874
Anal. Chem. 1981, 53, 874-877
Mixed Ligand Chelate Extraction of Lanthanides with 5,7-Dibromo-8-quinolinol Systems Osamu Tochiyama’ and Henry Freiser * Depatfment of Chemistty, University of Arizona, Tucson, Arizona 8572 I
The equllibrlum extractlon behavior Is studied for a series of representative tervalent ianthanlde ions, Pr, Eu, and Yb, uslng chloroform solutlons containing either 5,7-dlbromo-8quinolinol (HQ) alone or combined with 1,lO-phenanthrollne (phen) or tetra-n-heptylammonlum chlorlde (R,NCI). The results demonstrate that these lanthanides are extracted as LnQ3*HQor in the presence of phen as LnQ,*phen or In the presence of R4NCias R4NLnQ4. Although 8-qulnolinol forms more stable chelates wlth these metals, the higher acid dissociation constant of the dibromo compound results in increased extraction constants, maklng extractlon possible In lower pH ranges. Furthermore, the effect of phen or R,NCI on the extraction and separation of lanthanides by 5,7-dlbromo-8-quinollnol is more pronounced than thelr effect in the 8-qulnolinol system.
used without further purification. The extraction of lanthanides whose maximum initial conM was carried out in the presence o f 2 centration was 2 X X M each of succinic acid and tris(hydroxymethy1)aminomethane and various amount of sodium hydroxide or hydrochloric acid. In these experiments, extraction was carried out without sodium tartrate because the pH ranges of interest were sufficiently low that metal hydroxide complex formation was negligible. Equal volumes (10 mL) of an aqueous and an organic solution were brought into contact in a glass vial and mechanically shaken vigorously for 60 min at room temperature. After phase separation, an aliquot of the organic phase was pipetted out and brought into contact with 0.1 M sodium formate-formic acid buffer at pH 2.6. The concentration of lanthanide back-extracted into aqueous solution was determined by the arsenazo I11 method described previously ( I ) .
RESULTS AND DISCUSSION As a part of a systematic evaluation of the use of chelating extractants such as 8-quinolinol and its derivatives, the equilibrium extraction behavior of a series of representative tervalent lanthanide ions into chloroform solutions containing either 8-quinolinol (HQ) alone or combined with 1,lOphenanthroline (phen) or tetra-n-heptylammonium chloride (R4NC1)was studied in detail (1,2). Lanthanides were found to be extracted in the form of the self-adduct complexes LnQ3.2HQ or LnQ3-3HQ,except for La which is extracted as a simple chelate, LaQ,. In the presence of phen, mixed adduct formation was observed for all the lanthanides except La, having the formula LnQ3.2HQ.phen, which further enhanced their separation. In the presence of R4N+,lanthanides were extracted as the complex R4NLnQ4*HQexcept for La which was extracted as a simple ion pair, R4NLnQ4. The separation of La from the other lanthanides was enhanced in the presence of R4N+,but separation of the others was almost the same as in the absence of R4N+. We decided to conduct a comparativeextraction study with a substituted 8-quinolinol in order to evaluate the role of substituents that affect both chelate stability (electrostatic effects) and extractability. 5,7-Dibromo-8-quinolinol was an attractive choice for several reasons: First, because its lower pK, suggested that the lanthanides might be extractable at a lower pH range than that obtained with the parent compound. Second, it would be interesting to see how the poorer chelatingagent would affect both self-adduct and mixed ligand chelate formation (3). Finally, this provided an opportunity to study the effect of the increased lipophilicity of this ligand upon the extractability of the chelate. EXPERIMENTAL SECTION The apparatus and materials such as lanthanides, tetra-nheptylammonium chloride, arsenazo 111, and buffer solutions used in the experiment have been described previously ( I , 2). 5,7Dibromo-8-quinolinol (Eastman Organic Chemicals) was recrystallizedtwice from hot ethanol and was dissolved in chloroform just prior to use. All the other chemicals were A.R. grade and On study leave from the Faculty of Engineering,Tohoku University, Sendai, Japan. 0003-2700/81/0353-0874$01.25/0
In order to clarify the distribution behavior of lanthanides in the presence of 5,7-dibromo-8-quinolinol (HQ), the extraction data were used to obtain plots of log D vs. variables such as pH of the aqueous phase and logarithm of the concentration of 5,7-dibromo-8-quinolinol for representative tervalent lanthanide ions, Pr, Eu, and Yb. All slopes of log D vs. pH are close to 3, e.g., 3.2 for Pr, 2.8 for Eu, and 2.4 for Yb at M of HQ, which indicates that the extracted species contains three Q- ions because the predominant species of lanthanides is considered to be Ln3+ in the aqueous phase. Slopes a little smaller than 3 may be attributed to the contribution of water-soluble complexes of lanthanides with 5,7-dibromo-8-quinolinolsuch as LnQ2+and/or LnQ2+in the aqueous phase, which is expected from values of formation constants of 1to 1 and 1to 2 complexes of lanthanides and 8-quinolinol-5-sulfonicacid which are not small enough to be neglected in comparison with 1 to 3 complexes (4). Plots of log 0-3 pH vs. log [HQ],, indicate a slope of 3.7 for Pr, 3.8 for Eu and 3.8 for Yb. From these results, the extraction stoichiometry can be expressed as Ln3+
+ 4HQ(o)
LnQ3.HQ(o)
=i
+ 3H+
(1)
where subscript o refers to the species in the organic phase. The distribution ratio, D, of lanthanide is given by
(3) where K , and KDRare the acid dissociation constant, (5), and distribution constant, 104.16(5), of HQ, p3 and KDc are the overall aqueous formation constant of LnQ3 and its distribution constant, respectively, and PI,, is the adduct formation constant of the reaction LnQdo) + HQ(o) % LnQ3.HQ(o) (4) As shown in Table I, values of P H ~ for , ~ 5,7-dibromo-8quinolinol (DBQ) at M are lower than those for 8quinolinol, even when the latter is at a 10-fold higher concentration. This indication of a significant increase in the extraction constant is undoubtedly attributable to the influence of the Ks/KDR ratio which for 8-quinolinol is 10-12.60, 0 1981 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
875
Table I. Extracted Species of Lanthanides by 5,7-Dibromo-8-quininoland 8-Quinolinola and pH,,, Values extracted species
Pr
PHl, 2 Eu
6.44 5.95 5.29
6.13 5.25 4.83
Yb
conditions
5,7-Dibromo-8-quinolinol
LnQ,.HQ LnQ,.phen R,NLnQ,
5.46 4.32 4.35
[HQ] = lo-' [HQ] = lo-,, [phen] = [HQ] = [R,NCl] = lo-,, [NaCl] =
8-Quinolinol 8.62 8.01 7.50 [HQ] = lo-' LnQ ,. 2HQ 6.92 6.24 5.63 [HQ] = l o - ' LnQ,a3 HQ 6.78 6.07 5.44 [HQ] = lo-', [phen] = lo-' LnQ,-BHQ.phem 5.65 [HQ] = [R,NCl] = lo-,, fNaCl] = lo-, 6.54 R,NLnQ,. HQ a Values of pH,,, for 8-quinolinol systems are calculated from the data ( I , 2 ) by use of log P , = 6.9, 7.4, and 7.3 for tartrate complexes of Pr, Eu, and Yb (6). respectively. Table 11. Summary of Extraction Constants and Separations Factors extraction constants Pr Eu LnQ,.HQ LnQ,.phen R,NLnQ,
a
log KexP 1,o 1% KexKphen log K'ex,o
LnQ,. 2HQ LnQ,.3HQ LnQ3-2HQ.phen R,NLnQ,.HQ See footnote in Table I.
log KexP 2,o log KexP3,0 log KexP2,0Kphen log K e x . 1
5,7-Dibromo-8-quinolinol -11.32 -10.38 -8.86 -6.75 -12.17 -10.33 8-Quinolinola -15.86 -14.02 -14.75 -12.74 -13.34 -11.21 -20.17
smaller than the 10-11.46value for DBQ. Full exploitation of this advantage is limited by the low chloroform solubility of DBQ, 3.02 X M compared to 2.63 M for 8-quinolinol, making it impossible to otbtain extractions at lower pH values by utilizing high DBQ concentrations. The main difference between 8-quinolinol and DBQ is the number of neutral quinolinols adducting in the extracted species. Only one adduct molecule was found in the extracted species by use of DBQ whereas more (two or three) neutral molecules are contained in the extracted 8-quinolinol complexes. This may be attributed either to the steric effect of the 7-position bromine atom, even though the effect of 7position is said to be much less than that of 2-position, or to the weaker basicity of neutral 5,7-dibromo-8-quinolinol as an adduct-forming reagent than 8-quinolinol. In any event, we can expect better extraction and better separation by using stronger bases as adducting agents. The results of the distribution of Pr, Eu, and Yb when 5,7-dibromo-8-quinolinol is used in conjunction with 1 , l O phenanthroline clearly indicate that three Q- ions and one phen are contained in the extracted species rather than the four observed in the absence of phen. The slopes of log D vs. p H are 2.8,2.4, and 2.5, tlhose of log 0-3 pH vs. log [HQ] are 2.8, 2.9, and 3.4, and those of log 0-3 pH vs. log [phen] are 1.2,1.0, and 1.0 for Pr, Eu, and Yb, respectively. Hence, the stoichiometry of extraction can be expressed as Ln3++ 3HQ(o) + phen(o) + LnQ3.phen(o) 3H+ (5) and
(a,
+
D = -CLIl(0) - - 2KexKphen WQI03 [phenl CLtl
(6)
m+13
where Kphen
=
bQ3.phenI L n Q J [phenl
(7)
Yb
separation factors Pr/Eu Eu/Yb
-8.37 -3.95 -8.40
0.94 2.11 1.84
2.01 2.80 1.93
-12.50 -10.88 -9.32 -16.60
1.84 2.01 2.13
1.52 1.86 1.89 3.57
Table 111. Differences between Extraction Constants reacPr Eu Yb tion 5,7-Dibromo- 8-quinolinol log (KexKphen/Kex @ 1,o) 2.5 3.6 log (K' ex,olKex@1,o 1 -0.9 0.1 8-Quinolinol
4.4 0.0
1
2
(1) LnQ,.HQ(o) + phen(o) + LnQ,.phen(o) + HQ(o) (2) LnQ,.HQ(o) t R,NCl(o) f R,NLnQ,(o) + H+ t c1(3) LnQ3.3HQ(o) t phen(o) + LnQ3.2HQ.phen(o)+ HQio) (4) LnQ3.3HQ(o) t R,NCl(o) f R,NLnQ,.HQ(o) + HQ(o) t H' t Cl( 5 ) LnQ3.2HQ(o) + HQ(o) f LnQ3.3HQ(o) (6) LnQ3-2HQ(o)+ phen(o) + LnQ3-2HQ.phen(o) (7) LnQ3.2HQ(o)t R,NCl(o) f R,NLnQ,.HQ(o) + H+ t C1--A
1-
No adducting 5,7-dibromo-8-quinolinolmolecule was found in the extracted species in contrast with the 8-quinolinol system where two neutral 8-quinolinol molecules were adducting to LnQ3 along with one phen. The reason may be similar to that of the self-adduct system. In Table I, values of pH112 clearly show that phen has stronger effects to give rise to better extraction and better separation of lanthanides in the 5,7-dibromo-8-quinolinolsystem than in the 8-quinolinol system. These effects can be compared more clearly by their extraction constants and separation factors as shown in Table I1 and also by the differences between extraction constants
876
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
corresponding to the reactions as shown in Table 111. In the 5,7-dibromo-8-quinolinol system, as LnQ3.HQ(o) and LnQ3-phen(o)are the only confirmed extracted species in these experimental conditions, we can only estimate the constants of the exchange reaction 1 in Table 111. Values of log (KexKphen/K&l,,) in Table 111 clearly show that phen favors the extraction of lanthanides much more than 5,7-dibromo8-quinolinol alone. Also, the gradual increase of the value of log (KexKphen/KexPl,o)from Pr to Yb means that exchange of HQ by phen favors the separation of lanthanides throughout the series. As the trend of these values is the same as the usual complex formation constants of lanthanides in aqueous solution, it is probable that phen coordinates directly to the metal atom as a bidentate ligand. For comparison, we can estimate values of log (Kexp2,&,hen/Ked~,o) for reaction 3 for 8-quinolinol, which is the same type of exchange reaction as reaction 1 except the extracted species with 8-quinolinol has two more self-adducting molecules. We can easily see that the effect of phen is much larger in the 5,7-dibromo-8quinolinol system than in the 8-quinolinol system by comparing values of log (KexKphen/Ke&,o) and those of log (KexP2,0Kphen/Kex~3,0). Although the values of log (KeX&,&phen/Ke&,,) (which are for simple adduct formation reaction of phen expressed by reaction 6 in Table 111)show the effect to give better extraction and a little better separation in the 8-quinolinol System, Values of log (KexKphen/Kedl,o)in the 5,7-dibromo-8-quinolinolsystem are still larger than those values. If we assume that two adduct molecules of 8-quinolinol have no effect on the adducting reaction of phen to LnQ3 chelate, the difference of the effect of phen may be attributed to the acid-base characters of the metal-chelates, that is, the metal in the more stable chelates of 8-quinolinol is expected to have less tendency to react further with a base, phen, or to have less “residual” Lewis acid character than the less stable chelate of 5,7-dibromo-8-quinolinol ( 3 ) . The behavior of the lanthanide ions in the presence of both extractants is in sharp contrast to that of Zn (8) in which a whole series of extractable species was found ZnQ2.HQ, ZnQ2.phen,ZnQ(phen)2+C104-,and Zn(phen)32+.2C104-.Of course, the ability of Zn2+to complex with phen (log = 6.4, log & = 12.2, log p3 = 17.1) (4) far exceeds those of the lanthanides (log /3 = 1.8-2.4 (9)). In fact, because of the low formation constants of L n ( ~ h e n ) ~no + ,significant quantities of such complexes are formed in our experiments. It is remarkable then, that even when chelated by 3 mol of &-, there is sufficient “residual Lewis acidity” to form the mixed ligand chelate, LnQ3.phen. The formation constant of the organic phase reaction is about the order of magnitude of that of the aqueous phase chelation of Ln3+and phen. Even allowing for differences in solvents, one is tempted to hypothesize that chelation of the lanthanide, e.g., with 8-quinolinolor salicyclic acid (7), “softens” the ion sufficiently to enhance its reactivity with a soft ligand such as phen. Another route to improve extractability and selectivity lies in the use of ion pair complex formation in conjunction with chelation (2). Thus, by use of tetra-n-heptylammonium chloride in conjunction with 5,7-dibromo-8-quinolinol, the distribution ratios for Pr, Eu, and Yb change in the following manner: d log D/dpH is 3.3 for Pr, 3.7 for Eu, and 4.1 for Yb; d log D/d log [DBQ], is 4.0 for Pr, 4.0 for Eu, and 4.1 for Yb; the slopes of log D vs. log [R4NC1], and vs. log [Cl-] are 1.0 and -1.0, respectively, for all three ions. Therefore the extraction stoichiometry and the extraction constant can be expressed as Ln3+
+ 4HQ(o) + R4NCl(o) ==
+
R4NLnQ4(o) 4H+
+ C1-
(8)
Kk,, = [R4NLnQ~1,[H+14[C1-l/ b 3 + l [HQ1,4[R~NC110 (9) Again the difference in the number of adduct quinolinol molecules can be seen in this system. Although there was one 8-quinolinol adduct molecule in the extracted species by 8quinolinol and R4NC1,no adduct molecule was found in the extracted species by 5,7-dibromo-8-quinolinol and R4NC1. Also, a mixed synergistic effect of R4NC1and phen was not recognized, that is, M of R4NCl had no effect on the extraction of Yb as a function of phen in the presence of 5 X M 5,7-dibromo-8-quinolinol and M NaC1. Values of pH1p in Table I show that R4NC1also is more effective in obtaining better extraction and better separation in the 5,7dibromo-8-quinolinolsystem than in the 8-quinolinolsystem. The extraction constants, separation factors, and the differences between the extraction constants are also given in Tables I1 and 111. The values of log (K’ex,o/Kex~l,o) show that the separation of lanthanides becomes better only for Pr/Eu and not for Eu/Yb. In the 8-quinolinol system, however, there is very small gain or no gain in the separation of Pr and Yb as shown by the values of log (K~x,l/Kex~z,o) or log (KkX,1/ Ked3,,).The difference between the 5,7-dibromo-8-quinolinol and the 8-quinolinol system of the effect of R4NCl to give better extraction is quite big and larger than that of phen. In general, phen or R4NCl has a larger effect on the 5,7-dibromo-8-quinolinolsystem than on the 8-quinolinol system, which may be caused by the lesser stability of 5,7-dibromo8-quinolinol chelates than the other. As the separation factor (SF) is the ratio of the extraction constants of the two metal lanthanides, it can be expressed logarithmically as
+
log SF = A IOg KDC A log p3
+ A log K
(IO)
where K is the constant for further reaction, such as P,, &hen, &,Kphen, or the constant for reaction 2 or 7. On the other hand, we can estimate A log p3 from the values for 8quinolinol-5-sulfonicacid (3)assuming that changes of log p3 are smooth. From the log p3 values of 15.34,16.78,17.08,and 18.45 for Pr, Sm, Gd, and Er, respectively, 16.9 and 19.2 are estimated for Eu and Yb, respectively. Therefore, A log p3 can be estimated to be 1.56 (Pr/Eu) and 2.31 (Eu/Yb). Values of A log Kedl,oshown in Table I1 are smaller than these values, which suggests that the values of KDC have an opposite trend, that is, K ~ c ( y b Q C~ )KDC(EuQ3)C KDC(PrQ3),possibly because of the different degree of hydration of chelate. On the other hand, separation factors in the presence of phen, A log KexKphen, shown in Table I1 are significantly larger than these values. This indicates that phen molecule is directly coordinating to the metal atom. Nevertheless, the fact that all separation fadors in Table I1 are not very different from those expected from complex formation constants in the aqueous solution indicates that the controlling factor on the separation of lanthanides in these systems is the difference of chelate formation constants in the aqueous solution. These studies demonstrate the value of 5,7-dibromo-8quinolinol as a chelating extractant both alone and admixture with auxiliary ligands in separating lanthanide ions. In addition, they provide valuable clues, such as the possible change in the “hard a c i d character of the lanthanides that results on chelation which may lead to enhanced reactivity toward still softer ligands which, in turn, may lead to enhanced selectivity. Testing of this hypothesis is under way at this laboratory.
LITERATURE CITED (1) Hori, T.; Kawashima, M.; Freiser, H. Sep. Scl. Techno/. 1960, 75, 861. (2) Kawashlma, M.; Freiser, H. Anal. Chem. 1961, 53, 284. (3) Chou, F.; Freiser, H. Anal. Chem. 1968, 40, 34.
Anal. Chem. 1981, 53, 877-884 Smith, R. M., Martell, A. E., Eds. "Critical Stability Constants"; Plenum Press: New York, 1975; Vol. 2. Stary, J.; Freiser, H. "IUPAC Equilibrium Constants of Liquid-Liquid Distributlon Reactlon:s, Part IV: Chelating Extractants"; Pergamon Press: Oxford, 1978 Sillen, L. G., Martell, 14. E., Eds. Spec. Pub/.-Chem. SOC.1964, NO. 17. Zolotov, Yu. A. "Extrwtion of Chelate Compounds"; Ann Arbor Humphrey Science Publishers: London, 1970.
877
(8) Woodward, C.; Frelser, H. Anal. Cbern. 1968, 40, 345. (9) Makarchuk, TIL., Pyatnitskii, 1. V., Gavriiava, E. F. Ukr. Khim. Zb. 1979, 45, 656.
RECEIVED for review December 22, 1980. Accepted February 12, lg81*This work was supported by a research grant from the U.S. Department of Energy.
Precision of Assays Based on Liquid Chromatography with Prior Solvent Extraction of the Sample L. R. Snyder" and Sj. van der Wal Technicon Instruments Corporation, Tarrytown, New York 1059 1
A comprehenslve theory Is presented far the varlous contrlbutions to assay imprecision in procedures which are based on sample extraction followed by high-performance liquid chromatography (HPLC). Experlmental data (1200 assays, 4200 results) for the ll'echnicon FAST-LC system are used with this theory in an erffort to better understand and control assay preclslon for bolth automated and manual procedures for determlnlng various analytes In serum or other body flulds. This study provldes speclflc conclusions and recommendatlons In the following areas: standardization procedures and protocols, physlcal prolpertles requlred In Internal standards, the relative importance of dlfferent sources of lmpreclslon and means for lmprovlng precision, the value of different sampleInjection techniques in IHPLC, the relative Importance of temperature control in pretireatment and HPLC analysis, the preclsion obtalnable with small samples or with samples containing very low concenitratlons of analyte (ems.,In assays for free drugs), and other aspects of blochemical or clinlcal HPLC analysls. Finally, it appears that automation can lead to an approxlmately 3-fold greater assay preclslon, other factors equal, for procedures based on sample pretreatment plus HPLC analysls. Slmilarly, internal standardlzatlon when properly applled can reduce assay lmpreclslon by 2-fold.
Many samples requirle pretreatment prior to their analysis by HPLC, as discussed in (1). Cleanup procedures, which we discuss here, are often based on some form of solvent extraction. The precision of such HPLC assays with solvent extraction is usually poorer than for direct HPLC analysis, e.g., 5-10% coefficient of variation (CV) for assays requiring pretreatment vs. 1-3% CV for other HPLC assays. The general principles for imlproving the precision of HPLC assays per se have been discussed (2-7). However, little attention has so far been given to error analysis for procedures that combine sample pretreatment with HPLC analysis. In the present paper we discuss the sources of imprecision that can affect assays based on solvent extraction and HPLC analysis. Elsewhere (8) we illustrate the application of this treatment for the better understanding and control of imprecision in the determination of various therapeutic drugs in serum.
THEORY General Error Analysis. Each of the various terms introduced here is further: defined in the Glossary. A chro-
matographicassay generates a Gaussian band for each analyte in the sample. We assume that a photometric detector is used to quantitate analyte concentration on the basis of a measurement of peak height, i.e., maximum band absorbance, A, corrected for base line as in Figure la. The similar use of band area is discussed in (7). The quantity A will be proportional to the amount of analyte represented by the band. Error in the measurement of A (standard deviation, a) can be divided into contributions proportional to A (a,) and independent of A (a,,):
+
= u,2 : a = (cu,/100)2A2 :a (1) For a given assay, values of cu, and an will be constant. It is convenient to express the above variances in terms of equivalent coefficient of variations; multiplication of eq 1by (100/A)2 gives a2
cu2 = CU:
+
+ (100aJ2/A2
(la)
Here, cu is the coefficient of variation for a single measurement of A (e.g., points i, ii, or iii in Figure la). Often a final value of analyte concentration is the result of some number n absorbance measurements; e.g., base line absorbance for correction of A, measurement of calibrator and/or internal standard A values, etc. Furthermore, replicate measurement of the same quantity (e.g., an A value for a given band in the same sample) some number m times can be used to increase the precision of a final (average) value of A and analyte concentration. For an assay CV that is the result of n measured quantities, each of which is repeated mktimes (and averaged), we then have
where cuk is the coefficient of variation of an individual measurement. Equation 2 is approximate for mk C 10; see (9). Consider the base line correction of a final value of A , as in Figure la. Since the absorbance values (i and iii in Figure la) are used to determine the base line absorbance, and since cu, w ill be zero for this measurement (A equal zero), the value of cu2 is 0.5(100aJ2/A2 (eq l a and 2). Similarly, the value of cu2 for the peak absorbance (ii in Figure la) is given by eq la. The coefficient of variation for the corrected value of A is then (see also (2))
CV2 = cu:
0003-2700/61/0353-0877$01.25/00 1981 American Chemical Society
+ 1.5(100an)2/A2
(3)
