SXIX1

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CYANEX 923 AS AN EXTRACTANT FOR TRIVALENT LANTHANIDES AND YTTRIUM M.L.P. Reddy a; R. Luxmi Varma a; T.R. Ramamohan a; Sushanta K. Sahu b; V. Chakravortty b a Regional Research Laboratory (CSIR), Trivandrum, India b Department of Chemistry, Utkal University, Bhubaneswar, India Online Publication Date: 01 May 1998

To cite this Article Reddy, M.L.P., Varma, R. Luxmi, Ramamohan, T.R., Sahu, Sushanta K. and Chakravortty, V.(1998)'CYANEX 923

AS AN EXTRACTANT FOR TRIVALENT LANTHANIDES AND YTTRIUM',Solvent Extraction and Ion Exchange,16:3,795 — 812 To link to this Article: DOI: 10.1080/07366299808934553 URL: http://dx.doi.org/10.1080/07366299808934553

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SOLVENT EXTRACTION AND ION EXCHANGE, 16(3),795-812 (1998)

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CYAN EX 923 AS AN EXTRACTANT FOR TRIVALENT LANTHANIDES AND YTTRIUM

M.L.p.Reddy*, R.luxmi Varma and T.R. Ramamohan Regional Research laboratory (CSIR),Trivandrum-695 019, India Sushanta K.Sahu and V. Chakravortty Department of Chemistry, Utkal University, Bhubaneswar-751 004, India

ABSTRACT The extraction of yttrium and some trivalent lanthanides from thiocyanate and nitrate solutions using Cyanex 923 ( TRPO ) in xylene as an extractant has been investigated. It has been found that these trivalent metal ions are extracted from thiocyanate solution as M(SCN)3.n TRPO ; n in general having the values of 4 and 3 for the lighter and the heavier lanthanides respectively. On the other hand, from nitrate solutions these trivalent metal ions are extracted as M(N03)3' 3 TRPO. The equilibrium constants of the extracted complexes have been obtained by non-linear regression analysis. In both the thiocyanate and nitrate systems, the distribution ratios of trivalent lanthanides are found to increase with decreasing ionic radii and the distribution ratio of yttrium lies along with those of the middle lanthanides. The separation factors between these trivalent metal ions were evaluated and compared with those obtained using commercially important extraction reagents like tributylphosphate (TBP), trioctylphosphine oxide (TOPO) and di-2-ethylhexylphosphoric acid (DEHPA). The separation possibilities between yttrium and the trivalent lanthanides have also been discussed.

795 Copyright \0 1998 by Marcel Dekker, Inc.

796

REDDY ET AL.

INTRODUCTION

In spite of several reported procedures[1,2], separation of trivalent lanthanides as a group from actinides as well as separation of individual lanthanides from' each other still offers a formidable challenge in the field of separation science, Trivalent lanthanides are now-a-days usually

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separated using multistage extractions[3]. This procedure is tedious and time consuming. Thus, there is a growing interest in developing altemate procedures including the use of ion-specific compounds and development of new extraction systems for the separation of lanthanides as a group or from one another,

Hence it was decided to investigate the extraction

behaviour of trivalent lanthanides and yttrium employing new extraction systems, which might be of help in the design and development of economically viable solvent extraction processes for the separation of trivalent lanthanides.

Cyanex 923 ( TRPO) is a mixture of four trialkylphosphine oxides, which exhibit extraction properties similar to those of trioctylphosphine oxide (TOPO) and has been commercially produced by American Cyanamid Company, U.S.A.[4].

1 R2R P=Q

1

R2 RP=Q

1

R = hexyl and R = octyl

The above solvent mixture has the advantage of being a liquid and is completely miscible with all commonly used hydrocarbons. Recently, Cyanex 923 has been suggested as a potential extractant for the separation of zirconium and hafnium[5].

In the present study, the

extraction of trivalent lanthanides and yttrium has been investigated from thiocyanate and nitrate solutions using Cyanex 923 as an extractant.

797

CYANEX 923

EXPERIMENTAL

Cyanex 923, supplied by Cyanamid Canada Inc., was used as such without further purification.

The extractant contains about 93% of

trialkylphosphine oxides, including dioctyl-hexylphosphine oxide (40-44%), dihexyl-octylphosphine oxide (28-32%), trihexylphosphine oxide and

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trioctylphosphine oxide(6). Its average molecular weight is equal to 348.

Xylene, analytical reagent quality, was used as a diluent in the present work.

Stock solutions of yttrium and lanthanides were prepared from their oxides (Rare Earth Products, Cheshire, U.K., 99.99%) by dissolving in 3 concentrated hydrochloric acid and diluting to 100 cm with distilled water.

These solutions were then standardized by titration with a standard solution of EDTA at pH=5.0 using acetate buffer with xylenol orange as . . .. . . . .

4 the Indicator. Initial metal Ion concentration was maintained at 1x10

3 molldm for all extraction studies, Arsenazo I (Fluka, Switzerland) solution was prepared by dissolving 3 25 mg of this reagent in 250 cm of distilled water. Ammonium acetate 3 buffer (pH=7.5) was prepared by dissolving 19.25 gms in 25 cm of distilled water and adjusting the pH with HCI/NaOH.

All the other

chemicals used were of analytical grade.

A

Hitachi220

double

beam

microprocessor

based

spectrophotometer was used for measuring absorbances. An ECIL, India digital pH meter was used for pH value measurements. All the computer programs were written in FORTRAN 77 and executed on a 32-bit mini-computer (HCL, HORIZON III).

Solvent Extraction Procedure Distribution ratios were determined by shaking equal volumes of aqueous and organic phases for 60 min in a glass stoppered vial with the

REDDY ET AL.

798 help of a mechanical shaker at 303.:t1oK.

Preliminary experiments

showed that extraction equilibrium is attained within 5-10 min. allowing the phases to settle, 5 cm

3

After

aliquots of aqueous phase were

3 3 pipetted into a 25 cm beaker and 1 cm of ammonium acetate buffer and 3 5 cm of arsenazo I solution were added.

After adjusting the pH to 7.5,

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3 the solution was transferred into a 25 cm volumetric flask and made up to the mark. The absorbances of the solutions were measured at 575 nm and the metal concentrations were computed from the respective calibration graphs. The concentration of the metal ion the organic phase was obtained by material balance. These concentration values were used to obtain the distribution ratio, D. RESULTS AND DISCUSSION

The trivalent lanthanides or yttrium in the aqueous phase form a variety of complexes in the presence of thiocyanate or nitrate ions.

However, under the present experimental conditions, we have assumed the formation of the first two complexes defined by

M3+

where X -

+i

x- 8

MX(3-/)+ I

(1)

=SCN - or N0 3- and i =1 and 2 for SCN - and only 1 for the

nitrate system. Then the total metal ion in the aqueous phase (MI) is given by

CYANEX 923

799

The values of the stability constants ([31 and [32) were taken from literature[7,8).

The

extraction

reaction of a trivalent lanthanide or yttrium from

thiocyanate or nitrate solutions with Cyanex 923(TRPO) can be written as

Maq

K

3+

+ 3Xaq + n TRPO oll1 /E>n MX 3 .n TRPO oll1

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Where n=3 or 4. It was assumed that the

(3)

activity coefficients of the

species involved remain constant over the range of concentrations used and no significant dimerization of the extractant, Cyanex 923 occurs.

Then the distribution ratio, 0 of the metal can be written as

D=

Âą n=3

[X-]3[TRPO]n (1 + P1[X-] + P2[X-]2) Kex,n

(4)

Extraction of yttrium and trivalent lanthanides by Cyanex 923 from thiocyanate medium The effect of pH in the range 2.5 to 3.0 on the extraction of the . . 3 .

trivalent metal Ions from 1.0 molldm molldm

3

..

thiocyanate solution With 0.005

Cyanex 923 in xylene has been studied and it has been found

that the 0 values are constant in this pH range. Thus, it is clear that H

+

ions do not participate in the extracted species.

The effect of thiocyanate concentration (0.2 to 1.0 molldm

3)

on the

extraction of trivalent lanthanides and yttrium has been studied at constant pH = 3.0 and Cyanex 923 concentration (0.02 mol/dm

3

for La(lJl) and

3 3 Nd(III); 0.005 molldm for Eu(III), Y(III) and Tm(llI) and 0.002 molldm for Lu(III)). The ionic strength of the aqueous phase was maintained constant 3 (1=1.0 using 1.0 moll dm NaCI04). The extraction of trivalent lanthanides

800

REDDY ET AL.

and yttrium increases linearly with increasing thiocyanate concentration.

From the slopes of the plots, log {D(I + 13 1[SCN-] + 13 2[SCN-]2)} vs log 3, [SCN-] mol/dm it is clear that 3 molecules of thiocyanate ions are involved in the extracted complexes ( for the sake of conciseness, the data not included).

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The variation of D of trivalent lanthanides and yttrium with the

3) concentration of Cyanex 923 (0.001 to 0.02 mol/dm has been studied 3 from 1.0 molldm thiocyanate (pH=3.0) as the aqueous phase, and the results are. shown in Fig.1. From the slopes of the plots, log {D(1 + -

-2

3

13 1[SCN ] + 13 2[SCN 1 )} vs log [ TRPO ] mo 1/ dm , it is clear that 4 and 3 molecules of Cyanex 923 are involved in the extracted complexes of La(III), Nd(lIl) and Eu(III); and Y(III), Tm(lII) and Lu(III), respectively.

Similar solvation numbers have been reported elsewhere for the extraction of these trivalent lanthanides from thiocyanate solutions with neutral organophosphorus extractants such as trioctylphosphine oxide (TOPO) and

tributylphosphine

oxide

(TBPO)

[9]

and

also

with

dialkyl

sulphoxides[10]. The solvation numbers of the metal ions decrease in general from 4 for the lighter lanthanides to 3 for the heavier ones. It has been shown [11,12 ] that the coordination number of the lanthanides in a series of homologous lanthanide compounds decreases from lighter to heavier lanthanides as a result of a decrease in ionic radii. It looks likely therefore that the decrease in solvation number observed in the present study is a direct reflection of the decrease in coordination number of these trivalent lanthanides in the extracted complexes.

The stoichiometry of the extracted complex of the trivalent metal ions has been confirmed by analysing the equilibrium data using equation 4. The equilibrium constants of the extracted species were calculated by non-linear regression analysis as described in our earlier publication[13]. and are given in Table 1.

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CYANEX923

801

Fig.1. Effect of Cyanex 923 concentration on the extraction of trivalent lanthanides and yttrium.

Aqueous

phase:

3

1.0 molldm

thiocyanate solution at pH=3.0.

Table 1. Equilibrium constants for the extraction of trivalent lanthanides and yttrium from thiocyanate solutions with Cyanex 923 in xylene Extracted complex

Log equilibrium constant

La(SCN)3 . 4 TRPO

8.68±0.20"

Nd(SCN)3. 4 TRPO

10.02±0.20

Eu(SCN)3 . 4 TRPO

10.47±0.30

Tm(SCN)3. 3 TRPO

9.24±0.20

Lu(SCN)3. 3 TRPO

10.05±0.30

Y(SCN)3 . 3 TRPO

7.85±0.20

"The errors in the equilibrium constant values were calculated from the rootmean square fractional deviation between the experimental and calculated distribution ratios

REDDY ET AL.

802

Comparison of extraction behaviour of yttrium and lanthanides from thiocyanate medium The D values of trivalent Y, La, Nd, Eu, Tb, Ho, Tm and Lu have 3 been determined from 1.0 molldm thiocyanate solutions at pH=3.0 using 3 0.005 molldm Cyanex 923 in xylene as an extractant. For comparison, the D values for the above trivalent metal ions have also been determined

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3 with 0.005 mol/dm Tapa in xylene as an extractant. It is clear from the results reported in Fig.2 that the D values of these trivalent metal ions 3 increase with decreasing ionic radii of the Ln + ion with the Cyanex 923 extractant. On the other hand, the 0 value of Y lies along with those of the middle lanthanides (between Tb and Ho). A similar extraction behaviour for these trivalent metal ions has been observed with Tapa as an extractant. However, the 0 value of Y lies between that of Eu and Tb. The extraction of different lanthanides with Cyanex 923 in thiocyanate system appears to indicate the absence of any steric constraints in the coordination of the requisite number of the mono-dentate, linear NCSentities. The extractions show the expected increase with increasing electrostatic interaction between the cation and the ligand.

Similar

extraction behaviour has been reported by Reddy and coworkers[14] while extracting

these

metal

ions

from

thiocyanate

solutions

with

octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in xylene. Here, the D value of yttrium was found to be less than that of La, which is contrary to the present observations. Borkowski et al.[15] have also noticed similar trends in the extraction of trivalent lanthanides from thiocyanate solutions using 100% TBP. Here again they have reported that the D value of yttrium is less than that of La. Further, it is also clear from Fig.2 that the extraction efficiency of these trivalent metal ions with Cyanex 923 was found to be moderately lower than with the Tapa.

The separation factors (S.F.= DM,IDM,) between the trivalent metal ions have been calculated and are given in Table 2. The separation

803

CYANEX 923

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,et路r---------------------,

10

64

66

ATOMIC NUMBER,

60

62

Lnlt

70

Fig.2. Effect of atomic number on the distribution ratio of trivalent . .

lanthanides and yttrium.

3

Aqueous phase: 1.0 mol/dm

3 thiocyanate at pH=3.0; TRPO=0.005 mol/dm ; TOPO =0.005 mol/drn".

<0.01

0.02

0.01

0.02

<0.01

0.11

Lu

-

Tm

13 5.41

2.32

0.01

0.06

0.32

0.74

-

0.09

Ho

0.01

0.02

0.60

Tb

3.15

1.35

0.22

0.29

-

0.03

1.87 1.39

4.62

0.01

0.08

0.43

0.03

0.18

10.76

0.16

17

58

575

3.42

106

-

45.6

0.10

Eu

6.4

Nd

9.1

Tm

33.75

1.68

Ho

0.16

0.72

Tb

3

9.88

0.53

Eu

Cyanex 923 in xylene from 1.0 molldm

-

-

Nd

3

0.02

La

Y

6.3

M1

La

M2 y

Table 2. Separation factors (S.F. =DM1/DM2) with 0.005 molldm solution at pH=3.0

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6

34

78

106

56 3562 361

Lu

thiocyanate

r

>

~

gJ t::l -<

:<l

~

00

805

CYANEX923

3) factors among trivalent lanthanides (Lu/La = 3.56x10 with Cyanex 923 were found to be at least 3 orders of magnitude lower than the separation factor values reported for the same pair with di-2-ethylhexyl phosphoric 6) acid (DEHPA) in n-dodecane (SF = 1.2x10 from thiocyanate solution 3-fold [16). However, the present values are about 10 higher than those

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reported for the same pair with TBP (SF of LulLa =3.2)[16) and three times higher than that of the TOPO (S.F. of LulLa = 1.05x10\ Better selectivities between yttrium and heavy lanthanides (S.F. of LuIY = 56; TmlY = 9) have been achieved with Cyanex 923 as compared to DEHPA

(SF of LulY= 19.6; TmlY = 3.4)[17), which has been widely used in the Rare Earth Industry. Thus, it can be concluded from the above study that yttrium can be easily separated from heavy lanthanides in a few stages of .countercurrent extraction using Cyanex 923 as an extractant from solutions containing thiocyanate, as compared to the usually employed extractants like, DEHPA Extraction of vttrium and trivalent lanthanides with Cyanex 923 from nitrate medium The effect of pH in the range 2.5 to 3.0 on the extraction of the 3 3 trivalent metal ions from1.0 mol/dm nitrate solutions with 0.05 mol/dm

Cyanex 923 has been studied and it has been found that the extraction is independent of pH. The variation of nitrate ion concentration (0.2-1.0 molldm

3)

at constant Cyanex 923 concentration (0.05 molldm

3)

and the

variation of Cyanex 923 concentration (Fig.3) at constant nitrate solution concentration (1.0 rnol/dm" of pH=3.0), respectively, have been studied for the extraction of trivalent lanthanides and yttrium. The relevant log-log plots gave straight lines with slopes of 3 for all these trivalent metal ions, indicating the extraction of the complexes, M(N03)3.3 TRPO. Extraction of trivalent lanthanides by neutral organophosphorus extractants like TBP

REDDY ET AL.

806

2·t,r-----------... o Experimental - Calculated

1·6

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1·2 r--1 I

..,

0

0·6

~

Q.

0·4

0' ~O.O 0 ...J

- 0·4 -2.4

~·O

-1·6

-1·2

-0·8

Log [TRPO]mOlldm3

Fig.3.

Effect of Cyanex 923 concentration on the extraction of trivalent 3 lanthanides and yttrium. Aqueous phase: 1.0 molldm nitrate solution at pH=3.0.

from nitrate media has been extensively studied[18,19] and it has been established that the extracted species is the trisolvated complex like M(N03)3.3TBP. Recently, Reddy and coworkers[14

I have also reported

trisolvates for the extraction of lanthanides from nitrate solutions with octyl(phenyl)-N,N-diisobutyl carbamoylmethylphosphine oxide.

The equilibrium constants of the above extracted species were calculated by non-linear regression analysis (using equation 4) and are given in Table 3.

It can be seen from the table .that the equilibrium

807

CYANEX 923

Table 3. Equilibrium constants for the extraction of trivalent lanthanides

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and yttrium nitrate solutions with Cyanex 923 in xylene Extracted complex

Log equilibrium constant

La(N0 3)3 . 3 TRPO

4.42.!0.10 *

Nd(N03)3 . 3 TRPO

5.19.!0.20

Eu(N0 3)3 . 3TRPO

5.45.!0.10

Tm(N0 3)3 . 3 TRPO

6.35.!0.20

Lu(N0 3)3 . 3 TRPO

6.60.!0.20

3 TRPO

5.33.!0.10

Y(N0 3)3

'The errors in the equilibrium constant values were calculated from the rootmean square fractional deviation between the experimental and calculated distribution ratios.

constant values of the various trivalent lanthanides increase with decreasing ionic radii.

Corrioarison of extraction behaviour of yttrium with lanthanide series from nitrate medium The 0 values of trivalent La, Nd, Eu,Tb, Ho,Tm, Lu and Y have . 3 . .

been determined from 1.0 molldm molldm

3

.

nitrate solution (pH=3.0) usmq 0.02 3

Cyanex 923 or 0.02 mol/dm TOPO as an extractant and the

results are shown in Fig.4. It is clear from the above study that the 0 values of different trivalent lanthanides increases with increase of the atomic number and the 0 value of Y occupies the position of the middle lanthanides (i.e. between Eu and Tb). Further, it can be seen from Fig.4 that the 0 values of trivalent lanthanide series with TOPO system do not increase monotonically with increase in atomic number of these metal ions, but have a maximum at Eu. Here, the behaviour of yttrium is similar

REDDY ET AL.

808

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10

to o < a;

;:::

z

o

S (!J

1( I-

s'"

TOPO

r "

!

58

&0

62

64. ,~6

ATOMIC NUMBER,

Fig.4.

68

70

Ltr

Effect of atomic number on the distribution ratio of trivalent

3

lanthanides and yttrium. Aqueous phase : 1.0 moll d m solution at pH=3.0; TRPO =0.02 mOl/dm

3;

.

nitrate

Tapa = 0.02 mol/dm

3.

CYANEX923

809

to that of the heavy lanthanides (i.e. between Tm and Lu). The existence of a maximum in the extractions by Tapa indicates that once the cationic radius decreases beyond a certain size, steric considerations become more important than electrostatic effects.

The extraction of different

lanthanides in the nitrate system with Cyanex 923 indicates the absence of steric factors and the extractions show the expected increase with

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increasing electrostatic interaction between the cation and the ligand.

Table 4 gives the separation factors among the trivalent metal ions, 3 calculated at 1.0 molldm nitrate solution (pH=3.0) using 0.02 mol/drn" Cyanex 923 as an extractant. The S.F. values found in the present study with Cyanex 923 between the trivalent lanthanides (Lu/La=200) are definitely higher than those reported for TBP(S.F. of LuiLa = 3.2)[16] and CMPO (SF of LulLa =0.5) systems[14]. However, the S.F. values observed with Cyanex 923 between these trivalent lanthanides are lower than the S.F. values reported with widely used extraction systems such as DEHPA. The S.F. values found in the present study between yttrium and heavy lanthanides (SF of LulY =18; TmlY =10.9) are comparable to the S.F. values of DEHPA reported elsewhere[17].

As can be seen from Fig.4, the separations among the trivalent lanthanides with Tapa are hindered by the presence of characteristic extraction maxima towards the middle of the lanthanide series, whereas with Cyanex 923 the extraction of various lanthanides increases with increasing atomic number. Thus, using Cyanex 923 from nitrate solutions, better selectivities can be achieved among the trivalent metal ions as compared to the Tapa system.

CONCLUSION

The extraction equilibria of trivalent lanthanides and yttrium from thiocyanate and nitrate solutions have been investigated using Cyanex

0.14 0.07 0.05 0.01 0.01

1.50 0.79 0.60 0.09 0.06

Tb

Ho

Tm

Lu

0.21

2.36

Nd

Eu

-

11

La

y

M2

0.02

0.04

0.25 0.04

0.06

0.40

0.16 0.09

0.07

1.33

2.51

4.0

18.6

0.12

0.75

1.89

-

0.34

0.53

1.60

C.64

3.0

7.4

4.7

14

II

1.7

6.5

0.6

14

27

43

200

18

Lu

8.6

16

26

120

3 3 Table 4. Separation factors (S.F.= DM1/DM2) with 0.02 mol/dm Cyanex 923 in xylene from 1.0 mol/dm nitrate solution at pH=3.0 Tb Ho Tm Y La Nd Eu M1 1.69 10.9 0.09 0.43 0.67 1.27

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811

CYANEX 923

923 in xylene as an extractant. The results clearly demonstrate that belter selectivities and extraction efficiencies can be obtained for these trivalent metal ions when extracted from thiocyanate solutions as compared to the nitrate system. The separation factors observed between yttrium and the heavy lanthanides (LulY = 56; TmlY =9) with Cyanex 923 in thiocyanate system were found to be significantly higher than the separation factors reported with DEHPA, which is a commonly used extraction system in Downloaded By: [CSIR eJournals Consortium] At: 06:39 5 August 2008

rare earth separations. Thus, Cyanex 923 may be useful for the separation and purification of yttrium especially from heavy lanthanides, which exist together in minerals like xenotime, in fewer stages of countercurrent extraction as compared to the normally used extractants in the Rare Earth Industry.

REFERENCES 1. M.L.P. Reddy, T. Prasada Rao and A.D. Damodaran, Mineral Processing and Extractive Metallurgy, .1l, 91 (1995).

2. K.L. Nash, Solvent Extr. Ion Exch.,.11, 729 (1993).

3. L.Sherrington, Commercial Processes for rare earths and thorium, in Handbook of Solvent Extraction ,Edited by T.C. Lo, M.H.I. Baird and C. Hanson, Wiley Interscience, New York, p.717 (1983).

4. WA Rickelton and RJ. Boyle, Sep. ScLTechnol., 23, 1227 (1988).

5. E. EI.Ammouri and PA Distin, Solvent Extr. Ion Exch., 14,871(1986).

6. M. Wisniewski, M.B.Bogacki and J.Szymanowski, J. Radioanal.Nucl.

Chem. Articles, 208 ,195(1996).

7. RM. Smith and A.E. Martell, Critical Stability Constants, VolA, Plenum Press, New York (1976).

8. G.R Choppin and J. Ketels, J. Inorg. Nucl. Chem., 27,1335 (1965).

9. P.K. Khopkarand P. Narayanankulty, J. Inorg. Nucl. Chem., 34, 2617 (1972).

812

REDDY ET AL.

10.M.L.P. Reddy, P.B.Santhi, TR Ramamohan and AD. Damodaran, Radiochim. Acta, 64, 121 (1994).

11. J.L. Hoard, B. Lee and M.D. Lind, J. Am. Chem. Soc., 87, 1612 (1965).

12.D.G. Karraker, Solvent Extraction Research, Edited by A.S. Kertes and Y.Marcus, John Wiley, New York, p.113 (1969).

Downloaded By: [CSIR eJournals Consortium] At: 06:39 5 August 2008

13. P.B.Santhi, M.L.P. Reddy, TR Ramamohan and AD. Damodaran, Talanta, 41, 9(1994).

14. M.L.P. Reddy, AD. Damodaran, J.N. Mathur, M.S. Murali, MV Balarama Krishna and RH. Iyer, Solvent Extr. Ion Exch., 14, 793 (1996).

15. M. Borkowski, J. Krejzler and S. Siekierski, Radiochim. Acta,...§§, 99 (1994).

16. B.Ceccaroli and J. Alstad, J. Inorg. Nucl. Chem., 43, 1881 (1981).

17. TB. Pierce and P.F. Peck. Analyst, 88. 217 (1963).

18. D.F. Peppard, J.R Faris, P.R Gray and G.W. Mason, J. Phys.

Chem.•2, 294 (1953).

19. C.F.Best, E. Hesford and HAC. McKay, J. Inorg. Nucl. Chem., 12.

136 (1959).

Received by Editor August 5, 1997