Anal. Chem. 1987, 59,813-818 (6) Nieman. J. A.; Rcdgers, L. B. Sep. Sci. 1975. 70, 517-545. (7) Jentoft, R. E.: Oouw, T. H. Anal. Chem. 1972, 4 4 , 681-686. (8) Paulaiiis, M. E.; Krukonis, V. J.; Kurnik, R. T.: Reid, R. C. Rev. Chem. .Ens. 1983, 2, 179-249.
RECEIVED for review July 11,1986. Accepted November 10,
813
1986. This work was supported by the University of California Toxic Substances Program and by Cooperative Agreement CR812258-01between the University of California, Riverside, CA, and the U.S. Environmental Protection Agency’s Environmental Monitoring System Laboratory, Las Vegas, NV.
Solvent Extraction Studies of Europium(II I), Ytterbium(II I), and Lutetium(II I) with Ionizable Macrocyclic Ligands and Thenoyltrif luoroacetone V. K. Manchandal and C. Allen Chang*
Department of Chemistry, University of Texas at El Paso, El Paso, Texas 79968-0513
Solvent extraction behavlor of Eu( I II ) , Yb( II I), and Lu( III ) has been investigated by uslng thenoyltrtfiuoroacetone (TTA) as extractant In the presence of 1,7-dlaza-4,10,13-trloxacyclopentadecane-N,N’dlacetlc acid (DAPDA) aml 1,lO-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N’-dl~ceticacid (DACDA) as macrocyclic Ionophores. DAPDA and DACDA were chosen in this work In view of thelr unique complexation toward lanthanides. I t was observed that In the pnsence of DAPDA (L), Eu( I I I ) extracted predominantly as ternary complex [Eu(L)(lTA)], whereas Yb( 111) and Lu( 111) were extracted as mixed, binary Ln(lTA), and ternary [Ln(L)(lTA)] complexes. On the other hand, In the presence of DACDA, Eu( II I ) formed mixed binary and ternary complexes In the organic phase, whereas Yb( 111) and Lu( 111) formed predominantly binary complexes. I n contrast to the extraction In the presence of DAPDNDACDA, heavler lanthanldes, Le., Yb( II I ) and Lu( I I I),were extracted much less compared to lighter lanthanldes, Le., La( I I I ) and Nd( I I I), in the presence of ethylenediamine-N,N’-dlacetlc acid (EDDA), a structurally analogous noncyciic poiyamlnopolycarboxyllc acid.
In spite of several reported procedures (Id), separation of lanthanides as a group from trivalent actinides as well as separations of individual lanthanides from each other still offers a formidable challenge to analytical chemists (6, 7). In general, multistage extraction is carried out to achieve the desired purification of a particular lanthanide from mixtures (8,9), which is tedious and time-consuming. Thus, there is a growing interest in developing alternate procedures including the use of ion-specific compounds or crown ethers for separation of lanthanides as a group or from one another (10). In a n effort to develop lanthanide ion selective reagents, we initiated a systematic study of lanthanide complexes of macrocyclic ligands with ionizable, functional pendant arms. Two such ligands, i.e., 1,7-diaza-4,10,13-trioxacyclopentadecane-N,”-diacetic acid (K21DA or DAPDA) and 1,lO-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N’-diacetic acid (K22DA or DACDA) shown in Figure 1, have been characterized in terms of the thermodynamic complex formation stabilities (11,12),mixed ligand complex formation (13),and On leave from Radiochemistry Division, B.A.R.C., Bombay, India. 0003-2700/87/0359-0813$01.50/0
dissociation kinetics (14) of complexes of lanthanide ions. It was observed that in contrast to the open-chained EDTA, both ionizable macrocyclic ligands form stronger complexes with the lighter lanthanides. In particular, DAPDA, the 15membered ring compound, formed the strongest complex with europium(II1) among all lanthanide ions (12). In view of such unique selectivity, we have investigated the solvent extraction behavior of the lighter lanthanides, i.e., La(II1) and Nd(III), by using thenoyltrifluoroacetone (TTA) as extractant in benzene in the presence of the two macrocyclic reagents (15). It was found that the binary complex, Ln(TTA)3, was the dominant species extracted at pH 15.0 and the ternary complex, Ln(DAPDA/DACDA) (TTA) was the dominant species at pH -7.5. Extraction of the ternary complex of La(II1) was greater in the case of DAPDA and smaller in the case of DACDA as compared to the extraction of corresponding ternary complex of Nd(II1). The measured extraction cbnstants followed the order which could be explained on the basis of both metal ionic potential as well as steric effects of the resulting complexes. In the present work, we report the results of spectral as well as distribution studies carried out with heavier lanthanides, e.g., Eu, Yb, and Lu in the presence of DAPDA and DACDA as macrocyclic ionophores. TTA was employed as the organic extractant and benzene was used as the organic diluent. For comparison, studies have also been carried out in the presenoe of EDDA, a structurally analogous noncyclic polyaminbpolycarboxylic acid. These studies revealed significant differences in extraction mechanisms and thus the nature of extracted species for lighter and heavier lanthanides. EXPERIMENTAL SECTION Reagents. DAPDA and DACDA were synthesized and purified in our laboratory by the procedure reported earlier (11). Analytical reagent grade EDDA and EDTA were purchased from Spectrum Chemical Manufacturing Corp. and Mallinckrodt, Inc., respectively. Nitrate salts of europium, ytterbium, and lutetium used were supplied by Aldrich Chemical Co. Standard metal salt solutions (0.01 M) were prepared by titrating them against standard EDTA solution using xylenol orange as indicator. Thenoyltrifluoroacetone (TTA, laboratory reagent grade) was obtained from Aldrich and it was used after recrystallization from benzene/hexane mixture. Its purity was confirmed by melting point determination and measurement of acid dissociation constant. Spectroscopicgrade benzene was used as organic diluent. Distribution Studies. The aqueous phase contained 1.8 X lo4 M metal ion and 1.8 X lo4 M DAPDA/DACDA, and the ionic strength was adjusted to 0.2 M with tris(hydroxymethy1)aminomethane (Tris) buffer and tetramethylammonium chloride. 0 1987 American Chemical Society
814
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
the extraction constant, K,,, are given as follows: Ln3+(a)+ 3HA(o)
LnA3(o) + 3H+(a)
(2)
Kex = ~ ~ ~ ~ 3 1 , ~ ~ + l , 3 / ~ ~ ~ ~(3) 3+l,~H~l where subscripts o and a indicate organic and aqueous phases, respectively, and HA is protonated TTA. Considering the complexation of Ln3+with L2-,A-, and OHin the aqueous phase, eq 1, can be written as D = [LnA31,/([Ln3+l,11+ XI)
(4)
where
X
dapda ( K i l 3 A
Figure 1. Structures of DAPDA and DACDA.
Varying proportions of tris(hydroxymethy1)aminomethane and hydrochloric acid were used to form buffers in the pH range 6.0-9.0. It was necessary to adjust the ionic strength of Tris buffer to at least 0.2 M, in view of its low buffer capacity particularly in the region below pH 7.5 and above pH 8.5. The organic phase contained varying concentration of TTA in the range 8.3 X M to 1.0 X lo-' M in benzene. The volume of each phase was maintained at 6.0 mL. Equilibration was carried out for 10-12 h on a Burrell wrist-action shaker followed by settling of the two phases for 1 h. The two phases were then separated, carefully avoiding any cross contamination. Prolonged shaking was done for the purpose of convenience, though equilibrium was reached much earlier, and the equilibrium distribution ratio obtained did not change with the additional time allowed. The pH of the aqueous phase was measured with a Fisher combination pH electrode and a Fisher Model 825 MP pH meter. Acidity of the aqueous phase was adjusted to -0.1 M before metal analysis by adding 0.05 mL of concentrated HCI to approximately 5.0 mL of the aqueous phase. Concentration of lanthanide ion in the aqueous phase was measured by dc plasma emission spectroscopic technique, using a Beckman Spectrospan VI spectrometer equipped with an instrument computer and a dataspan computer. The spectrometer combines a high-energy dc plasma excitation source with a high-resolution echelle grating. Liquid samples were converted to aerosol form and introduced into the excitation region. The echelle grating and prism in the optics module separate the emitted light into its component wavelengths. In the present work, intensities of emitted light at 466.188 nm, 328.937 nm, and 261.542 nm were measured for the determination of europium, ytterbium, and lutetium concentrations, respectively. The dc plasma emission instrument was calibrated using five standards of the lanthanide ion in 0.1 M HC1 medium covering the concentration range of most of the samples analyzed (0.1-30 ppm). Occasionally, the organic phase was equilibrated with 1 M HC1 and the concentration of stripped lanthanide ion was obtained by the dc plasma emission technique. Material balance within *5% was obtained in all experiments carried out at pH 7.5 in the presence of DAPDA/DACDA. Spectrophotometric Studies. A Perkin-Elmer 552 spectrophotometer equipped with a microcomputer was used for ultraviolet absorption studies. Aqueous-phase spectra of 5 x M TTA were measured at different pH values. UV spectra of the binary system (Ln-TTA) in the presence of 4 X lov4M Ln(II1) in the and those of ternary systems (Ln-DAPDAIDACDA-"A) presence of 4 X M Ln(II1) + 4 X lo-* M DAPDA/DACDA were measured at pH 7.5 (Tris buffer) against appropriate reference solutions, respectively. Theory. Distribution ratio (D) is defined as (1) D = W n l , - [Lnl,)/[Lnla where [LnIt is the total initial concentration of Ln in the aqueous phase and [Ln], is the concentration of Ln in the aqueous phase at equilibrium. [Lnlt was obtained experimentally by equilibrating the benzene phase in the absence of T T A with the aqueous phase at a particular pH. (a)Determination of Experimental Extraction Constant (KeXP) for Binary Species LnA,+ The equilibrium and expression for
= ZP,[L2-]' + ZPJ[A-]' I
+ CPk[OH-lb k
(44
and z, J , and k denote the number of ligands that can form complexes with Ln with the corresponding overall formation constants p,, PJ, and Pk. It is noted that under present experimental conditions, X >> 1 and only the first term on the right-hand side of eq 4a is significant. From eq 3 and 4
K,, = D(1 + X)[H+],3/[HAIo3
(5)
The experimental extraction constant, KexP,evaluated in the present work is given as KexP= D[H+],3/[HA],3
(6)
which is related to K,, as K,, = KeXp(l+ X)
(7)
( b )Determination of Extraction Constant (K,;) for Ternary Species LnLA. Equilibrium involving ternary species LnLA and the expression for the extraction constant K,; are given as follows:
& LnLA(o) +H+(a)
(8)
Kex' = [LnLAI,[H+Ia/[LnL+I,[HAI,
(9)
LnL+(a) + HA(o)
Assuming (i) that only mononuclear metal species are present in the two phases and (ii) that lanthanide is present only as LnL+ in the aqueous phase at pH 7.5, as suggested by the relatively strong complex formation in our previous *ark (11,12),eq 1 can be written in a simplified manner for the extraction of ternary species
D,= [LnLA],/[LnL+],
(10)
From eq 9 and 10, one obtains
Kex' = D,[H+Ia/[HAlo
(11)
or
log D , = log [HA],
+ log Kex'+ pH
(12)
[HA], in eq 6 and 11 refers to the concentration of HA in the organic phase at equilibrium, which is related to [HAItotaland [HA],' as follows: [HA],' = IHAlt,,,, - n[LnA,I,; [LnA,I, = [LnIt,,~D/(D+ 1) [HA], = [HA],' PHA/(PHA + 1 + KHA/[H+I)
(13)
where n is the number of ligands A- on Ln3+,PHA is the partition coefficient of HA, P H A = [HA],/[HA], (40 in the present case), and K m is the acid dissociation constant of HA (-log Km4 = 5.87).
RESULTS AND DISCUSSION Spectrophotometric Studies. The chemistry of TTA at different p H values in the aqueous phase as well as in the benzene phase after two-phase equilibration has been discussed in our earlier work (15). Formation of ternary complexes of the type Ln(DAPDA/DACDA)TTA was suggested for La and Nd on the basis of bathochromic shifts of maxima corresponding to the UV spectrum of TTA at pH 7.5. As seen
ANALYTICAL CHEMISTRY, VOL. 59,NO. 6 , MARCH 15, 1987
815
Table I. Slopes of Plots of log D as a Function of log [TTAIoat pH 7 . P
DAPDA DACDA EDDA
Lab
Ndb
Eu
Yb
Lu
1.31 f 0.04 1.24 h 0.09 1.73 f 0.27
1.00 f 0.12 1.06 h 0.08 1.71 f 0.23
1.30 f 0.03 2.09 f 0.08 2.87 f 0.29
1.84 f 0.03 2.97 h 0.01 3.03 f 0.10
2.67 f 0.13 2.72 f 0.17 2.76 f 0.25
"Tris buffer solution was used. bData from ref 15. -1.o
1 0.51
-0.51
-4
-3
-2
Log [TTAb I
I
300
400
h (nm) Flgure 2. UV spectra of 5 X M thenoyitrifluoroacetone at pH 7.5 (Tris buffer) under different complexing conditions: (a) only TTA; (b) TTA 2.6 x 10-4 M E~(III); (c) TTA 2.6 x 10-4M EU(I) 2.6 X M DAPDA; (d) TTA 2.6 X M Eu(II1) 2 . 6 X lo-' M DACDA.
+
+
+
+
+
in Figure 2, the UV spectra of TTA at pH 7.5 showed similar shifts in the case of Eu for binary, Eu-TTA (curve b), as well as ternary, Eu-DAPDA-TTA (curve c), systems. (The spectrum of TTA alone is labeled as curve a.) Increase in intensities in the UV spectra (curves b and c) is expected as a result of complexation. On the other hand, there was no shift for the ternary, Eu-DACDA-'M'A (curve d) system and there was little decrease in spectral intensity. It appears that the ternary complex, Eu(DAPDA)TTA, of the smaller macrocycle DAPDA forms readily, but the one, Eu(DACDA)TTA, with the larger macrocycle DACDA does not form under identical conditions. Similar results have also been obtained with the Yb(DAPDA/DACDA)TTA system in the present studies and the Yb(DAPDA/DACDA) (acac) system in our previous work (13). This lack of ability to form strong ternary complexes with 0-diketones for the heavier lanthanide primary complexes of the larger macrocycle, DACDA, is apparently related to their smaller metal ionic radii. This is consistent with the differences in the coordination number of the complexes formed in the synergistic system Ln-TTA-TBP, reported previously for Eu(II1) and Tm(II1) ions (16). Distribution Studies. Effect of TTA Concentration. Logarithmic plots of variation of distribution ratio with TTA concentration in the organic phase at equilibrium for Eu(III), Yb(III), and Lu(II1) in the presence of DAPDA, DACDA, and EDDA ligands in aqueous solutions at pH 7.5 are straight lines with different slopes. The values of these slopes calculated using a least-squares method are given in Table I. According to eq 5 and 12, these slope values refer to the average number of extractant moieties on each extracted metal species. A slope of 1.3 obtained for the Eu-DAPDA-TTA system suggests that the dominating species extracted into the organic phase is Eu(DAPDA)TTA. This is similar to those observed for La(II1) and Nd(II1) systems (Table I). The extraction constant for Eu(DAPDA)TTA calculated from eq 10-13 is log Ke; = -4.34 f 0.08 which is about the same as that of La(DAPDA)TTA (log K e i = -4.31) which in turn was found to be
-
Flgure 3. Logarithmic variation of distribution ratio of Eu(II1) at pH 7.5 with TTA concentration in the presence of stoichiometric concentrations of DAPDA and DACDA: (+) 1.8 X M DAPDA; (0)1.8 X M DACDA, [Eu(III)] = 1.8 X M.
greater than the extraction constant for Nd(DAPDA)TTA (log
K e l = -4.59) in our previous work (15). As discussed earlier, the observed trend in the ternary complex extraction constants could be attributed to (i) variation in the stability of primary complex of macrocyclic ionophore, (ii) increasing ionic potential and steric hindrance of the resulting ternary complex with increase of atomic number of the lanthanide ion, and (iii) increasing partition coefficient of ternary complex with atomic number of lanthanide. In the Eu-DACDA-TTA system, since DACDA has one more coordinating ether oxygen and its ligand size is bigger, the ternary species, Eu(DACDA)'M'A, is not formed as readily as that for Eu(DAPDA)TTA, which is demonstrated by the spectrophotometric studies, vide supra. Thus, the slope value of 2.09 implies that mixtures of the binary species, Eu(TTA),, and ternary species, Eu(DACDA)TTA, are extracted under the experimental conditions. In the case of the Eu-EDDA-TTA system, a slope value of 2.87 suggests that a major extracted species is E u ( T T A ) ~ . This is probably due to the facts that EDDA ligand does not complex strongly with Eu(II1) and that the resulting complex is not hydrophobic enough to facilitate the extraction of mixed ligand complexes. It is of interest to observe that under identical experimental conditions, the straight line plots of log D vs. log [TTAIofor both Eu-DAPDA-TTA and Eu-DACDA-TTA systems cross each other at [TTAIo = 1.4 X M (Figure 3). Although the data were obtained from separate individual experiments, the implication is significant, Le., one can vary the [TTA],, to change the relative amount of extracted metal species. It seems that at high [TTA],, Eu(TTA), is favored and at low [TTA],,, Eu(DAPDA)TTA is preferred, if both Eu(DAPDA)+ and Eu(DACDA)+ are present for the extraction. Slope data for the extraction of Yb(II1) species (Table I) indicate that Yb(DAPDA)TI'A and Yb(TI'A), are coextracted in the case of the Yb-DAPDA-TTA system and only Yb(TTA), is extracted in both Yb-(DACDA/EDDA)-TTA systems. On the other hand, slope values in Table I for Lu(II1) extractions for all three ligands suggest that the binary Lu(TTA), species is the dominant one in the organic extract.
816
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6,MARCH 15, 1987
Table 11. Experimental Extraction Constants for Binary Complexes Ln(TTA), in the Presence of EDDA and DACDA" log Kelp
system
ELI-EDDA-TTA Yb-EDDA-TTA Lu-EDDA-TTA Yb-DACDA-TTA 1.u-D.4CDA-TT.4 X
-11.85 -13.05 -13.13 -12.62 -12.19
f 0.17 f 0.08
f 0.13 f 0.06 r 0.13
log K,'
log K2D
8.38 8.93 9.09 10.90 10.84
6.36 7.92 8.48
2 -
1 -
Log D
[Ln] = 1.8 X M, [EDDAJ = 1.8 X 10.' M, [DACDA] = 1.8 M. "Data taken from ref 11 and 19.
0-
x'
-1
YI
LD
h
m
m
F:
PH Flgure 5. Distribution of Yb(II1) as a function of pH in the presence of different cornplexing agents: ( X ) 1.8 X M Yb(II1) -t 1.8 X M EDDA; (+) 1.8 X M Yb(II1) 4- 1.8 X M DAPDA; (0) 1.8 X M Yb(II1) 4- 1.8 X M DACDA. [TTA]t,,,, = 3.3 X 10-3 M, I = 0.13.
ul
m
10
m
PH Figure 4. Distribution of Eu(II1)as a function of pH in the presence of different cornplexing agents: [lTA], = 3.3 X M, I = 0.13. (x) 1.8 x 10-4 M EU(III) 1.8 x I O ~ EDDA; M (+) 1.8 x 10-4 M EU(III) 1.8 x 10-4 M DAPDA; (0) 1.8 x 10-4 M E ~ ( I I I ) 1.8 x 10-4 M DACDA.
+
+
,
+
These two observations can be easily attributed to the phenomenon of lanthanide contraction and the lack of hydrophobicity of the EDDA ligand. No attempt was made to obtain the extraction constants in systems where mixed species appeared to be present in the organic extract. On the other hand, experimental extraction constants, KexP, for the binary species Ln(TTA), in the presence of a stoichiometric amount of DACDA and 10 times the stoichiometric amount of EDDA were calculated and are listed in Table 11. The decrease in extraction constants from the values reported in the literature (17, 18) is principally due to the complexation of Ln(II1) with DACDA or EDDA. As shown in our previous work a t pH 7.5, aqueous complexation of Ln(II1) with DACDA is quantitative under the experimental conditions (11). The same is observed in the case of EDDA since 10 times the stoichiometric amount of the ligand was used. It appears that the dissociation of these aqueous complexes precedes the formation of extractable complexes of the type Ln(TTA),. This is also evident from the values of the extraction constant which follow an inverse order of their aqueous complex formation stabilities (Table 11). Thus, it is not surprising to observe that in the case of the DACDA system, aqueous complexation facilitates the extraction of Lu(II1) over Yb(II1) whereas the selectivity is reversed in the case of EDDA. Except in the case of the Eu-DAPDA-TTA system, for which the ternary Eu(DAPDA)TTA species is present even a t pH 5.3, the rest of the systems have slope values close to 3 for the plots of log D vs. log [TTA], at pH -6.0 indicating the extraction of dominant, binary Ln(TTA)3 species.
Effect of p H . Figure 4 shows the variation of log D as a function of pH for Eu(II1) in the presence of different complexing agents. In the pH range between 6.0 and 7.0, extraction of Eu(II1) follows the order EDDA > DAPDA > DACDA which is parallel with the instability constants of their Eu(II1) complexes. This is similar to the extraction behavior of La(II1) and Nd(II1) (15). On the other hand, in the pH range between 7.2 and 8.4, extraction of Eu(II1) in the presence of DACDA is larger compared to that in the presence of DAPDA if other experimental conditions are equal. This is explained on the basis that the extracted species are different in the two cases. Higher extraction of Eu(II1) in the presence of EDDA is possibly due to the dominant presence of Eu(TTA), which may have greater solubility in the organic phase compared to that of the ternary species. Maxima in the curves for EDDA and DACDA appear at pH -7.5 and for DAPDA, at pH -7.0, respectively. Figure 5 shows the variation of log D with pH for Yb(II1) in the presence of EDDA, DAPDA, and DACDA. The order of extraction was found to change with the increase of pH, Le., the extraction in the presence of EDDA is higher at pH 6.0 compared to DACDA, it is lowered at pH 1 7 . 0 . This is due to the fact that the increase in aqueous complexation with increase of pH is greater with EDDA which was present in 10 times the stoichiometric amount and only the binary species, Yb(TTA),, is extracted into the organic phase. It is also observed that the maximum for DAPDA curve shifts to lower pH (-7.0) as compared to lighter lanthanide results whereas for DACDA and EDDA it remains at pH 7.5. Similar results were obtained in the Lu-(DACDA/DAPDA/ EDDA)-TTA systems with regard to pH effect except that the maximum for Lu-DAPDA-TTA is at pH -6.5. The shift of pH for maximum lanthanide extraction is worth noting. It appears that due to the presence of ternary species for Yb-DAPDA-TTA system in the organic phase, effective concentration of DAPDA to complex Yb(II1) ion in the aqueous phase is lower compared to either DACDA or EDDA where binary species dominate in the organic phase. The excess ligands, DACDA and EDDA, can form 2:l complexes with the lanthanides which prevent at least partially the hydrolysis of the resulting complexes. On the other hand, the hydrolysis effect seems to be more important for DAPDA systems. As discussed in our previous work (15),decrease in extraction with increasing pH may also be partly due to the
ANALYTICAL CHEMISTRY, VOL. 59,
LogD
A
.
-6'
LqD
----7/
1 -
-5
-1
0-
-2
2 -
-2
1 -
-I
-7
-3,
2 -
NO.6, MARCH 15, 1987 817
lo
-
0-
-4
-1
t ? f % W 8 ; M 8 8 g 8 6 X % R C E
La
Eu
Nd
Yb Lu
Flgure 6. Logarithmic variation of distribution at pH 6.0 with atomic
number of lanthanide in the presence of different complexing agents: (X) 1.8 X lo-' M Ln(II1) 1.8 X M EDDA; (+) 1.8 X lo-' M M Ln(II1) 1.8 X M DAPDA; (0)1.8 X Ln(II1) f 1.8 X lo-' M DACDA; (A)computed using literature data ( 18). [TTAItotal= 3.3 x 10-3 M, I = 0.13.
+
+
hydrolytic cleavage of TTA which is significant beyond pH 8.0. Effect of Atomic Number of Lanthanides. Distribution ratios at pH 6.0 in the present ternary systems increase with the atomic number of lanthanide, e.g., La < Nd < Eu < Yb < Lu (Figure 6). Qualitatively, this trend is consistent with the reported TTA-chelate extraction constants for various lanthanides as given by the dashed line curve in Figure 6 (18) although extracted species are not necessarily the same in the present work. The presence of the two macrocyclic reagents, Le., DAPDA and DACDA, does change the values of separation factors for m y two different lanthanide ions due to their selective nature in metal binding. However, the variation of the values of separation factors can sometimes but not always be more favorable than the normal T"A extraction selectivity. This can be illustrated by considering the following two examples: (a) The separation factors for Lu/Yb (DLu/Dm)at pH 6.0 in log units are 0.77 in the presence of DAPDA and 0.24 in the presence of DACDA, respectively. Both values are greater than that in the presence of 'TA alone, i.e., 0.15 log units. This is because the complexes of Lu(II1) with DAPDA and DACDA are respectively less stable by 0.43 log units and 0.06 log units than those of Yb(II1). Thus it is easy to rationalize that the macrocyclic ligands will complex Yb(II1) more readily and increase the values of separation factors. (b) The separation factors for Nd/La (DNd/DLa) at pH 6.0 in log units are 0.34 in the presence of DAPDA and 0.10 in the presence of DACDA, respectively. The two values are much lower than that in the presence of TTA alone, Le., 1.15 log units. The decrease in separation factors in these cases is not governed by the increase in complex stabilities for Nd(II1) over La(III), which incidentally with DACDA is the same. A more plausible reason for the observed decrease in separation factors is the extraction of predominantly ternary species of the type Ln(DAPDA/DACDA)TTA rather than the binary species Ln(TTA), for lighter lanthanides. Extraction constants reported for the ternary systems (15)are much closer to each other as compared to those reported for the binary systems (17, 18). A break near Nd(II1) in the curve obtained in the presence of EDDA (as shown in Figure 6) and lack of regular increase in distribution ratio could be explained on the basis of the
W f W W 8 ; M l f P 8 6 1 % R C P La Nd EU Yb Lu
Figure 7. Logarithmic variation of distribution ratio at pH 7.5 with atomic number of lanthanide in the presence of different complexing agents, I = 0.2: (X) 1.8 X M Ln(II1) 1.8 X M EDDA, [lTA],,l = 1.6 X M; (+) 1.8 X M Ln(II1) 1.8 X M DAPDA, [TTA]t,b, = 3.3 X M; (0) 1.8 X low4 M Ln(II1) 1.8 M; (A)computed using X M DACDA, [TTA],, = 3.3 X literature data (78).
+
+
+
larger concentration of EDDA (10 times the stoichiometric amount) and a steady increase in the Pz values of Ln(EDDA)< complex with atomic number (19). Typically, the separation factors in log units decrease from 0.15 (in the absence of EDDA) to 0.05 (in the presence of EDDA) for the Lu/Yb system and from 1.15 (in the absence of EDDA) to 1.01 (in the presence of EDDA) for the Nd/La system. This is in contrast to the extraction behavior of lanthanides in the presence of macrocyclic ligands. Figure 7 shows the variation of distribution ratio with atomic number a t pH 7.5. Whereas the shape of the DACDA curve is similar to the one obtained a t pH 6.0, curves for DAPDA and EDDA deviate significantly from those obtained a t pH 6.0. This is caused by the variation in the nature of extracted species and the increase of the values of conditional complex formation constants with increase of pH. It is interesting to observe that in the presence of DAPDA, neodymium extracts the least among all five lanthanides studied and in the presence of EDDA, it is extracted the most. The separation factor values obtained for the particular pairs of lanthanides can be rationalized using arguments similar to those presented for extraction curves at pH 6.0. It may be added that the lower extraction of heavy lanthanides, i.e., Yb(II1) and Lu(III), compared to lighter lanthanides, i.e., La(II1) and Nd(III), in the case of the Ln-EDDA-TTA system, though expected, has been reported in the present work for the first time.
CONCLUSIONS It has been demonstrated in the present work that the preferential extraction of Lu(II1) over La(II1) commonly observed with a variety of extractants including TTA could be reversed in favor of La(II1) at pH 7.5 in the presence of EDDA (10 times the stoichiometric amount). This work together with the previously reported one (15) also revealed significant differences in extraction mechanisms and the nature of extracted species for lighter lanthanides, e.g., La(II1) and Nd(III), and heavier lanthanides, e.g., Yb(II1) and Lu(III), in the presence of DAPDA and DACDA. It is interesting to point out that the rates of extraction are different for the two groups of lanthanides due to different extraction mechanisms and species. Whereas the formation of the ternary complex Ln(DAPDA/DACDA)TTA appears
Anal. Chem. 1987, 59,818-821
818
to be the rate-determining step for the lighter lanthanides, it is the dissociation of Ln(DAPDA/DACDA)+ complex which governs the rate of extraction for heavier lanthanides. There is a possibility of developing a separation procedure for lighter lanthanides from heavier lanthanides wherein equilibration time can be judiciously chosen to achieve the preferential extraction of either group of lanthanides from a given mixture. A systematic study on the kinetic control of extraction selectivities is in progress and will be reported elsewhere (20).
(8) Weaver, B. Ion Exchange and Solvent Extraction; Marinsky, J. A,, Marcus, Y . , Eds.; Marcell Dekker: New York, 1974; Vol 6, pp 189-277. (9) Powell, J. E. Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, Jr., Eyring, L., Eds.; North-Holland: Amsterdam, 1979; voi. 3, pp 81-108. E ~D, D,; ~ id, ~ ~ G, R,; , C, G, Anal, Chem, 1986, 58, 18 14- 18 16. (11) Chang, C. A.; Rowland, M. E. Inorg. Chem. 1983, 22, 3866-3869. (12) Chang, C. A.; Ochaya, V. 0.Inorg. Chem. 1988, 25, 355-358. (13) Chang, C. A.; Garg, B. S.; Manchanda, V. K.;Ochaya, V. 0.; Sekhar, V. C. Inorg. Chim. Acta 1988, 115, 101-106. (14) Sekhar, V. C.; Chang, C. A. Inorg. Chem. 1988, 25, 2061-2065. (15) Manchanda, V. K.; Chang, C. A. Anal. Chem. 1988, 58,2269-2275. (16) Healy, T. V.; Peppard, D . F.; Mason, G. W. J . Inorg. Nucl. Chem. 1982, 2 4 , 1429-1448. (17) Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes ; Wiley-Interscience: New York, 1969; p 509. (18) Alstad, J.; Augustson, J. H.; Farbu, L. J . Inorg. Nucl. Chem. 1974, 36, 899-903. (19) Thompson, L. C. J . Inorg. Nucl. Chem. 1982, 2 4 , 1083-1087. (20) Chang, C. A.; Manchanda, V. K.; Peng, J., submitted for publication.
LITERATURE CITED Weaver, B.; Kappelrnann, F. A . J . Inorg. Nucl. Chem. 1988, 30, 263-272. Fardy. J. J.; Pearson, J. M. J . Inorg. Nucl. Chem. 1973, 35, 25 13-2524. Natasi. M. J. C.; Lima, F . W. J . Radioanal. Chem. 1977, 35, 289-301. Bigelow. J. E.; Collins, E. D.; King, L. J. Actinide Separations; ACS Symposium Series 117; Navratii, J. D., Schulz, W. W., Eds.; American Chemical Society: Washington, DC, 1979; pp 147-155. Cecille, L.; Dworschak, H.; Girardi, F.; Hunt, B. A,; Mannone, F.; Mousty, F. Actinide Separations ; ACS Symposium Series 117; Navratil, J. D., Schulz, W. W., Eds.; American Chemical Society: Washington, DC, 1979; pp 427-460. Nakamura, S.; Suzuki, N. Inorg. Chim. Acta 1988, 114, 101-107. Motomizu, S.; Freiser. H. Solvent Exfr. Ion Exch. 1985, 3(5). 637-665.
RECEIVED for review August 4, 1986. Accepted November 7, 1986. This material was prepared with the support of U S . Department of Energy Grant No. DE-FG05-84ER13292; however, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE.
Capacity of Sampling and Preconcentration Columns with a Low Number of Theoretical Plates Per Lovkvist* and Jan Ake Jonsson Department of Analytical Chemistry, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden
Short sampHng and preconcentration columns often show very low plate numbers, in whlch case the usual slmple treatment of chromatography Is not appilcable and gives unreallstlc results. Several alternate equations for the breakthrough curve are compared and a realistic model of the breakthrough properties of short columns Is developed. The results suggest (in contrast to previous works) that a significant sampling capacity is retained even at very low plate numbers.
Short chromatographic columns are of considerable interest in many research areas, for example, the sampling of pollutants in air or water or preconcentration of samples before a separation (trace enrichment, purge and trap, etc.). Such applications differ from ordinary analytical gas or liquid chromatography in several respects: the sample enters the column as a front instead of a narrow plug; as no separation is intended, it is not necessary to have as many theoretical plates as for an analytical column; the retention must be large, in order to permit large sample volumes. There are several reasons why it may be advantageous to use short columns: A low pressure drop is required for many applications, due to limitations of the pumping equipment. This is especially the case for field sampling. A high flow rate is often desired, as it permits the collection of a larger sample. For trace analysis, this may be an important factor. A small physical size is advantageous for a desorption step following the preconcentration. Several of these points lead naturally to the use of short, large-diameter columns, packed with coarse packing materials. 0003-2700/87/0359-0818$01.50/0
Table I. Front Shapes‘
J l Ja
eq no.
1
@[n”’(T
- I)]
2
@[nl”(T
-
3
@[n’/’(, - l ) / T l / Z ]
4
@[n’/’(~ - l ) / ~ ’ / ’ ] [1/2(nT)’/’] p[n’/’(T - l ) / T ’ / ’ ]
5
@[n’/’(~ - 1)/d/’]+ exp(2n) x
4, 7
+ @[-n’”(T + 111
“Normalized flux (JIJ,)
+ l)/T’/’]
as a
5 6
+
O[-nli’(, T
I)]
ref
X
6, 8
8
function of relative retention time
= t / t D . T h e function cc is the normal distribution and @ is its
Such columns will often have only a few theoretical plates. Although this is a common case, it is not always recognized. In the Appendix, a few examples are given. It is usually assumed that the shape of the eluting front (the breakthrough curve) can be described as the integral of a Gaussian peak. With this assumption, several attempts have been made to find an expression which relates the capacity of a sampling column to the retention volume and the number of theoretical plates (1-4). Another frontal equation, very similar t o the integrated Gaussian, was also used in this context ( 5 ) . However, these equations apply strictly only to columns with relatively high plate numbers. Consequently, it is of interest to find more general expressions for the front shape which are valid also for low plate numbers. C 1987 American Chemical Society
