1ia
T. R. E. KRESSMAN
Discussion The positively charged -NH3+ group in the aspartic acid molecule can be expected to repel a positively charged ion from the adjacent carboxyl anion.47 Under the experimental conditions aspartic acid, insofar as its complexing action is concerned, can be considered to be the equivalent of a monocarboxylic acid anion. The addition of formaldehyde in the amounts stated a t constant pH increases the acid ionization constant of the amino group from 2.5 X lO-’O to about 1.6 X lo-’,& thus reducing the net positive charge. In addition, the formaldehyde reacts with the uncharged amino group.” The net result is t o increase markedly the bound fraction of Ca++or Sr++. Since the resulting formation constant becomes roughly equivalent to that of the malate anion with Ca++or Sr++rather than succinate, it appears that, the group formed on the a.mino acid molecule between HCHO and -NH2 exerts a complex forming action similar to the -OH group in malate. A further discussion of these points will be deferred until more data are obtained. The relatively minor effects of temperature on the complex formation constant of strontium citrate appears characteristic for the alkaline earth(47) E. J. Cohn, and J. Edsall, “Proteins, Amino Acida and Peptides,” Reinhold Publishing Corporation, New York, N. Y.,1943.
Vol. 56
organic acid complexes in the 3-55’ range. A systematic and more extensive study of the effects of temperature, ionic strength, pH and dielectric constant on the complexes found between the alkaline earths, transition elements, and organic acids is in progress. The technical assistance of Mr. R. C. Lesko is gratefully acknowledged. The writer is indebted to Dr. A. Lindenbaum for the development of the radiochemical assay method for 45Ca,and to Mr. R. Bane and Mr. R. Telford for the chemical analysis of the resin. REMARKS ,
I should like to point out that there are ISAAC FELDMAN: certain, as yet unstated, limitations on the method of Dr. Jack Schubert. Considering the com lex a M x Ay -c bMm ayA. A strai ht-line log plot using Zhubert’s method is obtained only wten a = b, and therefore x = M. This limitation becomes important for hydrolyzable ions such as beryllium, since in basic solutions beryllium may polymerize, even in tracer concentration. Even in the case where a =i b and x = m, one must still determine “m” or “x” by an indeE n d e n t method. That is, the original assumption that x Ay is equivalent to MAy/x is subject to limitations. These facts were first pointed out to me by Dr. Loft Toribara of our laboratory. RESPONSEADDED IN PROOF.-DI. Feldman’s comments apply generally to practically all methods used for the measurement of formation constants of complex ions. The extent of polymerization of a complex ion if any, would have to be determined in supplementary experiments.
+
ION EXCHANGE SEPARATIONS BASED UPON IONIC SIZE BY T. R. E. KRESSMAN The Permutit Co., La., London, England Received August 50, 1961
The molecular pores of “Zeo-Karb 225,” a sulfonated cross-linked polystyrene resin, are shown to be smaller than those of the phenol sulfonate resin “Zeo-Karb 215.” In spite of this the rate of exchange between the “,-form of “Zeo-Karb 225” and various quaternary ammonium salts is greater than that with “Zeo-Karb 215.” As with %eo-Karb 215” the kinetics of exchange are controlled by particle diffusion (P-kinetics). Whilst the smallest pores in “Zeo-Karb 215” are larger than the effective size of the phenyldimethylbenzylammonium ion, those in “Zeo-Karb 225” are smaller than the tetramethylammonium ion. Differences of rates of exchange on “Zeo-Karb 215” have already been employed to effect separations between cations of different size: when “Zeo-Karb 225” is used the degree of separation i s enhanced by virtue of the limited saturation capacity for the larger ions. The study is extended to the absorption of acids on “De-Acidite E” and on the highly orous “Decolorite.” Inorganic acids, e.g., HzS04,HCI are taken up rapidly by both resins from 0.01 N solution at 22”. gome dye acids are also taken up rapidly by “Decolorite,” and more slowly and to a limited extent by “De-Acidite E.” In every case the rate determining step is diffusion in the solid particle as indicated by the form of the kinetics and by interruption tests. The results can be used a8 a basis for the separation of inorganic salts from dye solutions with “De-Acidite E,” and the feasibility of separating dye acids one from another with “Decolorite” is also indicated.
of the phenolsulfonic acid resin “Zeo-Karb 215” and the sulfonated polystyrene resin (‘Zeo-Karb 225” are compared and their effect on separations based upon rates and total capacity is studied. The study is extended to the absorption of acids, having large molecules, on the anion exchanger “DeAcidite E” and the highly porous anion exchanger “Decolorite.” The application of the results to the separation of inorganic salts from dyes and to the separation of high molecular weight acids among themselves is indicated. Both the anion exchange resins are of the weakly basic type and the equilibrium absorption of acids, Acidite B”.Z In the present paper the pore size distributions if not the relative rate of absorption, is considerably influenced by the dissociation constant of the acid (1) T. 8,a. kressnilsn arid J. A. Kttchener, Disc. Flrradau Pos., in question. In order to eliminate this as far as ko,V I 9d 11949). poePible, dyestuff8 containing free SO&.i groups ( 9 ) n. Wt bl*hWdddd, NdftlPri $641 Rlb tl940) Since the mechanism of ion exchange is determined among other factors by the size of the exchanging ion, it is natural to suppose that its contribution could be exploited for effecting separations. The separation of ions by exploiting differences in rates of exchange has already been demonstrated in a preliminary manner by Kressman and Kitchener’ and the feasibility of separations based on differences in saturation capacities is suggested by the results of Richardson which showed that the saturation capacity for acids decreases with increasing ionic size in the anion exchanger “De-
Jan., 1952
IONEXCHANGE SEPARATIONS BASEDUPON IONIC SIZE
were used in the study since the acid strength of these groups is not greatly influenced by their number and by their environment (at least in the absence of adjacent NH2 groups).a
Experimental Materials.-“Zeo-Karb 225” is a unifunctional cation exchanger in bead form prepared from sulfonated crosslinked polystyrene and that used was a sample of “Permutit” laboratory grade resin sieved 16/30 B.S.S. mesh. It was converted to the H-form by flowing an excess of N HC1 through a column of the resin until sodium ions were only just detectable in the effluent with a flame photometer. The resin was washed with distilled water until the effluent was free of acid, then air-dried. The NH4-form of the resin was prepared by flowing a 0.5 N solution of ammonium chloride through the column of the H-form until no more acid appeared in the effluent and washing off the excess ammonium chloride. The resin was filtered at the pump and surface moisture removed by mopping between filter papers. The mopped resin was then kept in a tightly stoppered bottle. The H-form of the resin was used for the experiments illustrated in Fig. 5 and the NH4-form for those in Figs. 2 and 3. The “De-Acidite E ” and “Decolorite,” both weakly basic anion exchangers in granular form, were likewise samples of laboratory grade resins and they were converted to the free base forms by flowing a 3 N solution of ammonia through a column of the resin until chloride was undetectable in the effluent. The excess ammonia was washed from the resin with distilled water until the water emerging had a pH of about 7.5. The granules were graded by elutriation. The washed resins were ke t in distilled water and sufficient for immediate needs was Rtered at the pump, mopped between filter papers until surface dry and then kept in stoppered bottles. No mopped resin was used which was more than 30 hours old, a fresh quantity being taken from the large batch under distilled water, rinsed by several decantations with distilled water, then filtered and mopped as before. The resins were used in the free base form throughout. Determination of Effective Sphere Radius of the Swollen Resin Particles.-This was determined by the method previously described.1 Briefly the method consists in weighing a known number of water swollen particles and determining their density ( p ) with the aid of a density bottle. The effective sphere radius P is calculated from 4/3.rrap = w where w is the average weight of one particle. The “ZeoKarb 225” particles were, in fact, in the form of spheres and the method gives the average radius of the spheres. This was found to be 0.40 mm. The effective radii of the “DeAcidite E” and “Decolorite” granules were 0.39 and 0.42 mm., respectively. Determination of Exchange Velocity.-The technique alreadv described’ usine the limited bath method was adhered to eiactly, the speea of rotation of the stirrer being kept constant at 1000 r.p.m., a speed previously shown to be the optimum.’ In the experiments illustrated in Figs. 4 and 5, 2.92 meq. of swollen H-resin were used together with 125 cc. of solution originally 0.01 N in bpth NH,+ and NEt4+. I n Figs. 2 and 3, 2.50 meq. NH4-resin were used with 125 cc. solution, 0.02 N in total cations. In the experiments with “DeAcidite E” l g. of swollen resin was used and, because of its lower capacity, 2-g. lots of “Decolorite” were used. 125 cc. of solution containing 1.25 meq. acid was used for each experiment. Determination of Total Capacity.-The total number of exchangeable hydrogen ions in the “Zeo-Karb 225” was determined by stirring a known weight-l to 2 g.-of the resin with about50 cc.of watercontainingabout20mg.ofNaCl and titrating the total acid liberated with 0.1 N NaOH to methyl red end-point. The total number of exchangeable ammonium ions in the NH4-form was determined by distilling a known weight with dilute NaOH solution into standard acid and back titrating in the usual way. (a) Iandolt-B6matein Tabellen, Bond IT, 1187 at.
WQ.
(l936h
119
This method was not practicable for measuring the total capacity for organic cations and the method previously described4 was used here. The solution of the quaternary salt was flowed slowly through a column of H-resin and the acid in the effluent titrated with alkali. .The column was allowed to stand for 24 hours with the resin immersed in the quaternary salt solution and the flow of solution restarted and coptinued until no more acid appeared in the effluent: this acid was titrated. This was repeated until on running off the salt solution after the 24 hours standing no titratable amount of acid appeared in the effluent. The total acid liberated is a measure of the quaternary ion entering the resin. Since the two anion exchangers are weakly basic their capacities were measured as a function of pH. Two-gram samples of the water swollen resins were weighed into a series of stoppered bottles and known volumes of nitric acid solution added to each. The initial acid concentration was different in every bottle and ranged from 0.01 to 0.5 N . After allowing 36 hours for equilibrium to be attained the pH of the solution in each bottle was measured and the residual acid titrated in an aliquot with standard alkali. Figure 1 shows the curves obtained.
1 Fig. 1.-Capacity-pH
2 3 4 5 PH. curves for “Decolorite” and ‘‘DeAcidite E.”
Determination of Total Water Content of *e Fully Swollen Resins.-Samples of the fully swollen resins, blotted between filter papers, were weighed and dried to constant weight a t 110’. The values thus obtained for the water contents. exoressed as e. water Der e. oven dried resin. are: “Zeo-Karb ’225” H-f&m 0.86 gP/g.; “Zeo-Karb 225” NH4-form0.53 -g . / g-. ; “De-Acidite E” 0.47 g./g.; “Decolor- ite” 2.54 g./g. Preparation of Pure Free Dye Acids.-These were prepared from the impure sodium salts by ion exchange. A solution of the salt was prepared, containing about 30 meq. total cations in about 1 liter of water. This was passed through a column of “Zeo-Karb 225” in the H-form (25 cm. X 1.5 cm.) and then through a column of “De-Acidite E” (25 cm. X 1.0 cm.). The former converted all the salts present to the free acids and the latter absorbed the inorganic acids, while allowing the dye acid to pass through. The solution of the dye acid was then brought to 0.01 N by appropriate dilution and electrometric titration with standard alkali. 1 - Amino - 8 - naphthol - 3,6 - disulfonic acid was prepared from the monosodium salt by neutralizing a solution with sodium hydroxide and then passing this through a column of “Zeo-Karb 225” (H-form). It was then brought to 0.01 N by appropriate dilution. Dyes Used.-In addition to l-arnino-S-naphthol-3,6-disulfonic acid, the dyes studied were Chlorazol Sky Blue, [3,3’ - dimethoxydiphenyl - 4,4’ - bis-(2 - azo - 8 -amino 1hydroxynaphthalene - 5,7 - disulfonic acid)] ; Carmoisine [naphthalene - 1 - sulfonic acid - 4 - (2 - azo - 1 - hydroxynaphthalene - 4 - sulfonic acid)]; Orange I1 12 - hydroxynaphthalene - l - azobenzene - p - sulfonic acid] and Orange G [2 - hydroxynaphthalene - 6,s - disulfonic acid 1 - azobenzene]. The major diEmeters of these dyes are approximately 30,20,15 and 15 A., respectiyely; that of the aminonaphtholdisulfonic acid is about 10 A.
-
Discussion Ionic size contributes directly to the mechanism (4) T. R . E . K r w m a n and #Ib A. Kir,chearlr, J , C h m , OW,, j l 0 0 (1949).
T.R.E. KRESSM
120
of both cation and anion exchange reactions in two ways: (1) it d e c t s the position of equilibrium, and (2) it affects the rate of attainment of equilibrium. Above a certain minimum size (depending upon the exchanger) it contributes indirectly by determining the total or saturation capacity. For the exchange of the alkali metal, ammonium and hydrogen ions the size function determining the equilibrium is the distance of closest approach a0 in the DebyeHiickel equation,46 the f f i i t y decreasing with increasing ionic size, Le., increasing ao. This and other evidence suggest that the cations are bound to the resin by Coulomb forces. Large organic cations show higher ffiities, provided the pores of the exchangers are of sufficient size and, in contrast to the simple metal ions, the f f i i t i e s increase with increasing ionic size. This suggests that van der Waals forces contribute largely to the affinity, the Coulomb forces being less important. Qualitative support of this is provided by the observation that the afEnities of some quaternary ammonium ions increase with the number of atoms in contact with a surface.6 Considerably less work has been carried out with anion exchangers (see, however, refs. 7, 8, 9), and 1.0 0.8
i0.6
P 6
0.4
0.2
Vol. 56
it is impossiblefrom the data available to determine the contribution made by the size of the anion. The present work, however, indicates that in the absorption of free acids by a weakly basic, highly porous exchanger, the position of equilibrium is independent of ionic size. In so far as ionic size affects the rate of attainment of cation exchange equilibrium two mechanisms can be distinguished, where the rate is controlled by diffusion in the particles of the exchanger (Pmechanism) and in the bounding Nernst film (Fmechanism), respectively, Some evidence of an intermediate (I) mechanism where the rate is influenced by diffusional resistance in both phases has also been obtained.’ The form of the kinetics is often dependent upon circumstances; for example, with the sodium ion on a phenolsulfonate type resin P-kinetics a t relatively high concentration can give way to F-kinetics at lower concentrationlO* ll; and P-kinetics at lower temperature to F-kinetics at higher.12 With quaternary ammonium ions on “Zeo-Karb 215” the kinetics appeared to be controlled by particle diffusion under all circumstances’ and, as would be expected, the rate decreases as the size of the cation increases. The saturation capacity of a sulfonic acid resin is the same for all ions when the ions are very small. With increasing size of the ion, however, a critical size is reached a t which the ion is larger than some of the pores of the resin and a limited capacity only is exhibited. A resin of higher capacity will generally require a higher degree of cross-linking if similar swelling characteristics are to be maintained, because of the hydrophilic nature of the exchange active groups, and this results in smaller pores. Thus the smallest pores of “Zeo-Karb 215” (total capacity 2.8 meq./g. dry H-resin) are larger than the effective size of the phenylben 1dimethylammonium ion (major diameter 11.2 although they are smaller than that of the tetrabenzylammonium and the quininium ions? The smallest pores of “Zeo-Karb 225” (total capacity 5.2 meq./g.) on the other hand, are appreciably smaller than the tetramethylammonium ion (4.6 A. diameter) and are probably about 2 or 3 A. diameter only. Now the rates of exchange of several quaternary ammonium ions with the NH4-form of “Zeo-Karb 225” are found to be controlled by particle diffusion, as indicated by the initially straight lines obtained on plotting Qt/&.. against d t(Fig. 2) and direct evidence of the existence of a concentration gradient within the solid is provided by a discontinuity in the velocity-time curves (Fig, 3) after a period of interruption.12 It would be expected, therefore, that a greater diffusional resistance would be exhibited by “Zeo-Karb 225” for a given quaternary !!ion than by “Zeo-Karb 1215.” However, the times for half-change are, in fact, less with “Zeo-Karb 225” than with “Zeo-Karb 215,” as shown in Table I, suggesting that the larger
8,
1
2
3 4 5 6 4 (t in minutes). of mechanism--“Zeo-Karb 225.”
Fig. 2.-Testa 1.0
0.8
i0.6
e
> 0.4 0.2
5
10
15
20
t (minutes).
Fig. 3.-Rates of exchsnge--“Zeo-&rb
225.”
7
(6) G. E. Boyd, J. Schubert and A. W. Adamson, J . Am. Cham. SOC., 69, 2818 (1947). (10) G. E. Boyd, A. W. Adamson and L. 8. Myerr, iW., 6% 2836 (6) T. R. E. h m a n and J. A. Kitohener, J . Cham. SOC.,1208 (1947). (1949). (11) D. El. Hale and D. Reiohenberg. DSa. Faradau 808.. NO. I (7) W. C. Ba-, Quoted by J. Sohub&, And. CAa., H, 1367 79 (1949). (1051). (12) T. R. 1. -man and J. A. Xitohenu, 5656.. 100. I,101 (8) R. Knnin and R J. Myem, Tms JowxAL, 51, I111 (1947). (1949). ( 0 ) R. Xtrnia rmd R I . M i m , J. Am. Chmn. Sac., PI (1967).
Jan., 1952
ION EXCHANGE SEPARATIONS BASEDUPON IONIC SIZE
pores of the polystyrene resin are concentrated near the external surface of the spheres. The rates of exchange of the sodium ion are practically identical with both resins, aa would be expected where the rate is controlled by diffusion in the liquid film. (In this table the times for half change on. “Zeo-Karb 215” are taken from ref. 13 and are corrected for particle size (“Zeo-Karb 215” 0.45 mm. radius; “Zeo-Karb 225” 0.40 mm. radius) by multiplying the measured hi, for the quaternary ions (PditTusion) by (0.40/0.45)2 and for the sodium ion (FditTusion) by 0.40/0.45). TABLE I Ion
Na+ NMe4+ NEt4+ PhNMe&HrPh+
1.25 mins. 2.7 mins. 10.0 mins. ea. 3 weeks
20 30 Minutee. of N&+ and NE&+ for H + on ‘‘ZeoKarb 215.” 10
Fig. 4.-Exchange
Time for halfyhange “5m-Karb 215” Zeo-Karb 225”
121
1.25 mins. 1.75 mine. 3.0 mins. ca. 1 week
The equilibrium exchange values (i.e., Qo) for these four ions on “Zeo-Karb 225” deserve brief mention. Under the experimental conditions used the equilibrium amounts of NH4+ liberated by the four ions are: Na+ 1.17 meq., N M a + 0.5 meq., NEt4+ 0.74 meq., PhNMsCHzPh+ 1.6 meq. If the saturation capacities were the same for all 10 20 30 the ions these equilibrium values would increase Minutes. in the order Na+, NMe4+, NEt4+, PhNMe2CHz- Fig. 5.-Exchange of N&+ and NEb+ for H+ on “ZeoPh+. The observed order is explained by the Karb 225.” limited saturation capacities of the resin for the organic ions. That for NMe,+ is 76% and that for fold enrichment in 4 minutes occurs with %eoNEtr+ 6Zy0 of the maximum; however, that for Karb 215” the enrichment with “Zeu-Karb 225” PhNMenCHSh+ is unexpectedly high (74% of after the same time is 1.6-fold. the maximum) and the intrinsically high affinity The rates of absorption of acids on “De-Acidite of this ion for the resin enables a comparatively E” likewise decrease with increasing size of the high Qo value to be reached. The abnormally acid molecule, and a specific effect of valency of the high saturation capacity for the PhNMeaCHzPh+ anion is observed, sulfuric acid exchanging at a ion suggests either that some orientation of the greater rate than hydrochloric acid-Fig. 6. molecule occurs in its passage along the resin pores, This is consistent with the results of Kunin and its effective diameter thus being something between Myers* who supposed the effect to be due to the its major (11.2 b.) and its minor (4.6 b.) diameters; greater attractive force between the resin and the or that considerable distortion of the resin struc- more highly charged ion. This is in marked conture can occur if the attractive force is sufficiently trast to cation exchange where increased valency large. In this connection it is noteworthy that the results in a lower rate of exchange.’S bivalent ion [CHZN(CZHS)~]Z++ also exSulphuric acid hibits a higher saturation capacity (83y0 l.o (Interrupted) of the maximum) than its major diameter of 10.7 A. would-suggest. 0.8 The separation of inorganic ions from organic by exploiting their different rates of exchange on “Zeo-Karb 215” has been i dem0nstrated.l The demee of BeDaration Q’ is enhanced when “Zeo-hrb 225” is used d 0.4 L/ // by virtue of the limited saturation caI-omlno- 8 - nophthol-J.6 - dlrulphonk acid pacity for the larger ion. This is illusomngr I trated m Figs. 4 and 5, which show the . 0.2 concentration of H+, NE&+ and NEt4+ in a solution (initially equimolar with reI I I I I 1 I I spect to N H 4 + and NEt4+) during the 10 20 30 40 60 60 10 80 course of exchange experiments with :(minutea). “Zeo-Karb 215” and “Zeo-Karb 225,” Fig. 6.-Ratea of acid abeorption on ‘?)e-Acidite E.” resDectivelv. The solution becomes enriched in NEt4+ in each case, but whereas a 1.3The rates of absorption of l-Smino-8-naphthol3,g-disulf0nic acid, Orange csrmoisme (18) T. R. E. h m s n and J. A. Kitohenor, D h . Fa& Soc., ChlOra501 Sky Blue are very low and it ia evident 80. T, 96 (1949). Oe6
T.R. E. KRESSMAN
122
Vol. 56
with one isvery marked: tl/,values are not available but the times for ‘/c-change are 4.5 and 15 min., respectively. Although Carmoisine possesses two naphthalene rings and Orange G only one, the rates of excrhange are dmost identical: this is 1.o remarkable since, even if the molecule enters with its major axis roughly parallel with that of the pores, the two naphtha0.8 HydrQEhlOPIC acid lene rings would be expected to encounter a greater diffusional resistance than the * 0.6 one. 8 P The curves of Qt/Qm against 2/t: for both “DeAcidite E” and “Decolorite” all 0.4 exhibit an initial straight portion (Figs. 8 and 9) indicating that diffusion through the solid particles is the rate determining 0.2 step, and this is confirmed by the discontinuity in the rate curves exhibited 15 2o 25 30 35 40 45 after periods of interruption (Figs. 6 and 7). This is surprising in the case of the t (minutes). inorganic acids on “Decolorite” since the Fig. 7.-Rates of acid absorption on “Decolorite.” r>ores must be manv times larger than The specific effect of valency on the rate of acid the acid molecules and it woGd be expezed that absorption is observed also with “Decolorite” rapid movement through the pores would occur. (Fig. 7) and it is seen to apply not only to inorganic It suggests that the rate step is not controlled only acids like hydrochloric and sulfuric but also to the by simple diffusion. The effect of valency on the equilibrium position large organic dyes. Thus, Orange G and Orange I1 are of similar molecular structure differing only is very marked with “Decolorite,” where the acids in the positionrand number of SOIH groups. The can be divided into two groups-those with uniconsiderably greater rate of exchange of the former valent anions showing an equilibrium absorption 0.01 meq. per gram, and those with with its two S03Hgroups compared with khe latter of 0.45 bivalent showing an equilibrium absorption of 1.0 0.51 0.01 meq. per gram (see Table 11). The absence of any specific effect of structure is remarkable in view of the extent to which it influences 0.8 the equilibrium position in cation exchange. The comparatively low degree of cross-linkage in the i0.6 resin might, however, account for this. that -a- limited saturation capacity is available for these ions: with “Decolorite” the effect is observed only with the Chlorazol Sky Blue ion (cf. Table 11).
’
*
*
81
‘=
TABLE I1 EQUILIBRIUM ABSORPTIONON “DE-ACIDITEE” AND
0.4
“DECOLORITE” “De-Acidite E” (meq./g.)
0.2
1
2
3
4
5
6
7
4 (t in minutes). Fig. 8.-Tests
of mechanism-“De-Acidite
E.”
*
1.00 1.22
0.44 .50
0.68
.45 .52 .46 .45
..
.24 .36
