Anal. Chem. 1988, 58, 2269-2275 Sweileh, J. A.; Cantwell, F. F. Anal. Chem. 1985, 57, 420. Gallego, M.; Valdrcel, M. Anal. Chlm. Acta 1985, 769, 161. Gallego, M.; Silva, M.; Valdrcel. M. Fresenlus’ Z . Anal. Chem. 1986, 323, 50. Silva, M.; Gallego, M.; Valdrcel, M. Anal. Chim. Acta 1986, 779, 341. Le Bihan, A,; Courtot-Coupez. J. Bull. Soc. Chim. Fr. 1970, 7 , 406. Le Bihan, A.; Courtot-Coupez, Anaiusis 1973-1974, 2, 695.
2289
(24) Wdf, W. R.; Stewart, K. K. Anal. Chem. 1979, 57, 1201. (25) Standard Methods for Examlnafbn of Wafer and Waste Waters, 15th ed.;American Public Health Association: Washington, DC, 1980; pp 530-532.
for review September 257 1985* Resubmitted February 4,1986. Accepted April 15, 1986.
Solvent Extraction Studies of Lanthanum(I I I) and Neodymium(I I I) with Ionizable Macrocyclic Ligands and Thenoyltrif luoroacetone V. K. Manchanda’ and C. Allen Chang* Department of Chemistry, University of Texas at El Paso, El Paso, Texas 79968-0513
Extraction behavior of La( III)and Nd( III) has been investigated by wing thenoyltrifluoroacetone (“A) as extractant in the presence of 1,7-diaza-4,10,13-trioxacyciopentadecane-N,N’diacetk acld (DAPDA) and 1,10diaza-4,7,13,16tetraoxacyclooctadecane-N,N’diacetic acid (DACDA) as macrocyclic ionophores. DAPDA and DACDA were chosen in this work In view of their unique complexation toward lanthanides. Uitravlolet spectra were adduced for the formation of ternary complex La(DAPDA)TTA In the aqueow, phase. I t was observed that Ln(TTA), is the dominating species extracted at pH 15.0 and Ln(DAPDNDACDA)TTA Is the dominating species at pH -7.5. Extraction of ternary complex of La(II1) was fwnd to be greater in the case of DAPDA and smaller in the case of DACDA as compared to the extraction of the correspondingternary complex of Nd( III).By use of the stoichiometric concentration of DAPDNDACDA and following distribution as a function of pH, lanthanide extraction maxima were observed at pH 7.5-8.0. Finally, the analytical slgnlficance of the work is discussed.
Development of suitable separation procedures of trivalent lanthanides from actinides has drawn the attention of analytical chemists since the advent of nuclear industry (1,2). Their role in the disposal of nuclear waste to the environment was recognized during the early years of nuclear power production (3,4).Subsequently, this area received further impetus with the increasing application of transuranium elements in industry and research (5). On the other hand, continued studies on the properties of lanthanide compounds have revealed their interesting applications as laser materials (6),as catalysts and as shift reagents in nuclear magnetic resonance spectroscopy (8) and in magnetohydrodynamics (9). Multistage extraction is usually necessary for the separation of adjacent lanthanides in solvent extraction methods employing neutral phosphorus reagents, monoacidic orthophosphates, @-diketones,and tertiary/quaternary ammonium ion species. One of the most common extractants employed a t present for the separation of lanthanides from each other as well as from actinides is bis(2-ethylhexy1)phosphoric acid (HDEHP), which suffers from another drawback due to its
(a,
‘On leave from Radiochemistry Division,B.A.R.C., Bombay, India. 0003-2700/86/0358-2269$01.50/0
limited solubility in the organic phase (10). During the last few years interest has been focused in our laboratory on the development of ionizable macrocyclic ionophores selective toward lanthanides. It was found that both 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N’-diacetic acid (K21DA or DAPDA) and 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,”-diacetic acid (K22DA or DACDA) shown in Figure 1form stronger lighter lanthanide complexes with the former being slightly selective toward the Eu(II1) ion (11, 12). These reagents form monovalent cationic complexes with trivalent lanthanides, and in a recent communication (13)we have demonstrated the possibility of ternary complex formation involving these cationic species and the acetylacetonate anion. It is therefore of particular interest to develop extraction procedures employing ion-specific macrocyclic compounds capable of selective complexation of lanthanides. Due to the favorable partition coefficient and dissociation constant, thenoyltrifluoroacetone(TTA) was used as the more suitable extracting agent instead of acetylacetone (14).La(II1) and Nd(II1) were chosen as metal ions in this work as typical representatives of early members of the lanthanide series. Apart from using macrocyclic ionophores as selective complexing agents, ethylenediamine-N,”-diacetic acid (EDDA), a noncyclic polyaminopolycarboxylicacid with similar functional groups, was also included in the present study for comparison purposes.
EXPERIMENTAL SECTION Reagents. DAF’DA 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 ManufacturingCorp. and Mallinckrodt,Inc., respectively. Nitrates of lanthanum and neodymium used were supplied by Aldrich Chemical Co. and Alfa Products, respectively. Standard metal salt solutions (0.01M) were prepared by titrating them against standard EDTA solution using xylenol orange as indicator. Thenoyltrifluoroacetone (TTA, laboratory reagent grade) was obtained from Aldrich Chemical Co. and its purity was confirmed by melting point determination and measurement of acid dissociation constant. Spectroscopic grade benzene was used as organic diluent. Distribution Studies. The aqueous phase contained 1.6 X lo4 M metal ion, 1.6 X M DAPDA/DACDA and the ionic strength was adjusted to 0.2 M with tris(hydroxymethy1)aminomethane (Tris) buffer and tetramethylammonium chloride. Varying proportions of tris(hydroxymethy1)aminomethane and hydrochloric acid were used to form buffers in the pH range 6.5-9.0. It was required to adjust the ionic strength of Tris buffer 0 1986 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
1 dacda(K22DAl
2 dapdo(K213A)
Flgure 1. Structures of DAPDA
TTA were measured at different pH values. UV spectra of the M La(II1) binary system (La-TTA) in the presence of 4 X and those of ternary systems (La-DAPDA/DACDA-TTA) in the M La(II1) + 4 X loT4M DAPDA/DACDA presence of 4 X were measured at pH 7.5 (Tris buffer) against appropriate reference solutions, respectively. Organic phase spectra were measured after equilibrating 1.66 X M TI'A/benzene solutions with aqueous phases containing 1.66 X M La(II1) or 1.66 X M La(II1) + 1.66 X M DAPDA, at pH 7.5 (Tris buffer). After equilibration, benzene solutions were always diluted with heptane (50 times). Calculations. The neutral form of TTA is designated as HA and the fully protonated form of DAPDA/DACDA as H4L2+. ( a ) Determination of Extraction Constant ( K e Xfor ) Binary Species LnA3. The equilibrium and the expression for Kegare given as follows:
and DACDA.
to at least 0.2 M, in view of ita low buffer capacity particularly in the region below pH 7.5 and above pH 8.5. Acetic acid/ tetramethylammonium hydroxide buffer was employed for adjusting the pH of aqueous phase in the range 4.7-6.0 (acetate concentration of -0.01 M and ionic strength of 0.1 M, adjusted with (CHJ4N+Cl-was used). The organic phase contained varying M to 1.0 X lo-' concentrations of TTA in the range 8.3 X M in benzene. A volume of each phase was maintained at 6.0 mL. Equilibration was carried out for 10-12 h on a Burrell wrist shaker followed by settling of the two phases for 1h. Two phases were separated thereafter avoiding carefully 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 decrease with additional time allowed. The pH of the aqueous phase was measured by using a Fisher combination pH eledrode and a Fisher Model 825 MP pH meter. Acidity of the aqueous phase was adjusted to -0.1 M by adding 0.05 mL of concentrated HCl 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 ita component wavelengths. In the present work, intensites of emitted light at 408.672 nm and 406.109 nm were measured for the determination of lanthanum and neodymium concentrations, respectively. The dc plasma emission instrument was calibrated using five standards of the lanthanide ion in 0.1 M HCl medium covering the concentration range of most of the samples analyzed (0.1-30 ppm). Acetate was found to interfere seriously with La (408.672 nm) as well as with Nd (406.109 nm) peaks. When the total lanthanide concentration in the aqueous phase was 1.8 X M, the EDTA titration method was used to determine the remaining lanthanide concentration after the distribution experiment using xylenol orange as the indicator. Occasionally,the organic phase was equilibrated with 1M HC1 and the concentration of stripped lanthanide ion was obtained by the dc plasma emission technique. Material balance within 15% was obtained in all experiments carried out at pH 7.5 in the presence of DAPDA/DACDA. Distribution ratio (D) is defined as
Ln3+(a)+ 3HA(o) Kex
=
LnA3(o) + 3H+(a)
(2)
~ ~ ~ ~ ~ 1 0 ~ ~ + l , 3 / ~ ~ ~ 3 + l , ~ ~ ~ l , 3
where subscripts o and a indicate organic and aqueous phases, respectively. The experimentally observed distribution ratio, D, is defined as total metal in organic phase D = total metal in aqueous phase
(3)
Assuming (i) only mononuclear species are present in the two phases and (ii) complexation of Ln3+with A- and OH- can be neglected at pH 15.0,eq 3 can be written in a simplified manner as D = [LnA3],/([Ln3+],+ C[L~AC~(~-"'I,)
(4)
1
The second term in the denominator refers to the complexation of lanthanide ion with acetate in aqueous phase. From eq 2 and 4 it is obtained
where pi is the overall stability constant for complex L ~ A c ~ ( ~ - ~ ) + and [Ac-] is the free acetate ion concentration at equilibrium. [HA],, in eq 2 and 5 refers to the concentration of HA in the organic phase at equilibrium, which is related to [HA],, and [HA],,' as follows: [HA],' = [HA],,
- n[LnAn],; [LnAn],= [Ln],,D/(D
+ 1)
where n is the number of ligands A- on Ln3+,PHA is the partition coefficient of HA = [HA],/[HA], (40, in the present case), and KHAis the acid dissociation constant of HA (-log KWA=5.87). (b) Determination of Extraction Constant (Ke;) for Ternary Species LnLA. LnL+(a) + HA(o)
LnLA(o) +H+(a)
(7)
Assuming (i) only mononuclear species are present in two phases and (ii) lanthanide is present only as LnL+ in aqueous phase at pH -7.0, as suggested by the nature of potentiometric titration curves in our previous work (11, 12), eq 3 can be written in a simplified manner for the extraction of ternary species
D 1= [LnLA],/[LnL+], 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. [Ln], was obtained experimentally by equilibrating the benzene phase in the absence of T T A with the aqueous phase at a particular pH. Spectrophotometric Studies. A Perkin-Elmer 552 spectrophotometer equipped with a microcomputer was used for ultraM violet absorption studies. Aqueous phase spectra of 5 X
(1)
(9)
From eq 8 and 9 or log D1 = log [HA], + log K,;
+ pH
(11)
( c ) Determination of Two-Phase Stability Constants (P,K) for Ternary Species LnLA. Considering the presence of LnLA
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
1
I_
I
I
d
I
I
200
300
400
Flgm 2. UV spectra of 5 X lod M ttwonytbiflwroacetoneat different pH: (a) at pH 1.0; (b) at pH 12.0; (c) at pH 7.5 (Tris buffer): (d) at pH 8.0 (Tris buffer).
species in aqueous phase along with LnL', eq 9 can be written as D2 = [LnLA],/([LnL+], + [LnLA],) or
+ K[A-],)
(12)
Defining P, as the partition coefficient of ternary complex (LnLA), P, = [LnLA],/[LnLA],, and K as the adduct formation constant of ternary complex (LnLA),K = [LnLAl,/([LnL+l,[A-l,), eq 12 is transformed into D2 = PS[A-Ia/(1 + KlA-1,) or
_1 D2
1
P,K[A-],
+ -P1m
Thus, a plot of 1/D2vs. l/[A-], gives a slope value l/(P,K) and an intercept 1/P, where [A^], is calculated from [HA]: employing the relation [A-la = [HAI,'KHA/((~ + PHA)[H+I+ KHA~
(14)
(concentration of A- in LnLA present in aqueous phase has been neglected). ( d ) Determination of Equilibrium Constant (Kz) f o r the Formation of LnLz-. Considering the presence of LnL2-species along with LnL+ in aqueous phase, eq 9 can be written as D3 = [LnLA],/([LnL+], + [LnL2-],) or 0 3
= [LnLAlo/([LnL+la(l+ K2[L2-la)l
(15)
From eq 9 and 15 0 3
= Dl/(l
+ K2[L2-la)
or K2 = [ ( 4 / 0 3 ) - 11/[L2-la
I
A h )
I
h (nm)
D2 = [LnLA],/[LnL+],(l
\I
(16)
where [L"], is the concentrationof L present in the anion form at equilibrium.
RESULTS AND DISCUSSION Spectrophotometric Studies. UV Spectroscopy of TTA in Aqueous Phase. Figure 2 shows the ultraviolet spectra of 'ITA under different pH conditions. Curve a obtained at pH 1.0 refers to the ketohydrate form of TTA (15) and curve b obtained at pH 12.0 refers to the decomposition products of TTA, Le., acetylthiophene and sodium trifluoroacetate (16). Curves c and d measured at pH 7.5 and 8.0, respectively,
Figure 3. UV spectra of 5 X lo-' M thenoyltrifluoroacetone at pH 7.5 (Tris buffer) under dlfferent complexing conditions: (a) only TTA; (b) TTA 4 x 10-4 M L ~ ( I I I ) 4 x 10-4 M DAPDA.
+
+
correspond to the enolate anion (16). As expected on the basis of the pK, value of TTA (5.87), enolate anion concentration at pH 8.0 (curve d) is nearly the same as that at pH 7.5 (curve C).
Figure 3 shows the spectra of TTA at pH 7.5 (Tris buffer) in the uncomplexed form as well as in the presence of La(II1) and macrocyclic ionophore, DAPDA. It is clear that peak maxima at 338 nm as well as 262 nm corresponding to uncoordinated TTA anion at pH 7.5 (curve a) are shifted toward longer wavelengths on complexation (curve b). Similar shifts have been observed in various systems which follow the order La-DAPDA (11nm) > La-H20 (8 nm) > La-DACDA (4 nm). Similar order has also been observed when La(II1) was replaced by Nd(II1). These shifts have been proposed to be related to the relative complexation of p-diketonate anion to the lanthanide cation in our earlier studies on similar systems (13). Spectra corresponding to enolate anion of TTA at pH 7.5 as well as at pH 8.0 were found slowly transforming to the spectrum corresponding to acetylthiophene and sodium trifluoroacetate. The rate of hydrolytic cleavage has been reported (16) to be a function of pH and ionic strength of buffer medium. Similar transformation has also been seen in the present work even for coordinated TTA in the systems LaH20, La-DAPDA, and La-DACDA. Though a significant decrease in the intensities of all spectra given in Figure 3 has been observed as a function of time, there was little shift observed in the peak maxima over a period of few days. A typical spectral change of ternary system, La-DAPDA-TTA, is shown in Figure 4 where the spectrum was scanned every hour for the same solution. Nevertheless, spectra recorded after 500 h in all the cases (in Figure 3) at pH 7.5 resembled acetylthiophene and sodium trifluoroacetate spectrum measured at pH 12.0 in every respect. It is obvious from these studies that in the aqueous phase at pH 7.5, TTA anian is in the coordinated form as La(DAPDA)TTA species in equilibrium with decomposition products of TTA. UV Spectroscopyof TTA in Organic Phase. Figure 5 shows the spectra of benzene solutions of TTA diluted in heptane, after equilibrating the benzene solutions for about 12 h with aqueous phases adjusted at pH 1.0 (b), pH 7.5 (c), and pH 12.0 (d) in three separate experiments. The spectrum of unequilibrated solution of TTA is also shown in Figure 5a for comparison purpose. As suggested by King and Reas (15), these organic-phase spectra appear predominantly to be due to the enol form of TTA. Decrease in concentration of T T A in the organic phase at pH 7.5 and at 12.0 is expected in view of its dissociation and decomposition, respectively, in the aqueous phase.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
A 30C
200
400
i(nmr
Flgure 4. UV spectra of La-DAPDA-TTA (Figure 3b) as a function of time: (a)spectrum obtained 2 h after adjusting pH at 7.5 (Tris buffer); (b) spectrum obtained 17 h after adjusting pH at 7.5 (Tris buffer).
(nm'
spectra of 1.66 X M theonyltrifluoroacetone in organic phhse on complexation: (a)TTA in benzene equilibrated with aqueous phase at pH 7.5, diluted 50 times in heptane; (b) TTA in benzene equllibrated with aqueous phase containing 1.66 X M La(II1) at pH 7.5,dlluted 50 times in heptane; (c) TTA in benzene M La(II1)+ equilibrated with aqueous phase containing 1.66 X 1.66 X M DAPDA at pH 7.5,diluted 50 times in heptane. Figure 6, UV
2.01
1 0.51
400
300 A(nrn)
spectra of 1.66 X M theonyltrifiuoroacetone in organic phase under different conditions: (a) TTA in benzene, diluted 50 times in heptane; (b) TTA in benzene equilibrated with aqueous phase at pH 1.0,diluted 50 times in heptane; (c) TTA in benzene equilibrated with aqueous phase at pH 7.5,dlkRed 50 times in heptane; (d) TTA in benzene equllbrated with aqueous phase at pH 12.0,diluted 50 times in heptane. Flgure 5. UV
Figure 6 shows the spectra of benzene solutions of TTA diluted in heptane, after equilibrating the TTAIbenzene solutions for about 12 h with aqueous phases containing La(II1) (b), La(II1) with DAPDA (c), and a blank experiment corresponding to pH 7.5 (Tris buffer) alone (a). Figure 6 parts b and c clearly suggest that TTA is present in these two cases in a different form (complexed) than in the case of Figure 6a (uncomplexed). Analogous to what was observed in the aqueous phase (Figure 3), organic phase spectra are also similar for both La-TTA and La-DAPDA-TTA systems in terms of peak maxima though spectral intensities of the latter are much smaller than those of the former. Relative intensities of the peaks corresponding to La-TTA (Ama 346 nm) and La-DAPDA-TTA (Ama 349 nm) in the spectra of aqueous phases correspondingto the organic phases included in Figure 6 have been reversed as compared to their organic phase intensities. This reversal suggests the lower extraction of La(DAPDA)TTA compared to La(TTA)3toward the organic phase. Identical distribution experiments carried out with Nd(II1) in place of La(II1) revealed similar peak maxima for coordinated TTA in binary as well as ternary systems in both aqueous and organic phases. It was rather interesting to
I
6
8
7
9
PH
Figure 7. Distribution of La(II1)as a function of pH in the presence M, I = 0.13: of different complexing agents, [TTA],, = 3.3 X (e)1.6 X M La(II1); (0) 1.6 X M La(II1)i- 1.6 X M
+
EDDA; (A)1.6 x 10-4M L ~ ( I I I ) 1.6x 10-4M DAPDA; (x) 1.6x M La(II1) 1.6 X low4 M DACDA.
+
observe that in the binary system the intensity of Nd-TTA peak is larger in the organic phase and smaller in the aqueous phase compared to the corresponding La-TTA peaks. On the other hand, in the ternary system, intensity of the La-DAPDA-TTA peak is larger in the organic phase and smaller in the aqueous phase compared to the corresponding NdDAPDA-TTA peak. These studies suggest that whereas neodymium extracts more than lanthanum in the binary system, Ln-TTA, lanthanum extracts more than neodymium in the ternary system, Ln-DAPDA-TTA. A similar reversal in the distribution ratios measured by the dc plasma emission spectroscopic technique has been observed for lanthanum and neodymium in binary and ternary systems as discussed in Distribution Studies. No changes in intensities of the organic-phasespectra were observed with time indicating the stability of the extracted species. This is in contrast to the behavior of species in the aqueous phase, which were found quite susceptible to decomposition in basic media. Distribution Studies. Effect of p l i . Variation of distribution ratio (D)for the extraction of La(II1) and Nd(II1) ions was investigated as a function of pH in the range 6.0-9.0.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
l
0
2.0
2273
2'ol 1.0 -
log D
0-
-1.0-
I
i -3
-2
-1
log [TTAI. PH
Flgure 8. Distribution of Nd(II1) as a function of pH in the presence M; I = 0.13: of different complexing agents, [TTA],, = 3.3 X (0) 1.6 X lo-' M Nd(II1); (0)1.6 X M Nd(II1) + 1.6 X M EDDA; (A)1.6 X M Nd(II1) 1.6 X M DAPDA; (X) 1.6 X
lo-'
M Nd(II1)
+ 1.6 X
+
M DACDA.
It is seen from Figures 7 and 8 that the distribution ratio increases with pH up to 7.5 and decreases thereafter presumably due to hydrolysis of the species LnL+ (where L = EDDA, DAPDA, or DACDA). This decrease may also be partly due to the decompositionof W A in the aqueous phase which is accelerated a t high pH as discussed in the previous section under Spectrophotometric Studies. Similar variation of distribution ratio with pH was observed by Zolotov et al. (17)while investigating the extraction of mixed complexes of In(II1) with 1-(2-pyridylaz0)-2-naphthol and acetate at different pH values. Extraction maximum was observed at lower pH for In(II1) (6.0-6.5)compared to that of lanthanides in the present studies (7.5-8.0). This may be principally due to the ease of hydrolysis of In3+compared to that of La3+or Nd3+ (14).
At a particular pH, the D value decreased for various extraction systems in the order (Ln-H20) > (Ln-EDDA) > (Ln-DAPDA) > (Ln-DACDA). This can be explained on the basis of the availability of uncomplexed trivalent lanthanide ion as well as the relative complexation of TTA anion (A-) with the central metal cation. Due to stronger complexation of DACDA with Ln3+,a relatively less amount of uncomplexed Ln3+is available for the formation of species of the type LnA3 Extraction systems involving larger proportion of LnA3species are expected to result in higher distribution ratio values due to larger partition coefficients of these species compared to the ternary LnLA species (18). It was also observed that D decreased in the case of DAPDA after pH 8.0 whereas for DACDA, it started decreasing a t about pH 7.5. This is also possibly due to the stronger complexation of A- with Ln(DAPDA)+cation compared to that with Ln(DACDA)+cation. Concentration of EDDA was maintained at 10 times the stoichiometric amount of the metal ion since the experiments with an equal stoichiometric amount revealed the tendency of metal ion to hydrolyze a t pH >7.0. On comparison of the curves in Figures 7 and 8,it was further observed that generally extraction of Nd(II1) is larger in all cases except with DAPDA where La(II1) extracts more up to pH 8.0. Due to the varying amount of [TTA], at different pH, slope of log D - pH curve for Ln3+as well as LnL+ species changes continuously. This is due to the fact that pH values are high which makes [HA],, change drastically as a function of pH. In addition, the change in relative proportion of Ln3+ and LnL+ species with pH may affect the slope of these curves. Effect of TTA Concentration. Extraction of Nd(II1) at a concentration of 1.8 X M was studied as a function of
Figure 9. Variation of log D as a function of log [TTA],: (0)Ln = La (1.6X M), L = DAPDA (1.6X M), pH 7.5,I = 0.2;(0) Ln = Nd (1.6X lo4 M), L = DACDA (1.6X lo4 M), pH 7.5,I = 0.2; (A)Ln = La (1.6 X lo-' M), L = DACDA (1.6X lo4 M), pH 7.5,I = 0.2;(A)Ln = Nd (1.8X lo3 M), pH 4.7,I = 0.1;(0) Ln = Nd (1.8 X lo3 M), L = EWA (5 X M), pH 5.1,I = 0.1;(0) Ln = Nd (1.8 x 10-3 MI, L = DAPDA (2 x 10-3 MI, PH 5.1, I = 0.1.
Table I. Equilibrium Constants for the Extraction of Ternary Complexes ( I = 0.2 M) system La(DAPDA)TTA La(DACDA)TTA Nd(DAPDA)TTA Nd(DACDA)TTA
log ke; -4.31 -4.86 -4.59 -4.71
f 0.07 f 0.06 f 0.02 f 0.04
log (P,K) 2.95 2.46 2.78 2.72
f 0.02 f 0.03 f 0.03 f 0.04
log K,a 10.11 12.21 11.6 12.21
a D a t a taken from'ref 11and 12.
TTA concentration, varying in the range 1.4 X M to 4.8 X M at pH 4.7,using acetate buffer (I = 0.1 M). Figure 9 shows that linear plot of log D vs. log [TTA], results in a slope of -3.0 confirming that the extracted species is NdM EDDA to the aqueous (TTA)3. On addition of 5 X phase, a similar log-log plot was obtained at pH 4.7 and 5.1 except that distribution ratio values were marginally decreased for a particular combination of TTA concentration and H+ concentration. Slope of -3.0 in the presence of EDDA suggests that the dominating species extracted at pH 5.1 is still Nd(TTA)3. Lowering of the extraction constant value from -8.54 f 0.12 (log Kex)for Nd-TTA at pH 4.7to -9.54 f 0.10 (log Kex)for Nd-EDDA-TTA at pH 5.1 indicated aqueous complexation of Nd3+with EDDA. Note that the values for log K,, were calculated by using eq 4 and 5 with the consideration of acetate complexation which is significant in the present case. Distribution experiments carried out at pH 5.1 in the presence of 2 X M DAPDA showed further decrease in the distribution ratio values. Slopes of log D vs. log [TTA], plots for NdLA system in the pH range 5.9 to 7.3 were found to vary between 2.0and 1.0suggesting that both binary, NdA3, as well as ternary, NdLA, species were coextracted into the organic phase. Figure 9 also shows plots of log D vs. log [TTA], at pH 7.5 for La(DAPDA)TTA, La(DACDA)TTA, and Nd(DACDA)TTA systems. Marginal deviation from unity slope was observed in one case with increasing value of [TTA],, suggesting the presence of species of the type LnA3 or LnLA-HA along with ternary species LnLA. Binding of chelating extractant as neutral molecule as well as in the anionic form has been observed by Kawashima et al. (19) in their study of lanthanide extraction and by Tamhina et al. (20) in their investigation on thorium extraction. Points falling on lines of unit slope in Figure 9 were further considered for the calculation of extraction constants (K,;)
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
2274
Table 111. Distribution Ratios of La(II1) and Nd(II1) with Varying Concentration of TTA in the Presence of EDDA at pH 7.5, [Ln] = 1.66 x M, [EDDA] = 1.66 X M
- 10
"i 1 -
/c
0
D
1
1O4[TTA],w, M
D(La)
D(Nd)
3.33 6.66 10.0 13.32 16.66
0.19 0.34 1.05 1.36 3.35
0.37 0.72 2.31 2.99 6.72
1
t
,o/
is
Figure 10. Variation of 1/D as a function of l/[A-] at pH 7.5 to determine log ( P a ) : (0)La(III)-DACDA-llA (0)La(II1)-DAPDATTA; (A)Nd(II1)-DAPDA-TTA; (0)Nd(II1)-DACDA-TTA.
1
6.
I D
'
4-
Table 11. Observed Separation Factors between Nd(II1) and La(II1) with Different Complexing Agents at pH 7.5, [Ln] = 1.6 X lo-* M
complexing agent DAPDA" DACDA" EDDAb
1igand:metal ratio 1:l 1:l 1O:l
separation factor [D(Nd)/D(La)l 0.55 f 0.08 1.7 f 0.05 2.08 f 0.20
2-
01
%
0
20
10 [ DAPDA It x
IO4
M and 1.0 X lo-* M. [TTAIbw varied between 3.3 X M. [TTAIbw varied between 3.3 X 10"' M and 1.6 X
Flgufe 11. Variation of D with [DAFDAImI,[La] = 1.6 X lo4 M, pH 7.5, and I = 0.2 M: (0)[TTA],,,, = 3.33 X lo3 M; (A)[TTA],,, = 5.0 X lo3 M; (0)[lTA],o,, = 6.66 X M; ( X ) [TTA],,, = 8.33 X M; (0)[TTA]total = 1.0 X lo-* M.
of the ternary complexes using eq 10. Table I shows the KeL values obtained for various extraction systems investigated in this work. An attempt was made to calculate log (PmK) values from the slopes of linear plots of 1/D vs. l/[A-], (Figure 10) as given in eq 13 (Table I). It should be noted that the P, values could not be obtained from the values of intercepts of plots 1/D vs. l/[A-], due to large errors. Also, log K1 values listed in Table I have been obtained earlier in our laboratory by the potentiometric technique (11, 12). The highest value of log (PmK) for La(DAPDA)TTA appears to be due to the stronger binding of TTA with the relatively weaker complex La(DAPDA)+. Similarly, the lowest value of log (P,K) is observed for the stronger La(DACDA)+ complex. The higher value obtained for the Nd(DACDA)+ complex, in spite of its similar stability with La(DACDA)+, could possibly be explained on the basis of a favorable partition coefficient of Nd(DACDA)TTA compared to the corresponding lanthanum complex. Macrocyclic ionophores employed in the present study offer interesting possibilities of extracting La(II1) more in the case of DAPDA and less in the case of DACDA compared to Nd(111). Table I1 summarizes the mean separation factors [D(Nd)/D(La)] obtained by varying [TTAIbM in the range 3.3 x M at pH 7.5. Extraction of La(II1) was M to 1.0 X about 1.8 times that of Nd(II1) in the case of DAPDA whereas in other cases of DACDA and EDDA, simple TTA extraction behavior of Nd(II1) extracting more than La(II1) was observed (14). Also in the latter two cases, separation factor decreased significantly from the one observed with TTA alone as the extractant. The separation factor (-100) reported in the literature for binary systems (14) in favor of Nd(II1) at low pH (-5.0) could not be reproduced in the present studies a t pH 7.5. Though an exact value could not be assigned due to the complex nature of extraction, i.e., hydrolysis, it appeared to decrease by at least more than an order of magnitude
(separation factor 5.0) in this work. It should be noted that there is no report available on the role of ionizable macrocyclic ionophores on the extraction of lanthanides by organic extractant. No attempt was made to calculate extraction constant with EDDA as primary ligand in the present work for the following reasons: (i) concentration of EDDA was 10 times the stoichiometric requirement, thus there was some possibility of formation of anionic species of the type Ln(EDDA)r, and (ii) [TTA], dependency on the distribution ratio ( 0 ) was -1.7 (from the slope of log D-log [TTA], plot) suggesting significant contribution of Ln(TTA), species even at pH 7.5. Distribution ratio data for varying concentrations of TTA are given in Table 111. Effect of DAPDA Concentration. Distribution experiments were also carried out on La(II1) at fixed TTA concentration, fixed pH, and various concentrations of DAPDA. Figure 11 shows the exponential decrease of distribution ratio (D) with increase of DAPDA concentration for several fixed [TTA],d values. This decrease suggests the presence of an additional equilibrium involving the species LnL+. It appears that on adding increasing amounts of DAPDA, anionic species of the type L A 2 - are formed. A second molecule of L may bind only through acetate anionic groups. Employing eq 16, log K2 was obtained as 3.66 -+ 0.08 in the present work for La(DAPDA)i, which is consistent with what would be expected using acetate as a model. Analytical Significance. During the distribution of lighter lanthanides with thenoyltrifluoroacetone in the presence of macrocyclic ionophores, i.e., DAPDA and DACDA, ternary complexes of the type Ln(DAPDA)TTA appear to be the dominant extracting species at pH 7.5. The distribution ratio of lanthanides in the ternary system decreases significantly compared to simple TTA extraction, presumably due to the lower partition coefficient of ternary complexes compared to that of binary complexes. Nevertheless, it has been found
Anal. Chem. 1906, 58, 2275-2278
2275
possible to extract quantitatively lanthanides from the ternary system by adjusting reagent concentration. Extraction of ternary complexes with DACDA as a macrocyclic ionophore was always lower compared to that with DAPDA. Selective extraction of Nd(II1) over La(II1) reported for simple TTA extraction was though maintained in the case of DACDA; it was reversed in the case of DAPDA. These extraction results have very significant implications. Two common procedures are followed to extract lanthanide species at low pH (i.e., 3 or 4). With extractant such as "TA, the heavier lanthanide complexes are extracted more because they form stronger P A complexes due to their greater charge density. The lanthanide species can also be extracted in the presence of a strong complexing agent such as EDTA. The resulh are that the lighter lanthanide ions are extracted more because the heavier lanthanides are all tied up in the aqueous solution with EDTA and are less available for extraction. The ternary complexes of lanthanides with open-chained, flexible primary ligands such as EDTA and secondary extractants are not found in the organic phase. Thus the presence of ternary macrocyclic lanthanide complexes in the organic phase indicates their improved hydrophobicity. Because of the various possibilities of forming ternary macrocyclic complexes with extractants of different sizes and steric requirements, one can further tune the selectivity for the extraction. Certainly, the subtle differences in solubility of the resulting ternary species are also significant for determining the final selectivity.
Bilal, E. A.; Herrmann, F.; Metscher, K.; Muhlig, E.; Reichmuth, Ch.; Schwarz, E. Actinide Separatlons; Navratii, J. D., Schulz, W. W., Eds.; American Chemical Society: Washington, DC, 1979; ACS Symp. Ser. NO. 117, pp 561-569. Seaborg, G. Man made Transuranium Elements ; Prentice-Hall: Englewood Cliffs, NJ, 1963; p 58. Chinn, S.R.; Hong, H. Y.-P.; Pierce, J. W. Laser Focus Electro-Opt. Mag. 1076, 12, 64. Eiattar, A.; Wallace, W. E.; Craig, R. S. The Rare Earths in Modern Science and Technology; McCarthy, J. G., Rhyne, J. J., Eds.; Plenum Press: New York, 1977; Vol I , p 87. Reuben, J.; Elgavish, G. A. Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring, L., Eds.; North-Holland: Amsterdam, 1979; Vol 4, p 483. Marchant, D. D.; Bates, J. L. The Rare Earths in Modern Science and Technology; McCarthy, J. G., Rhyme, J. J., Sllben, E. H., Eds.; Plenum Press: New York, 1977; Vol 11, p 553. Cecille, L.; Dworschak, H.; Glrardi, F.; Hunt, E. A,; Mannore, F.; Mousty, F. "Actinide Separations"; Navratil, J. D., Schulz, W. W., Eds.; American Chemlcal Society: Washington, DC, 1979; ACS Symp. Ser. NO. 117, pp 427-440. Chang, C. A.; Rowland, M. E. Inorg. Chem. 1063, 22, 3866-3869. Chang, C. A.; Ochaya, V. 0. Inorg. Chem. 1086, 25, 355-358. Chang, C. A.; Garg, E. S.;Manchanda, V. K.; Ochaya, V. 0.; Sekhar, V. C. Inorg. Chlm. Acta 1086, 115, 101-106. Marcus, Y.; Kertes, A. S . Ion Exchange and Solvent Extraction of Metal Complexes: Wiley-Interscience: New York, 1969; Chapters 3 and 8. King, E. L.; Reas, W. H. J. Am. Chem. SOC. 1051, 7 3 , 1606-1808. Cook, E. H.; Taft, R. W. J. Am. Chem. SOC. 1052, 7 9 , 6103-6104. Zolotov, Y. A.; Seryakova, I. V.; Vorobyeva, G. A. Talanta 1087, 14, 737-743. Zolotov, Y. A. Extraction of Chelate Compounds; Ann-Arbor Humphrey Science Publishers: Ann Arbor, MI, 1970; p 36. Kawashima, M.; Frelser, H. Anal. Chem. 1081, 5 3 , 284-286. Tamhina, E.; Gojmerac, A.; Herak, M. J. J. Inorg. Nuci. Chem. 1078, 4 0 , 335-338.
Registry NO.TTA, 14529-33-0;DAPDA, 81963-61-3;DACDA, 72912-01-7; La, 7439-91-0; Nd, 7440-00-8.
RECEIVED for review December 2, 1985. Accepted May 12,
LITERATURE CITED (1) Surls, J. P.; Choppin, G. R. J. Inorg. Nucl. Chem. 1057, 4 , 62. (2) Leuze, R. E.; Lloyd, M. H. Process Chem. 1070, 4 , 597. (3) Weaver, E.; Kappelmann, F. A. ORNL-3559, 1964.
1986. This material was prepared with the support of U.S. Department of Energy Grant No. DE-FG05-84ER12392; however, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE.
Determination of Sub-Part-per-Trillion Amounts of Cobalt by Extraction and Photoacoustic Spectroscopy Takehiko Kitamori,*' Kazumichi Suzuki,' Tsuguo Sawada? Yohichi Gohshi,2and Kenji Motojirna'P Energy Research Laboratory, Hitachi, Ltd., 1168 Moriyama, Hitachi, Ibaraki 316, Japan, and Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan
Ultratrace amounts of cobalt at parts-per-trllllon (ppt) levels were extracted Into m-xylene from aqueous solutlon as nltrogonaphthol chelates, and colorimetric determlnatlon of the m-xylene phase was carrled out wlth photoacoustic spectroscopy (PAS). Coexlstent metals and excessive reagents were completely ellmlnated by acldlc and alkallne washings of the m-xylene phase, and the background level became constant and k w enough for ultrasenslthre PAS determlnatlon. Concentratlons of forelgn heavy metals, such as Iron, nlckel, chromium, and copper, lo5 thnes more than cobalt caused no Interference. The determlnatlon llmlt was obtalned as 0.64 ppt based on twice the standard devlatlon. The reagent 2nltroso-l-naphthdwas more stittable than l-nltroso-2-naphthd from the vlewpolnt of stablllty.
'Hitachi, Ltd.
University of Tokyo. Present address: Laboratory of Analytical Chemistry, Kaken Co., Ltd., Shinden 1044 Hori, Mito, Ibaraki 310, Japan. 0003-2700/86/0358-2275$01.50/0
In recent years, water and reagents used in technical fields, such as semiconductor,biochemical, and nuclear engineering, are being highly purified, and some kinds of impurities are demanding parbper-trillion (ppt) (pg/mL) level analyses. For example, the amounts of cobalt that are neutron activated in nuclear power reactor coolant and cause an increase of radiation dose rate around the coolant system have been reduced to extremely low levels and are estimated to be on the order of parts per trillion (1). Hence a determination method for ultratrace cobalt is required to clarify the species behavior in the coolant. Photoacoustic spectroscopy (PAS) for liquid samples using a piezoelectric transducer as an acoustic signal detector has been shown to be quite sensitive, and the detection limit absorbance of the equipment ( S I N = 2; N , noise of the equipment) has reached the order of lo4 cm-' when employing an optimized photoacoustic (PA) cell ( 2 , 3 ) . Therefore, determination with PAS seems to be suited to ultratrace analysis, and indeed its potentiality has been shown experimentally 0 1986 American Chemical Society
