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The extraction of yttrium(III) and lanthanum(III) from hydrochloric acid solutions by di-(2-ethylhexyl)phosphoric acid
J. inorg, nucl. Chem., 1973, Vol. 35, pp. 1003-1010.
Pergamon Press.
Printed in Great Britain
THE EXTRACTION OF YTTRIUM(Ill) AND LANTHANUM(III) FROM HYDROCHLORIC ACID SOLUTIONS BY DI-(2-ETHYLHEXYL)PHOSPHORIC ACID TAICH1 SATO and M A S A H I R O U E D A Department of Applied Chemistry, Faculty of Engineering, Shizuoka University, Hamamatsu, Japan (Received 29 April 1972) Al~tract-The partition of yttrium(lll) and lanthanum(Ill) between hydrochloric acid solutions and solutions of di-(2-ethylhexyl)phosphoric acid (HDEHP) in kerosene has been investigated under different conditions. I.R. and high-resolution NMR spectral studies have been carried out for the organic phases. The mechanism of the extraction is discussed on the basis of the results obtained. INTRODUCTION
THE EXTRACTIONof rare earths from acid solutions by dialkyl phosphoric acids has been studied by several investigators[I-6]. One of the present authors has previously investigated the extraction of uranium(VI), thorium(IV), zirconium(IV) and iron(Ill) from hydrochloric acid solutions by di-(2-ethylhexyl)phosphoric acid (HDEHP)[7-10], and this study extends the work to the extraction of yttrium(I I I) and lanthanum(III). EXPERIMENTAL Reagents H D E H P (Union Carbide Corp.) was purified with aqueous sodium carbonate solution[l 1]; kerosene used as diluent was washed successively with conc. sulphuric acid, dilute sodium hydroxide and water. Aqueous yttrium and lanthanum solutions were prepared by dissolving the chlorides (YCl3. 6H20 and LaCIa.7H20) in hydrochloric acid of required concentrations. Other chemicals were of analytical reagent grade. Extraction and analytical procedures The procedure for obtaining the partition coefficient (the ratio of the equilibrium concentration of yttrium or lanthanum in the organic phase to that in the aqueous phase) was as described previously [I 1], except that trivalent metals in the organic phase were stripped with 1 M hydrochloric acid solu1. E.g., T. Moeller, D. F. Martin, L. C. Thompson, R. Ferrics, G. R. Feistel and W. J. Ramdall, Chem. Rev. 65, 1 (1965); C. F. Baes, Jr.,J. inorg, nucl. Chem. 24, 707 (1962). 2. D. F. Peppard, G. W. Mason and S. W. Moline, J. inorg, nucl. Chem. 5, 141 (1957). 3. D. F. Peppard, G. W. Mason, W. J. Driscoll and R. J. Sironen,J. inorg, nucl. Chem. 7,276 (1958). 4. D. F. Peppard, G. W. Mason and 1. Hucker, J. inorg, nucl. Chem.24, 881 (1962). 5. G. Duyckaerts, P. Drize and A. Simon, J. inorg, nucl. Chem. 13, 332 (1960). 6. O. B. Michelsen and M. Smutz, Proc. Int. Solvent Extraction Conf., The Hague 1971, Vol. II, p. 939. Soc. Chem. Ind., London, (1971). 7. T. Sato,J. inorg, nucl. Chem. 27, 1853 (1965). 8. T. Sato, Z. anorg, allgem. Chem. 358, 296 (1968). 9. T. Sato,Anal. Chim. Acta 52, 183 (1970). 10. T. Sato and T. Nakamura, Proc. Int. Solvent Extraction Conf., The Hague 1971, Vol. I, p. 238. Soc. Chem. ind., London, (1971). I I. T. Sato, J. inorg, nucl. Chem. 24, 699 (1962). 1003
1004
T. SATO and M. U E D A
tion (preliminary experiments showed that equilibrium between the phases was complete in 10 min). The concentration of yttrium and lanthanum in the aqueous solutions were determined by titration with E D T A using xylenon orange (XO) as indicator[12]. The chloride concentration in the organic phase was determined by Volhard's method with nitrobenzene, and the water content of the organic phase by Karl-Fischer titration.
Infrared and NMR spectral measurements The spectra of the compounds, obtained by evaporating the Organic phases from the extraction of the aqueous solutions with H D E H P in n-hexane, were determined for a capillary film between thallium halide plates by a Nujol mull method. NMR spectra were measured as described previously [13].
Dependence of extraction of yttrium(Ill) and lanthanum(Ill) on acid and solvent concentration The extraction of aqueous solutions containing 1 g/l of yttrium chloride in hydrochloric acid at various concentrations by H D E H P in kerosene at 20°C gave the results shown in Fig. 1, compared with those for similar extraction by pure TBP (tri-n-butyl phosphate). Figure 1 shows that the partition coefficient for yttrium(I I I) decreases with increasing aqueous acidity below about 5 M, and above this acidity the extraction curve rises. The variation of the partition coefficient is interpreted as follows: at low aqueous acidity, yttrium is extracted by a cation-
O'(.v,
0'1 I Initial aqueous hydrochloric acid ¢oncn,
I0 M
Fig. 1. Extraction of yttrium(ill) from hydrochloric acid solutions by H D E H P in kerosens (numerals on curves are H D E H P molarities; the molar line denotes the extraction with pure TBP). 12. J. Kinnunen and B. Wennerstrand, Chem. Analyst 46, 92 (1957). 13. T. Sato and K. Adachi, J. inorg, nucl. Chem. 31, 1395 (1969).
The extractionof yttrium(Ill)and lanthanum(Ill)
1005
exchange reaction in which hydrogen is liberated, and at high aqueous acidity by a solvating reaction similar to that with non-ionic reagents. If it is assumed that the initial decrease in the partition coefficient is governed by an ion-exchange reaction, viz. Y3+(a) + 3(HX)2(o) ~ YXrHa(o) + 3H+(a)
(1)
where X represents the anion (CsH~70)2PO2-, (HX)2 refers to the dimeric solvent, and (a) and (o) are aqueous and organic phases, respectively, then the following relationship must hold: log 8E~ = log K + 3 log (Cs -- 6Cv)/Cn
(2)
in which E] is the partition coefficient, K the equilibrium constant, Cs the total H D E H P concentration, Cy the yttrium concentration in the organic phase and CH the aqueous acidity. Log-log plots of E~ vs (Cs-6Cy)/CH at constant aqueous acidity indicate that Eqn (2) is satisfied at hydrochloric acid concentrations in range 0.05-5 M, but not below 0-05 M. It is therefore postulated that extraction under the latter condition involves the formation of a polymeric species: YX6H3(o) + Y3+(a) + 2(HX)2(o) ~ YsXloH4(o) + 3H+(a).
(3)
The overall reaction 2YZ(a) + 5(HX)2(o) ~ Yzxl0H4(o ) q- 6H+(a)
(4)
leads to the relationship log 8E: = log K1 + 3 log (Cs - 3 Cy)/C H
(5)
where K1 is a constant. A log-log plot o f E ] vs (Cs-3Cy)/CH shows that Eqn (5) is satisfied at hydrochloric acid concentrations below 0.05 M. We thus infer that, although the monomeric species is formed when H D E H P is present in excess, the increase in the yttrium concentration in the organic phase involves the formation of a polymeric y t t r i u m ( l l l ) - H D E H P complex. Accordingly, the following general equation, expressed as an ion-exchange reaction involving the formation of polymeric species, describes extraction of yttrium(Ill)from hydrochloric acid solutions by H D E H P: nYZ+(a) + (2n + 1)(HX)2(o) ~ Y,,X2~2,,+I)Hn+2(o)+ 3nH+(a)
(6)
where n i> 1. At high aqueous acidity, if we assume that extraction involves the combination of m molecules of H D E H P dimer (HX)2, which is bonded as monomer units with the yttrium ion by a reaction similar to that for TBP, viz. Y'~+(a) + 3Cl-(a) + m(HX)2(o) ~-- YCI3.2mHX(o),
(7)
1006
T. SATO and M. U E D A
then we have log 2mE°a = log K~ + m log (Cs - 2mCy)
(8)
where K2 is a constant. Log-log plots o f E ao vs ( C s - 2 m C y ) in 5, 7 and 10 M gave slopes of 1.80, 1.83 and 1.83 for m = 1.5-4, respectively. However, since reactions (1) and/or (3) will also occur to some extent at higher acidities, the values of the partition coefficients obtained are probably higher than Eqn (7) with m = 1-5, i.e. Y3+(a) +
3Cl-(a) + ~ ( H X ) 2 ( O )
~
YCI3.3HX(o),
(9)
would indicate. In the extraction of yttrium chloride solution (1 g/1) containing hydrochloric acid with pure TBP, the value of the partition coefficient shows a third-power dependence, indicating the formation of a trisolvate, YCI3.3TBP. Therefore the equilibrium (9) is also supported by the reaction Y3+(a) + 3Cl-(a) + 3TBP(o) ~ YCI33TBP(o).
(10)
The results for lanthanum are given in Fig. 2. As the partition coefficient for lanthanum(Ill) decreases monotonically with the aqueous acidity, it seems that the extraction of lanthanum is principally dominated by the ion-exchange reaction. p^^
'2
O.
Initial aqueous hydrochloric acid conch, M
Fig. 2. Extraction of lanthanum(Ill) from hydrochloric acid solutions by H D E H P in kerosene (numerals on curves are H D E H P molarities).
The extraction of yttrium(l I I ) and lanthanum( I I 1)
1007
From the dependence of the partition coefficient on the concentration of H D E H P , we deduce that the following equation holds for the extraction of lanthanum(III) from hydrochloric acid solutions: Laa+(a) + 3(HX)z(o) ~ LaXoH3(o)
(ll)
3H+(a).
+
Lanthanum(III) is less extractable than yttrium(III). At a constant hydrochloric acid concentration of 0-02M, the variation in the lanthanum concentration of the organic phase as a function of the initial aqueous lanthanum concentration and the H D E H P concentration at a fixed total concentration of 0-25 M was examined by the method of continuous variation. A maximum was found at a molar ratio of [HX]/[La] = - 6, implying the formation of the complex LaXrHa. As regards the constitution of the complexes, it is proposed that at low aqueous acidity the metal displays a coordination number of six, being chelated to three HX2- groups in the monomeric species and forming a polymeric chain in the polymeric species; at high acidity the composition of the yttrium complex is analogous to that formed with TBP. Extraction in the presence of lithium chloride The extraction of yttrium(III) and lanthanum from mixed HCI/LiCI solutions (see Fig. 3) is only slightly influenced by the chloride ion concentration, but decreases with increasing hydrogen ion concentration. The gradual fall in the upper continuous curves in Fig. 3 is explained by chloride complexing in the aqueous phase. It is assumed that the extracted species contains no chloride, as evidenced by the fact that at low aqueous acidity the chloride ion concentration in the organic
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I t l~Jll[
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Initial aqueaus total chloride ¢oncn,
*Jill[
I I0
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Fig. 3. Extraction of yttrium(III) and lanthanum(Ill) from hydrochloric acid solutions containing lithium chloride by H D E H P in kerosene (numerals on curves are H D E H P molarities; figures in parentheses indicate initial aqueous hydrochloric acid molarities; continuous and broken lines represent extraction from mixed HCI/LiCI solutions and from HCI solutions, respectively. AY, &La).
1008
T. SATO and M. UEDA
phase varies little with the amount of yttrium extracted. At high chloride concentration, the partition coefficient increases continuously with the chloride ion concentration, as in the extraction in the presence of hydrochloric acid only. This probably corresponds to the formation of the extractable species YCls by Eqn (9).
Temperature effect The extraction of yttrium and lanthanum chloride solutions at temperatures between l0 ° and 50°C gave the results in Fig. 4, where TBP results are given for comparison. The values of the heat of reaction (change in enthalpy in kcal/mol) in Eqns (6), (9), (10) and (1 l) are estimated as: yttrium(Ill), 5.4 in 0"2 M HCI, 5"5 in 1 M HCI, - 5 . 9 in 7 M HCI, - 5 . 7 in 7 M HCI by TBP; lanthanum(III), 1.6 in 0.02 M HCI.
Infrared spectra Organic extracts from aqueous solutions containing yttrium chloride at 3, 5, 10, 20 and 50g/l in 0.1 M hydrochloric acid with 0.1 M H D E H P in n-hexane or lanthanum chloride of 3, 5, 10, 20 and 50 gJl in 0.02 M hydrochloric acid with 0.3 M H D E H P in n-hexane at 20°C were examined by i.r. spectroscopy. Some representative spectra are given in Fig. 5, along with that of the organic extract saturated with yttrium, prepared by extracting several times successively from aqueous chloride solution. The i.r. spectrum of H D E H P shows the P --~ O stretching band at 1230 cm -1, the OH stretching bands at 2680 and 2350 cm -1, which arise from the hydrogen
I..~=~d,,====~ 0• -=====~
c
rt
O
OOI
3.0
3.1
3~ 3.3 3-4 3"5 3.6 (I/T)xlO 3, ,,K-I
Fig. 4. Temperature dependence of partition coefficient for the extraction from hydrochloric acid solutions of yttrium(Ill) by HDEHP in kerosene (©, 0 and t3, 0.2, 1 and 7 M HCI, respectively) or pure TBP (V, 7 M HCI) and of lanthanum(llI) by H D E H P in kerosene (0, 0.02 M HCI) (numerals on curves are H D E H P molarities).
The extraction of yttrium(I ! 1) and lanthanum(l l I)
4000
:5200
2400
1900
1700
1500
Frequency,
1300
I I00
1009
900
700
crn-I
Fig. 5. Infrared spectra of organic extracts from aqueous solutions containing yttrium chloride in 0-1 M hydrochloric acid with 0.1 M H DEH P in n-hexane or lanthanum chloride in 0.02 M hydrochloric acid with 0-3 M H D E H P in n-hexane (A, H D E H P ; B and C, yttrium chloride solutions at 3 and 10 g/l; D, yttrium-saturated organic extract; E and F, lanthanum chloride solutions at 3 and 20 g/I).
bond in the formation of dimer, the O H bending band at 1690 cm -1, and the [P-O] - C stretching band at 1030 cm -I. In the spectra of the extracts, as the concentration of yttrium or lanthanum increases, the intensities of the OH stretching and bending bands decrease, and the P ~ O absorption band bonded to the metal ion shifts to lower frequencies at 1200-1185 cm -~, and accordingly the intensity of the free P ~ O decreases. In the extraction of yttrium, the absorptions at 1060 and 985 cm -1 appear as very shallow shoulders to the peak at 1030 cm -1 in H D E H P . Simultaneously the absorptions at 1160 and 1100 c m -1, assigned to P O O - asymmetric and symmetric vibrations respectively [10, 14], appear and progressively increase in intensity with increasing yttrium concentration in the organic phase. These changes probably arise from the formation of the polymeric species[15]. Furthermore, a very broad band in the region 1700-700cm -~, which is observed in the organic extract from yttrium chloride solution at an initial aqueous concentration of 3-5 g/l, decreases in intensity at higher yttrium loadings, and then its absorption disappears in the spectrum of the compound saturated with yttrium. According to Whateley e t a/.[16,17], the background 14. 15. 16. 17.
J. R. Ferraro, J. inorg, nucl. Chem. 24,475 (1962). T. Sato, J. inorg, nucl. Chem. 26, 311 (1964). T. L. Whateley, Nature 212, 279 (1966). L.E. Smythe, T. L. Whateley and R. L. Werner,J. inorg, nucl. Chem. 30, 1553 (1968).
1010
T. SATO and M. U E D A
absorption in the region 1700-700 cm -1 indicates the presence of short and strong symmetrical or near-symmetrical hydrogen bonding. In the spectra of organic extracts from the extraction of lanthanum chloride solutions however, there is no sign of the characteristic absorption in the region 1700-700 cm -1. Hence it is considered that hydrogen bonding in the yttrium(III)-HDEHP complex is more symmetric than that in the lanthanum(III)-HDEHP complex[18]. Thus the i.r. results confirm that yttrium or lanthanum extracted into H D E H P by cationexchange is bonded to the phosphoryl oxygen bond. N M R spectra Organic phases from the extraction of yttrium chloride solutions (3, 5 and 10 g/l) containing 0-1 M hydrochloric acid and lanthanum chloride solutions (3, 20 and 50 g/l) containing 0-02 M hydrochloric acid with 0.2 M H D E H P in carbon tetrachloride at 20°C were examined by N M R spectroscopy. The spectrum of watersaturated H D E H P shows a sharp peak at z -- 0.91 in a doublet due to the methyl protons, a strong peak at 0.67 assigned to methylenic protons, a triplet at 6.12 arising from methylenic protons attached to the carbon atoms immediately adjacent to oxygen atoms, and a hydroxyl proton band at -0.40. In the extraction of yttrium chloride solutions at initial aqueous concentrations of 3, 5 and l0 g/l, the hydroxyl proton signals appear at - 0 . 9 7 , - 2 . 0 0 a n d - 2-70, respectively, but the other absorptions are little influenced. This suggests that hydrogen bonding in the complex formed by the ion-exchange reaction is stronger than that in the dimer of H D E H P , in agreement with the i.r. result which shows the presence of a broad band in the region 1700-700 cm -1. In contrast, the organic phases from the extraction of lanthanum exhibits a shift of the water proton resonance to higher fields at - 0 . 1 , 0 and 0-15 with increasing initial aqueous concentration of lanthanum chloride. Hence it is presumed that hydrogen bonding in the species extracted from yttrium chloride solutions at low aqueous acidity is stronger than that from lanthanum chloride solutions. Acknowledgement- We wish to thank Mr. F. Ozawa for assistance with N M R spectral measurement. 18. R. G. Snyder and J. A. Ibers, J. chem. Phys. 36, 1356 (1962).
Pergamon Press.
Printed in Great Britain
THE EXTRACTION OF YTTRIUM(Ill) AND LANTHANUM(III) FROM HYDROCHLORIC ACID SOLUTIONS BY DI-(2-ETHYLHEXYL)PHOSPHORIC ACID TAICH1 SATO and M A S A H I R O U E D A Department of Applied Chemistry, Faculty of Engineering, Shizuoka University, Hamamatsu, Japan (Received 29 April 1972) Al~tract-The partition of yttrium(lll) and lanthanum(Ill) between hydrochloric acid solutions and solutions of di-(2-ethylhexyl)phosphoric acid (HDEHP) in kerosene has been investigated under different conditions. I.R. and high-resolution NMR spectral studies have been carried out for the organic phases. The mechanism of the extraction is discussed on the basis of the results obtained. INTRODUCTION
THE EXTRACTIONof rare earths from acid solutions by dialkyl phosphoric acids has been studied by several investigators[I-6]. One of the present authors has previously investigated the extraction of uranium(VI), thorium(IV), zirconium(IV) and iron(Ill) from hydrochloric acid solutions by di-(2-ethylhexyl)phosphoric acid (HDEHP)[7-10], and this study extends the work to the extraction of yttrium(I I I) and lanthanum(III). EXPERIMENTAL Reagents H D E H P (Union Carbide Corp.) was purified with aqueous sodium carbonate solution[l 1]; kerosene used as diluent was washed successively with conc. sulphuric acid, dilute sodium hydroxide and water. Aqueous yttrium and lanthanum solutions were prepared by dissolving the chlorides (YCl3. 6H20 and LaCIa.7H20) in hydrochloric acid of required concentrations. Other chemicals were of analytical reagent grade. Extraction and analytical procedures The procedure for obtaining the partition coefficient (the ratio of the equilibrium concentration of yttrium or lanthanum in the organic phase to that in the aqueous phase) was as described previously [I 1], except that trivalent metals in the organic phase were stripped with 1 M hydrochloric acid solu1. E.g., T. Moeller, D. F. Martin, L. C. Thompson, R. Ferrics, G. R. Feistel and W. J. Ramdall, Chem. Rev. 65, 1 (1965); C. F. Baes, Jr.,J. inorg, nucl. Chem. 24, 707 (1962). 2. D. F. Peppard, G. W. Mason and S. W. Moline, J. inorg, nucl. Chem. 5, 141 (1957). 3. D. F. Peppard, G. W. Mason, W. J. Driscoll and R. J. Sironen,J. inorg, nucl. Chem. 7,276 (1958). 4. D. F. Peppard, G. W. Mason and 1. Hucker, J. inorg, nucl. Chem.24, 881 (1962). 5. G. Duyckaerts, P. Drize and A. Simon, J. inorg, nucl. Chem. 13, 332 (1960). 6. O. B. Michelsen and M. Smutz, Proc. Int. Solvent Extraction Conf., The Hague 1971, Vol. II, p. 939. Soc. Chem. Ind., London, (1971). 7. T. Sato,J. inorg, nucl. Chem. 27, 1853 (1965). 8. T. Sato, Z. anorg, allgem. Chem. 358, 296 (1968). 9. T. Sato,Anal. Chim. Acta 52, 183 (1970). 10. T. Sato and T. Nakamura, Proc. Int. Solvent Extraction Conf., The Hague 1971, Vol. I, p. 238. Soc. Chem. ind., London, (1971). I I. T. Sato, J. inorg, nucl. Chem. 24, 699 (1962). 1003
1004
T. SATO and M. U E D A
tion (preliminary experiments showed that equilibrium between the phases was complete in 10 min). The concentration of yttrium and lanthanum in the aqueous solutions were determined by titration with E D T A using xylenon orange (XO) as indicator[12]. The chloride concentration in the organic phase was determined by Volhard's method with nitrobenzene, and the water content of the organic phase by Karl-Fischer titration.
Infrared and NMR spectral measurements The spectra of the compounds, obtained by evaporating the Organic phases from the extraction of the aqueous solutions with H D E H P in n-hexane, were determined for a capillary film between thallium halide plates by a Nujol mull method. NMR spectra were measured as described previously [13].
Dependence of extraction of yttrium(Ill) and lanthanum(Ill) on acid and solvent concentration The extraction of aqueous solutions containing 1 g/l of yttrium chloride in hydrochloric acid at various concentrations by H D E H P in kerosene at 20°C gave the results shown in Fig. 1, compared with those for similar extraction by pure TBP (tri-n-butyl phosphate). Figure 1 shows that the partition coefficient for yttrium(I I I) decreases with increasing aqueous acidity below about 5 M, and above this acidity the extraction curve rises. The variation of the partition coefficient is interpreted as follows: at low aqueous acidity, yttrium is extracted by a cation-
O'(.v,
0'1 I Initial aqueous hydrochloric acid ¢oncn,
I0 M
Fig. 1. Extraction of yttrium(ill) from hydrochloric acid solutions by H D E H P in kerosens (numerals on curves are H D E H P molarities; the molar line denotes the extraction with pure TBP). 12. J. Kinnunen and B. Wennerstrand, Chem. Analyst 46, 92 (1957). 13. T. Sato and K. Adachi, J. inorg, nucl. Chem. 31, 1395 (1969).
The extractionof yttrium(Ill)and lanthanum(Ill)
1005
exchange reaction in which hydrogen is liberated, and at high aqueous acidity by a solvating reaction similar to that with non-ionic reagents. If it is assumed that the initial decrease in the partition coefficient is governed by an ion-exchange reaction, viz. Y3+(a) + 3(HX)2(o) ~ YXrHa(o) + 3H+(a)
(1)
where X represents the anion (CsH~70)2PO2-, (HX)2 refers to the dimeric solvent, and (a) and (o) are aqueous and organic phases, respectively, then the following relationship must hold: log 8E~ = log K + 3 log (Cs -- 6Cv)/Cn
(2)
in which E] is the partition coefficient, K the equilibrium constant, Cs the total H D E H P concentration, Cy the yttrium concentration in the organic phase and CH the aqueous acidity. Log-log plots of E~ vs (Cs-6Cy)/CH at constant aqueous acidity indicate that Eqn (2) is satisfied at hydrochloric acid concentrations in range 0.05-5 M, but not below 0-05 M. It is therefore postulated that extraction under the latter condition involves the formation of a polymeric species: YX6H3(o) + Y3+(a) + 2(HX)2(o) ~ YsXloH4(o) + 3H+(a).
(3)
The overall reaction 2YZ(a) + 5(HX)2(o) ~ Yzxl0H4(o ) q- 6H+(a)
(4)
leads to the relationship log 8E: = log K1 + 3 log (Cs - 3 Cy)/C H
(5)
where K1 is a constant. A log-log plot o f E ] vs (Cs-3Cy)/CH shows that Eqn (5) is satisfied at hydrochloric acid concentrations below 0.05 M. We thus infer that, although the monomeric species is formed when H D E H P is present in excess, the increase in the yttrium concentration in the organic phase involves the formation of a polymeric y t t r i u m ( l l l ) - H D E H P complex. Accordingly, the following general equation, expressed as an ion-exchange reaction involving the formation of polymeric species, describes extraction of yttrium(Ill)from hydrochloric acid solutions by H D E H P: nYZ+(a) + (2n + 1)(HX)2(o) ~ Y,,X2~2,,+I)Hn+2(o)+ 3nH+(a)
(6)
where n i> 1. At high aqueous acidity, if we assume that extraction involves the combination of m molecules of H D E H P dimer (HX)2, which is bonded as monomer units with the yttrium ion by a reaction similar to that for TBP, viz. Y'~+(a) + 3Cl-(a) + m(HX)2(o) ~-- YCI3.2mHX(o),
(7)
1006
T. SATO and M. U E D A
then we have log 2mE°a = log K~ + m log (Cs - 2mCy)
(8)
where K2 is a constant. Log-log plots o f E ao vs ( C s - 2 m C y ) in 5, 7 and 10 M gave slopes of 1.80, 1.83 and 1.83 for m = 1.5-4, respectively. However, since reactions (1) and/or (3) will also occur to some extent at higher acidities, the values of the partition coefficients obtained are probably higher than Eqn (7) with m = 1-5, i.e. Y3+(a) +
3Cl-(a) + ~ ( H X ) 2 ( O )
~
YCI3.3HX(o),
(9)
would indicate. In the extraction of yttrium chloride solution (1 g/1) containing hydrochloric acid with pure TBP, the value of the partition coefficient shows a third-power dependence, indicating the formation of a trisolvate, YCI3.3TBP. Therefore the equilibrium (9) is also supported by the reaction Y3+(a) + 3Cl-(a) + 3TBP(o) ~ YCI33TBP(o).
(10)
The results for lanthanum are given in Fig. 2. As the partition coefficient for lanthanum(Ill) decreases monotonically with the aqueous acidity, it seems that the extraction of lanthanum is principally dominated by the ion-exchange reaction. p^^
'2
O.
Initial aqueous hydrochloric acid conch, M
Fig. 2. Extraction of lanthanum(Ill) from hydrochloric acid solutions by H D E H P in kerosene (numerals on curves are H D E H P molarities).
The extraction of yttrium(l I I ) and lanthanum( I I 1)
1007
From the dependence of the partition coefficient on the concentration of H D E H P , we deduce that the following equation holds for the extraction of lanthanum(III) from hydrochloric acid solutions: Laa+(a) + 3(HX)z(o) ~ LaXoH3(o)
(ll)
3H+(a).
+
Lanthanum(III) is less extractable than yttrium(III). At a constant hydrochloric acid concentration of 0-02M, the variation in the lanthanum concentration of the organic phase as a function of the initial aqueous lanthanum concentration and the H D E H P concentration at a fixed total concentration of 0-25 M was examined by the method of continuous variation. A maximum was found at a molar ratio of [HX]/[La] = - 6, implying the formation of the complex LaXrHa. As regards the constitution of the complexes, it is proposed that at low aqueous acidity the metal displays a coordination number of six, being chelated to three HX2- groups in the monomeric species and forming a polymeric chain in the polymeric species; at high acidity the composition of the yttrium complex is analogous to that formed with TBP. Extraction in the presence of lithium chloride The extraction of yttrium(III) and lanthanum from mixed HCI/LiCI solutions (see Fig. 3) is only slightly influenced by the chloride ion concentration, but decreases with increasing hydrogen ion concentration. The gradual fall in the upper continuous curves in Fig. 3 is explained by chloride complexing in the aqueous phase. It is assumed that the extracted species contains no chloride, as evidenced by the fact that at low aqueous acidity the chloride ion concentration in the organic
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-"~.,~
\\{/"
\\
I t l~Jll[
i ~I
Initial aqueaus total chloride ¢oncn,
*Jill[
I I0
I
M
Fig. 3. Extraction of yttrium(III) and lanthanum(Ill) from hydrochloric acid solutions containing lithium chloride by H D E H P in kerosene (numerals on curves are H D E H P molarities; figures in parentheses indicate initial aqueous hydrochloric acid molarities; continuous and broken lines represent extraction from mixed HCI/LiCI solutions and from HCI solutions, respectively. AY, &La).
1008
T. SATO and M. UEDA
phase varies little with the amount of yttrium extracted. At high chloride concentration, the partition coefficient increases continuously with the chloride ion concentration, as in the extraction in the presence of hydrochloric acid only. This probably corresponds to the formation of the extractable species YCls by Eqn (9).
Temperature effect The extraction of yttrium and lanthanum chloride solutions at temperatures between l0 ° and 50°C gave the results in Fig. 4, where TBP results are given for comparison. The values of the heat of reaction (change in enthalpy in kcal/mol) in Eqns (6), (9), (10) and (1 l) are estimated as: yttrium(Ill), 5.4 in 0"2 M HCI, 5"5 in 1 M HCI, - 5 . 9 in 7 M HCI, - 5 . 7 in 7 M HCI by TBP; lanthanum(III), 1.6 in 0.02 M HCI.
Infrared spectra Organic extracts from aqueous solutions containing yttrium chloride at 3, 5, 10, 20 and 50g/l in 0.1 M hydrochloric acid with 0.1 M H D E H P in n-hexane or lanthanum chloride of 3, 5, 10, 20 and 50 gJl in 0.02 M hydrochloric acid with 0.3 M H D E H P in n-hexane at 20°C were examined by i.r. spectroscopy. Some representative spectra are given in Fig. 5, along with that of the organic extract saturated with yttrium, prepared by extracting several times successively from aqueous chloride solution. The i.r. spectrum of H D E H P shows the P --~ O stretching band at 1230 cm -1, the OH stretching bands at 2680 and 2350 cm -1, which arise from the hydrogen
I..~=~d,,====~ 0• -=====~
c
rt
O
OOI
3.0
3.1
3~ 3.3 3-4 3"5 3.6 (I/T)xlO 3, ,,K-I
Fig. 4. Temperature dependence of partition coefficient for the extraction from hydrochloric acid solutions of yttrium(Ill) by HDEHP in kerosene (©, 0 and t3, 0.2, 1 and 7 M HCI, respectively) or pure TBP (V, 7 M HCI) and of lanthanum(llI) by H D E H P in kerosene (0, 0.02 M HCI) (numerals on curves are H D E H P molarities).
The extraction of yttrium(I ! 1) and lanthanum(l l I)
4000
:5200
2400
1900
1700
1500
Frequency,
1300
I I00
1009
900
700
crn-I
Fig. 5. Infrared spectra of organic extracts from aqueous solutions containing yttrium chloride in 0-1 M hydrochloric acid with 0.1 M H DEH P in n-hexane or lanthanum chloride in 0.02 M hydrochloric acid with 0-3 M H D E H P in n-hexane (A, H D E H P ; B and C, yttrium chloride solutions at 3 and 10 g/l; D, yttrium-saturated organic extract; E and F, lanthanum chloride solutions at 3 and 20 g/I).
bond in the formation of dimer, the O H bending band at 1690 cm -1, and the [P-O] - C stretching band at 1030 cm -I. In the spectra of the extracts, as the concentration of yttrium or lanthanum increases, the intensities of the OH stretching and bending bands decrease, and the P ~ O absorption band bonded to the metal ion shifts to lower frequencies at 1200-1185 cm -~, and accordingly the intensity of the free P ~ O decreases. In the extraction of yttrium, the absorptions at 1060 and 985 cm -1 appear as very shallow shoulders to the peak at 1030 cm -1 in H D E H P . Simultaneously the absorptions at 1160 and 1100 c m -1, assigned to P O O - asymmetric and symmetric vibrations respectively [10, 14], appear and progressively increase in intensity with increasing yttrium concentration in the organic phase. These changes probably arise from the formation of the polymeric species[15]. Furthermore, a very broad band in the region 1700-700cm -~, which is observed in the organic extract from yttrium chloride solution at an initial aqueous concentration of 3-5 g/l, decreases in intensity at higher yttrium loadings, and then its absorption disappears in the spectrum of the compound saturated with yttrium. According to Whateley e t a/.[16,17], the background 14. 15. 16. 17.
J. R. Ferraro, J. inorg, nucl. Chem. 24,475 (1962). T. Sato, J. inorg, nucl. Chem. 26, 311 (1964). T. L. Whateley, Nature 212, 279 (1966). L.E. Smythe, T. L. Whateley and R. L. Werner,J. inorg, nucl. Chem. 30, 1553 (1968).
1010
T. SATO and M. U E D A
absorption in the region 1700-700 cm -1 indicates the presence of short and strong symmetrical or near-symmetrical hydrogen bonding. In the spectra of organic extracts from the extraction of lanthanum chloride solutions however, there is no sign of the characteristic absorption in the region 1700-700 cm -1. Hence it is considered that hydrogen bonding in the yttrium(III)-HDEHP complex is more symmetric than that in the lanthanum(III)-HDEHP complex[18]. Thus the i.r. results confirm that yttrium or lanthanum extracted into H D E H P by cationexchange is bonded to the phosphoryl oxygen bond. N M R spectra Organic phases from the extraction of yttrium chloride solutions (3, 5 and 10 g/l) containing 0-1 M hydrochloric acid and lanthanum chloride solutions (3, 20 and 50 g/l) containing 0-02 M hydrochloric acid with 0.2 M H D E H P in carbon tetrachloride at 20°C were examined by N M R spectroscopy. The spectrum of watersaturated H D E H P shows a sharp peak at z -- 0.91 in a doublet due to the methyl protons, a strong peak at 0.67 assigned to methylenic protons, a triplet at 6.12 arising from methylenic protons attached to the carbon atoms immediately adjacent to oxygen atoms, and a hydroxyl proton band at -0.40. In the extraction of yttrium chloride solutions at initial aqueous concentrations of 3, 5 and l0 g/l, the hydroxyl proton signals appear at - 0 . 9 7 , - 2 . 0 0 a n d - 2-70, respectively, but the other absorptions are little influenced. This suggests that hydrogen bonding in the complex formed by the ion-exchange reaction is stronger than that in the dimer of H D E H P , in agreement with the i.r. result which shows the presence of a broad band in the region 1700-700 cm -1. In contrast, the organic phases from the extraction of lanthanum exhibits a shift of the water proton resonance to higher fields at - 0 . 1 , 0 and 0-15 with increasing initial aqueous concentration of lanthanum chloride. Hence it is presumed that hydrogen bonding in the species extracted from yttrium chloride solutions at low aqueous acidity is stronger than that from lanthanum chloride solutions. Acknowledgement- We wish to thank Mr. F. Ozawa for assistance with N M R spectral measurement. 18. R. G. Snyder and J. A. Ibers, J. chem. Phys. 36, 1356 (1962).