Ion exchange

Development of ion-exchange processes for isolating individual lanthanons and yttrium has been prompted by: (1) a need to separate small amounts of rare earths from each other to facilitate analyses of mixtures; (2) a need to isolate gross quantities of highly purified individual lanthanides and yttrium, to supply increasing demands of research and technology. One might expect that a single procedure would perform both tasks equally well; but, unfortunately, this is not so. For analytical purposes, separations of microgram and milligram quantities

SEPARATION CHEMISTRY 87 are p e r f o r m e d m o s t rapidly and efficiently by the m e t h o d o l o g y of "elution c h r o m a t o g r a p h y " and by " e x t r a c t i v e c h r o m a t o g r a p h y " . F o r p r o d u c t i o n of useful quantities, m e t h o d s related to " d i s p l a c e m e n t c h r o m a t o g r a p h y " h a v e b e e n found to be the m o s t economical.

5.1. C a t i o n elution c h r o m a t o g r a p h y

Plate t h e o r y was a d a p t e d to partition c h r o m a t o g r a p h y by Martin and Synge (1941) and to ion-exchange c h r o m a t o g r a p h y b y Mayero,~lT~,,e~,~i~947). In the d e v e l o p m e n t of this t h e o r y , three a s s u m p t i o n s w e r e made: (1) an i o n - e x c h a n g e column can be c o n s i d e r e d to be a s t a c k of hypothetical c o m p a r t m e n t s of uniform length, each comprising an effective theoretical plate; (2) the solute concentrations in the e x c h a n g e r and in the interstitial solution are u n i f o r m and at equilibrium with each other within the confines of a given plate; (3) the column loading and elution conditions are such that a distributing cation constitutes only a minute f r a c t i o n of the total n u m b e r of e x c h a n g e a b l e ions in any plate, so that its sorption c a n be considered a linear function of its c o n c e n t r a t i o n in the interstitial solution, i.e., its distribution coefficient can be regarded as a constant.

It was pointed out b y G l u e c k a u f (1955) that the shape of the elution c u r v e m a y be r e p r e s e n t e d by e x p r e s s i o n s :

c = c* e x p [ - N ' ( v * -

v)Z/2vv],*

^{logl0 c}⁼ I O g l 0 C * - - 0.217N'(v* - v ) 2 / 1 ) / ) *

and that the m a x i m u m c o n c e n t r a t i o n of a c o m p o n e n t A is related to the total a m o u n t (in meq) of A loaded by the expression:

c* = ( m / v * ) ( N ' / 2 1 r ) 1/2,

where c = c o n c e n t r a t i o n of A in the effluent solution (meq/ml), c* = m a x i m u m concentration of A in the effluent (meq/ml), v = volume of effluent at which c o n c e n t r a t i o n c is noted (ml), v* = v o l u m e of effluent at which the m a x i m u m c o n c e n t r a t i o n c* is noted (ml), N ' = ½(N - No) = the n u m b e r of theoretical plates f r o m the center of the original b a n d to the b o t t o m of the column (actually with light loading, N ' ---- N ) , m = a m o u n t of A loaded (meq).

The n u m b e r of theoretical plates in a given column under a given set of conditions is c u s t o m a r i l y c o m p u t e d f r o m an e x p e r i m e n t a l elution curve. In t e r m s of k n o w n and m e a s u r a b l e quantities,

N = L / h = 2 7 r ( c * v * / m ) 2-~ 8 ( v ' / w ) 2,

where L = length of bed, h = theoretical plate height (same units as L), w = band width on a volume of effluent basis (ml) m e a s u r e d where c = c * / e = 0.368c*, i.e., the m e a n band width.

N o t e that h (and c o n s e q u e n t l y N and w) d e p e n d s on the kinetics of e x c h a n g e rate of the species in question, as well as the set of e x p e r i m e n t a l conditions chosen.

If Kd r e m a i n s c o n s t a n t for all c o n c e n t r a t i o n s of the c o m p o n e n t under consi- deration, a c o n s t a n t fraction of the c o m p o n e n t is always in solution and being

88 J.E. P O W E L L

transported by the mobile phase. Consequently, the separation factor or distribution coefficient ratio can be expressed in terms of the respective volumes at which the maxima in individual component concentrations appear in the elution curve.

ot A = K d A / K d B = ( V * -- VO)/(V~( -- Vo) = Vf~/V~,

where v0, the free volume in the column, includes both the interstitial volume of the bed and other dead space in the system (i.e., that between the bottom of the bed and column outlet).

Performances of elution chromatographic systems are frequently compared by the resolution of individual components, defined as twice the difference between peak volumes divided by the sum of the approximated peak widths, WA+ Ws, on a volume basis, by Ambrose et al. (1960). That is: R = 2(v]] - v * ) / ( W A + Ws).

~ v 2 -

I ~ V B

Others, e.g., Dybczynski (1970), favor a statistically more exact relationship:

R = ( v ~ - v ~ ) l n ( ~ r A + ~ B ) , where: o - = w l 2 x / 2 is the standard deviation of the particular peak width (see definition of w given previously).

Much use has been made of cation-exchange elution chromatography, since the subject was comprehensively reviewed by Powell (1964), as a means of analyzing traces of rare earths encountered in rocks and minerals and in fission-produces mixtures. For the most part, however, few improvements have been made in the basic techniques.

Gradient-elution techniques date back to a 9 h, pH-gradient elution of complex lanthanon mixtures with lactate described by Nervik (1955), a 30min, pH- gradient resolution of a Y - T b - E r - T m - L u mixture with glycolate by Stewart (1955), and a 5 min, concentration-gradient separation of Yb and Lu activities by Preobrazhenskii et al. (1957) using lactate. Further application of the pH- gradient technique, employing ammonium o~-hydroxyisobutyrate (which is superior to either glycolate or lactate) has been reported by Wolfsberg (1962), Massartl-~c,~'¢e(1963), and Foti et al. (1967). The latter performed a tracer-level separation of 9 rare earth elements in 14 h with 0.25 Moe-hydroxyisobutyrate in the pH range 3.83-8.34, on a - 2 0 0 + 4 0 0 mesh, ammonium-form, Dowex 50W- X4, resin bed, 47 inches long and 3 mm in diameter, at room temperature, using a flow rate of 0.125 ml/min.

With fission-product mixtures and in some cases of activation analysis it is expedient, even imperative, to complete an analysis within a short time span.

Campbell (1973), working with a H I B in pressurized Dowex 50W-X8 systems,

SEPARATION CHEMISTRY 89 has exploited very small resin particle sizes, elevated temperature, and the concentration-gradient elution technique to accomplish resolutions of 15- component, rare earth mixtures in less than 2 h. In one experiment, 75 mg of lanthanon mixture (5 mg of each) was loaded on a 33 cm long, 0.9 cm diameter, -400 mesh, Dowex 50W-X8, resin bed and eluted with excellent results at a flow rate of 10 ml/min with pH 4.4, ammonium o~HIB, employing a stepwise increase in eluant concentration from 0.1 to 1.0 molar. The elution time required was

100 rain.

In confirmation of the work of Nishi and Fujiwara (1964), Karol (1973) found that 2-hydroxy-2-methylbutyrate, the next higher homologue, provides a larger separation for most Ln 3+ pairs than a-hydroxyisobutyrate.

5.2. Anion elution chromatography

Separation of lanthanons on anion-exchange columns depends upon the ability of tervalent cations to form anionic complexes of differing stability with negatively charged ligands. The art dates back to a report by Hoffman,O~,,,~l~1950) than ~47pm and 154Eu tracers could be resolved on a Dowex 1 anion-exchange column, pretreated with citric acid, by sorption from and elution by 0.0125 M citric acid solution at pH 2.1. The rough separation factor, calculable from their observed Pm and Eu peak volumes was only 1.3; and the order of elution was P m - E u . . . the reverse of that observed in cation-exchange elutions.

Lanthanons are not sorbed to any appreciable extent on anion exchangers from aqueous HCI, HNO3 and H2SO4, and are only weakly sorbed from solutions of salts such as sulfites, sulfates, thiosulfates, nitrates, nitrites and thiocyanates. M a r c u s , ~ ~ - ~ 9 5 9 ) , however, demonstrated that gradient elution, from 6 M to 3 M, with LiCI at 78°C would elute cations in the sequence C s - B a - Y b - E u - S m - N d - P r - C e - L a . From t h i s it is apparent that the lighter lanthanons form the most stable anionic species with CV.

Hamaguchi et al. (1965), using 3 M Mg(NO3)2, demonstrated that the lighter lanthanons could be resolved in the order G d - E u - S m - N d - P r - C e - L a (as with C1-), but that the heavier rare earths, E r - L u , separated very little.

K o r k i s c h o ~ Y'~¢~(1961) and Faris ,~q~.~t~1962) demonstrated that adding methanol to aqueous HNO3 systems markedly improved the distribution coefficients for sorption of lanthanons in anion-exchange systems; and Faris et al. (1962) listed separation factors relative to gadolinium from which the following separation factors for adjacent lanthanons were estimated

Other water-miscible solvents also enhance Ln distribution coefficients, includ- ing: higher alcohols, acetone, cellosolve, dioxane and tetrahydrofuran. Methanol is the solvent of choice, however, simply because it is a better solvent for rare earth nitrates than most of the others.

Molnar et al. (1967) reviewed previous anion-exchange separations and improved the distribution coefficients somewhat by employing nitrate-form A m - berlite IRA-400 and 80% MeOH-20% aqueous NH4NO3. Log Kd increased

90 J.E. P O W E L L TABLE 22.3

Separation factors for D o w e x 1 with 90% M e O H - 1 0 % (1 M HNO3).

Pair a z z+l Pair ct z+t Pair a z+l

L u - Y b 1.0 D y - T b 1.1 P m - N d 2.6 Y b - T m 1.0 T b - G d 1.3 N d - P r 2.2 T m - E r 1.0 G d - E u 1.6 P r - C e 1.7 E r - H o 1.0 E u - S m 1.9 C e - L a 1.7 H o - D y 1.0 S m - P m 2.3

linearly with NH4NO3 concentration in a parallel manner, so that the separation factors remained virtually constant.

Faris (1967) studied the elution of rare earths, on a-hydroxyisobutyrate- charged Dowex 1-X4, with 0.0125 M a H I B in 25% a q u e o u s methanol (propanol, ethanol, isopropanol, tetrahydrofuran, acetone, cellosolve, dioxane, etc.). The elution order was L a - C e - P r - N d - S m - E u - G d - T b - D y - H o - E r - T m - Y b - L u . . . the reverse of that observed in cation-exchange elution experiments with a H I B . Yttrium eluted with Dy. From the distribution data, the individual separation factors appear to diminish from about 2.8 for L a - C e to about 1.1 for Yb-Lu.

Dybczynski (1959) initiated an investigation of the anion-exchange elution behavior of lanthanides in ethylenediamine-N,N,N',N'-tetraacetate systems and continued this work in collaboration with Minczewski (1962). They proposed that the separations observed stem from the varying ability of the hydrated LnCh- species to undergo the exchange: 2LnCh- + H2Ch2-~ 2LnCh- + H2Ch 2-.

Their assumption was that associations of LnCh with -N(CH3)~ on the resin lattice are somewhat analogous to the solubilities exhibited by NaLnCh salts in water given by Marsh (1955) and dependent on the hydration of the cations. The distribution coefficients first increase from La to Eu and then decrease from Eu through Lu, while the solubilities of the NaLnCh salts decrease from La to Sm or Eu and then increase. The elution sequence at room temperature is: L u - Y b - T m - E r - - - Y - H o - L a - D y - C e - T b - P r - N d - G d - P m - S m - E u , with light Ln species interspersed among the heavy ones. Dybczynski (1964) next demonstrated that raising the temperature effected an increase in the quality of the separation. Not only did the exchange kinetics and theoretical plate heights i m p r o v e . . , values of the distribution coefficients underwent changes, so that fewer light Ln species were interspersed in the heavy Ln sequence than at room temperature. The sequence at 92°C became: L u - Y b - T m - E r - Y - H o - D y - T b - G d - L a - E u - S m - C e - P m - P r - N d . That is, La shifted its position three places as the distribution maximum changed from Eu to Nd. An additional benefit of elevated temperature was that reduced viscosity diminished the hydraulic resistance and permitted greater flow rates without the necessity of a large increase in pressure.

Dybczynski (1967) expanded upon the various phenomena observed with temperature changes. He cautioned that increasing the temperature does not necessarily lead to improved separations. Both the separation factor and plate

SEPARATION CHEMISTRY 91 height can either decrease or increase (or remain constant) with an increase in t e m p e r a t u r e . . , depending on the species to be separated and the ion-exchange system employed.

According to D y b c z y n s k i (1970), optimum resolutions in anion-exchange separations of lanthanon pairs on D o w e x 1 are achieved with nominally 4%

crosslinked resin (Dowex l-X4). Although separation factors increased rather regularly with increased crosslinkage (from X2 to X16), the plate height was the factor that determined the quality of the resolution and was lowest in 4%

crosslinked resin beds. Plate height increased by two orders of magnitude in going from X4 to X16 resin. A sieve effect, heralded by a sudden drop in distribution coefficients, was noted at the higher crosslinkings; because a resin that c a n n o t b e c o m e highly swollen with w a t e r cannot utilize all of its potential capacity in sorbing large species. On the other hand, resins that swell ex- cessively dilute their capacity to sorb species by ion exchange.

Papers by W o d k i e w i c z o , ~ b , ~ s ~ 9 6 7 , 1972) and D y b c z y n s k i ~ d W~,lk~,,~

regarding the merits of D C T A

(trans-l,2-diaminocyclohexane-N,N,N',N'-

tetraacetate), c o m p a r e d to E D T A , as an eluant in the anion-exchange separation of lanthanons, reveal that D C T A is inferior to E D T A for this purpose in most respects. The individual distribution coefficients are (for the most part) lower;

the separation factors are smaller; and more sluggish exchange kinetics cause larger theoretical plate heights than are o b s e r v e d with E D T A under comparable conditions. Similar to the n o n m o n o t o n i c sequence o b s e r v e d with E D T A , the elution order with D C T A is: L u - Y b - T m - E r - Y - H o - D y - T b - L a - G d - E u - C e - P r - S m - N d - P m , at 25 °. As with E D T A , the best c o m p o n e n t resolutions were obtained with D o w e x l-X4 resin.

The sieve effect due to high resin crosslinkage was even more p r o n o u n c e d with D C T A than with E D T A chelate s p e c i e s . . , probably because of greater bulk.

S c h o e b r e c h t s et al. (1973) s~tudied the distribution of trivalent lanthanides between

N~-hydroxyethylethylenediamine-N,N,N'-triacetate-

and nitrilotriacetate-charged D o w e x l-X4 resin and solutions of H E D T A or E D T A . The experimental results indicated that the extraction mechanism is a chelating

• reaction b e t w e e n H E D T A or N T A sorbed on the anion-exchange sites of the resin and the 1:1 complex, L n ( H E D T A ) or L n ( E D T A ) - , in the aqueous phase.

The kinetics of reaction were rapid, and the theoretical plate heights obtained from column elution experiments were in good agreement with this chelation model (according to the investigators). T h e y remarked that Kd of the lanthanons, in that system in which N T A was on the resin a n d E D T A was in the aqueous phase, increased in order T m - T b - G d , in agreement with the trend r e p o r t e d by Geier~M/4~-k~(1971) for formation constants of the mixed L n ( E D T A ) ( I D A ) 3 species f o r m e d from L n ( E D T A ) - and IDA 2-. T h e y further c o m p a r e d Geier's data trend with --zig values given by Brficher et al. (1972) for the p r e s u m e d reaction:

H2(EDTA) 2 + 2 L n ( E D T A ) ~ 2 L n ( E D T A ) + Hz(EDTA) 2-

92 J.E. POWELL

and noted a r e m a r k a b l e correlation which might be an indication that the m o d e of distribution in the E D T A and D C T A s y s t e m s studied by M i n c z e w s k i (1962) and W o d k i e w i c z , I h ~ 9 6 7 ) w a s analogous to that in their mixed e x c h a n g e systems.

As a m a t t e r of fact, on a tracer scale, the slope of -½ o b s e r v e d b y M i n c z e w s k i in log-log plots of Kd vs. [H2(EDTA) 2-] does not differentiate b e t w e e n the reaction f a v o r e d b y Minczewski,

2 L n ( E D T A ) - + H2(EDTA) a ~- 2 L n ( E D T A ) - + H2(EDTA) 2- and the reaction

2 L n ( E D T A ) - + 3H2(EDTA) 2 ~,~ 2LnH2(EDTA)~- + H2(EDTA) 2

required to a c c o u n t for sorption of a singly charged lanthanon chelate on a diprotonated E D T A anion already on the resin. T h e first r e a c t i o n is d i s f a v o r e d by y o u r writer, h o w e v e r , on the grounds that the o b s e r v e d K s values would imply that the anion e x c h a n g e r was exhibiting a p r e f e r e n c e f o r m o n o v a l e n t anions o v e r divalent anions. In the latter case, t e r v a l e n t anions being f a v o r e d o v e r divalent anions agrees with the usual o b s e r v a t i o n . This reaction is, of course, c o m p l e t e l y analogous to the m e c h a n i s m p r o p o s e d by S c h o e b r e c h t s et al.

(1973) for L n ( E D T A ) f r o m the a q u e o u s phase associating with r e s i n - b o u n d H ( N T A ) 2-.

2 L n ( E D T A ) - + 3 H ( N T A ) 2 ~ 2 L n H ( E D T A ) ( N T A ) 3- + H ( N T A ) 2

In the case of L n ( H E D T A ) associating with resin bound H E ( H E D T A ) - and H ( H E D T A ) 2-, h o w e v e r , the aqueous 1 : 1 chelate species bears no charge and the reactions:

L n ( H E D T A ) + H 2 ( H E D T A ) - ~ L n H E ( H E D T A ) ~

L n ( H E D T A ) + 2 H 2 ( H E D T A ) - ~ L n H ( H E D T A ) ~ + H 3 ( H E D T A ) L n ( H E D T A ) + H ( H E D T A ) 2- ~ L n H ( H E D T A ) ~ -

2 L n ( H E D T A ) + 3 H ( H E D T A ) 2- ~ 2 ( L n ( H E D T A ) , 3- + H 3 ( H E D T A )

are stoichiometrically feasible. T h e last one could, in tracer-scale e x p e r i m e n t s at 2.25, a c c o u n t for the -½ slope in log-log plots of Ks vs. total H E D T A c o n c e n t r a t i o n o b s e r v e d b y S c h o e b r e c h t s et al. (1973); but none of the a b o v e a c c o u n t s for the o b s e r v a t i o n that the slope changed f r o m -½ to - 1 w h e n the p H changed f r o m 2.25 to 6.00. On the other hand, a slope of - 1 would be e x p e c t e d if the e x c h a n g e reaction at p H 6 (where H ( H E D T A ) 2- c o m p r i s e s nearly 100% of the anions p r e s e n t in b o t h phases) were:

H L n ( H E D T A ) 2- + H ( H E D T A ) 2- ~ H L n ( H E D T A ) 2 + H ( H E D T A ) 2-

This course of e v e n t s would s t e m f r o m prior f o r m a t i o n (almost 100%) of the H L n ( H E D T A ) 2- species f r o m L n ( H E D T A ) and H ( H E D T A ) 2- in the a q u e o u s phase.

SEPARATION CHEMISTRY 93 W h a t e v e r the actual m e c h a n i s m in a n i o n - e x c h a n g e distributions of rare earths b e t w e e n an a m i n o p o l y c a r b o x y l a t e - c h a r g e d resin and an a q u e o u s solution of the same (or different) a m i n o p o l y c a r b o x y l a t e anion, the L n distribution c u r v e (log Ko vs. Z ) p e a k s at (or near) the same Z as does the second step chelate f o r m a t i o n constant. F o r e x a m p l e ; in the case of H2(EDTA)2-/H2(EDTA) 2-, M i n c z e w s k i et al. (1962) f o u n d the Ka a m a x i m u m at a b o u t Z = 63 (Eu), c o r r e s p o n d i n g rather well with the K2 value m a x i m u m o b s e r v e d b y Briicher et al. (1975) at Z = 62 (Sm).

The order, Kacd>KaTb'>KaTm, given b y S c h o e b r e c h t s et al. (1973) in the H ( N T A ) 2 - / H z ( E D T A ) 2 and H ( H E D T A ) 2 - / H ( H E D T A ) 2- s y s t e m s at a b o u t p H 6.00, reflects a similar increase to a m a x i m u m followed by a decline in Kz as Z increases, which has b e e n r e p e a t e d l y o b s e r v e d in such cases as:

L n ( N T A ) + ( N T A ) 3 ~ L n ( N T A ) 3- L e v y et al. (1961)

L n ( E D T A ) - + ( N T A ) 3 - ~ L n ( E D T A ) ( N T A ) 4 Geier a,~d I-kxr|~,~ C l q l l ' ~ L n ( E D T A ) - + (IDA) 2- ~ L n ( E D T A ) ( I D A ) 3 Geier ~ 1 [ t o r | ~ n Clel'll) L n ( H E D T A ) + ( H E D T A ) 3- ~ L n ( H E D T A ) 3- N e d d e n et al. (1973) L n ( E D T A ) - + ( H E D T A ) 3- ~ L n ( E D T A ) ( H E D T A ) 4- Briicher et al. (1975) L n ( E D T A ) - + ( E D T A ) 4- ~ L n ( E D T A ) ~ - Brficher et al. (1975)

T h e c o n s e n s u s regarding such s y s t e m s is that, although separation f a c t o r s f o r a d j a c e n t e l e m e n t s are in s o m e cases large enough to be of interest in a n a l y s e s b y anion e x c h a n g e , the n o n m o n o t o n i c trend (of Ka as Z increases) causes separation f a c t o r s f o r intermediate lanthanons to be too low (near the Ka m a x i m u m ) and causes elution p e a k s of the lighter lanthanons to be interspersed a m o n g the h e a v i e r ones.

5.3. Displacement chromatography

D i s p l a c e m e n t c h r o m a t o g r a p h y m a k e s it possible to utilize a large fraction of the e x c h a n g e c a p a c i t y of a given resin bed as well as the entire driving potential of the eluant. In the simplest case, B and C, the species to be s e p a r a t e d , are sorbed on an e x c h a n g e bed which has p r e v i o u s l y been loaded with species A (which either B or C will readily displace by a f a v o r a b l e m a s s - a c t i o n r e a c t i o n . . , due to their greater affinities f o r the e x c h a n g e sites), and eluted with a solution containing a fourth species, D, which readily displaces A, B and C. If the equilibrium c o n s t a n t for the e x c h a n g e reaction:

C + 1] ~,-~- (~ + B

e x c e e d s unity, C displaces B while D is displacing both B and C. Since A was choSen such that B will displace A (the s o r b e d A species), B will rapidly segregate as I] at the front of the sorbed B - C band. This t e n d e n c y , coupled with f a v o r a b l e f r o n t - e n d and r e a r - e n d d i s p l a c e m e n t reactions, p r o m o t e s d e v e l o p m e n t of a s t e a d y - s t a t e condition, w h e r e i n a d j a c e n t bands of B and C, of

94 J.E. POWELL

c o n s t a n t length, p r o g r e s s d o w n the s y s t e m head-to-tail u n d e r the displacing influence of D as it e n c o u n t e r s the r e a r m o s t sorbed species C. Due to the fixed c a p a c i t y of a given s y s t e m , A is f o r c e d to v a c a t e the bed as D deposits, at a c o n c e n t r a t i o n equivalent t o the c o n c e n t r a t i o n of D in the eluant. W h e n species A has all b e e n displaced f r o m the s y s t e m , first B, then C, and finally D a p p e a r in the effluent. By collecting a p p r o p r i a t e fractions of the effluent solution as it issues f r o m the bed s y s t e m , it is possible to isolate substantial a m o u n t s of all desired p r o d u c t s in a high state of purity. While a certain a m o u n t of overlapping is inherent, the relative degree of o v e r l a p can be controlled v e r y effectively by adjusting the s y s t e m d i a m e t e r a n d the eluant flow rate to the a m o u n t of mixture to be separated.

Column p e r f o r m a n c e d e p e n d s upon two p a r a m e t e r s : the s e p a r a t i o n factor, and the height equivalent to a theoretical plate ( H E T P ) . The separation f a c t o r is defined as the ratio of the r e s p e c t i v e distribution coefficients, that is,

ot~ = Kac/KaB = [B][C]/[C][B]

In this definition: [B], [C], [13] and [C] r e f e r to the total c o n c e n t r a t i o n s of B and C in the mobile and static phases, r a t h e r than to individual ions or c o m p l e x species. It will be seen that c o m p l e x i n g or chelating agents can alter the distribution c o n s t a n t s m a r k e d l y , thus affecting the separation factor.

H E T P is the distance by which attainment of the condition of d y n a m i c equilibrium, r e p r e s e n t a t i v e of a static s y s t e m , is displaced by the m o t i o n of one phase with r e s p e c t to the other. T h a t is to say, it is the distance, m e a s u r e d along the bed, b e t w e e n points at which the ratio of B to C in the solution and the ratio of B to C in the resin p h a s e differ by one separation factor. H E T P is a function of the time of c o n t a c t and the rates of e x c h a n g e of species; hence, it d e p e n d s upon flow rate, particle size, c o n c e n t r a t i o n , p H , t e m p e r a t u r e , and even the various stabilities of individual chelate species which m a y be present. Although H E T P is difficult to predict with precision, it can be m e a s u r e d accurately for virtually a n y set of e x p e r i m e n t a l conditions if the separation f a c t o r is known.

The f u n d a m e n t a l equations which a p p l y to the separation of binary mixtures by d i s p l a c e m e n t c h r o m a t o g r a p h y are:

u = (1 + ~No)/~ log (B/C)m L log a (B/C)0 - m log a - h

where v = the m i n i m u m n u m b e r of d i s p l a c e m e n t s of the s o r b e d mixture required for separation, e = a m o u n t a e x c e e d s one (e = a - 1), No = mole fraction of the first c o m p o n e n t in the mixture, (B/C)0 = ratio of c o m p o n e n t s B and C at s o m e r e f e r e n c e point in the s o r b e d band, (B/C),, = ratio of c o m p o n e n t s B and C at a n o t h e r point in the band m theoretical plates or L c m f u r t h e r d o w n the resin bed, m = n u m b e r of theoretical plates b e t w e e n the points w h e r e ratios (B/C)0 and (B/C),, are o b s e r v e d , h = H E T P , the height equivalent to a theoretical plate in cm, L = length m e a s u r e d along c o l u m n . . , equal to m theoretical plates.

Powell (1964) cautions that the relation, v = (1 + eNo)e, applies only when: (1) the H E T P is sufficiently small; and (2) the value of e is sufficiently large that the

SEPARATION CHEMISTRY 95 region of inherent gross overlap b e t w e e n resolved bands is small c o m p a r e d to their lengths. Adjusting the bed diameter to the a m o u n t of rare earths to be separated makes it possible to apply this treatment even though H E T P is of some magnitude and the value of ~ is fairly small.

If one defines the region of gross overlap as the region wherein the percentage of one c o m p o n e n t falls from 99.9 to 0.1, log(B/C)m/(B/C)0 b e c o m e s equal to six;

and m = 6/logoz plates and L = 6h/log ~ then comprise the region of gross overlap under steady-state conditions. Care must be exercised in designing equipment to ensure that the length of the sorbed band e x c e e d s 6h/log o~

sufficiently that substantial yields of pure c o m p o n e n t s result.

If the yield is poor in a given experiment, a larger load and a p r o p o r t i o n a t e l y longer system will generally improve the separation efficiency. Reduction of H E T P is also advantageous. The most practical ways to accomplish this are: (1) decrease the diameter of the resin particles; and (2) elevate the t e m p e r a t u r e to increase reaction and diffusion rates. Decreasing the flow rate d e c r e a s e s the H E T P , to be sure, but unfortunately increases the elution time. Decreasing resin Crosslinkage allows more rapid diffusion of species in the static phase, but much of the advantage is offset by the r e d u c e d exchange capacity of a more swollen media and less dimensional stability in a packed column (low crosslinked resins expand and contract r e m a r k a b l y b e t w e e n exchange cycles an6 with changes in concentration of the mobile phase).

The above t r e a t m e n t (applicable to a binary system) has been adapted to ternary mixtures by Powell et al. (1968) and James et al. (1968); and to y e t more complex systems by Powell et al. (1971) and Helfferich et al. (1970). While the solutions to more complex systems are quite straightforward, the mathematical expressions rapidly b e c o m e tedious.

Simple cation-exchange separation of neighboring rare earths by displacement c h r o m a t o g r a p h y is not generally feasible, since all the lanthanons exhibit the same affinity for most cation-exchange media (chelating resins excepted). That is, separatiod factors for adjacent rare earths undergoing simple cation exchange are nearly unity. The reader is r e f e r r e d to the data of Surls et al. (1957). This circumstance has necessitated the use of chelating agents to e n h a n c e separation factors.

To be useful in the separation of rare earths by displacement c h r o m a t o g r a p h y , a chelatant must possess the following characteristics: (.1) the reagent and its metal chelates must be r e a s o n a b l y soluble in some inexpensive but compatible solvent (preferably water); (2) the reagent must be selective in its chelating action; (3) the reagent must form rare earth chelates of sufficient stability to p r o m o t e clean-cut displacement of rare earths from the resin bed by a m m o n i u m or alkali-metal ions; (4) the reagent must form labile chelate species which must not be of such great stability that the a c c o m p a n y i n g cation-exchange process is h a m p e r e d unnecessarily.

Citrate, nitrilotriacetate (NTA), and many p o l y a m i n o p o l y c a r b o x y l a t e anions form 1 : 1 chelate species with yttrium and the lanthanons that are either neutral or negatively charged, so that they can be exploited in an aqueous mobile phase

Dalam dokumen PREFACE (Halaman 91-108)

v)Z/2vv*],

(trans-l,2-diaminocyclohexane-N,N,N',N'-

N~-hydroxyethylethylenediamine-N,N,N'-triacetate-

v)Z/2vv],*