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Anal. Chem. 1981, 53, 2115-2117
Mixed Ligand Chelate Extraction of Lanthanide Ions in Systems Involving 74 1-Vinyl-3,3,6,6-tetramethyl hexyl)==8=quinolinol, 8-Quinolino1, and 1,IO-Phenanthroline Etsu Yamada' and Henry Frelser" Department of Chemistry, University of Arizona, Tucson, Arizona 8572 1
A study of the equlllbrlum extractlon behavlor of a serles of representatlve tervalent lanthanlde Ions, Pr, Eu, and Yb, was carrled out wlth chloroform solutions containing either 7-dodecenyl-8-quinollnol (DDQ or HK) alone or comblned wlth 1,lO-phenanthroline (phen). The results demonstrated that the lanthanldes are extracted as a slmple chelate, LnK3, or In the presence of phen, as LnK,*phen. Solvent extraction of lanthanldes has been studied further wlth mlxtures of HK and 8-qulnolinol (Ha) In chloroform. The mixed complexes In chloroform were considered to be LnK,Q, and at hlgher 8qulnollnol concentratlon LnKQ,, but the additlon of 0.1 M phen changes the specles to LnK,Q3-,*phen complexes. The formatlons of mixed complexes and addition of phen Improve both the extractlon and separation.
As a part of a systematic evaluation of the use of chelating extractants such as 8-quinolinol and its derivatives for the separation of individual lanthanides, the equilibrium extraction behavior of a series of representative tervalent lanthanides into chloroform solutions containing either 8quinolinol alone or combined with phenanthroline or tetran-heptylammonium chloride (R4NCl) was studied in detail (1,2).
We decided to conduct a comparative extraction study with a substituted 8-quinolino~lin order to evaluate the role of substituents that affect both chelate stability and extractability. The reagent Kelex 100, 7-(1-vinyl-3,3,6,6-tetramethylhexyl)-8-quinolinol (DDQ), is commercially available (3). The applications of DDQ to the solvent extraction of Co, Cd, Zn, Fe, and Ni have been previously reported (4-81, but there is no study on the solvent extraction of tervalent lanthanides with DDQ. The extraction of Pr, Eu, and Yb with DDQ in the absence and presence of phen, respectively, is here reported. Furthermore, since it was found that a small amount of 8-quinolinol has a large synergistic effect on the extraction of lanthanides by DDQ, it was decided that an investigation of the reason for the enhancement would be worth pursuing as a possible means of further increasing the separation of individual lanthanide ions. Several studies of the extraction with mixtures of 0-diketones have been carried out (9-11). I t would be very interesting to determine the mechanism of the extraction of lanthanides by mixtures of DDQ and 8quinolinol. Will lanthanides extract as mixed complexes, LnK,Q3, or adduct complexes, LnK3.aHQ? Will phenanthroline adduct to those chelates? EXPERIMENTAL SECTION The apparatus and materials such as lanthanides, Arsenazo 111, and phenanthroline used in the extraction have been previously described ( 2 ) . 'On study leave from: The Faculty of Science, Kyoto University,
Kyoto, Japan.
Kelex 100 (kindly supplied by Sherex Chemical Co.) was p u rified according to Lakshmanan's method (4). It was purified by shaking a solution in toluene several times with 1.0 N hydrochloric acid, until the aqueous acid extract no longer exhibited an absorption peak at 212 nm, which was due to water-soluble pro. tonated 8-quinolinol. After the mixture was washed with water until no chloride ions could be detected in the aqueous phase, the solvent was evaporated and purified DDQ was dried in vacuo over CaC12. It was dissolved in chloroform just prior to use Buffer solutions included either 0.0125 M succinic acid alone or mixed with 0.0125 M tris(hydroxyamino)methane (THAM) Tartaric acid, used in the earlier study (I), was unnecessary because pH ranges were sufficiently low that the metal hydroxide complexes were of no consequence. All the other chemicals were of Analytical Reagent grade and used without further purification Extraction Procedure. A 10-mL portion of buffered lanthanide solution and an equal volume of the reagent solution were equilibrated in a vial by vigorous shaking for 1h, a time period which was found adequate for attainment of equilibrium. The pH value of the aqueous phase after extraction was taken as the equilibrium pH value. After phase separation, equal volume1 aliquots of both phases >werepipetted out. The lanthanide in the aqueous phase was determined by the Arsenazo I11 method after adjusting pH within 2.6 f 0.1 with formic acid. The lanthanide in thie organic extract was back-extracted in 10 mL of pH 4.05formate buffer solution by shaking for 10 min. After the mixture was washed once with 10 mL of pure chloroform, an aliquot of aqueous solution was determined according to the previous procedures.
RESULTS AND DISCUSSION (1) Extraction of Lanthanides by DDQ Alone or Corn.. bined with Phenantknroline. The extraction behavior of Pr, Eu, and Yb by DDQ into chloroform was studied in the absence and presence of phenanthroline, respectively. The ploh of log D vs. variables such as the pH of the aqueous phase andl the logarithm of the concentrations of DDQ and of phenanthroline were obtained to determine the stoichiometry andl equilibrium constants of the extraction. The observed slopes of log D vs. pH are somewhat less than 3.0 for Yb a t 0.1 M DDQ but close enough to indicate that, the extracted species contains three ligand anions. Plots of' log D[H+I3vs. log ["lo all indicate a slope very close to 3.0. From these results, the extraction stoichiometry can be expressed as Ln3+ 4- 3HK(o) + LnK3(o)
+ 3H+
(1)
where o refers to the species in the organic phase. The distribution ratio, D, of lanthanide is given by
where K,, = @ & ~ K ~ / K DK R , and ~ , KDRare the acid dissociation constant, 10-10.4,and distribution constant, 106.62 (12)) of HK, and Ps and KI,c are the overall aqueous phase formation constant and the distribution constant of chelate LnK*
0003-2700/81/0353-2115$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
Table I. Summary of Extraction Constants and Separation Factors
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.
separation factors Q
extraction constants Yb Pr Eu logKex log
LnK, LnK,. phen (I
Pr/ Eu/ Eu Yb
O I
m
KexKphen
_I
The main difference between DDQ and 8-quinolinol is that no self-adducts are formed with DDQ in the undissociated form whereas two or three neutral molecules are contained in the extracted 8-quinolinol complexes. This may be attributed to the steric effect of 7-position alkenyl substituents. We previously observed that a 7-substituted derivative, 5,7-dibromo-8-quinolinol (DBQ), formed lanthanide chelates with only 1 mol of adducted ligand (13). This would lend weight to the steric factor; the 7-substituent in DDQ is much larger than that in DBQ. Extraction in the presence of phenanthroline was also evaluated by slope analysis. The slopes of log D vs. pH are 2.5, 2.4, and 2.5; those of log D[H+I3vs. log [HK], are 3.2,2.9, and 2.7 for Pr, Eu, and Yb, respectively. The plots of log D[H+I3vs. log [phen], exhibit a linear range of unit slope. These results suggest that one phenanthroline adducts to LnK3 Hence, the stoichiometry of extraction can be expressed as K!h + phen(o) e LnK3.phen(o)
-20.0
1
-21.0
I -3.0
-2.0
-1.0
0
L o g [8-QUINOLIhOLlo
Figure 1. Distribution of Eu(II1) with mixtures of Kelex 100 and 8quinolinol as a function of log [8-quinoilnol]. [Kelex lOO], = 0.10 M.
(2) Extraction of Lanthanides with Mixtures of DDQ and 8-Quinolinol. The extraction of Pr, Eu, and Yb with DDQ-8-quinolinol mixtures in chloroform was studied. In the present paper, the extraction of Eu(II1) was studied in detail. The general expression for the extraction of Ln3+ with mixtures of HK and HQ is
Ln3+
+ UHK(0) + bHQ(O) + LnKaQbHa+b-3(0)+ 3H' (6)
(3)
where Kphenis the adduct formation constant of phenanthroline. From this, it can be shown that
the extraction constant of LnK3.phen, K,:
I
0
log Kphen 2.2 2.3 2.4 Calculated from individual extraction data.
LnK3(o)
1!
0 I
-19.45 -17.87 -16.77 1.58 2.10 -17.21 -15.59 -13.34 1.62 2.25
Ka,b
(7) = [ L ~ K C I Q ~ H Q + & ~ I O[Ln3+I [ H + I ~[HKIoQIHQI,b / D =
c [LnKaQbH~+b-do/b 3 + 1
(8)
here a + b 1 3. In the case where the [HK],/[HQ], ratio is held at a constant value, C , [HQ], = [HK],C1 can be introduced into eq 7 and 8 and then
defined as
(5) From the extraction data, using eq 2 and 4, it is possible to calculate values of log K, and log K,'. The difference between these two values corresponds to the adduct formation constant of phenanthroline, log Kphen. These values and separation factors are listed in Table I. In the absence of phenanthroline, the separation of individual lanthanides was almost the same as with 8-quinolinol. Phenanthroline enhanced the extraction but gave only slightly better separation. The effect of phenanthroline is smaller in the DDQ system than in 8-quinolinol system as may be seen from the comparison of values of log Kphen and those of log (&xKphen/ KexPz,o)* In the 8-quinolinol system, two neutral 8-quinolinol molecules were adducted to LnQ3 along with one phenanthroline. If it is assumed that two adduct molecules of 8-quhohol have no effect on the adducting reaction of phenanthroline to LnQ3 chelate, the difference of the effect of phenanthroline may be attributed to either the acid-base character of metal chelates or the steric effect of chelating extractants. Inasmuch as the values of the acid dissociation constants of DDQ and of 8-quinolinol are close, 10.4 and 9.9, respectively, the smaller adduct formation constant of phenanthroline in the DDQ system is probably attributable to a steric effect of the 7alkenyl substituent of DDQ.
[LnKQQbHa+b-310[Ln3+] - KQ,bc-b[HK],(l+b[H+13(9) D[H+I3
+ CKQ,bC-b[HK],(l+b
(10)
The plots of log D[H+I3vs. log [HK], when the ligand ratio [HK],/[HQ], has a constant value (about 25) gives a fairly straight line of slope 3. Hence, from eq 10 we see that the average number of the chelating acid molecules in the complexes is three, which indicates that no adducts are formed in this system. Plots of log (D- Dox)[H+]3vs. log [HQ] when various amounts of 8-quinolinol are added to organic phases containing, in all cases, 0.1 M DDQ, is shown in Figure 1. D, is the distribution constant by 8-quinolinol alone. D, is much smaller than D, so log (D- Dox)is almost equal to log D. M, with The slope becomes close to unity above 4 X a tendency to increase further to a slope of 2. In Figure 2 are shown the plots of log D[H+I3vs. log [DDQ], at 0.01 M 8quinolinol, which is the concentration region that the slope of log (D - DoX)[H+l3 vs. log [HQ] is 1. Here the slope is 2. Figure 3 shows the plots of log D[H+l3vs. log [DDQ], at 0.1 M 8-quinolinol, which is the concentration region where the slope of log (D- DoX)[H+l3 log [HQ], is 2, and here the slope is close to unity. The slopes of log D vs. pH are close to 3 at 0.01 M 8-quinolinol and 0.1 M 8-quinolinol containing, in both cases, 0.1 M DDQ. These results suggest that extracted species of Eu(II1) can be expressed as mixed complex EuK2Q, and at higher 8quinolinol concentration EuKQ2.
ANALYTICAL CHEMISTRY, VOL.
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~~~~
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Table 11. Extraction of Lanthanides by Kelex 100, or Combined with 8-Quinolinol and pH,,, Values
I
extracted species
1
LnK, LnK,. phen LnK, Q LnKQ, LnK,Q,,.phen
- 2 . il
-1.0
0
L3G [DOLI1
Figure 2. Distribution of Eu(II1) with mixtures of Kelex 100 and 8quinolinol as a function of log [Kelex]. [8-Quinolinol], = IO-* M. -17.0
.
-la.’
i
-
PHI,,
Pr
Eu
Yb
conditions
7.45 6.94 6.24 [HK], = 0.1 M 7.03 6.54 5.76 [HK], = 0.1 M, [phen], = 0.1 M 7.10 6.62 5.86 [HK], = 0.1 M, [HQ], = lo-’ M 6.62 6.12 5.32 [HK], = 0.1 M, [HQ], = 0.1 M 5.81 [HK], = 0.1 M, [HQ], = 0.1 M [phen], = 0.1 M
In the mixed complex system containing DDQ, the donating ability of HQ is not sufficient to form an adduct. The effect of phenanthroline on the extraction of Eu(II1) with DDQ-Bquinolinol mixtures was examined. The plota of log D vs. pH for Eu in the presence of 0.1 M phenanthroline at 0.1 M DCIQ and 0.1 M 8-quinolimol is shifted toward the low pH region compared with similm plots in the absence of phenanthroline. Hence, we may assume that extraction reaction in the presence of phenanthroline is Ln3+
+ aHK(o) + (3 - a)HQ(o) + phen + LnK,Q3-,phen
+ 3Ht
(13)
From these results, it would appear that the formation of mixed complexes with a mixture of two chelating extractants and adduct formation to mixed complexes may further enhance separation calpability in the lanthanide series.
LITERATURE CITED -20.0
, -2.0
-1.0
0
Log (DOQlo
Figure 3. Distribution of Eu(III) with mixtures of Kelex 100 and 8quinolinol as a function of log [Kelex]. [8-Quinollnol], = 0.1 M.
The ext,raction of Pr and Yb with DDQ-8-quinolinol mixtures in chloroform was also studied. From the results obtained, the extraction reaction can be expressed as Ln3”-= 2HK(o) t HQ(o) + LnK2Q(o) 3H+ (11) a t higher 8-quinolinol concentration 3Ht (12) Ln3”’ HK(o) 2HQ(o) + LnKQ,(o) In Table 11, values of pH1p.clearly show that the formation of mixed complexes results in better extraction and better separation of lanthanides.
+
+
+
+
(1) Horl, T.; Kawashima, M.; Freiser, H. Sep. Sci. Techno/. 1980, if5, 861-875. (2) Kawashlma, M.; Frcsiser, H. Anal. Chem. 1981, 53, 284-286. (3) Hartlage, J. A., Society of Mining Engineers Meetlng (AIME), Salt Lake City, UT, Sept 1961). Lawson, G. J. J. Inorg. Nucl. Chem. 1973, 35, (4) Lakshmanan, V. I.; 4285-4294. (5) Lakshmanan, V. I.; Lawson, G. J.; Nyhoirn, P. S., Proceedings of ISEC, Lyon, France, Sept 1974, pp 699-709. Lawson, G. J. Proceedings of ISEC, Lyon, France, (6) Lakshrnanan, V. I.; Sept 1974, pp 116’3-1183. (7) Flett, D. S.; Cox, M.; Heels, J. D. J. Inorg. Nucl. Chem. 1975, 37, 2197-2201. (8) Ashbrook, A. W. Coord. Chem. Rev. 1975, 16, 285-307. (9) Newman, L.; Kiotz, P. “Solvent Extraction Chemistry”; Dyrssen, I)., Liljenzln, J. O., Rydtrerg, J., Eds.; North-Holland Amsterdam, 1967; pp 128- 134. (10) Sekine, T.; Dyrssen, D. J. Inorg. Nucl. Chem. 1964, 26, 2013. (11) Sekine, T.; Dyrssen, D. J. Inorg. Nucl. Chem. 1967, 29, 1489. (12) Bag, S.; Freiser, H., unpublished data.
RECEIVED for review :March 30,1981. Accepted July 20,1981. This research was supported by the Department of Energy.
