 1.0 M each in Cu(NO& and Cr(NOs)s, is pipetted carefully onto the top of the column. Elution with 1.0 M nitric acid will give a clear separation of blue and violet bands attributable to the eopper(I1) and chromium(II1) cations, respectively. After the copper ion has been eluted from the column, addition of 6.0 M nitric acid will displace the chromium ion. Results and Discussion T h e resin product is a pale tan solid that swells significantly upon suspension in water. With care the recovery of polymer; a s thesulfonate, should be quantitative a n d t h e weighed yield dependent on two factors: the extent of sulfonation and the hygroscopic nature of the product. Since the product cannot be separated from the reactant polymer, extent of reaction is determined hy a simple acid-base titratiun uf the sulfonic acid groups. T h e degree of hydration is less well defined and will denend unon the careindrvingand exposure t o moisture during weighing. Yield and &b;titution values given above represent ranges obtained by a typical class of undergraduates. Decreasing the extent of diviuylbenzene cross-linking will result in a large increase in volume swelling upon sulfonation and washing with water. Yields greater than 100% are common, and students should be informed of ~ ~ bvdration ~ nroblems. ~ ~ An interesting dilemna arises in calculating the percent yield and substitution, i.e., the molecular weight of the poly~~
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Thls oaoer . . was Dresented at the 1987 Pacific Conference on Chemishy and ~peciroscopy.Irvine, CA, Oct. 28-30, 1987 ' Author to whom correspondence should be adoressed. Volume 66
Number 7 July 1989
813
mer reactant is not known and the stoichiometry is best discussed in terms of a reactive unit. The polymer is composed of 96% styrene and 4% divinylbenzene yielding an average weight of 105.2 g/molecular unit; 1g of polymer will contain 9.51 mmol of phenyl units. Substitution of one sulfonic acid per phenyl moiety will give a molecular weight of 185.2 g/unit or 5.40 mmol monopositive cationic sites per gram of resin. Commercial resins are frequently characterized in terms of milliequivalents of sulfonic acid per gram of solid or per milliliter of wet material. The ion exchange property of the resin is colorfully demonstrated by the separation of a mixture of copper(I1) and chromium(II1) cations. I t can be carried out by combining resin products from several students and, although timeconsuming, is a recommended addition to the experiment.
614
Journal of Chemical Education
Acknowledgment The authors wish to thank The W. M. Keck Foundation for its support of this work. Literature Clted 1. Fieser, L. F.:Williamson, K. L. Orgonie Ezperiments, 6th ed.; Heath: Lexington, MA, 1987;pp 281-291. 2. Pavie, D.L.; Lampmao. G. M.: Kriz. G. S., Jr. Infroduetion Lo Orgonie Lobordory Techniques, A Confemporory Appmoch, 2nd ed.; Saunders: Philadelphia, 1982: pp 77F-191 .. .. ...
3. Baer, M. U.S. Pat. 2 533 210,1950: Chem Abst. 1951,45,3ffilh. 4. R0th.H. H.; C0wherd.E. R.:Baumsn. W. C.Blit.9M)502,196$ChomAbstr 1964.61, 7194h. 5. Roth, H. H.;Smith, H.B.U.S.Pat, 26637W.1953:Chem.Abstr. 1954,4&6161h. 6. Pinner. S. H.APlocfieolCourroin Polymer Chamislry:Pergamon: New York, 1961:pp 1617. . J. Appl. Cham. 1951,1,12&132. 7. P e p p e ~K. W
