ml sample of this dialyzed solution was used giving a 3.06-mm column in the cell. Procedure. The pairs of “locations” (q,, 4,‘ = q n Q) selected for fringe shift measurements must be converted to actual positions in the centrifuge cell ( r , = q,’/2, r,’ = qnf’/z) and using the magnification factor, F, these are converted to distances on the photograph (R%,Rn’)measured from the reference wire image. If the reference wire is located a distance ro from the axis of the rotor, then R = ( r - ro)F. For a 3-mm solution column, qb - q. s 4 cm2 and it is convenient to take Q = 2 cm2. By choosing values of q for which all digits beyond the third are zero, the square roots can be read directly from standard tables. Table I shows an appropriate set of values for q and q’ along with the associated values of r and r ‘ . These particular values can be used without modification in all experiments involving a column height in the neighborhood of 3 mm. For other column heights, a similar approach can be used to choose the pairs of locations for which fringe shifts are to be measured. Thus for a 6-mm column q b - q. G 8 cm2,one can take Q = 4 cm2, and if 10 points are desired the successive values of q are taken at intervals of 0.40 cm2 so that the pairs of “locations” (q,,, 9,’) are : (43.80, 47.80), (44.20, 48.20), . , ., (47.40, 51.40).
+
Table I. Pairs of Locations Suitable for Measurement of Fringe Shifts in 3-Mm Columns and Observed Shifts for Ribonuclease A at Sedimentation Equilibrium n a 1 2 3 4 5 6 7 8 9 10 b
(11) W. F. Harrington and J . A. Schellman, Compt. Rend. Trau. Lab. Carlsberg, Ser. Chim., 30, 21 (1956).
47.47 47.60 47.80 48.00 48.20 48.40 48.60 48.80 49.00 49.20 49.40
4I,,’, cm2
49.60 49.80 50.00 50.20 50.40 50.60 50.80 51.00 51.20 51.40 51.78
rn, cm 6.890 6.8993 6.9138 6.9282 6.9426 6.9570 6.9714 6.9857 7 . m 7.0143 7.0285
r,,’, cm 7.0427 7.0569 7.0711 7.0852 7.0993 7.1134 7.1274 7.1414 7.1554 7.1694 7.196
(A?J)obs
fringes 6.97 7.28 7.59 7.90 8.23 8.63 9.06 9.44 9.85 10.26
yields M = 1.375 X l o 4in excellent agreeme:t with the known value 13,683. With a = 1.2 cm, X = 5460 A, and k = 1.9 X 10-3 dl/g, the observed intercept and slope lead to c(0) = 1.07 x g/dl and 5: = 0.50 g/dl.
RESULTS
Included in Table I are the fringe shifts, AQJ,observed in the sedimentation equilibrium experiment with RNase A. When these are used together with the associated values of qn to construct a graph of log AQJ us. q, a straight line is obtained (Figure l), with slope 0.0937 and intercept -3.617. Under the given experimental conditions, with p = 1.006 g/ml, and taking D = 0.695 ml/g ( I I ) , the observed slope
qn, cm2
ACKNOWLEDGMENT
The author thanks C. C. Chang for technical assistance and Henry Klostergaard for calling attention to the Guggenheim method in connection with transient state studies in the ultracentrifuge and for his continued interest. RECEIVED for review April 12, 1968. Accepted June 22, 1968. Work supported by U S . Public Health Service Research Grant GM 10832 from the National Institute of General Medical Science.
Anion Exchange in DilmethyI SuIfoxide Alan M. Phipps Department of Chemistry, Boston College, Chestnut Hill, Mass, 02167
The use of conventional strong base polyvinyl styrene divinylbenzene anion exchange resins with dimethyl sulfoxide under essentially anhydrous conditions has been investigated. The rate of swelling is generally much slower than in aqueous solution but the resin is eventually swollen to a greater degree. Selectivity coefficients of CI-, Br-, I-, SCN- and C104- with reference to NOs- have been determined by batch and column techniques. The effects on selectivity of the resin phase composition and the water content of the A selectivity solvent system have been studied. sequence in order of resin preference of B r - > CI- > I> NOa- > SCN- > CIO1-which is radicallydifferent from the selectivity sequence of these ions in aqueous solution has been found.
THE USE OF MIXED SOLVENT systems with polyvinylstyrenedivinylbenzene type resins is a well established technique in the field of ion exchange separations. While the influence of nonaqueous solvents on ion exchange selectivity is often considerable, the reasons for it have not been well characterized. This is partly due to the fact that a consideration of the strength of solvent-ion interactions is a relatively recent approach to the phenomenon of selectivity, and partly because mixed solvent systems very often introduce a superimposed liquid-liquid partition process.
Investigations of nonaqueous systems, while limited by very slow exchange rates, may provide useful information in regard to the effect of the solvent on the various factors which determine ion exchange selectivity with resins of this type. The choice of solvents is rather restricted because a solvent must have the ability to swell the resin and a reasonably high dielectric constant to ensure dissociation of electrolytes. These two properties are not necessarily concomitant. Caticn exchange behavior in absolute methanol has been studied ( I ) , and selectivities were found to be very sensitive to the presence of small amounts of water. This is probably true for anion exchange as well, in view of the successful chromatographic separation of glucosides on anion exchange columns in alcohol/water solutions (2). Anion exchange selectivity in anhydrous liquid ammonia has been studied ( 3 ) although comparison with aqueous systems is perhaps partially obscured by the large difference in temperature involved. Dimethyl sulfoxide is a relatively inert solvent with a high dielectric constant which has been found to swell some (1) R. W. Gable and H. A. Strobel, J. Phys. Chem., 60, 513 (1956). (2) H. Ruckert and 0. Samuelson, Acra Chem. Scand., 11, 315 (1957). (3) A. M. Phipps and D. N. Hume, ANAL.CHEM., 39,1755 (1967)r VOL. 40, NO. 12, OCTOBER 1968
0
1769
ion exchange resins. Janauer and coworkers have studied some cation exchange systems in DMSO and DMSO-water ( 4 , 5). Fritz and Gillette (6) have studied the DMSO-water mixed solvent system and determined selectivities for several metal halide complexes. An investigation of the ion exchange selectivities of the simple univalent anions has, therefore, been undertaken in anhydrous DMSO solutions. EXPERIMENTAL Reagents. Dowex 1-X8 200-400 mesh anion exchange resin was washed with ethanol and several times alternately with 1 Msolutions of hydrochloric acid and sodium hydroxide. The chloride, bromide, iodide, nitrate, thiocyanate, and perchlorate forms of this resin were prepared by exhaustive washing on a column with aqueous solutions of the appropriate sodium salt, followed by washing with deionized water until a negative flame test was obtained. The same lot of Dowex-1 was used in all cases. The resin was dried in a vacuum oven at 50 "C over Pz06for a period of one week. The weight capacity for each anionic form was determined before and after equilibration with DMSO by standard methods. Dimethyl sulfoxide (Baker Analyzed Reagent) was for the most part used as supplied. Several representative selectivity determinations were made with DMSO purified by a method based on the distillation described by Payne (7). No significant differences between solvents were found. Procedure. The swollen- OS. dry-volume ratio was determined as a n index of resin swelling. The volume occupied by a 3- to 4-gram column of each anionic form of the resin after a three day equilibration with DMSO was taken as the swollen volume. The dry volume was obtained from the same operation with n-octane. Another estimate of the degree of swelling and measurements of swelling rates were obtained microscopically with a technique similar to that described by Freeman and Scatchard (8). The microscope used was a Bausch and Lomb Model PG252 equipped with an Abbe condenser and built-in illuminator, a 1OX flat field achromatic objective and a 1OX filar micrometer eyepiece. The resin beads were placed in a "micro culture slide'' (A. H. Thomas No. 7046), covered with a No. l l / z cover glass, and sealed with paraffin. Batch equilibrations of 1-gram samples of resin, a mixture of electrolytes containing the same number of milliequivalents as the resin sample, and 25 ml of DMSO were carried out in paraffin sealed 50-ml Erlenmeyer flasks kept in a constant temperature bath at 45 + 0.1 "C for 1 to 2 weeks with intermittent agitation. The resin and electrolyte samples were chosen so that the molarity of the external solution was 0.10 =k 0.01M in each case. The halides and thiocyanate were determined in samples of the external solution by volumetric and gravimetric argentimetry, perchlorate was precipitated and weighed as the tetraphenylarsonium salt (9). The total electrolyte content of the external solution was determined by diluting an aliquot approximately 10-fold with water, passing through a hydrogen form Dowex 50w-X8 cation exchange column and titrating the liberated hydrogen ions with standard sodium hydroxide. The volume of the external solution was corrected for solvent imbibed by the resin as determined from swelling measurements. (4) G. E. Janauer, J. T. Carrano, and M. VanWart, 3rd Middle
Atlantic Regional Meeting; Amer. Chem. SOC.,Philadelphia, Pa., February 1968. ( 5 ) G. E. Janauer, J. T. Carrano, and M. VanWart, 155th National Meeting; Amer. Chem. SOC., San Francisco, Calif., April 1968. (6) J. S. Fritz and M. L. Gillette, Talanta, 15, 287 (1968). (7) R. Payne, J. Amer. Chem. SOC.,89,489 (1967). (8) D. H. Freeman and G. Scatchard, J. Phys. Chem., 69, 70 (1965). (9) D. J. Glover and J. M. Rosen, ANAL.CHEM., 37, 306 (1965). 1770
ANALYTICAL CHEMISTRY
Time, Minufss
Figure 1. Rate of swelling of Dowex 1- X8 anion exchange resin in DMSO Unswollen diameter 0.09-0.10 mm. Ionic form: A = C104-, A = NO;-, = CI-,o = Br-
=
SCN-,
Column chromatographic experiments were accomplished with a 20-cm West condenser fitted with a drying tube at the top and a stopcock made of Teflon (Du Pont) at the bottom. The column was maintained at 45 f 0.1 "C with a Haake Model F constant temperature circulator. RESULTS AND DISCUSSION Swelling. The swollen- us. dry-volume ratios for each form of Dowex 1-X8 used, as determined by the column method are given in Table I. These values are reproducible within a range of 2%. Because the assumption is made that the ratio of the void volume in the swollen column us. the void volume in the dry column is unity, two theoretically more accurate methods of determining resin swelling were attempted. The average diameter of approximately 25 resin beads was measured with a microscope with a micrometer eyepiece before and after a 24-hour equilibration in DMSO. Volume ratios determined in this way were in general 5-10% lower than the values given in Table I but were not reproducible better than + l o % in groups of 25. The resin beads are quite spherical, as shown by making 4 measurements at 90" intervals, but are apparently not uniform in regard to distribution of fixed ionic groups. This irregularity was also indicated in swelling rates which were obtained from measurements of bead diameter at intervals beginning immediately after addition of DMSO to the culture slide. Representative swelling curves in terms of percentage of total swelling us. time are shown in Figure 1. Swelling rates differing by as much as 25% in the time required to reach one half of the final swollen volume were observed even between beads of the same initial diameter less than 0.2 mm apart on the culture slide. A pycnometric determination of resin swelling was attempted in which suction dried, DMSO swollen beads were compared to dry beads in terms of the displacement of noctane. The volume ratios obtained by this procedure were in general 5-10% higher than those given in Table I but were not reproducible, and the technique was considered unsuitable for 200-400 mesh resin beads. The aqueous volume ratios obtained with the column technique for the resin forms listed in Table I fall in the range of 1.31 i .03 with the exception of the perchlorate form which is only slightly swollen. The swelling is essentially
t
3-'
ml 0.1 M
1
0
.I
.2
A
.3
.5
6
I o n i c Composition 01
Resin
.7
.E
NH,NO,
Eluenl
I
.9
Ti,
Figure 3. Elution of anions from a Dowex 1- X8 200-400 mesh column with 0.1Mammonium nitrate in DMSO
Figure 2. Dependence of the selectivity coefficient on the ionic composition of the resin phase Ion A: A
=
SCN-,
=
Clod-,
=
CI-, 0
=
Br-
the same in liquid ammonia as in water. Somewhat greater swelling is observed in methanol and somewhat less in acet onitr ile . No consistent physical explanation can be given for the degree of swelling which is exhibited by ion exchange resins. Electrostatic repulsion of the fixed ionic groups, solvation of these groups, the counterions and the resin matrix, and association between the fixed groups and the counterions have all been convincingly suggested for specific cases. The polyvinylstyrene-divinylbenzene resin matrix is assumed not to be swollen by DMSO because the cation exchange resin Dowex 50w-X8, which is prepared from the same polymer beads as Dowex 1-X8, shows no swelling by the column method in either the sodium or potassium forms. (The hydrogen and ammonium forms are swollen to a considerable extent.) This latter behavior is of particular interest, for on the basis of conductivity data (IO, I l ) , anions are commonly held to be unsolvated in DMSO while sodium and potassium are highly solvated. Selectivity. The experimental index of selectivity was taken to be the mass law concentration product ratio for the assumed exchange reaction : RB
+ A - e R A + B-
resins of this type, very slow rates of exchange were encountered. In some DMSO solutions containing bromide ion, equilibrium was found to be incomplete after two weeks at 25 "C. Use of higher temperatures is limited by the tendency of the resin to decompose on heating above 60 "C. The value of 45 "C was chosen on the basis of a discontinuity at approximately this temperature in the plot of refractive index us. temperature reported by Schlafer and Schaffernicht (12). This discontinuity has been interpreted as an indication of the break up of associated chains of DMSO. DMSO has a viscosity of 1.45 centipoise at 45 "C us. 2.20 c.p. at 25 "C (12). Selectivity coefficients determined at = 0.5 are given in Table 11. The aqueous values given for comparison were determined at 45 "C with the same procedure and the same lot of resin. Ammonium ion was employed as the co-ion in all of the exchanges listed in Table 11. The same K8 value was obtained for Br-/NO,- exchanges in which sodium or potassium ion was used as the co-ion. In all cases except (12) H. L. Schlafer and W. S. Schaffernicht, Ai7gew. Chem., 72, 618 (1960). ~
(1)
This selectivity coefficient is defined as
Table I. Swelling of Dowex 1 -X8 in Dimethyl Sulfoxide Ionic form Volume (swollen)/vol (dry) c104SCNNO$-
where and X refer to the ionic fraction in the imbibed internal solution (resin phase) and in the external solution, respectively. Values of the selectivity coefficient were determined for chloride, bromide, iodide, thiocyanate, and perchlorate as ion A- with the nitrate ion as ion B- . Equilibrium distributions of these anions between Dowex 1-X8 and 0.10 f .01M solutions were determined at 45 "C. As in most cases involving strictly nonaqueous ion exchange with (10) P. G. Sears, G. R. Lester, and L. R. Dawson, J. Phys. Chem., 60, 1433 (1960). (11) J. S. Dunnett and R. P. H. Gasser, Trans. Faraday Soc., 61, 922 (1965).
1.68 1.68 1.60 1.53 1.49
Brc1-
xA
Table 11. Selectivity Coefficients at = 0.5, 45 "C AKe A-/NOa- (DMSO) Ks A-/NOa- (H20) Br2.81 0.89 c1ISCNc104a = 0.61. b X A = 0.67.
2.54 1.4 0.63 0.48
0.31 5.2a 7.8b 19
ZA
VOL. 40, NO. 12, OCTOBER 1968
1771
the I-/NO,- exchange, K, values obtained from equilibrations beginning with the resin initially in the nitrate form were checked with equilibrations beginning with the A- form resin. The iodide form of Dowex 1 reacts with DMSO to a significant extent producing iodine and dimethyl sulfide. Therefore, only the nitrate form resin was used for the I-/NOIexchange. At 45 “C, 0.1M DMSO solutions of ammonium iodide appear to be stable indefinitely, but a slight yellow color is produced by the addition of nitrate form Dowex 1. Determinations of the total ionic content of the external solution in each exchange indicate that equivalent exchange between phases is achieved to within a range of 2 z . This is considered to be a fair estimate of the reliability of the calculated K, values. The variation of K, with 3, is shown in Figure 2. The concentration of the external solution was maintained at O.lOM, so that a different analytical method would be required for the determination of reliable selectivity coefficients values higher or lower than those shown. A similar at behavior is observed for halide and nitrate exchanges in aqueous solution (Dowex-2) but aqueous exchanges involving thiocyanate and perchlorate are markedly dependent on the composition of the resin (I.?). Although the low exchange rate and high solvent viscosity would severely limit the usefulness of anhydrous DMSO in chromatography, the column chromatographic behavior of these anions was investigated in the interests of further characterization of exchange phenomena in this solvent. A composite of representative elution curves obtained under conditions matching those of the batch equilibrations is shown in Figure 3. Milliequivalent amounts of ammonium perchlorate, thiocyanate and chloride were eluted from a 14-cm nitrate form Dowex 1-X8 column maintained at 45 + .1 “C. The eluent was 0.1Mammonium nitrate at a flow rate of 0.59 ml/cm/min. By making use of an equation originally formulated by Mayer and Tompkins (14) in the application of the plate theory to ion-exchange chromatography, an estimation of the selectivity coefficient may be obtained from the position of the peak in the elution curve. In this way K, values (GS. nitrate ion) of 0.52, 0.71, and 1.5 were calculated for Clod-, SCN-, and C1-, respectively. A comparison of these values with those obtained from the batch equilibrations indicates that the attainment of equilibrium in each “plate” is essentially complete for perchlorate and thiocyanate and about 60 in the case of the chloride exchange. When a mixture of the four anions under investigation is eluted from the same column with 1.OM ammonium nitrate only one peak is observed in the elution curve. With the concentrated eluent, a K, value of 1.2 is calculated for both the SCN-/NOa- and C1-/No3- exchanges and a value of 1.5 is obtained for the Br-/N03- exchange. The elution curves for chloride and bromide are shown in Figure 4. These curves exhibit the conventional appearance of the chromatographic elution of ions with about the same (Cl-) and greater (Br-) affinity for the resin phase than the ion used as eluting agent. The flow rate for the 1.0 M eluent was 0.32 ml/cm/min and is as high as is possible for this column without the application of pressure. Since this rate represents less than 0.2 ml/min, a longer column is impractical with 200-400 mesh resin.
-
NOS- > Clod- > Br- > C1- > I-. This is different from the sequence predicted by aqueous conductance but equally inapplicable to observed selectivities. In the theory of anion exchange selectivity outlined by Reichenberg (16) the counter ion must lose some molecules of solvation in order to become associated or influenced by the fixed ionic group. Selectivity, therefore, is governed by the energy of solvation, and a correlation is made between aqueous selectivity and hydration enthalpy. In the absence of solvent-ion interaction, selectivity should be dependent (15) F. Helfferich, “Ion Exchange,” McGraw-Hill., New York,
(13) H. P. Gregor, J. Belle, and R. A. Marcus, J. Amer. Chem. SOC., 77, 2713 (1955).
(14) S.W. Mayer and E. R. Tompkins, ibid., 69, 2866 (1947). 1772
0
ANALYTICAL CHEMISTRY
1962, Chapter 5. (16) D. Reichenberg in “Ion Exchange Vol. I,” J. Marinsky, Ed., Marcel Dekker, New York, 1966, Chapter 7. (17) R. M. Diamond and D. C. Whitney, ibid., Chapter 8.
\
mide ions. These two ions have nearly identical electrostatic attraction values calculated from crystal radii. Their DMSO selectivity coefficients are very close and, in fact, undergo a reversal in the order of resin preference above = 0.8. The selectivity sequence observed in DMSO is the same as is observed in anhydrous liquid ammonia at -74 "C. Moreover, the numerical values of K8 are in close correspondence. The influence of added water on the exchange behavior of anions in DMSO solution may be seen in Figure 5. Selectivity coefficients were determined from batch equilibrations of Dowex 1-X8 with solutions containing various proportions of DMSO and water. If K, values were determined only by electrostatic forces, the influence of the solvent would be primarily environmental. There is at least a superficial similarity between the variation in K , for the Br-/N03- and Cl-/NOs- exchanges and a plot of the dielectric constant at 25 "C us. the solvent composition for the DMSO-water system from the data of Lindberg and Kenttamaa (18). Considering the number of factors involved in the overall phenomenon of ion exchange, a simple relationship between K, and the dielectric constant is not be be expected. [Calculation of the product (K,D2 loF3)gives a value of 7.1 i 0.6 for the Br-/NO,- exchange and a value of 5.5 f 2.0 for the Cl-/NOI- exchange. In the latter case the variation of K, with is significantly greater, as is indicated in Figure 2.1
xA
I 0
.I
.2
.3
?)
Mole
.5
Froction o f
.7
.6
.E
I .9
1.0
DMSO
Figure 5. Dependence of the selectivity coefficient on solvent composition in the water-DMSO mixed solvent system. Ion A: A = SCN-,
W =
clod-,
=
C1-, 0 = Br-
only on electrostatic forces. On the basis of X-ray crystallographic data, Reichenberg lists an electrostatically controlled sequence of C1- > Br- > I- > c104-.Omitting the nitrate and thiocyanate ions, for which data of this type do not appear to be available, this is the selectivity sequence observed in DMSO except for the reversed order of the chloride and bro-
xTA
RECEIVED for review April 22, 1968. Accepted June 28, 1968. (18) J. J. Lindberg and J. Kenttamaa, Suomen Kemistiiehti, B33, 104 (1960).
Chemical Separations Using a-Hydroxyisobutyric Acid Solutions and Both Cation and Anion Exchange Resin Hazel D. Perdue, Alice Conover, Nancy Sawley, and Ruth Anderson Lawrence Radiation Laboratory, Unicersity of California, Livermore, CaI$. Rapid chemical separations of some cationic species were achieved in tracer studies with a-hydroxyisobutyric acid solutions adjusted to pH 5 on 9-cm columns of Dowex-50-X8, 100-200 mesh ion exchange resin. Column capacity permitted use of 10 to 20 mg of a single carrier. Elution sequence is included for Y, Zr, Th, the rare earths, Fe, Pu, Am, Pb, Ca, Sr, and Ba (separated from Ra). Zirconium has been found to elute rapidly in a-hydroxyisobutyric acid solutions (pH 1,8), giving quantitative separations from rare-earth and actinide elements which do not elute under these conditions. The procedure also separates from95-97 Nb daughters.
SIMPLERAPID CHEMICAL separations were achieved with ahydroxyisobutyric acid solutions on a small column of Dowex 50-X8, 100-200 mesh cation-exchange resin, using tracers and up to 20 mg of a single carrier. This report covers studies of the behavior of calcium, iron, plutonium, zirconium, niobium, thorium, americium, lead, the rare earth group, strontium, yttrium, barium, and radium when eluted from Dowex 50-X8, 100-200 mesh cation resin with a-hydroxyisobutyric acid solutions (a-HIBA) adjusted to pH 5 with NH4OH. Studies with tracer 95Zr-95Nb and carrier Zr suggested a complex elution behavior in pH 5 , a-HIBA solutions. The
95Nb daughter was separated by elution with 0.25M a-HIBA (pH 5). Further recovery of Zr suggested reactions dependent upon pH and time of contact with the eluent. However, zirconium was separated and recovered quantitatively from 95-97Nbtracer by elution with 0.5M a-hydroxyisobutyric acid (a-HIBA) solutions (pH 1.8), after the 95-g6Nbdaughters were removed with 5 ml of 0.125M a-HIBA solution (pH 1.8). Rare earths, actinides, alkaline earths, and Fe were not eluted under these conditions. Zirconium-95,-97 was also isolated (using 10-mg Zr carrier) from solutions containing fission products from 1016 fissions and gram quantities of Fe3+,AI3+,and other soil constituents. Combination of simple hydroxide precipitation steps and use of Dowex 21K anion exchange resin in 6 M HCI acid system reduced total cations to the capacity of the 9-cm column of Dowex 50-X8 resin used. Final precipitation as ZrP207gave 70% yield of the Zr carrier when pH 1.8 a-HIBA solutions were used for Dowex 50-X8 elution, but only ~ 3 5was recovered when pH 5 solutions were used, Niobium, insoluble in dilute HNO, solutions, is not chemically bound to the resin. Tracer Nb deposits as the hydrated oxide on glass and resin surfaces; however, it complexes rapidly with dilute solutions of a-HIBA. The separation from Zr is good (decontamination factors of 100).
-
VOL. 40, NO. 12, OCTOBER 1968
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