hol waa possible in the presence of tartrate and fluoride (> 100 mg.), thorium and zinc (2 mg.), titanium and aluminum (1.8 mg.), gallium (560 pg.), uranium (50 pg.), tungstate (360 pg.), molybdate (200 pg.), zirconium (200 wg.), and cerium (280 pg.) ions. Indium, copper, and nickel interfered due to the formation of insoluble precipitates with the reagent. The interferencesof C U +(160 ~ p g . ) , Ni+*(lOOpg.), In+3(2000pg.) could be eliminated by carrying out the absorption measurements after filtration. Traces of iron and vanadium produced interfering colors. It was observed that thallium could be estimated in the presence of appreciable amounts of fluoride ions. Attempts were, therefore, m d e to eliminate the interferences due to ferric, thorium, titanium, zirconium, and aluminum ions by adding sodium fluoride solution. The
Table 11.
Wed of Diverse Ions in Presence of Fluoride
Concn. of T1+’= 200 pg. added = 2.0 ml. of 5% F- soh.
Ions added
Concn. tolerated sources
(ag.)
Precision and Accuracy. Precision data were obtained from standard solutions prepared and estimated on different days. The precision varied from 3.2% at 144 pg. per ml. to 0.1% at 444 pg. per ml. Accuracy of the method wm justified by measuring some standard solutions. The relative error waa 1.5% at 144 pg. per ml. and 0.9% at 444 pg. per ml. LITERATURE CITED
results in Table I1 indicate that large amounts of iron, thorium, aluminum, titanium, and zirconium could be tolerated if the spectrophotometric measurements were carried out in the presence of fluoride ions.
(1) Meyer, A. S., Ayrea, G. H.,J . Am. Chem. SOC.79,49 (1957). (2)Sandell E. B., “Colorimetric De termination of Traces of Metals,” p. 97,Interscience, New York, 1959. (3) Ibid., p. 83. (4) Shome, S. C., Current Sa‘. (India) 13, 257 (1944). (5) Sogani, N. C., Bhattacharya, S. C., ANAL.CHEM.28, 81, 1616 (1956). RECEIVEDfor review January 13, 1966. Accepted June 28, 1966.
Spectrophotometric Investigation and Analytical Application of the Synergic Solvent Extraction of Rare Earths with Mixtures of 2 -Thenoyltriflu o roaceto ne and Tri-n-octyI Phosphine Oxide TOMITSUGU TAKETATSU and CHARLES V. BANKS Institute for Atomic Research and Department of Chemistry, lowa State University, Ames, lowa The absorption spectra of rare earth-TTA-TOP0 complexes, that were synergically extracted into toluene from an aqueous solution, were measured from 430 through 800 mp. One absorption band each for neodymium, holmium, and erbium was remarkably enhanced and the molar absorptivities at the wavelengths of maximum absorption were about 7, 26, and 2 2 times greater, respectively, than the same quantities in chloride medium. The formulae of these complexes were estimated spectroF1lotometrically to be LII(TTA)~ TOPO and Ln(TTA)a 2TOPO. The spectrophotometric determination of neodymium, holmium, and erbium was investigated using the analytically significant absorption bands at 584.1, 451 -8,and 520.2 mM, respectively. a
I
that the sharpness and complexity of absorption spectra of solutions of rare earths arise from pure electronic transitions involving the 4f subshell, which is shielded by the 5s and 5p outer shells from interaction with the ionic field in the solutions. If the strength of the ionic field surrounding T IS WELL KNOWN
1524
ANALYTICAL CHEMISTRY
the rare earth ions is sufficient to penetrate the shielding, some absorp tion bands may be shifted in wavelength and enhanced in sensitivity. Such effects have been observed with rare earth complexes of (ethylenedinitrilo) tetraacetic acid (EDTA) (6, 9, IO) and of related compounds (IO) in aqueous solutions. Rare earth complexes with 5,7-dichloro-8-quinolinol (6) in chloroform also show these same effects. Recently, it has been observed that the absorption bands of the complexes of the rare earths are particularly enhanced in the following cases: with acetylacetone in chloroform and benzene (7), with carbonate in aqueous solution (8),with nitrate in tetrabutylammonium nitrate-nitroethane (a), and with thenoyltrifluoroacetone in benzene ( 4 ) . It is expected that strong enhancement of the intensity and a shift in wavelength of certain bands in the absorption spectra would be observed for cases in which synergic complexation of the rare earths influences the ionic field surrounding the metal ions more than would the chelating acid or the neutral donor done. Healy and Ferraro (1) studied the absorption spectra of the uranyl,
thorium, and neodymium complexes extracted into organic solvents containing a mixture of 2-thenoyltrifluoroacetone (TTA) and tri-n-octyl phosphine oxide (TOPO) or tri-nbutyl phosphate (TBP); and Ihle et al. (8) reported a spectrophotometric study of the uranyl-di(2-ethylhexy1)phosphoric acid-TOP0 complex. The absorption spectra of the TTATOPO complexes of rare earths which are synergically extracted into toluene from aqueous acetate solution and which show significant absorption in the visible region are recorded in this paper. The formulae and spectrophotometric analytical applications of the neodymium, holmium, and erbium TTATOPO complexes are discussed. EXPERIMENTAL
Reagents. Standard solutions of scandium and the rare earths, except cerium, were prepared by dissolving in dilute hydrochloric acid the 99.9% pure oxide, which had been purified by ion exchange and analyzed spectrographically in this laboratory. The solution of cerium(II1) was prepared by dissolving in dilute hydrochloric acid the reagenbgrade cerous chloride. The concentrations of these
-
Pr-TTA-TOW COMPLEX
I
WAVELENGTH, my
Figure 1.
Absorption spectra of cerium
elements were determined by titration with EDTA using Xylenol Orange as the indicator. TTA, obtained from the J. T. Baker Chemical Co. or Peninsular ChemResearch Inc., was purified by recrystallization from heptane. All other chemicals used were of analytical reagent-grade quality. Sodium acetate buffer solutions (pH 0.9 through 6.0) were prepared by adding various amounts of 1M hydrochloric acid to 500 ml. of 1M sodium acetate solution and diluting to 1000 ml. with water. Apparatus. All absorption spectra were measured with a Cary Model 14 recording spectrophotometer. The measurements of absorbance a t a single wavelength were made with a Beckman Model DU quartz spectrophotometer. One-centimeter quartz cells were used in both cases. The hydrogen ion concentration was determined with a Beckman Model GS p H meter. Recommended Procedure. A hydrochloric acid solution containing a sample was placed in a beaker, and the solution was evaporated to near dryness on a water bath. The residue
was dissolved in 10 ml. of acetate buffer solution (pH 5) and transferred to a separatory funnel. Ten milliliters of a toluene solution containing 1 mmole TTA and 1 mmole TOPO was placed in contact with the aqueous solution. After the separatory funnel was shaken for 20 minutes, the organic layer was transferred into a centrifuge tube and centrifuged to remove water. The absorbance of the organic solution was measured on the spectrophotometer a t 584.1, 451.8, and 520.2 mp for neodymium, holmium, and erbium, respectively. Pure toluene was used as the reference solvent. The amount of the rare earth that was extracted was determined by titrating the rare earth remaining in the aqueous solution with EDTA. At the concentrations of TTA and TOPO used in these experiments, the small amounts of TTA and TOPO transferred to the aqueous phase gave no interference with the EDTA titration. RESULTS AND DISCUSSION
Absorption Spectra. One of the rare earths or scandium was quantitatively extracted from the acetate
I
solution into toluene as the TTATOPO complex using the recommended procedure. The absorption spectra of the toluene solutions of these complexes were recorded from 430 through 800 mp. Measurements at wavelengths below 430 mp were not carried out because of the strong absorption band displayed by TTA in that region. Scandium, yttrium, lanthanum, gadolinium] and lutetium showed no absorption in the region investigated. Though samarium, terbium, dysprosium, and ytterbium showed absorption bands in the region from 430 through 500 mp, the spectra were featureless and the molar absorptivities were less than about 3. Typical absorption spectra for the TTATOPO complexes of cerium, praseodymium, neodymium, europium, holmium, erbium, and thulium dissolved in toluene are given in Figures 1 through 7 , respectively, and for comparison, the data for aqueous solutions containing the corresponding rare earth chlorides are included. The strong orange-yellow color of the broad absorption band below 600 mp in an organic solution containing the cerium complex probably originates from the tetravalent oxidation state of cerium. In the case of praseodymium, complex formation causes an increase in the intensity of the absorption spectrum to some extent and shifts the bands t o longer wavelengths relative to the spectrum of an aqueous solution of the chloride salt. The absorption spectrum of the neodymium complex in toluene has a strong band in the visible region with a maximum a t 584.1 mp, while the spectrum of the aqueous solution of the chloride has a band a t 576 mp. The molar absorptivity of neodymium in the
Eu-TTA-TOW COMPLEX
450
WAVELENGTH, m y
Figure 3.
Absorption spectra of neodymium
Figure 4. ropium
SO0
550 WAVELENGTH, my
600
Absorption spectra of eu-
VOL 38, NO. 1 1 , OCTOBER 1966
1525
120.41 103.2
86.0
68.8
51.6 34.4
I
I
I
I
1
I
I
7
1
I
n o -TTA-TOP0 COMPLd
,450
500
Figure 5.
'
1
62.4
&
,
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I
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I
1
-
6.8-
m
a
I
WAVELENOTH ,my
E
8
'
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A 650
I
550 600 WAVELENCTH, m y
Absorption spectra of holmium
I
I
Ef-TU-TWO COMPLEX
Figure 6. co.2
1
Tm-TTA-TOPO COMPLEX
-
Absorption spectra of erbium
the acetate buffer solutions. The values of the p H of the aqueous solutions were measured after equilibration. The results are shown in Figures 8 and 9. It is known that above
0.6 W 0 2
0.4 0 u)
m
a
I
1526
I
I
ANALYTICAL CHEMISTRY
1
1
0.2
I
2
3
4
5
6
I
I
1
A,A ; Nd3
z 80;
a
c X
6ow W
s
40;
a W
a 20 WMLENQTH, mp
1.0 2.0 3 .O MOLE RATIO OF TTA TO EACH RARE EARTH
Figure 10. Relationship between absorbances or extracted amounts of rare earths and the concentration of TTA in the presence of excessive amount of TOPO
ratio of T O P 0 : L n = 1 : l and 2 : l . These data show that the values of x are 1 and 2 depending upon the concentration of TOPO. The absorption spectra of these rare earth complexes in toluene at the mole ratios of TOPO: Ln = 1:1 and 2: 1 are given in Figures 12, 13, and 14, respectively. Because the rare earths are completely extracted at both points with the exception of neodymium] which is 98% extracted at the mole ratio TOP0:Ln = 1: 1, it is obvious that the differences in the complexity and intensity of the absorption spectra originate from the extent of solvation by TOPO. From these data, it is felt that the complexes of rare earths are Ln(TTA)3 TOPO and Ln(TTA)3*2TOPO. I t is interesting that the absorbance of the holmium complex decreases with increasing solvation by TOPO but a t the same time the
absorption spectrum increased in complexity. As a reference the absorption spectra (A) of the rare earths in the presence of a n excess of TTA only are given in Figures 12, 13, and 14. Because the rare earth-TTA complexes are not completely extracted into toluene from a n aqueous acetate solution of pH 5 and because part of the complex precipitates in the organic solution, the absorbance does not correspond to the amount extracted. The absorption spectra were measured after removing the precipitate by centrifugation. From these figures, it is apparent that the bands in the absorption spectra of the TTA complexes are of lower complexity than that of these TTA-TOP0 complexes. Stability of the Complexes Extracted. After extraction] the absorbances of the neodymium, hol-
Figure 12. Effect of TOPO added on absorption spectra of the neodymium complexes in the presence of an excessive amount of TTA
mium, and erbium TTA-TOP0 complexes were measured a t various intervals of time a t the appropriate wavelength. No variation of absorbances was observed for 4 hours after extraction. Although these complexes are very stable, T T A in the organic solvent gradually decomposes, especially when exposed to light. Therefore, the organic solution of TTATOPO mixture is preferably prepared each day before using and the measurement of the absorbance of the complexes should be carried out within 4 hours. Calibration Experiment. The extraction and measurement of absorbances for samples containing various known amounts of neodymium] holmium, or erbium was carried out and the data are shown in Figure 15. The figure shows that the absorbances
0.7
-
W
I
- 2
E
0
w-
a
c ‘ XW
a W
0.5
z
- 2 W 0
4
0 W
a
O . 1
1.0 21) 3.0 4.0 5.0 MOLE RATIO OF TOPO TO EACH RARE EARTH
Figure I. Relationship between absorbances or extracted amounts of rare earths and the concentration of TOPO in the presence of the excessive amount of TTA
0.1
-
-WIWELENOTH, mp
Figure 13. Effect of TOPO added on absorption spectra of the holmium complexes in the presence of an excessive amount of TTA VOL 38, NO. 1 1 , OCTOBER 1966
1527
I
0.3 WPVLLENGTH. my
1
Figure 14. Effect of T O P 0 added on absorption spectra of the erbium complexes in the presence of an excessive amount of TTA
I
follow Beer’s law. However, because the peaks are very sharp, the wavelength and slit width should be fixed during the measurement.
Table 1.
+letal ions La+3 Ce+3 Pr +a Nd +3
Sm+3 Eu +a Gd +3
T b +3 Dy + 3 HO+~ Er + 3
Tm+a Yb+3
Lu +3 Y+3
sc+3 ~ 1 + 3
co
+2
Cr+3 c u +2
Fe + 3
hln + 2 Ni+2 Pb+2
Th +4
UO +% Zr + I
Zn +2
e
Figure 15.
I
I
1.0 Nd’?
I
1.2
Hots
I
I
1.4 1.6 1.8 AND E,+) (mp / ml)
1
1
1
21)
2.2
2.4
Calibration curves for neodymium, holmium, and erbium
Determination of Neodymium, Holmium, and Erbium in Synthetic Samples Containing Diverse Metal Ions Nd Er Found, Error, Tttken, Found, Added Taken, Taken, Error, (mg.) (w.1 (w.1 (w.1 (me.) (w.1 (mg.1’ (w.1 7.35 10.00 6.30 zto.00 15.1 7.35 6.30 5.65 5.65 10.00 6.30 5.65 4.5 7.35 7.85 +0.50 Interference Interference 6.30 5.65 10.8 7.35 7.50 +0.15 7.18 f0.88 5.65 fO.OO +o. 10 6.40 6.30 5.65 12.1 ... 6.10 +0.45 6.57 +0.27 6.30 5.65 18.3 7.35 7.35 10.00 5.68 +0.03 6.50 +0.20 6.30 5.65 19.6 7.35 5.70 $0.05 7.35 f 0 . 00 6.30 6.30 10.00 +0.05 5.70 5.65 22.2 7.35 7.35 10.00 19.8 7.35 6.42 +o. 12 6.30 5.65 7.35 10.00 5.74 +0.09 6.30 5.65 21.1 7.35 6.52 +0.22 7.35 zto. 00 5.70 +0.05 16.6 7.35 5.65 5.74 f0.09 7.35 10.00 ... ... 6.54 +0.24 22.8 7.35 6.30 ... 7.35 1 0 ,00 19.5 7.35 6.38 +0.08 5.65 6.30 7.38 +0.03 6.34 +0.04 29.9 7.35 6.30 5.65 7.30 -0.05 6.30 10.00 6.30 5.65 14.8 7.35 7.35 10.00 6.30 fO.OO 6.30 5.65 8.6 7.38 7.35 *o. 00 6.30 f 0 . 00 6.30 5.65 5.7 7.35 7.35 zto.00 6.30 2to.00 6.30 5.65 5.1 7.35 7.35 fO.00 5.65 Interference 6.30 1.9 7.35 Interference Interference 5.65 f0.00 5.65 6.30 10.00 6.30 2.0 7.35 7.35 f0.00 5.65 5.93 +0.28 Interference 6.30 2.9 7.35 Interference 5.65 Interference Interference 6.30 Interference 2.0 7.35 5.65 Interference Interference 6.30 7.64 +0.29 8.2 7.35 5.65 10.00 5.65 Interference 7.62 +0.27 6.30 6.3 7.35 5.65 *o.oo 5.65 6.30 10.00 6.30 7.33 -0.02 21.8 7.35 5.65 10.00 5.65 6.30 10.00 6.30 27.0 7.35 7.35 fO.00 5.65 6.20 f0.55 Interference 6.30 7.33 -0.02 25.1 7.35 5.65 5.80 +O. 15 6.48 +0.18 7.35 10.00 15.2 7.35 6.30 6.30 5.65 10.00 rto.00 5.65 7.35 10.00 11.8 7.35 6.30
Effects of Diverse Ions. The effects of 26 metal ions on the determination of neodymium, holmium, and erbium were investigated. The results are shown in Table I. Cerium displays an orange-yellow color as described before and gives a serious interference in the determination of holmium and erbium, but the interference in the determination of neodymium is relatively small. Generally, the colored metal ions give an interference in the determination of rare earths, especially for holmium. Although a precipitate
1528
I
0.6 0.8 CONCENTRATION OF
ANALYTICAL CHEMISTRY
appears in the aqueous solution when zirconium is shaken with the organic solution, the absorbances of the rare earths are not affected appreciably. Chromium(II1) , which did not extract, gave no interference. LITERATURE CITED
(1) Healy, T. V., Ferraro, J. R., J . Inorg. Nucl. Chem. 24, 1449 (1962). (2) Ihle, H., Michael, H., Murrenhoff, A,, Ibid., 25, 734 (1963). (3) Maeck, W. J., Kussy, M. E., Rein, J. E., ANAL.CHEM.37, 103 (1965). (4) I\lishchenko, V. T., Lauer, R. S.,
Efryushina, N. P., Poluektov, N. S., Zh. Anal. Khim.20, 1073 (1965). (5) Moeller, T., Brantley, J. C., J . Am. Chem. SOC.72, 5447 (1950). (6) Moeller, T., Jackson, D. E., ANAL. CHEM.22, 1393 (1950). (7) Moeller, T., Ulrich, W. F., J . Inorg. Nucl. Chem. 24, 1449 (1962). (8) Poluektov, N. S., Kononenko, L. I., Zh. Neorgan. Khim. 6 , 938 (1961). (9) Vickery, R. C., J . Chem. SOC.,1952, 421. (10) Vickery, R. C., Nature 179, 626 (1957). RECEIVEDfor review June 10, 1966. Accepted August 8, 1966.
