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The extraction of thorium (IV) from hydrochloric acid solutions by tri-n-octyl phosphine oxide
J. inorg, nucl. Chem., 1969, Vol. 31, pp. 3633 to 3642.
Pergamon Press. Printedin Great Britain
THE EXTRACTION OF THORIUM (IV) F R O M HYDROCHLORIC ACID SOLUTIONS BY TRI-nOCTYL PHOSPHINE OXIDE T A I C H I SATO and M A S A H I R O Y A M A T A K E Department of Applied Chemistry, Faculty of Engineering, Shizuoka University, Hamamatsu, Japan (First received 14 October 1968; in revised form 11 March 1969) A b s t n ~ t - T h e partition of thorium (IV) between hydrochloric acid solutions and kerosene solutions of tri-n-octyl phosphine oxide (TOPO) has been investigated. The organic phases have been examined by i.r. and high resolution nuclear magnetic resonance spectroscopy. The mechanism of thorium extraction is discussed on the basis of the results obtained. INTRODUCTION
phosphine oxides have received increasing attention for the solvent extraction of metal ions from acid solutions [ 1]. Several papers [2] have reported the extraction of metals by tri-n-octyl phosphine oxide (TOPO), but there are few observations on the extraction mechanism and on the composition of the complex formed. One of the authors has previously investigated the extraction of uranium (VI) and thorium (IV) from hydrochloric acid solutions by tri-n-butyl phosphate (TBP) [3]. This work is undertaken to obtain further information on the extraction of thorium (I V) from hydrochloric acid solutions by TOPO. TRI-ALKYL
EXPERIMENTAL Reagents. Highly pure TOPO (Dojindo & Co., Ltd.) was used without further purification. It was dissolved in kerosene which had been purified by washing several times successively with concentrated sulphuric acid, dilute sodium hydroxide solution and water. Thorium hydroxide was precipitated with aqueous ammonia from thorium nitrate solution (prepared from Th(NO3)4. 4H20, Yokozawa Chemical Co.), washed thoroughly with ammonium nitrate solution and distilled water, and dissolved in hydrochloric acid solutions to produce solutions of ThC14. Other chemicals were analytical reagent grade. Extraction and analyticalprocedures. Partition coefficients were obtained as described in Ref. [3]. Thorium (IV) was determined by titration with E D T A using Xylenol orange as indicator[4], the water content of the organic phase by Karl Fischer titration, and the chloride concentration in the organic phase by Volhard's method using nitrobenzene [5]. I.R. and N M R spectral measurements. I.R. spectra of the separated organic phase were determined relative to that of kerosene using a spectrophotometer with KCI prisms and a cell with thallium halide windows (thickness of sample 0-1 mm). 1. C. A. Blake, Jr., C. F. Baes, Jr., K. B. Brown, C. F. Coleman and J. C. White, Proc. 2nd Int. Conf. peaceful Uses atom. Energy, Geneva, 1958, Vol. 28, p. 289, United Nations (1958). 2. W. J. Ross and J. C. White, U.S~A.E.C. Rep., ORNL-2627 (1958); J. C. White and W. J. Ross, U.S.A.E.C. Rep., ORNL-2326, 2382 (1957), 2498 (1958); B. Martin, D. W. Ockended andJ. K. Foreman, J. inorg, nucl. Chem. 21, 96 (1961); A. H. A. Heyn and Y. D. Soman, ibid, 26, 287 (1964). 3. T. Sato, J. appl. Chem. 16, 53 (1966). 4. J. Korble and R. Pribil, Chemist Analyst 45, 102 (1956). 5. T. Sato, J. inorg, nucL Chem. 28, 1461 (1966). 3633
3634
T. SATO and M. Y A M A T A K E
Nuclear magnetic resonance (NMR) spectra were obtained in carbon tetrachloride solution on a Hitachi Perkin-Elmer Model R-20 High Resolution NMR spectrometer with a permanent magnet of 14,192 G. Tetramethylsilane was used as an internal reference. RESULTS AND DISCUSSION
Dependence on concentration of hydrochloric acid Figure 1 shows that for solution containing 1 g ThC1J1 at 20°C the partition coefficient rises with aqueous acidity above 0.5 M, passes through a maximum at
laO
1::
!
o.1
I IO Initial aqueous hydrochloric acid cohen., M
Fig. 1. Extraction of thorium (IV) from hydrochloric acid solutions by TOPO in kerosene at 20°C (numerals on curves are TOPO concentrations, M).
Extractionof thorium (IV)fromhydrochloricacidsolutions
3635
about 7 M and then lhlls at higher acidities. The shape of this curve closely resembles that for the extraction of uranium (VI) by TBP[3], and it is interpreted as follows: at low acidities salting-out by chloride ion causes the curve to rise, while the formation of chloride complexes and the competition with hydrochloric acid for the available TOPO check the rise at higher acidities. This explanation is supported by the following fact. In the extraction of hydrochloric acid at 20°C from aqueous solutions of varying acidity with kerosene solutions of TOPO at constant concentration, the acidity of the organic phase increases with the aqueous acidity, and in particular the organic solution from the extraction at 7 M aqueous acidity is found to contain hydrochloric acid: T O P O : w a t e r in the ratio 1: 1 : 1 , indicating the formation of the compound HC1-TOPO.H20. Extraction in presence of lithium chloride Figure 2 shows the extraction by 0-02 and 0.1 TOPO in kerosene at 20°C of thorium chloride solutions (1 g/l) containing 0.1 M hydrochloric acid and lithium chloride at various concentrations, compared with data for the extraction from hydrochloric acid solutions of varying acidity. While the partition coefficient in the presence of hydrochloric acid passes through a maximum at 6 M acid, in the presence of lithium chloride it increases continuously with the total concentration of chloride ion. This implies that the controlling factor in the extraction of thorium (IV) from hydrochloric acid solutions is the total chloride ion concentration; at higher acidities, when part of the hydrochloric acid in the aqueous phase is replaced by lithium chloride, the partition coefficient is increased because of the removal of hydrochloric acid competition for available TO PO. Dependence on concentrations of solvent and thorium If we assume that the extraction of thorium (IV) from hydrochloric acid solutions with TOPO is governed by a solvating reaction similar to that with TBP[3], viz. ThCI4 (a)+ n TOPO (o) ~ ThCI4 • n TOPO (o)
(1)
where (a) and (o) represent aqueous and organic phases respectively, the following relationship would be expected: log Eft, = log K + n log (Cvovo-n Crh)
(2)
in which Eft is the partition coefficient, K the equilibrium constant, CToPo the total TOPO concentration and CTh the thorium concentration of the organic phase. For the extraction of thorium chloride solutions (1 g/l) containing 3 and 4 M hydrochloric acid, and/or 0.1 M hydrochloric acid in the presence of 3 and 4 M lithium chloride the log-log plots of Eft vs. (Cs-n Crh) at constant hydrochloric acid concentration gave the slopes in Table 1 when the value of n was varied. Hence Equation (2) is satisfied for n = 2.5 in 3 and 4 M HCI, and for n = 3 in 0.1 M HC1 + 3 and 4 M LiC1. Assuming n = 2.5 as the mean of 2 and 3, we consider that the value of the partition coefficient 'in the presence of hydro-
3636
T. SATO and M. Y A M A T A K E
I [OC
! .s_
o-ol
I 1
Ih
i
I nit ial o q u e ~ J $ t o t a l
i
i
i i ill Io
chloride
conch., M
Fig. 2. Salting-out effect of both hydrocMoric acid and lithium chloride for the extraction of thorium (IV) by 0-02 and 0-1 M TOPO in kerosene at 20°C (numerals on curves are TOPO concentrations, M; A in the presence of 0.1 M hydrochloric acid and lithium chloride;C) in the presence of hydrochloric acid only).
Extraction of thorium (IV) from hydrochloric acid solutions
3637
Table 1. Slopes of log-log plots of E~t vs. (C-HCTh) at different aqueous hydrochloric acid concentrations
HC! 3M 4M 2 2-5 3 4
2.52 2-57 2-61 2.60
Slope 0- l M HCl + LiCl 3M 4M
2.66 2.56 2.44 2-47
3-38 3.31 3-26 3"64
3"27 3-16 3-06 3.20
chloric acid alone shows a mixed second- and third-power dependence, indicating the formation of a disolvate ThCI4.2TOPO and trisolvate ThCI4.3TOPO. In contrast, when the hydrochloric acid is partly replaced by lithium chloride, the value of the partition coefficient depends on the third power alone. This trisolvate corresponds to the complex formed in the extraction by TBP, viz. ThCI4.3TBP [3]. Thus we infer that n = 2 and 3 in Equation (1), i.e. ThCI4 (a) + 2TOPO (o) ~ ThCI4.2TOPO (o)
(3)
ThCI4 (o)+ 3TOPO (o) ~ ThC14.3TOPO (o).
(4)
and In the extraction of thorium chloride solutions of various concentrations containing 0.1 M hydrochloric acid in the presence of 4 M lithium chloride with 0-04 M T O P O in kerosene, the mole ratio of the concentration of thorium in the organic phase to that of TOPO, plotted as a function of the initial aqueous thorium concentration, approaches a limiting value of 0.3 (Fig. 3.), supporting the formao.4
a. 0 . 3 O
U
.._e o.I o :E
o
'i o.1
O~Z.
I
o-~
Initiol aqueous
I
o'.6
o'~ thorium
conch.,
o'-7
M
Fig. 3. Variation in mole ration of thorium concentration of organic phase to T O P O concentration with initial aqueous thorium concentration, for the extraction of thorium from 0.1 M hydrochloric acid solution in the presence of 4 M lithium chloride by 0.04 M T O P O in kerosene. Temp. 20°C.
3638
T. SATO and M. YAMATAKE
tion of a thorium chloride trisolvate. Furthermore, for those organic phases it was found that the increase in the chloride concentration and the water content relative to the concentration of thorium extracted are as follows: the former is nearly 4 based on the extraction of thorium chloride; the latter approaches a limiting value ( - 3 ) as the initial aqueous thorium concentration is increased, suggesting that three molecules of water one combine with T h C I 4 . 3 T O P O (Table 2.). Table 2. Karl Fischer titration of the organic solutions from the extraction of thorium chloride solutions containing 0.1 M hydrochloric acid and 4 M lithium chloride with 0.04 M TOPO in kerosene Mole ratio, Initial [Th]aq, [HzO]o~, [H20]o~ M M [Th]o~ 0-0134 0-0670 0"134 0"268 0"335 0"536 0"670
0-0175 0"0220 0.0260 0'0292 0.0314 0-0348 0-0348
2"36 2"55 2'70 2-80 2"86 2-90 2.90
I.R. spectra
T h e organic phases from the extraction of thorium chloride solutions (5 and 200 g[ 1) containing 0.2 M hydrochloric acid and 4 M lithium chloride with 0.1 M T O P O in kerosene gave the spectra illustrated in Fig. 4, compared with that from the extraction of 0.2 M hydrochloric acid in the presence of 4 M lithium chloride but in the absence of thorium. T h e spectrum for 0.2 M hydrochloric acid solution in the presence of 4 M lithium chloride closely resembles that of the water-saturated T O P O solution: the O H vibrations (stretching and bending bands at 3380 and 1620 cm -1 respectively) and the P --~ O stretching band at 1185 cm -1 (actually a doublet at 1185 and 1170 cm -1) [6] indicate the presence of weak hydrogen bonds in the compound T O P O . H20. T h e spectrum of the organic solution from the extraction of an aqueous solution containing 5 g/1 of thorium chloride in 0.2 M hydrochloric acid and 4 M lithium chloride shows the O H stretching band at 3380 and 3 1 6 0 c m -1 and the bending band at 1620 cm -1. At the same time, the absorption due to the P - * O stretching band of T O P O , bonded with thorium ion, is observed at 1100 cm -1, and the intensity of the free P--~ O band of T O P O decreases. In the organic phase from the extraction of an aqueous solution containing 200 g/l of thorium chloride, the spectrum exhibits a stronger absorption due to the chloro-complex of thorium, since the thorium concentration of the organic phase is higher than that in an organic phase derived from the equilibration of T O P O solution with an aqueous 6. J. Kennedy and A. M. Deane,J. inorg, nucl. Chem. 19, 142 (1961).
Extraction of thorium (IV) from hydrochloric acid solutions I
I
I
I
I
I
I
I
I
3639 I
I
1 I, OOO
I
I
I
I
I
i
I
f
3(.00
3,?..o0
2eO0
I'/00
IGO0
1~00
P400
I 300
I IZO0
f
I
! I O0
! 0o0
~00
F r e q u e n c y , crn-I
Fig. 4. I.R- spectra of the organic solutions from the extraction of thorium chloride solutions containing 0.2 M hydrochloric acid and 4 M lithium chloride with 0.1 M TOPO in kerosene (A: 0-2M HCl+4M LiCl alone; B and C: thorium chloride solutions of 5 and 200 g/l respectively). phase containing 5 g ThC14/l. In this case, the free P - * O absorption band of T O P O almost disappears, and the bonded P--> O band shifts slightly to a lower frequency. H e n c e the infra-red results confirm that thorium extracted into T O P O solution is bonded to the phosphoryl oxygen atom. As presumably one or two molecules of water are coordinated to thorium, it is thought that the thorium chloro-complex possesses the stoichiometry. ThC14(H20)3TOPO(OH2)2
or
ThCI4(2H20)3TOPO(OH2),
with thorium displaying a co-ordination number of eight.
N M R spectra T h e organic phases from the extraction of thorium chloride solutions (0, 5, 50 and 200 g/l) containing 0-2 M hydrochloric acid and 4 M lithium chloride by 0.1 M T O P O in carbon tetrachloride were examined by N M R spectroscopy. T h e spectra are given in Fig. 5, compared with that for a carbon tetrachloride solution of 0-1 M T O P O saturated with water. T h e N M R spectrum for water-saturated T O P O shows a peak at 9.11 (~"value) in a triplet due to the methyl protons, the sharp peak at 8.68 arising from methylenic protons, and the peak at 8.53 from methylenic protons attached to carbon atoms immediately adjacent to a phosphors atom, and in addition the water proton signal at 7-58, indicating the formation of the compound T O P O . H z O . In the organic solutions from the extraction of aqueous solutions containing 0, 5, 50 and
T. SATO and M. Y A M A T A K E
3640
S
6
7
#
,~
Fig. 5. N M R spectra of the organic solutions from the extraction of thorium chloride solutions containing 0.2 M hydrochloric acid and 4 M lithium chloride with 0.1M TOPO in carbon tetrachloride (A: T O P O solution saturated with water; B; 0.2M H C I + 4 M LiCl alone; C, D and E: thorium chloride solutions of 5, 50 and 200 g/l respectively).
200 g/1 of thorium chloride in a mixture of 0.2 M hydrochloric acid and 4 M lithium chloride, the water proton resonances appear at values of 6.63, 6.28, 5-69, and 5.39 respectively, and the methylenic protons attached to the carbon atoms immediately adjacent to a phosphorus atom at 8.53, 8-42, 8-27, and 8-20 respectively. In the U ( V I ) - H N O z - T B P system, the formulae UO2(NO3)~'2TBP and HNOz.TBP suggest that two molecules of nitric acid in the organic phase are displaced by one molecule of uranyl nitrate which is extracted
Extraction of thorium (IV) from hydrochloric acid solutions
3641
under suitable conditions, if no special reaction occurs between uranyl nitrate and nitric acid in the organic phase [7]. A similar relationship is also expected in the present system. H o w e v e r , since the uranyl n i t r a t e - T B P complex does not contain a water molecule [8], it is expected that the N M R spectra of the organic solutions from the extraction of uranium will show the shift of the water proton signal to a higher field; this has been seen in the extraction of zirconium nitrate by T B P [9]. In this study, the water proton signal for the extraction of hydrochloric acid solution in the presence of lithium chloride but in the absence of thorium is shifted to a lower field (from 7.58 to 6.63), due to the formation of a stronger hydrogen bond in replacing part of T O P O . H 2 0 by T O P O . H C I . H z O . When thorium is extracted into the organic phase, the water proton resonance should shift to a higher field if the complex contains no water. Nevertheless shifts to a lower field are observed on increasing the concentration of thorium. This is probably ascribed to the increased concentration of the thorium chlorocomplex ThCI4(HzO)3TOPO(OH2)2, in good agreement with the i.r. data. T e m p e r a t u r e effect
T h e extraction of thorium chloride solution (1 g/l), containing 4 m hydrochloric acid and/or 0.1 M hydrochloric acid in the presence of 4 M lithium chlo-
11 fit.
3"1
I
I
I
3":.
.~,~
3"4
I
3-,r
3-<;
I/TXIO s, OK-I Fig. 6. Temperature-dependence of partition coefficient for the extraction of thorium from hydrochloric acid solutions by 0"02M TOPO in kerosene (O 4M HCl; A 0-1 M HCI+4M LiC1). 7. T. V. Healy and H. A. C. Mckay, Trans. Faraday Soc. 52, 633 (1956); T. Sato, J. inorg, nucl. Chem. 9, 188 (1959). 8. T. Sato, J. appl. Chem. 15,489 (1965). 9. T. Sato, Z. anorg, aUg. Chem. In press.
3642
T. SATO and M. Y A M A T A K E
ride, with 0.02 M T O P O in kerosene at temperatures between 10 and 40°C provided the data shown in Fig. 6. The partition coefficients decrease with rising temperature, the estimated heats of reaction (change in enthalpy, kcal/mol) being 6.7 in 4 M HC1 and 4.7 in 0.1 M HCI + 4 M LiCI respectively. Acknowledgement-We wish to thank Mr. F. Ozawa for assistance with N M R spectral measurement.
Pergamon Press. Printedin Great Britain
THE EXTRACTION OF THORIUM (IV) F R O M HYDROCHLORIC ACID SOLUTIONS BY TRI-nOCTYL PHOSPHINE OXIDE T A I C H I SATO and M A S A H I R O Y A M A T A K E Department of Applied Chemistry, Faculty of Engineering, Shizuoka University, Hamamatsu, Japan (First received 14 October 1968; in revised form 11 March 1969) A b s t n ~ t - T h e partition of thorium (IV) between hydrochloric acid solutions and kerosene solutions of tri-n-octyl phosphine oxide (TOPO) has been investigated. The organic phases have been examined by i.r. and high resolution nuclear magnetic resonance spectroscopy. The mechanism of thorium extraction is discussed on the basis of the results obtained. INTRODUCTION
phosphine oxides have received increasing attention for the solvent extraction of metal ions from acid solutions [ 1]. Several papers [2] have reported the extraction of metals by tri-n-octyl phosphine oxide (TOPO), but there are few observations on the extraction mechanism and on the composition of the complex formed. One of the authors has previously investigated the extraction of uranium (VI) and thorium (IV) from hydrochloric acid solutions by tri-n-butyl phosphate (TBP) [3]. This work is undertaken to obtain further information on the extraction of thorium (I V) from hydrochloric acid solutions by TOPO. TRI-ALKYL
EXPERIMENTAL Reagents. Highly pure TOPO (Dojindo & Co., Ltd.) was used without further purification. It was dissolved in kerosene which had been purified by washing several times successively with concentrated sulphuric acid, dilute sodium hydroxide solution and water. Thorium hydroxide was precipitated with aqueous ammonia from thorium nitrate solution (prepared from Th(NO3)4. 4H20, Yokozawa Chemical Co.), washed thoroughly with ammonium nitrate solution and distilled water, and dissolved in hydrochloric acid solutions to produce solutions of ThC14. Other chemicals were analytical reagent grade. Extraction and analyticalprocedures. Partition coefficients were obtained as described in Ref. [3]. Thorium (IV) was determined by titration with E D T A using Xylenol orange as indicator[4], the water content of the organic phase by Karl Fischer titration, and the chloride concentration in the organic phase by Volhard's method using nitrobenzene [5]. I.R. and N M R spectral measurements. I.R. spectra of the separated organic phase were determined relative to that of kerosene using a spectrophotometer with KCI prisms and a cell with thallium halide windows (thickness of sample 0-1 mm). 1. C. A. Blake, Jr., C. F. Baes, Jr., K. B. Brown, C. F. Coleman and J. C. White, Proc. 2nd Int. Conf. peaceful Uses atom. Energy, Geneva, 1958, Vol. 28, p. 289, United Nations (1958). 2. W. J. Ross and J. C. White, U.S~A.E.C. Rep., ORNL-2627 (1958); J. C. White and W. J. Ross, U.S.A.E.C. Rep., ORNL-2326, 2382 (1957), 2498 (1958); B. Martin, D. W. Ockended andJ. K. Foreman, J. inorg, nucl. Chem. 21, 96 (1961); A. H. A. Heyn and Y. D. Soman, ibid, 26, 287 (1964). 3. T. Sato, J. appl. Chem. 16, 53 (1966). 4. J. Korble and R. Pribil, Chemist Analyst 45, 102 (1956). 5. T. Sato, J. inorg, nucL Chem. 28, 1461 (1966). 3633
3634
T. SATO and M. Y A M A T A K E
Nuclear magnetic resonance (NMR) spectra were obtained in carbon tetrachloride solution on a Hitachi Perkin-Elmer Model R-20 High Resolution NMR spectrometer with a permanent magnet of 14,192 G. Tetramethylsilane was used as an internal reference. RESULTS AND DISCUSSION
Dependence on concentration of hydrochloric acid Figure 1 shows that for solution containing 1 g ThC1J1 at 20°C the partition coefficient rises with aqueous acidity above 0.5 M, passes through a maximum at
laO
1::
!
o.1
I IO Initial aqueous hydrochloric acid cohen., M
Fig. 1. Extraction of thorium (IV) from hydrochloric acid solutions by TOPO in kerosene at 20°C (numerals on curves are TOPO concentrations, M).
Extractionof thorium (IV)fromhydrochloricacidsolutions
3635
about 7 M and then lhlls at higher acidities. The shape of this curve closely resembles that for the extraction of uranium (VI) by TBP[3], and it is interpreted as follows: at low acidities salting-out by chloride ion causes the curve to rise, while the formation of chloride complexes and the competition with hydrochloric acid for the available TOPO check the rise at higher acidities. This explanation is supported by the following fact. In the extraction of hydrochloric acid at 20°C from aqueous solutions of varying acidity with kerosene solutions of TOPO at constant concentration, the acidity of the organic phase increases with the aqueous acidity, and in particular the organic solution from the extraction at 7 M aqueous acidity is found to contain hydrochloric acid: T O P O : w a t e r in the ratio 1: 1 : 1 , indicating the formation of the compound HC1-TOPO.H20. Extraction in presence of lithium chloride Figure 2 shows the extraction by 0-02 and 0.1 TOPO in kerosene at 20°C of thorium chloride solutions (1 g/l) containing 0.1 M hydrochloric acid and lithium chloride at various concentrations, compared with data for the extraction from hydrochloric acid solutions of varying acidity. While the partition coefficient in the presence of hydrochloric acid passes through a maximum at 6 M acid, in the presence of lithium chloride it increases continuously with the total concentration of chloride ion. This implies that the controlling factor in the extraction of thorium (IV) from hydrochloric acid solutions is the total chloride ion concentration; at higher acidities, when part of the hydrochloric acid in the aqueous phase is replaced by lithium chloride, the partition coefficient is increased because of the removal of hydrochloric acid competition for available TO PO. Dependence on concentrations of solvent and thorium If we assume that the extraction of thorium (IV) from hydrochloric acid solutions with TOPO is governed by a solvating reaction similar to that with TBP[3], viz. ThCI4 (a)+ n TOPO (o) ~ ThCI4 • n TOPO (o)
(1)
where (a) and (o) represent aqueous and organic phases respectively, the following relationship would be expected: log Eft, = log K + n log (Cvovo-n Crh)
(2)
in which Eft is the partition coefficient, K the equilibrium constant, CToPo the total TOPO concentration and CTh the thorium concentration of the organic phase. For the extraction of thorium chloride solutions (1 g/l) containing 3 and 4 M hydrochloric acid, and/or 0.1 M hydrochloric acid in the presence of 3 and 4 M lithium chloride the log-log plots of Eft vs. (Cs-n Crh) at constant hydrochloric acid concentration gave the slopes in Table 1 when the value of n was varied. Hence Equation (2) is satisfied for n = 2.5 in 3 and 4 M HCI, and for n = 3 in 0.1 M HC1 + 3 and 4 M LiC1. Assuming n = 2.5 as the mean of 2 and 3, we consider that the value of the partition coefficient 'in the presence of hydro-
3636
T. SATO and M. Y A M A T A K E
I [OC
! .s_
o-ol
I 1
Ih
i
I nit ial o q u e ~ J $ t o t a l
i
i
i i ill Io
chloride
conch., M
Fig. 2. Salting-out effect of both hydrocMoric acid and lithium chloride for the extraction of thorium (IV) by 0-02 and 0-1 M TOPO in kerosene at 20°C (numerals on curves are TOPO concentrations, M; A in the presence of 0.1 M hydrochloric acid and lithium chloride;C) in the presence of hydrochloric acid only).
Extraction of thorium (IV) from hydrochloric acid solutions
3637
Table 1. Slopes of log-log plots of E~t vs. (C-HCTh) at different aqueous hydrochloric acid concentrations
HC! 3M 4M 2 2-5 3 4
2.52 2-57 2-61 2.60
Slope 0- l M HCl + LiCl 3M 4M
2.66 2.56 2.44 2-47
3-38 3.31 3-26 3"64
3"27 3-16 3-06 3.20
chloric acid alone shows a mixed second- and third-power dependence, indicating the formation of a disolvate ThCI4.2TOPO and trisolvate ThCI4.3TOPO. In contrast, when the hydrochloric acid is partly replaced by lithium chloride, the value of the partition coefficient depends on the third power alone. This trisolvate corresponds to the complex formed in the extraction by TBP, viz. ThCI4.3TBP [3]. Thus we infer that n = 2 and 3 in Equation (1), i.e. ThCI4 (a) + 2TOPO (o) ~ ThCI4.2TOPO (o)
(3)
ThCI4 (o)+ 3TOPO (o) ~ ThC14.3TOPO (o).
(4)
and In the extraction of thorium chloride solutions of various concentrations containing 0.1 M hydrochloric acid in the presence of 4 M lithium chloride with 0-04 M T O P O in kerosene, the mole ratio of the concentration of thorium in the organic phase to that of TOPO, plotted as a function of the initial aqueous thorium concentration, approaches a limiting value of 0.3 (Fig. 3.), supporting the formao.4
a. 0 . 3 O
U
.._e o.I o :E
o
'i o.1
O~Z.
I
o-~
Initiol aqueous
I
o'.6
o'~ thorium
conch.,
o'-7
M
Fig. 3. Variation in mole ration of thorium concentration of organic phase to T O P O concentration with initial aqueous thorium concentration, for the extraction of thorium from 0.1 M hydrochloric acid solution in the presence of 4 M lithium chloride by 0.04 M T O P O in kerosene. Temp. 20°C.
3638
T. SATO and M. YAMATAKE
tion of a thorium chloride trisolvate. Furthermore, for those organic phases it was found that the increase in the chloride concentration and the water content relative to the concentration of thorium extracted are as follows: the former is nearly 4 based on the extraction of thorium chloride; the latter approaches a limiting value ( - 3 ) as the initial aqueous thorium concentration is increased, suggesting that three molecules of water one combine with T h C I 4 . 3 T O P O (Table 2.). Table 2. Karl Fischer titration of the organic solutions from the extraction of thorium chloride solutions containing 0.1 M hydrochloric acid and 4 M lithium chloride with 0.04 M TOPO in kerosene Mole ratio, Initial [Th]aq, [HzO]o~, [H20]o~ M M [Th]o~ 0-0134 0-0670 0"134 0"268 0"335 0"536 0"670
0-0175 0"0220 0.0260 0'0292 0.0314 0-0348 0-0348
2"36 2"55 2'70 2-80 2"86 2-90 2.90
I.R. spectra
T h e organic phases from the extraction of thorium chloride solutions (5 and 200 g[ 1) containing 0.2 M hydrochloric acid and 4 M lithium chloride with 0.1 M T O P O in kerosene gave the spectra illustrated in Fig. 4, compared with that from the extraction of 0.2 M hydrochloric acid in the presence of 4 M lithium chloride but in the absence of thorium. T h e spectrum for 0.2 M hydrochloric acid solution in the presence of 4 M lithium chloride closely resembles that of the water-saturated T O P O solution: the O H vibrations (stretching and bending bands at 3380 and 1620 cm -1 respectively) and the P --~ O stretching band at 1185 cm -1 (actually a doublet at 1185 and 1170 cm -1) [6] indicate the presence of weak hydrogen bonds in the compound T O P O . H20. T h e spectrum of the organic solution from the extraction of an aqueous solution containing 5 g/1 of thorium chloride in 0.2 M hydrochloric acid and 4 M lithium chloride shows the O H stretching band at 3380 and 3 1 6 0 c m -1 and the bending band at 1620 cm -1. At the same time, the absorption due to the P - * O stretching band of T O P O , bonded with thorium ion, is observed at 1100 cm -1, and the intensity of the free P--~ O band of T O P O decreases. In the organic phase from the extraction of an aqueous solution containing 200 g/l of thorium chloride, the spectrum exhibits a stronger absorption due to the chloro-complex of thorium, since the thorium concentration of the organic phase is higher than that in an organic phase derived from the equilibration of T O P O solution with an aqueous 6. J. Kennedy and A. M. Deane,J. inorg, nucl. Chem. 19, 142 (1961).
Extraction of thorium (IV) from hydrochloric acid solutions I
I
I
I
I
I
I
I
I
3639 I
I
1 I, OOO
I
I
I
I
I
i
I
f
3(.00
3,?..o0
2eO0
I'/00
IGO0
1~00
P400
I 300
I IZO0
f
I
! I O0
! 0o0
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F r e q u e n c y , crn-I
Fig. 4. I.R- spectra of the organic solutions from the extraction of thorium chloride solutions containing 0.2 M hydrochloric acid and 4 M lithium chloride with 0.1 M TOPO in kerosene (A: 0-2M HCl+4M LiCl alone; B and C: thorium chloride solutions of 5 and 200 g/l respectively). phase containing 5 g ThC14/l. In this case, the free P - * O absorption band of T O P O almost disappears, and the bonded P--> O band shifts slightly to a lower frequency. H e n c e the infra-red results confirm that thorium extracted into T O P O solution is bonded to the phosphoryl oxygen atom. As presumably one or two molecules of water are coordinated to thorium, it is thought that the thorium chloro-complex possesses the stoichiometry. ThC14(H20)3TOPO(OH2)2
or
ThCI4(2H20)3TOPO(OH2),
with thorium displaying a co-ordination number of eight.
N M R spectra T h e organic phases from the extraction of thorium chloride solutions (0, 5, 50 and 200 g/l) containing 0-2 M hydrochloric acid and 4 M lithium chloride by 0.1 M T O P O in carbon tetrachloride were examined by N M R spectroscopy. T h e spectra are given in Fig. 5, compared with that for a carbon tetrachloride solution of 0-1 M T O P O saturated with water. T h e N M R spectrum for water-saturated T O P O shows a peak at 9.11 (~"value) in a triplet due to the methyl protons, the sharp peak at 8.68 arising from methylenic protons, and the peak at 8.53 from methylenic protons attached to carbon atoms immediately adjacent to a phosphors atom, and in addition the water proton signal at 7-58, indicating the formation of the compound T O P O . H z O . In the organic solutions from the extraction of aqueous solutions containing 0, 5, 50 and
T. SATO and M. Y A M A T A K E
3640
S
6
7
#
,~
Fig. 5. N M R spectra of the organic solutions from the extraction of thorium chloride solutions containing 0.2 M hydrochloric acid and 4 M lithium chloride with 0.1M TOPO in carbon tetrachloride (A: T O P O solution saturated with water; B; 0.2M H C I + 4 M LiCl alone; C, D and E: thorium chloride solutions of 5, 50 and 200 g/l respectively).
200 g/1 of thorium chloride in a mixture of 0.2 M hydrochloric acid and 4 M lithium chloride, the water proton resonances appear at values of 6.63, 6.28, 5-69, and 5.39 respectively, and the methylenic protons attached to the carbon atoms immediately adjacent to a phosphorus atom at 8.53, 8-42, 8-27, and 8-20 respectively. In the U ( V I ) - H N O z - T B P system, the formulae UO2(NO3)~'2TBP and HNOz.TBP suggest that two molecules of nitric acid in the organic phase are displaced by one molecule of uranyl nitrate which is extracted
Extraction of thorium (IV) from hydrochloric acid solutions
3641
under suitable conditions, if no special reaction occurs between uranyl nitrate and nitric acid in the organic phase [7]. A similar relationship is also expected in the present system. H o w e v e r , since the uranyl n i t r a t e - T B P complex does not contain a water molecule [8], it is expected that the N M R spectra of the organic solutions from the extraction of uranium will show the shift of the water proton signal to a higher field; this has been seen in the extraction of zirconium nitrate by T B P [9]. In this study, the water proton signal for the extraction of hydrochloric acid solution in the presence of lithium chloride but in the absence of thorium is shifted to a lower field (from 7.58 to 6.63), due to the formation of a stronger hydrogen bond in replacing part of T O P O . H 2 0 by T O P O . H C I . H z O . When thorium is extracted into the organic phase, the water proton resonance should shift to a higher field if the complex contains no water. Nevertheless shifts to a lower field are observed on increasing the concentration of thorium. This is probably ascribed to the increased concentration of the thorium chlorocomplex ThCI4(HzO)3TOPO(OH2)2, in good agreement with the i.r. data. T e m p e r a t u r e effect
T h e extraction of thorium chloride solution (1 g/l), containing 4 m hydrochloric acid and/or 0.1 M hydrochloric acid in the presence of 4 M lithium chlo-
11 fit.
3"1
I
I
I
3":.
.~,~
3"4
I
3-,r
3-<;
I/TXIO s, OK-I Fig. 6. Temperature-dependence of partition coefficient for the extraction of thorium from hydrochloric acid solutions by 0"02M TOPO in kerosene (O 4M HCl; A 0-1 M HCI+4M LiC1). 7. T. V. Healy and H. A. C. Mckay, Trans. Faraday Soc. 52, 633 (1956); T. Sato, J. inorg, nucl. Chem. 9, 188 (1959). 8. T. Sato, J. appl. Chem. 15,489 (1965). 9. T. Sato, Z. anorg, aUg. Chem. In press.
3642
T. SATO and M. Y A M A T A K E
ride, with 0.02 M T O P O in kerosene at temperatures between 10 and 40°C provided the data shown in Fig. 6. The partition coefficients decrease with rising temperature, the estimated heats of reaction (change in enthalpy, kcal/mol) being 6.7 in 4 M HC1 and 4.7 in 0.1 M HCI + 4 M LiCI respectively. Acknowledgement-We wish to thank Mr. F. Ozawa for assistance with N M R spectral measurement.