other coniplesed species) should be complcte n ithout any cross contamination. The exploratory experiments were carried out with cations of elements of the principal periodic groups. -4s a consequence, the procedures described for the separation of silver and of thallium ions from very complex mixtures (Figures 6 and 7) should be useful with mixtures of virtually all cations except those of the alkali metals, which are not complexed anionically by a n y of the reagents tested thus far. As Evans and Strain (8) have given approlimate comparative mobilities of the alkali metal ions in solutions of acids and ammonium salts, one may estimate the location of silver and thallium ions relative t o those of the alkali metal ions. For example, the mobility of the silver ion is about 1260 in acids (1000 X em. migrated +- potential gradient X hours). T h a t for cesium (and potassium) is 1750 and that for lithium is 1000. Consequently silver ions in acid solutions should separate with sodium
between lithium and potassium. The mobility of thallium is 1760; hence this ion should separate with potassium and cesium ahead of sodium. I n the ammoniacal oxalate solutions, silver and thallium exhibit similar mobilities (1160 and 1110) which are nearly the same as those of cesium and potassium (1200) in solutions of ammonium salts. These investigations indicate t h a t i t should be possible to develop rapid, sensitive, systematic, electrochromatographic procedures for the isolation of particular ions from complex mixtures. Should enough selective background electrolytes be found, a new, rapid, widely applicable system of analysis will have been developed. LITERATURE CITED
(1) Engelke, J. L., Strain, H. H., Wood, S. E.. ASAL. CHEY.26. 1864 (1954). (2) Evans, G. H., Strain, H. H., I & d , 28, 1560 (1956).
(3) Lederer, M., “Introduction to Pap:: Electrophoresis and Related Methods, Elsevier, Xew York, 1955.
(4) McDonald, H. J., Lappe, R. J., Marbach:‘E. P.. Spitzer, R. H., Urbin, R.1. C., Ionography. Electrophoresis in Stabilized Media,” Year Book Publishers, Chicago, Ill., 1955. ( 5 ) Ribeiro, L. P , Llitidieri, E., Affonso, 0 . R., “Electroforese em Papel e Metodos relacionados,” Servicio Grafico do I.B.G.E., Rio de Janeiro, 1958. (6) Sato, T. R., Kieieleski, W.E., Yorris, W. P., Strain, H. H., ANAL. CHEM. 25, 438 (1953). ( 7 ) Sato. T. R.. Sorris. \Ti. P..‘ Strain. ‘ H. H.,’Ibid., 26, 267 (1954). (8) Zbid., 27, 521 (1955). (9) Strain, H. H., Ibid., 31, 818 (1959). (10) Zbid.,. 32., 3R (1960); Ibid., 30. 620 (1958). (11) Strain, H. H., “Chloroplast Pigments and Chromatographic Analysis,” 32nd ilnnual Priestley Lectures, The Pennsylvania State University, University Park, Pa., 1958. (12) Strain, H. H., Sato, T. R., ANAL. CHEM.28, 687 (1956). (13) Tuckerman, M. M., Strain, H. H . , Ibid., 32, 695 (1960). (14) Wood, S. E., Strain, H. H., Ibid., 26, 1869 (1954). RECEIVEDfor review October 4, 1960. Accepted January 3, 1961. Based on work performed under the auspices of the C. S. Atomic Energ. Commission.
Use of Chlorine in Cation Exchange Separations W. J. BLAEDEL and EUGENE D. OLSEN’ Chemistry Department, University o f Wisconsin, Madison, Wis.
b Chlorination of strong acid resins results in a significant decrease in the number of sulfonic acid groups, and is accompanied b y an almost equal increase in the number of weak acid groups. Other effects of chlorination seem to b e nuclear chloride substitution and the formation of pH-sensitive chromophoric groups. The consequences of these effects in using chlorinated resins for analytical separations are discussed,
R
of metal ion species by cation exchange resins has been inferred or alluded to repeatedly in the chemical literature. For example, Fritz and Karraker (7) stated that Fe(II1) is reduced to Fe(I1) during passage through Dowex 50W, prohibiting good separation of Fe(II1) from divalent metal ions. Reduction of gold, platinum metals, and strong oxidants like dichromate, permanganate, molybdate, etc., has IS). also been reported (4, Lenskaya (9) showed t h a t oxidizing systems Fvith oxidation potentials below 0.60 volt are not reduced b y t h e sulfonic acid resin NSK, while systems EDVCTION
Present address, Chemistry De artnient, Franklin and Marshall Cotege, Lancaster, Pa.
with oxidation potentials above 0.77 volt are reduced. Lenskaya also claimed that the capacity of t h e resin was diminished by oxidation. I n attempts to prevent reduction and to improve separations, holding oxidants have been used b y some workers. Thus, in separations on Dowex 50W, Blaedel, Olsen, and Buchanan (4) used chlorine as a holding oxidant for t h e platinum metals, and found t h a t i t was necessary to pretreat the resin with chlorine in order to maintain chlorine in solution as a holding oxidant. I n analytical separations on Dower; 50, AlacKevin and MeKay (11) also found chlorine necessary in maintaining iridium in the oxidized state. For separations on cation resins, chlorine appears to have advantages over other, stronger oxidants, like dichromate or permanganate. It is capable of oxidizing a good number of metals, resin-chlorine systems are fairly stable after proper pretreatment, and the small amounts of chloride formed by reduction of chlorine are usually tolerable in separation schemes (4). In spite of the apparent uscfulncss of chlorine, not much has been reported on t h e nature of the reaction b e t w e n chlorine and sulfonic acid resins. Bauman, Skidmore, and Osmun (5) and later Braithwaite, D’Smico, and
Thompson (6) showed qualitatively that chlorine is adsorbed on strong acid resins, but they indicated t h a t the properties of the resin were little affected. I n a n accelerated oxidative test with an electrolytic procedure, Frisch, McGarvey, and Moffett (6) found that although the salt-splitting capacity did not change significantly, a definite increase in the moisture content and total exchange capacity occurred. These phenomena were interpreted to be due t o a reduction in the cross linking and the formation of rveak acid groups on the ends of broken chains. These findings were not completely in accord n i t h early qualitative studies made in this laboratory; so a more detailed study was undertaken. This paper describes the effects of chlorine uptake by the resin upon exchange capacity, moisture content, and weak acid groups. The practical consequences of these effects on analytical separations are also discussed. EFFECTIVENESS OF CHLORINE IN PREVENTING REDUCTION OF FE(III)
A tracer sample of about 0.1 pc. of FeiIII)b6-59 in 0.0161 HCIOA was adsorbed‘ on a small column Eontaining about 20 meq. of Don-ex 50W (100-200 mesh, X8, 15 em. long and 12 mm. VOL. 33, NO. 4, APRIL 1961
531
Figure 1.
Reduction of ferric ion by Dowex 50W
* Except in peak region, where a lower sensitivity of 10,000c.p.m.
i d . ) , and allowed to stand overnight to accentuate the formation of Fe(I1). Before adsorption of the sample, the column had been converted to the sodium form with 231 NaCl, mashed with water, and then acidified slightly with 10 ml. of 0.01M HC10, t o prevent hydrolysis of the Fe(II1). Elution was performed at 1.9 ml. per minute with 5y0 malonic acid adjusted to p H 2.5 with NaOH. The counting rate of the column effluent was measured (4) to give the record shown in Figure 1,A. Qualitative proof that the second peak n-as due to Fe(I1) was obtained by repeating the run with 1 mg. of Fe(II1) and testing the column effluent with o-phenanthroline. For short times of standing, the Fe(I1) peak does not appear, but tailing is still marked and reduces the effectiveness of separations. Figure 1,B, is a record of a similar run, except that the tracer sample and resin were treated with chlorine under conditions described previously (4). No significant differences were noted whether the Fe(II1) was allowed to stand in the column for only 1 hour or overnight before elution. It is apparent that Fe(II1) is reduced by Dowex 50W, and the chlorine prevents the reduction of Fe(II1) to Fe(I1) by the resin. CHLORINE UPTAKE BY RESIN
The work of Bauman et al. (3) and Braithwaite et al. (5) was concerned mainly with chlorine concentrations of about 100 p.p.m. or lower. They simply noted that the amounts of chlorine removed from solution decreased with time after extended passage, so i t was of interest to determine roughly how much chlorine could be taken up by the resin before saturation, and t o differentiate between the chlorine which was merely adsorbed and that which reacted chemically with the resin.
532
ANALYTICAL CHEMISTRY
full scale was used
An exploratory column experiment showed that 40 ml. of saturated chlorine water (ea. 0.1N) could be passed through 10 ml. of Dowex 50 in the acid form before chlorine could be detected in the effluent with o-tolidine, a sensitive spot test reagent for chlorine (1). After breakthrough, the chlorine concentration in the effluent rose fairly rapidly to the influent concentration, indicating no further reaction between the resin and chlorine. However, when the resin stood in the column for 4 hours after the passage of over 1 liter of saturated chlorine water, 15 to 20 ml. more could be passed before chlorine again appeared in the effluent. This work indicated a slow reaction between the resin and chlorine. I n batch experiments, the total chlorine uptake to saturation was determined. Ten milliliters of Dowex 50W (X8) in the acid form was shaken with successive 500-ml. batches of saturated chlorine water for periods of 1 to 2 hours in a glass-stoppered container. The chlorine in the supernate of each batch was titrated iodometrically with 0.1N Na?S2O3 after addition of excess K I . Five batches were required to saturate the resin, as indicated by a chlorine titer for the fifth batch that was only 1% less than that for the fourth batch. Calculations showed that 31.0 meq. of chlorine had been taken up by the 10 ml. of resin. To determine the unchanged chlorine adsorbed by the resin a t saturation, the chlorinated resin was transferred to a column, and excess supernate from the fifth batch of chlorine water was drained just to the top of the resin bed. Then 10-ml. portions of water were passed through the column and the effluents titrated for chlorine until chlorine-free. A total of 220 ml. of water was required, and 1.30 meq. of
chlorine Rere removed in this way. About 0.38 meq. of this chlorine was attributed to the void volume of the fifth batch supernate transferred into the column with the resin, which gave 0.92 meq. of chlorine in adsorbed form removable by a water $-ash. Lastly, the additional chlorine still available in the resin for oxidation. but not removable with water, was determined by passing a reducing agent through the column. Five-milliliter portions of 0.25N HzS03 were passed through the column and the effluents were titrated with 0.04N iodine, until the effluent titer was the same as the influent within 1%. Seventy milliliters of HzS03were needed, with '/z to 1hour reaction periods allowed for each of the last five portions. From the H2S03 used, the resin contained 2.90 meq. of oxidizing capacity. If i t is assumed that all of this oxidizing capacity was chlorine, the maximum amount of chlorine as such in the resin after saturation with chlorine &-as0.92 2.90, or 3.82 meq. Since a total of 31.0 meq. had been adsorbed, at least 27.2 meq. of chlorine must have reacted chemically with the resin. The stoichiometry of the chlorination reaction may be calculated in terms of the number of sulfonic acid exchange groups on the resin. The total exchange capacity is 1.9 meq. per ml., or 19 meq. per 10 ml. of resin. Therefore, 27.2/19, or 1.4. meq. of chlorine react per mole of sulfonic acid groups. The mechanism of this reaction is postulated later.
+
EFFECT OF CHLORINATION ON MOISTURE CONTENT AND EXCHANGE CAPACITY
Many workers have concluded that the exchange capacity of strong acid rpsins does not change upon treatment with low concentrations of chlorine
( 3 , 5 , 8). Frisch et al. (6) and Kunin (8) indicate that while the strong acid capacity (or "salt-splitting" capacity) does not. change, the tot'al capacity actually increases, and this is attributed t,o the format'ion of !Teak acid groups. Furthermore, they indicate that the capacity change is accompanied by Y reduction in cross linking, as evidenced by :in increase in the moisture curitelit, of the resin. To determine how higher leyels of chlorine might affect the crchange capacity and the moisture content, experiment's were devised which twre similar to those of Kunin ( 8 ) , except that the acid-base titrations were performed with methyl orange instead of phenolphthalein, to minimize the carbonate error. First, the moisture contents of two samples each of Dovies 50 and SOW in the wid forms were determined. Two 10-gram portions of each resin were converted iii columns to the acid form by washing \\-ith 1 liter of l X HC1. After rinsing free of excess acid, each resin T W S air-dried on a Huchner funnel with niild suction for 1i2 hour. Each of the four samples was divided into two pnrts :inti wighed to the nearest niilligmm. One portion of each resin sample was used to determine the moisture content by drying 12 hours a t 105' C. (An additional 2-hour drying period gave a weight c>hangc lt'ss than 0.3%. At highcsr trniperaturc.s, considerable decomposition took place, since constant weight n-as not attainctl even aft'er 24 hours.) The samples used to determine the water content were discarded. The total exchange capacity was determined on the remaining four airdried samples by adding an excess of st,andard 0.LV KaOH (containing 5% KaC1 to speed the exchange), allowing 12 hours for reaction, and then titrating aliquots of the supernate 1%-ithstandard 0.08AVIlCl to the methyl orange end point. Reaction was shown to be coniplcte by titrating aliquots 24 hours latcr, with no change in titer. The total capacity was determined by difference as the niilliequivalents of acid lilieratcd by KaOH per gram of original resin in the H form (dry basis). After the tots1 capacity had been determined, the salt-splitting capacity was dctermined for the same four resin samples by converting them in columns to the acid form, washing free of excess acid with distilled water, and then passing neutral 4% Ka2SO4through each column until no more acid was eluted. The S n l S 0 4 effluent solution was collcctcd and titrated wit'h standard 0.06.V NnOH to the methyl orange end point. The salt-splitting capacity was calculated as the milliequivalents of acid liberated by Ka2S04 per gram of original resin in the H form (dry basis). The same resin samples were then chlorinated in two different ways: (1) A stream of chlorine was passed continuously through one sample each of Dowes 50 and 50W suspended in water for 15 hours. (2) Another sampleeach
of Dowex 50 and 50W was treated with two successive 500-ml. batches of saturated chlorine water for 12 hours per batch. After washing free of chlorine, the moisture content, total capacity, and salt-splitting capacity were determined as before. Results for the Dowex resins are summarized in Table I. Less thorough studies indicated similar effects with Amberlite IR-120. All titration errors were kept within a few tenths of lyo,so t h a t differences in capacity greater than 0.5 to 1% are definitely significant for a particular resin sample. Differences of the order of 1 or 2y0 among different samples of the same resin are attributed to heterogeneity of the resin. Kunin (8) found an increase in the moisture content of the resin after chlorination, which he attributed to possible reduction in the cross linkage. T h e results shown in Table I indicate a definite decrease in the moisture content. These results should not be compared with those of Kunin, whose resin was chlorinated in the N a form, and less extensively than the resin in this paper. Table I also shows that the total capacity decreases only 1 to 3% upon chlorination, while 12 to 15% of the strong acid capacity-Le., sulfonic acid Addition of groups - disappear. Ba(KO& to t h e acidified supernate remaining after chlorination of the resin resulted in a dense white precipitate of BaS04, confirming the loss of sulfonic acid groups from the resin upon chlorination. Also, the qualitative FeC13 test (10) gave no color change for the unchlorinated resin, but gave a definite color change from orange to brown on the chlorinated resin, indicating the presence of phenolic groups. Koller (12) indicates t h a t displacement of sulfonic acid groups by chlorine can occur.
Table 1.
FORMATION OF WEAK ACID GROUPS UPON CHLORINATION
Table I shows t h a t before chlorination, the salt-splitting capacity is about 1% lower than the total capacity for all resin samples, indicating t h a t about 1% of the exchangeable groups on the unchlorinated resin are weak acid groups. The existence of weak acid groups on Dowex 50W strong acid cation exchanger and of weak base groups on Dowex 1 strong base anion exchanger has been inferred from tailing phenomena by Strelow (15) and by Sen Sarma, Anders, and Miller (14). The weak acid groups on Dowex 50W are ones with pK,'s in the region 7 to 13, and are neutralizable only in rather strongly basic solution. Upon running 2.5M NaCl a t pH's of 7.0 and 10.0 through unchlorinated Dowex 50W in the acid form, the effluent pH rose rather uniformly through the end of the p H region, indicating that a variety of weak acid groups is present, a conclusion which seems reasonable in view of the many different sites t h a t are available for weak acid groups on the resin matrix. The data in Table I show that upon chlorination of the resins, more weak acid groups were formed. Since the salt-splitting capacity after chlorination was 10 to 15% lower than t h e total capacity, and since t h e total capacity remained virtually unchanged, i t may be concluded that chlorination caused a conversion of sulfonic acid groups to weak acid groups on t h e resin. Again, these weak acid groups are ones with pK.'s in the region 7 to 13, and are neutralizable only in rather strongly basic solution. Chlorination of the resin also results in a n increase in the number of weak acid groups having pK,'s around 3. Figure 2 shows the changes in effluent p H that occur when 23-11 KH4C1 (at
Moisture Content and Exchange Capacity" of Chlorinated and Unchlorinated Dowex 50 and 50W
Resin type Dowex 50 Dowex 50W Sample 1 2 1 2 Sample wt., g., dry basis 4.761 5,162 5,127 4.864 Unchlorinated Moisture content, yo 54 1 53.8 j3.8 53.3 5.12 Total capacity (TC), meq./g. 5 19 4.95 4.95 Salt-splitting capacity (SSC),meq./g. 5.14 5.06 4.90 4 92 yo difference in capacities, 100 (TCSSC)/TC 1 .o 1 1 0.6 1 .o Chlorinated Chlorination treatment Continuous Eatch Continuous Eatch 46.4 46.2 47.0 47.2 RIoisture content, yo Total capacity, meq./g. 5.10 5.08 4,79 4.82 Salt-splitting capacity, meq./g. 4.35 4.46 4.20 4.31 %I; difference in capacities, 100 (TC12.2 10.6 SSC)/TC 14 7 12.3 7, change in capacities upon chlorination Total capacity -1 7 -0.8 -3.2 -2.6 -14.6 Salt-splitting capacity -15.4 -11.8 -12.0 a All capacities referred to \veight of original, unchlorinated resin in H form (dry basis)
VOL 33 NO. 4, APRIL 1961
533
2.5
M NH4CI
Figure 2. Evidence for formation of weak acid groups upon chlorination of Dowex 50 W 0 Clp.
Unchlorinated resin 15M. Clz. Chlorine gas passed through resin slurry in 0.1M HCI for 15 minutes 4H. Cla Resin chlorinoted for 4 hours I
I
1
I
I
I20 160 EFFLUENT VOLUME, ML.
80
its natural p H of 4.2) is passed through 10.8 ml. of Dowex 50W samples with different degrees of chlorination. The method of obtaining these data has been described (4). The p H leveling at 40 to 80 ml. is indicative of the removal of fairly strongly held acid from the resin. The laggard p H rise for the chlorinated resins compared t o the unchlorinated indicates a greater number of weak acid groups on the chlorinated resins. Titration of the 40- to 80-ml. portion of the effluent NH4C1 with standard 0.1N NaOH to p H 4 indicates that these weak acid groups comprise about 0.5y0 of the exchange capacity of unchlorinated resin. Substantially t h e same results were obtained when 2.5111 NaC1, adjusted t o p H 4.2 with HC1, was subindicating t h a t this stituted for “&I, behavior is not specific with NH4C1. This method of determining the weak acid content of the resin was used in preference to the more conventional p H titration, which is very slow and sometimes inaccurate, especially a t high pH. COLOR CHANGES CAUSED BY RESIN CHLORINATION
Several interesting properties of chlorinated resins were noted. Upon chlorination of Dowex 50W in the acid form, the color changes from t a n to a rather bright orange, which turns to dull orange on aging. Sunlight speeds the transition. Upon treatment of the resin with base, particularly after the resin has aged for about a week, the color changes reversibly from dull orange to dark brown. The acid-base color change remains reversible through many cycles from acid t o basic form, and is unaffected even by treatment with sulfur dioxide. (After sulfur dioxide was bubbled through chlorinated Dowex 50W in the acid form for over 3 hours, the resin exhibited the same acid and basic colors as before.) These color changes are proof that pH-sensitive chromophoric groups are formed on chlorinating the resin, but no relationship was established between these and 534
ANALYTICAL CHEMISTRY
l
l
200
the weak acid groups that n rre formed. Since exchange of weak acid groups is generally very slon , n hereas the color changes described are very rapid, the two phenomena may be independent of each other. Similar but less pcrceptiblc color changes were found to occur with the darker Don.e.; 50 rrsins.
resin bed, it has been showi (4) that neither the slight decrease in capacity nor the formation of weak acid groups will significantly affect a separation procedure, provided buffered eluents are used. I n addition, the formation of pH-sensitiue chromophoric groups by chlorine on the newer “white” resins is sometimes advantageous, since the conversion of the resin column from the acid form to the salt form can clearly be seen by the dar haling bnnds moring down the column. I n a prei-ious paper (4), for example. this eypedient helped to determine the amount of buffer necessary to convert a column from the acid to the ammonium form during a sequence of elutions a t suecessively higher pH’s. ACKNOWLEDGMENT
Financial support in the form of a fellowship from the Xlinnesota Mining and Manufacturing Co. and a research grant from the U. S. Atomic Energy Commission is gratefully acknonledged.
CONCLUSIONS
The preceding work shows that a large amount of chlorine reacts chemically with Dowex 50 and 50JV (1.4 meq. of chlorine per meq. of exchange capacity). Saturation with chlorine converts only about 15% of the strong acid groups to weak acid groups. T o explain these data, i t is postulated that the major part of the chlorine taken up reacts by nuclear substitution. It may be calculated from the data of Bauman and Eichhorn (2) that about 88% of the benzene rings in Dowex 50 (X8) contain a sulfonic acid group; therefore, a good fraction of the benzene rings could become chlorinated. Noller (12) indicates that such a substitution is possible, with the positions ortho to the methyl groups and meta to the sulfonic acid groups being favored. If nuclear substitution occurs upon chlorination, the hydrophilic nature of the resin matrix might change, and so might the moisture content. The decrease in salt-splitting capacity that was found in these studies does not necessarily disagree with the findings of Bauman et al. (3) and Braithwaite et al. ( 5 ) , who worked TTith the low levels of chlorine often found in city water supplies. On the contrary, it is remarkable that saturating the resin with high levels of chlorine will remove only about 15% of the sulfonic acid groups. There is every reason t o believe that chlorine can be used safely and advantageously as an osidizing agent in some analytical separations on cation resins. Although i t is necessary to pretreat the resins with chlorine to prevent influent chlorine from bcing removed from solution a t the top of the
LITERATURE CITED
( 1 ) Ani. Public Health A4ssoc., “Standard
Methods for Examination of Water, Setvage, and Industrial Wastes,” p. 66. 10th ed., TTaverly Press, New Yorli, 1955. j 2) Bauman, IT. C., Eichhorn, J., J . ~ 4 m . Chem. Sac. 69, 2830 (1947). (3) Bauman, IT. C.. Skidmore, J. K., Osmun, R. H., I n d . Eng. Chev2. 40, 1350 (1948). (4) Blaedel, TI-. J . . Olsen, E. D., Buchanan, R. F...$SAL. CmM. 32, 1866 (1960). (5) Braithwaite, D. G.. Il’hmico, J. S., Thompson, 11. T.. I n d . Eng. Chem. 42, 312 (1950). (6) Frisch, S.,McGarvcy, F., hfoffett, J., 16th. AAnn. TTater Conf., Proc. Engineering Soc. Kestern Penn., 1955. (7) Frit,z, J. S., Karrakcr, S. K., Ax.4~. CHEM.32, 957 (1960). (8) Kunin, R., “Ion Exchange Resins,” pp. 342, 359, 367, Wile)-, Sew York, 1958. ~. ~.
( 9 ) Lenskaya, V. 3 . . T r u d y Koniissii Anal. Khim., Sftad. r a u k S.S.S.R., Inst. Geokhim. i. -4nal. Khiin. 6, 333 (1955); Chenl. Abstr. 50, 12732 (1956). (10) Linstead, R. P., Keedon, B. C. L., “Guide t o Qualitative Organic Chemical Analysis,’’ p. 18! .Academic Press, Kern York. 1956.
(li)-hIa%e$i&
