Langmuir 1995,11, 3766-3771
3766
The Effect of High-speed Stirring on the Distribution Equilibria of Neutral Metal Chelates Mark L. Dietz" and Roger Sperline Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received November 8, 1994. I n Final Form: J u l y 25, 1995@ When a two-phase system comprising an organic solution of any of various interfacially active neutral metal chelates in contact with an appropriate buffered aqueous phase is vigorously agitated, a reversible decrease in the organic phase concentration of the chelate is observed. This decrease gives rise to a shift in the pHln value of the metal ion (Le., the pH corresponding to 50% extraction) from that observed in the corresponding unstirred system. The magnitude of this shift is shown to vary with the distribution constant of the chelate, the interfacial area generated upon stirring, and the organic solvent. In systems in which a pair of extractable metal chelates differing in interfacial activity is present, the shifts in pH112 are shown to alter the apparent selectivity of the chelating extractant and t o enhance the separation of the metal ions.
Introduction Chemical separations constitute an indispensable part of numerous industrial processes and an essential first step in many chemical analyses. In many separation methods, surface and interfacial phenomena can play a significant role. In the separation of metal ions by liquidliquid extraction, for example, it has long been recognized that the structural features which enable a molecule to function as a metal ion extractant can also render it interfacially active.' Thus, the nature of the liquid-liquid interface and the extent to which an extractant is adsorbed can play an important part in determining the rate and mechanism of metal ion transfer between the phases.'-3 The interface takes on particular significancein liquidliquid systems in which mixing conditions are such as to generate a dispersion of one phase in another and thus, a substantial interfacial area.3-6 Earlier work in this l a b ~ r a t o r yfor , ~example, has shown that under conditions ofvigorous agitation &e., high-speed stirring),pronounced shifts may be induced in the two-phase distribution equilibria associated with interfacially active extractant molecules (e.g., alkyldithizones), the extent of which is dependent, in part, upon the interfacial area generated. Previous work here has also established that interfacial activity is not confined to extractant molecules but is characteristic of a number of simple neutral metal chelates as In this report, we examine the effect of interfacial adsorption of such chelates on their two-phase distribution equilibria. Specifically, we demonstrate that shifts in chelate distribution equilibria can be induced by highspeed stirring and explore the effect of these shifts on the apparent selectivity of a metal ion extractant. Finally, we examine the possible application of such stirringinduced shifts to metal ion separations.
* To whom correspondence should be addressed. Present address: Chemistry Division,Argonne National Laboratory,Argonne, IL 60439. Abstract published inAdvanceACSAbstracts, October 1,1995. (1)Danesi, P. R.; Chiarizia, R. CRC Crit. Rev. Anal. Chem. 1980,10, 1. (2) Freiser, H . Acc. Chem. Res. 1984,17, 126. (3) Freiser, H . Chem. Rev. 1988,88, 611. (4) Watarai, H . TRAC 1993,12, 313. ( 5 ) Freiser, H. Bull. Chem. SOC.Jpn. 1988,61, 39. (6) Danesi, P. R. In Principles and Practices of Solvent Extraction; Rydberg, J., Musikas, C., Choppin, G. R., Eds.; Marcel Dekker: New York, 1992. (7) Watarai, H.; Freiser, H. J.Am. Chem. SOC.1983,105, 191. (8) Dietz, M. L.; Freiser, H. Langmuir 1987,3,467. @
Experimental Section Reagents. 8-Quinolinol (Aldrich),2-methyl-8-quinolinol(Aldrich), and 5,7-dibromo-8-quinolinol (Eastman Kodak) were recrystallized twice from ethanol. Chloroform was washed three times with deionizedwater prior to use, while carbon tetrachloride and toluene were used as received. Perchloric acid, acetic acid, sodium hydroxide, sodium acetate, and tris(hydroxymethy1)aminomethane (TRIS)were used for pH adjustment. The ionic strength of aqueous solutions was maintained at 0.1 (for 8-quinolinolates) or 1 (for 2-thenoyltrifluoroacetonates) by addition of sodium perchlorate. All materials used were analytical reagent grade. Stock solutions of metal ions were prepared by dissolving the appropriate reagent grade perchlorate in deionized water and standardizing against EDTA.g Apparatus. The automated high-speed stirring apparatus has been described in detail previously. For selective sampling of the organic phase, a microporousTeflon membrane phase separator was employed. For samplingthe aqueous phase, this device was replaced by an aqueous phase separator consisting of a Teflon support atop which was affixed a filter element comprising a disk of cellulose acetate microfiltration membrane ('ICE Micro Filter", Spectrum Medical Industries, Los Angeles, CAI and a disk of Whatman No. 1filter paper. This unit was epoxied to the end of a 14 cm length of glass tubing held in place in the reaction vessel with a rubber stopper. With either phase separator,withdrawal of sample from the vessel was carried out using Acidiflex tubing and a peristaltic pump. With this arrangement, the volume of liquid removed from the vessel at any given time is no more than 2 mL ( 51%of the total solution volume). Procedures. Distribution Experiments: Unstirred Systems. A series of 10 mL aliquots of an organic solution of the ligand and an equal volume of an aqueous phase buffered to an appropriate pH value (chosen so as to yield a metal ion distribution ratio of between 0.1 and 10)and initially containing ca. 3 ppm of nickel ion were shaken in 40 mL vials on an Eberbach box-type shaker until equilibrium was reached. (The exact time required was determined from previous kinetics experiments.) The vials were then allowed to stand undisturbed in a constant temperature bath (2' = 25 "C) to permit phase separation. The aqueous phase was drawn off from each and the metal ion content determined via atomic absorption. For each sample, the organic phase metal complex concentration was calculated from the difference between the equilibrium aqueous phase metal ion concentration and the initial value. The distribution ratio, D, defined as [ML,I,,~[Mn+].q,was calculated for each sample and (9) Schwarzenbach, G.; Flaschka, H . Complexometric Titrations; Methuen and Co.: London, 1969; Part 2. (10) Watarai,H.; Cunningham,L.; Freiser,H.Anal. Chem. 1982,54, 2390. (11)Aprahamian, E.; Cantwell,F. F.;Freiser,H . Langmuir 1986,I , 79.
0743-7463/95/2411-3766$09.00/0 0 1995 American Chemical Society
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a plot of log D vs pH prepared. The pH at which D = 1 (correspondingto equal concentrationsofthe metal in the aqueous and organic phases) was taken as ~ H v 2 . l ~ Distribution Experiments: Stirred Systems. Equal volumes (100mL) of an organic solution of the ligand and a buffered aqueous solution of the metal ion were introduced into a 500 mL Morton flask which was immersed in a water bath maintained at 25 i 0.1 "C. The mixture was vigorously agitated by stirring at high speed (5000 rpm). During this time, the organic phase was continuously withdrawn and circulated through the flow cell of a Cary 219 UV/visible spectrophotometer. The absorbance ofthis phase was monitored at or near the absorbance maximum ofthe chelate. When a stable absorbancewas reached, indicating complete formation ofthe chelate, a series of absorbance readings were collected at 2 s intervals. A second series of absorbance readings was collected after the stirringwas stopped and a stable absorbancewas again reached. The extent ofchelate adsorption was determined by calculating the absorbance decrement, AA, the difference between the organic phase absorbance in the unstirred and stirred systems. From this, the concentration of the chelate present in the organic phase during stirring was calculated from the expression
ratio of the metal is simply defined as the ratio of its total concentration in the organic phase to its concentration in the aqueous phase at equilibrium. For the extraction of a metal ion by some chelating extractant, HL, to form a metal chelate, ML,, under conditions for which the predominant form of the metal in the aqueous phase is simply the hydrated metal ion, Mn+, this definition corresponds to the following expression:
(3) If the extracted chelate is interfacially active, vigorous agitation, by generating a substantial interfacial area, will reduce its concentration in the bulk phases. Of course, elementary thermodynamic considerations require that, at equilibrium, the ratio of the chelate concentrations in the two bulk phases be fixed in a given system, regardless of the extent of agitation. Thus, the fraction, F , of the total adsorbed chelate entering the interface from the aqueous and organic phases upon stirring is given by
Faq= l/(KDc wheref is the fractional absorbancedecrement (AA/Aunstirred) and [ML,lOrgis the organic phase chelate concentration in the correspondingunstirred system. Changes in the aqueousphase composition were monitored simultaneously by withdrawing small (ca. 2 mL) aliquots of that phase using the aqueous phase separator and determining the metal ion content by atomic absorption. From the value obtained, designated as [Mn+l'aq, the distribution ratio of the metal ion under high-speed stirring conditions was calculated using the expression
D' = [ML,l ',,J[M"+I
laq
(2)
A plot of log D' versus pH was prepared by repeating these
measurements at severalpH values. Comparison ofthe apparent pH112 value (hereafter designated as pH'l/z) to that determined in the unstirred systemyielded ApHyz, an indication of the extent to which the extraction equilibrium has been shifted by stirring. Stripping of Interfacial Adsorbates. The effect of partial withdrawal of the organic phase during high-speed stirring from a two-phase mixture initially comprising equal volumes of an organic solution of a metal chelate in contact with a buffered aqueous phase was determined as described previ0us1y.l~The organic phase withdrawn was collected in a series of small (e.g., 5 mL) aliquots, and each aliquot was equilibrated with dilute (pH 1)perchloric acid to back-extract any metal ions present. Their concentrationswere then determined by atomic absorption.
Results and Discussion StirringInduced Shifts in Metal Chelate Distribution. In a n earlier report in this journal,8 we showed that if a two-phase system consisting of anorganic solution of any of a variety of simple neutral metal chelates (e.g., nickel 8-quinolinolate) in contact with an appropriate aqueous phase is vigorously agitated, a reversible decrease in the organic phase concentration of the chelate will occur. This decrease was found to arise from adsorption of significant amounts of the chelates at the greatly increased liquid-liquid interface generated by stirring, thereby implying participation of the interface in the distribution equilibria of the metal ion. To quantitatively assess the extent of this participation, it is necessary to consider the effect of a substantial interfacial area upon the overall distribution between the bulk phases of a metal ion whose neutral chelate is interfacially active. In the absence of a n appreciable liquid-liquid interface, the distribution (12) Martell, A. E.; Calvin, M. Chemistry of the Metal Chelate Compounds; Prentice-Hall: New York, 1952. (13) Chamupathi, V. G.; Freiser, H. Langgmuir 1988, 4 , 49.
+ 1)
(4)
and
where KDCis the distribution constant of the chelate (defined as [ML,Ior$[MLnIa,). This indicates that for chelates whose distribution constant is significantly greater than 1(as is the case for the chelates of interest here and for many others of practical value in metal ion separations), adsorption from the organic phase will predominate. Therefore, the interfacial adsorption of a neutral chelate is well represented by considering only a single interfacial adsorption constant, K', defined as K' =
lim
[ML,li/[ML,lorg
(6)
[MLlorg+J
Mass balance considerations therefore require that the amount of chelate adsorbed at the interface be essentially equal to that lost from the organic phase:
where [ML,Ii is the interfacial concentration of the adsorbed chelate in mol/cm2,Aiis the interfacial area (cm2), and Vorgis the organic phase volume. It has been shown previously8 that the adsorption of neutral chelates at a liquid-liquid interface conforms to the Langmuir isotherm. Thus, the interfacial concentration of the chelate may be expressed as follows:
where a and b are constants, with a corresponding to the interfacial excess of the adsorbed chelate at saturation, and the product ab corresponding to the interfacial adsorption constant, K', for the chelate. Rewriting eq 7 in terms of [ML,Ii and equating it with eq 8 yields, after rearrangement:
If we now define the distribution ratio of the metal ion under conditions of high-speed stirring in a manner analogous to the definition in an unstirred system (i.e.,
Dietz and Sperline
3768 Langmuir, Vol. 11, No. 10, 1995
6
6.2
6.4
6.6
6.8
7
PH
Figure 1. Effect of high-speed stirring on the pH dependence of the distribution ratio of nickel between a toluene solution of 2-methyl-8-quinolinol(0.02 M) and TRIS buffers.
D' = [ML,I',,$[Mn+]',,)
and ifwe assume that the interfacial adsorption constant for the metal ion is l,l4-I7 so that agitation will have no appreciable effect upon the concentration of the metal ion in the aqueous phase (an assumption borne out by experimental measurements), the distribution ratios of the metal in the stirred and unstirred systems are related by the expression
K'A,
DID' = - [ l / ( l Vorg
+ b[ML,]',,,)] + 1
nickel ion between solutions of 2-methyl-8-quinolinol in toluene and a buffered aqueous phase. As can be seen, agitation produces a decrease in the nickel distribution ratio a t any given pH, a n upward (i.e., toward higher pH) displacement of the pH dependency plot, and as expected, a shift in the apparent pHln value for the chelate. That this effect is not confined to the nickel 2-methyl-Squinolinolate/toluene system is demonstrated by the results for several other chelate/solvent combinations, presented in Table 1. In each case, vigorous stirring produces an increase in pH1/2, with the magnitude of the upward shift depending on the chelate and the organic solvent.
Effect of StirringInducedShiftsin Metal Chelate Distribution on Apparent Extractant Selectivity. When two metal ions form extractable chelates with a given ligand, the extent of their separation under a particular set of conditions is governed by their respective extraction constants (eq 14) and is reflected in the magnitude ofthe difference in their respective pHv2 values (eq 15). For metal ions of the same charge (whose DMvs pH dependencies therefore have the same slope), small differences in pHl/z values indicate little difference in the extraction behavior of the two metal ions and thus, poor separation. Conversely, large differences indicate a more facile separation.
(10) Mniaq
+ nHLorg= ML,
+ nH+aq
Ifwe designate the pH a t which D = 1as pH112 and that a t which D' = 1 as pH'l12, the change in the observed distribution ratio induced by stirring can be expressed instead as a shift in the apparent pHl/z value. That is, since log(D/D') = n(pH',/, - pH,,,) = nApH,,
(11)
where n is the number of protons transferred for each extracted metal ion (2 for the nickel systems considered here); we can rewrite eq 10 as follows:
KA;
(lOnApHvn) - 1= -1141 Vorg
+ b[ML,]',,,)]
(12)
From this equation, it can be seen that the apparent pHlIz value upon stirring will be higher than that observed in the absence of stirring by an amount which depends upon the interfacial activity of the metal chelate (as reflected in its interfacial adsorption constant, K'), the specific interfacial area (i.e., the interfacial area generated per unit volume of organic phase), and the chelate concentration. At sufficiently low concentrations, this expression reduces to 10nAPHllZ - 1 = K'A,N 1 org
(14)
(13)
a n equation essentially identical to that derived by Watarai and Freiser' to describe the shiR in the distribution equilibrium of an alkyl-substituted dithizone extractant upon high-speed stirring. Thus, equations ofthe same form describe the effect of introducing a substantial interfacial area in a liquid-liquid system containing either an interfacially active ligand anion or neutral metal chelate upon their respective distribution equilibria. Figure 1 depicts the effect of high-speed (5000 rpm) stirring on the pH dependency of the distribution ratio of (14)Watarai, H.; Freiser, H. J.Am. Chem. SOC.1983,105, 189. (15)Haraguchi, K.; Freiser, H. Inorg. Chem. 1983,22,1187. (16)Aprahamian, E.; Freiser, H. Sep. Sci. Technol. 1987,22,233. (17) Dietz, M.L.; Freiser, H. Langmuir 1991,7, 284.
Vigorous stirring of a liquid-liquid system containing a pair of extractable chelates of differing interfacial activity, by shifting the pHv2 value associated with each metal ion, can therefore alter the relative extractability of the two metal ions, hence, the apparent selectivity of the extractant. Clearly, for this effect to be of any practical utility in metal ion separations, it is essential that stirring increase the difference in the pH112 values of the pair of ions. To determine if the change in the relative extractability of two metal ions induced by stirring will enhance their separation, it is first necessary to establish the relationship between the interfacial activity of a neutral metal chelate and its distribution constant, KDC.To do this, the transfer free energy of the chelate from the organic phase to the aqueous phase is first divided into portions corresponding to the transfer of the polar (p) and nonpolar (np) portions of the molecule. Each of these portions is further divided into organic phase-to-interface and interface-to-aqueous phase steps, yielding the following expression: AGt(o-a) = AGnP(o-i)
+ AGP(o-i) + AGnP(i-a) + AGP(i-a)
(16)
Assuming that the polar portion of the adsorbed molecule faces the aqueous side of the interface and that the nonpolar portion faces the organic side, AGP(i4-a)and AGnp(o+i)are zero. Taking AGY0-a) as -RT l n ( l / K d and AGP(o-i) as -RT In K' yields
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Langmuir, Vol. 11, No. 10, 1995 3769
Table 1. pH1m Values for Several Nickel Chelates under Quiescent and Vigorously Stirred (SO00 rpm)Conditions PHUZ
chelatea 8-quinolinolate(8-HQ) 8-quinolinolate 2-methyl-8-HQ 2-methyl-8-HQ 2-methyl-8-HQ
solvent
stirred
unstirred
chloroform carbon tetrachloride chloroform
3.54 3.97 6.50 6.38
3.57
ApHi/z 0.03
4.08 6.57 6.49 6.52 6.76
0.11 0.07 0.11 0.20 0.26
toluene carbon tetrachloride 6.32 5,7-dichloro-8-HQ carbon tetrachloride 6.50 Conditions: 8-HQkhloroform: 1.00 x M in 0.02 M ligand. 8-HQharbon tetrachloride: 1.17 x 10-5M in 0.02 M ligand. 2-methyl8-HQhhloroform: 2.07 x M in 0.01 M ligand. 2-methyl-8-HQholuene: 7.09 x M in 0.02 M ligand. 2-methyl-8-HQharbon M in 0.02 M ligand. 5,7-dichloro-8-HQ/carbon tetrachloride: 5.05 x M in 1.01 x M ligand. tetrachloride: 8.45 x
1
Ni
j
10’4:
extraction constant correspond to lower pHm values. Thus, those metal ions forming chelates possessing the lowest distribution constants will have the highest pH112 values. As shown above, all else being equal, the chelates with lower distribution constants will also exhibit greater interfacial activity. Taken together, this indicates that the larger the value of pH112 associated with a particular metal ion, the greater will be the shift in pH112 observed for its chelate upon stirring. This, in turn, indicates that in a system containing a pair of extractable metal chelates of differing interfacial activity, vigorous agitation will magnify the pH112 difference between the metal ions, thereby enhancing their separation. As will now be shown, this may have practical significance.
Metal Ion Separations by Adsorbate Stripping.
If it is assumed that the nonpolar portion of the chelate includes neither the metal atom nor the bonding atoms of the ligand, it follows that AGnP(i-a) will be constant for a group of chelates consisting of a given ligand in combination with a series of metal ions. Thus, for a given interfacial area, there will be a n inverse relationship between the interfacial activity of such a chelate &e., the magnitude of its interfacial adsorption constant) and its distribution constant. This is illustrated by the results presented in Figure 2, which shows the relationship betweenK’Ai and KDCfor the 2-methyl-8-quinolinolates of copper(II), nickel(II), and zinc(I1) and for the 2-thenoyltrifluoroacetonates of copper(II), zinc(II), and cobalt(I1). As can be seen, in each case, the expected straight line of near-inverse unit slope is obtained. The larger K‘Ai values for the 2-methyl-8-quinolinolates can be explained by the greater hydrophobicity of 2-methyl-8-quinolinol relative to that of 2-thenoyltrifluoroacetone and by differences in the interfacial areas generated in the two systems. From classical solvent extraction principles,ls it can be shown that the extraction constant (eq 14) €or a given metal ion with a particular chelating extractant is directly proportional to the distribution constant of the chelate, KDC.As indicated in eq 15 above, larger values of the
In a n earlier report, Chamupathi13showed that if a liquidliquid system comprising a n organic solution of an interfacially active extractant in contact with a n appropriate aqueous phase is vigorously agitated and portions of the organic phase are simultaneously withdrawn from the system using a microporous Teflon membrane phase separator (MTPS), a gradual increase in the absorbance of the remaining organic phase, hence the organic phase extractant concentration, will be observed. This observation is explained by the fact that species adsorbed a t the liquid-liquid interface do not accompany the organic phase through the phase separator during its withdrawal.13J9 Rather, the adsorbed material is “stripped”from the interface as the phases separate on the MTPS and the total interfacial area decreases and is returned to distribute within the mixture remaining in the flask. As will now be shown, such a configuration (i.e., one in which the organic phase is gradually withdrawn during stirring) provides a means by which the differences in interfacial activity of metal chelates can be employed to effect the separation of two metal ions. That is, if the organic phase is withdrawn from a vigorously stirred two-phase system containing a pair of chelates, one interfacially active, the other not, the solution withdrawn should be depleted in the interfacially active component. Conversely, the solution remaining in the extraction vessel should be enriched in this component. At the same time, the concentration of the inactive (i.e., nonadsorbed) component should be unaffected by the stirring and organic phase withdrawal. The result will be a partial separation of the two metal chelates. Figure 3 shows the results obtained when, following equilibration of the two phases, the organic phase is gradually withdrawn from a vigorously stirred system initially comprising a solution of nickel (1.64 x M) M) perchlorates in a pH 7 TRIS and copper (4.43 x buffer in contact with a n equal volume (100 mL) of a 0.1 M solution of 2-methyl-8-quinolinol in toluene. This particular system was chosen because, under the experi-
(18)Sekine, T.; Hasegawa, Y. Solvent Extraction Chemistry: Fundamentals and Applications; Marcel Dekker: New York, 1977.
2.
KDC
Figure 2. Inverse relationship between interfacial activity and distribution ratio for several 2-methyl-8-quinolinolates (A) and 2-thenoyltrifluoroacetonates (B).(Organic solvent = chloroform.) Values O f & for the copper and zinc 8-quinolinolates are from refs 22 and 23, respectively. That ofthe corresponding nickel chelate was calculated from the value for nickel 8-quinolin01ate~~ assuming that methyl substitution increases the value Of& by 0.67 units.25Values for the copper and zinc 2-thenoylacetonates are from refs 26 and 27, respectively. That of the cobalt chelate was calculated from the corresponding value in the carbon tetrachloridelwater system assuming that AKDC= 2hKDM, where AKDMis the difference between the ligand distribution constants in carbon tetrachloridelwater and chlor~formlwater.~~
(19)Persaud, G.; Xu-min, T.; Cantwell, F. F. Anal. Chem. 1987,59,
Dietz and Sperline
3770 Langmuir, Vol. 11, No. 10, 1995 a
been depleted of the adsorbed chelate can now be expressed as
5 1 ' " " " ' ' " " " ' " ' . I
percent depletion
(PI= 100[1 - SO Corg dV/CorgV] V
(19)
Volume of Organic Phase Withdrawn, mL
Figure 3. Copper and nickel content of the permeate vs the volume of the organic phase removed from mixtures of copper and nickel 2-methyl-8-quinolinolates in toluene vs pH 7 TRIS. (Key: A, copper, stirred system; B, nickel, unstirred system; C, nickel, stirred system.) mental conditions, the extraction of both nickel and copper is essentially complete. Thus, as ordinarily practiced, simple liquid-liquid extraction would not provide a satisfactory separation of these two ions. In addition, previous work* has established that nickel 2-methyl-8quinolinolate is approximately 2 orders of magnitude more interfacially active than is the analogous copper chelate. Under the experimental conditions, the interfacial activity of the copper chelate is, in fact, barely detectable. As can be seen, the concentration of copper in the organic phase withdrawn (hereafter referred to as the permeate) is found to be indistinguishable from the initial (i.e., postequilibration, unstirred) organic phase value throughout the withdrawal, as expected for a species not exhibiting interfacial activity. In contrast, a significant reduction in the nickel concentration present in the permeate is observed until nearly three-fourths of the organic phase has been removed. (It is important to note that despite the substantial decrease in the organic phase nickel chelate concentration over this range, no significant change in the aqueous nickel ion concentration is seen, a n observation consistent with assumptions made in the derivation of the equation relating ApHl/z and K'Ai, above.) At ca. 80%withdrawal, the nickel concentration in the permeate begins to rise steeply, finally exceeding its initial concentration at ca. 95%removal. (Further removal of organic phase without concomitant withdrawal of significant amounts of aqueous phase was not possible.) Qualitatively, the rise in the nickel concentration in the permeate can be understood by assuming that, as was the case for interfacially active extractants,13adsorbed chelates do not accompany the organic phase through the phase separator during withdrawal. The behavior of nickel can be treated quantitatively by first noting that the amount ofinterfacially active chelate originally present is given by CorgVorg.In the absence of any appreciable chelate adsorption (i.e,, in a n unstirred system), a volume, V, of the organic phase withdrawn would contain C,,,Vmoles of chelate, while Corg!Vorg- V) moles would remain in the vessel. Under high-speed stirring conditions, however, the organic phase concentration is initially reduced to some new value Cor,. Because of the gradual increase in CO,which accompanies the withdrawal, the number ofmoles of chelate withdrawn is not simply CorgVbut rather moles withdrawn =
V
Corg dV
(18)
The extent to which the organic phase withdrawn has
Such a calculation for nickel 2-methyl-8-quinolinolate up to the point a t which curves b and c (Figure 3) intersect indicates that the amount of the chelate present in the permeate is reduced by 80%in the stirred system, while the copper concentration is unaffected. Obviously, all of the nickel chelate not withdrawn must be present in the retentate, that is, in the organic phase remaining in the vessel. Its total concentration in the retentate must therefore be given by
Such a calculation for the nickel 2-methyl-8-quinolinolate indicates that its concentration in the retentate is approximately 16 times its original value. It is important to note here that the exact shape of the plot relating Cg, to the volume of organic phase withdrawn (and, therefore, the extent of organic phase depletion/ enrichment observed with a given chelate) will depend on its initial concentration and its interfacial adsorption constant. That is, it is the decrease in interfacial area caused by organic phase withdrawal and the accompanying displacement of adsorbed chelate once interfacial saturation is reached which causes Cor,to rise. Therefore, the longer that interfacial saturation can be avoided, the greater the volume which can be withdrawn from the vessel before Corg begins to increase appreciably. This implies that adsorbate stripping will prove most effective in the removal of traces of interfacially active materials from mixtures whose other constituents are much less interfacially active. In this respect, the method is similar to classical adsorptive bubble separation methods (e.g., foam fractionation20and their adsorptive droplet analogs.21 In considering the possible application of "adsorbate stripping" to a n actual analytical or process-scale metal ion separation, several limitations of the approach, both real and potential, must be borne in mind. First, while we have not yet performed a n analysis of the factors governing the energy costs required to generate and maintain high interfacial area dispersions, it is reasonable to expect that adsorbate stripping may not prove cost effective in high-process-volume applications involving the recovery of inexpensive metals. Next, the formation of interfacial precipitates (so-called "interfacial cruds") is not an uncommon occurrence in liquid-liquid extraction systems. While we have not observed such materials in any of the systems examined here, it is clear that adsorbate stripping, like any separation scheme which relies upon interfacial adsorption, would be adversely affected by their presence. Finally, the Langmuir adsorption isotherm (eq 8 ) indicates that for any given concentration of adsorbed (20) Schnepf, R. W.; Gaden, E. L., Jr.; Mirocznik, E. Y.; Schonfeld, E. Chem. Eng. Prog. 1969,55,42. (21)Lemlich, R. Znd. Eng. Chem. 1968,60, 16. (22) Mottola, H. A.; Freiser, H. Talanta 1966,13,55. (23) Chou, F. A.; Fernando, Q.; Freiser, H. Anal. Chem. 1965,37, 361. (24)Oki, S.Anal. Chim. Acta 1970,49,455. (25)Bhatki, K.S.;Rane, A. T.; Freiser, H. Indian J. Chem. 1977, 15A,983. (26) Imura, H.;Suzuki, N. Talanta 1985,32,785. (27) Sekine, T.;Murai, R.; Niitsu, M.; Ihara, N. J.Znorg. Nucl. Chem. 1974,36,2569.
Langmuir, Vol. 11, No. 10, 1995 3771
Effect of Stirring on Equilibria of Metal Chelates chelate, [ML,]i, some finite organic phase chelate concentration, [MLnl’,,,g,will always remain. Thus, regardless of the experimental conditions, the organic phase withdrawn will not be entirely free of the interfacially active chelate. As a result, a satisfactory separation oftwo metal chelates is unlikely to be achieved using only a single organic phase withdrawal step.
Conclusions The results of this study clearly demonstrate that highspeed stirring can induce apparent shifts in the two-phase distribution equilibria associated with interfacially active neutral metal chelates. In liquid-liquid systems comprising more than one extractable chelate differing in interfacial activity, these shifts will alter the apparent selectivity of the chelating extractant and will enhance the separation of the constituent metal ions. Through
adsorbate stripping, it may be possible to exploit these stirring-induced shifts to achieve improved resolution of metal ions whose chelates differ in interfacial activity. Further work is needed to better define the extent of separation achievable under various conditions, to test the applicability of the approach to species other than neutral metal chelates (e.g., surface-active ion pairs), and to investigate the use of multistaging as a means of improving the separations obtained.
Acknowledgment. The authors thank Dr. Henry Freiser of the Department of Chemistry a t the University ofArizona for several helpful discussions. This work was supported by a grant from the National Science Foundation. LA9408861
