The extraction of U(VI) and selected M(III) cations by bis n-octyl phosphoric acid in two different hydrocarbon diluents

J.inorg. nucl.Chem., 1970,Vol. 32, pp. 3899to 3909. PergamonPress. Printedin Great Britain

THE EXTRACTION OF U(VI) AND SELECTED M(III) CATIONS BY BIS N-OCTYL PHOSPHORIC ACID IN TWO DIFFERENT HYDROCARBON DILUENTS* G. W. MASON, N. L. S C H O F E R and D. F. P E P P A R D Chemistry Division, Argonne National Laboratory, Argonne, i11.60439 (First received 29 April 1970; in revised form 27 May 1970)

A b s t r a c t - T h e extraction of Ce a+, Pm a+, Eu a+, Tm a÷, y3+, Am3+, Cma÷ and UOz z÷ from an aqueous chloride phase into a solution of (n-CsHIrO)2PO(OH), HDOP, in benzene or in n-heptane has been studied as a function of concentration of extractant in the organic phase and of hydrogen ion in the aqueous phase. From these data the stoichiometries of extraction have been deduced as: U(VI), benzene or n-heptane: U O ~ + 2(HY)zo -~ UO2(HY~)zo + 2Ha+ M(Ill), benzene: Ma d+ + 3(HY)z o ~ M(HY2)% + 3HA + M(I II), n-heptane: Ma ~++ 2'5(HY)2 ° ~ MY(HY2)% + 3HA+ where the subscripts A and O refer, respectively, to mutually equilibrated aqueous and organic phases. (The formulation of extracted entities is based solely upon established ratios and is in no sense to be interpreted in terms of established structure). Corresponding to the stoichiometries, values of K, F and [H +] obtained from the hydrogen ion dependency plots were inserted in the equation: K = K~Fa/[H+]b

to obtain values of Ks. (K is the observed distribution ratio, F the concentration of H D O P in the organic phase and [H ÷] the concentration of hydrogen ion in the aqueous phase following equilibration, and a and b the respective extractant and hydrogen ion dependencies. The equation is applied specifically to a system embodying 1"0 F (NaC1 + HC1) at 22 +__2°C). The K, (n-heptane) value is higher than the Ks (benzene) value for each of the cations studied. However, although the ratio of the two K, values is large for each of the seven M(III) cations, ranging from 50 to 200, it is only 10 for U(V1). From these observations, it is apparent that the term carrier diluent is preferable to the term inert diluent. Comparisons with systems employing [C~H~CH(C2H~)CH~O]2PO(OH) and (n-CaHIv)zPO(OH), respectively symbolized as H D E H P and H[DOP], as extractants are made in terms of: (1) acidity of the extractant; (2) structure of extractant, that is C - - P or C---O--P bonds; (3) steric effects within the extractant molecule; and (4) effect of diluent (aromatic or saturated hydrocarbon). INTRODUCTION

EXTRACTION f r o m an aqueous phase into a solution of a monoacidic phosphorusbased extractant has become a well established technique for the mutual separation of certain metallic cations [1]. In the interpretation of data the assumption is * Based on work performed under the auspices of the U.S. Atomic Energy Commission. 1. D. F. Peppard, In Advances in Inorganic and Radiochemistry (Edited by H. J. Emeleus and A. G. Sharpe), Vol. 9. p. 1. Academic Press, New York (1966). 3899

3900

G . W . MASON, N. L. SCHOFER and D. F. PEPPARD

f r e q u e n t l y m a d e t h a t t h e d i l u e n t e m p l o y e d is inert. T h a t this a s s u m p t i o n is n o t generally valid has been amply demonstrated[2-7]. For example, under comparable conditions both Eu(III) and Am(III) are extracted from aqueous chloride o r p e r c h l o r a t e p h a s e s b y ( C r H 1 3 O C z H 4 0 ) 2 P O ( O H ) with d i s t r i b u t i o n r a t i o , K , v a l u e s a p p r o x i m a t e l y 500 t i m e s g r e a t e r in c y c l o h e x a n e t h a n in b e n z e n e [6]. I n a n a t t e m p t to d e t e r m i n e t h e effect o f a m i l d d i l u e n t v a r i a t i o n u p o n t h e e x t r a c t i o n o f U ( V I ) a n d s e l e c t e d M ( I I I ) e l e m e n t s a s t u d y o f bis n - o c t y l p h o s p h o r i c a c i d ( n - C s H 1 7 0 ) 2 P O ( O H ) , in t w o h y d r o c a r b o n d i l u e n t s , b e n z e n e a n d n - h e p t a n e , was made. These two diluents were chosen because of the absence of functional groups which might interact strongly with the OH group and/or the PO group of t h e e x t r a c t a n t m o l e c u l e ; a n d this p a r t i c u l a r e x t r a c t a n t w a s c h o s e n b e c a u s e o f its r e l a t i v e l y l o w w a t e r s o l u b i l i t y , its r e l a t i v e l y high a c i d s t r e n g t h , a n d its f r e e d o m f r o m significant s t e r i c h i n d r a n c e . A s i m i l a r s t u d y o f bis n e o - o c t y l p h o s p h o r i c a c i d , a n i s o m e r i c e x t r a c t a n t w i t h significant s t e r i c h i n d r a n c e , will b e r e p o r t e d later.

EXPERIMENTAL The extractant, (n-CsH170)2PO(OH), was prepared and purified by the method described for the preparation and purification of (CrH13OC2H40)2PO(OH)[6], substituting octanol-1 obtained from Eastman Organic Chemicals Distillation Products Industries, for CoH13OC2H4OH. Anal. Calcd. for (CsH1~O)zPO(OH), C~rH3504P: C, 59-60; H, 10.94; P, 9"61; equivalent weight, 322.4 g. Found: C, 59.34; H, 11.02; P, 9.94; equivalent weight, 324 g. The C, H and P analyses were performed by Micro-Tech Laboratories, Skokie, Illinois. The equivalent weight was determined by titration as described previously [8]. No diacidic component was detected. The pKa of the extractant in 75 per cent ethanol at 22 _+2°C, determined as described previously [8], was found to be 3.30. The state of aggregation in the dry diluents was determined from isothermal distillation data obtained with a Thomas Isothermal Molecular Weight Apparatus at 30"4°C, triphenyl phosphate, (CrHsO)3PO, being employed as a monomeric standard. From the data for solutions approximately 0-1 F the values of 7/in (HY)n were calculated to be: benzene, 2.00; n-heptane, 2" 19. Following previous usage [9] in which H represents a theoretically ionizable hydrogen atom and G a generalized organic group a di-ester of orthophosphoric acid, PO(OH)3, derived from the alcohol or phenol, GOH, is represented as (GO)zPO(OH) and further symbolized as HDGP. Similarly the phosphinic acid (G)2PO(OH) is represented as H[DGP], the brackets distinguishing it from the phosphoric acid ester. The extractant presently reported, (n-CsH~70)zPO(OH), is therefore represented as HDOP, where D and O signify "di-" and "octyl" (specifically n-octyl), respectively. The two extractants for which comparative data are presented in Tables 1 and 2 are his 2-ethyl hexyl phosphoric acid, (2-C2H5 • CrHx20)2PO(OH), and his n-octyl phosphinic acid, (n-CaH~7)~PO(OH). They are symbolized respectively as HDEHP and H[DOP]. For ease of discussion these compounds are also written in the general formulation HY, where Y represents all of the empirical formula of a monomer except the ionizable hydrogen atom which is represented by H. Correspondingly, these extractants are represented in the dimeric form as (HY)~. In order to avoid ambiguity, the concentration unit employed is formality, F, here defined as the number of formula weights of solute per liter of solution. 2. 3. 4. 5. 6. 7. 8. 9.

D. Dyrssen and F. Krasovec,Acta chem. scand. 13, 561 (1959). D. Dryssen and D. H. Liem, Acta chem. scand. 14, 1100 (1960). D. Dryssen and D. H. Liem, Acta chem. scand. 18,224 (1964). G.W. Mason, S. Lewey and D. F. Peppard, J. inorg, nucl. Chem. 26, 2271 (1964). D. F. Peppard, G. W. Mason and G. Griffin, J. inorg, nucl. Chem. 27, 1683 (1965). G. W. Mason, A. F. Bollmeierand D. F. Peppard, J. inorg, nucl. Chem. 29, 1103 (1967). D. F. Peppard, G. W. Mason and C. M. Andrejasich, J. inorg, nucl. Chem. 27, 697 (1965). D. F. Peppard, G. W. Mason and I. Hucher, J. inorg, nucl. Chem 24, 881 (1962).

The extraction of U(VI) and selected M(I I I) cations

3901

The alpha-active isotopes 470 yr 241Am, 18 yr 244Cm and 1"6 × 105 yr 233U (23su : 233U mass ratio less than 0.03) were obtained from Argonne National Laboratory stocks. The beta-active isotopes 285 d 14~:e, 2.6yr l~TPm, [13, 16]yr 152'~S4Eu, 129d ~7°Tm and 5 8 d 91y were obtained and purified as described previously [ 10]. Phillips Petroleum Company "pure grade" n-heptane and Mallinckrodt Chemical Works benzene were used as carrier diluents. The distribution ratio, K, of a given nuclide is defined as the ratio of the concentration of nuclide in the upper (organic) divided by the concentration of nuclide in the lower (aqueous)of the two mutuallyequilibrated liquid phases, the concentration of nuclide being on an atomic basis as reflected in alpha or beta counting rates. The K values were determined by techniques described previously [11]. All data were obtained at 22 -+ 2°C. RESULTS AND CONCLUSIONS

The state of aggregation study, as reported in Table l, shows the extractant, H D O P , to be essentially dimeric in both benzene and n-heptane in the absence of water, in agreement with the findings of Kolarik e t a/.[12]. For purposes of discussion it is assumed that it is dimeric in the wet diluents also, under the conditions of the extraction studies. Partial justification of this assumption rests upon the report [ 13] that "The molecular weights of the monobasic acids are not water sensitive...", in reference to the molecular weight data for several (GO)EPO(OH) compounds in benzene and naphthalene. In place of (HDOP)2, the general formulation (HY)2 is used in representing the dimer in the discussion of extraction stoichiometries. The extraction of each M(III), studied as M s+, from a 1-0 F (NaCl + HCI) aqueous phase is seen to be inversely third-power hydrogen ion dependent in each of the diluents: benzene, Figs. 1 and 2; n-heptane, Figs. 5 and 6. Correspondingly, the extraction of U(VI) as UO2 2+ is inversely second-power hydrogen ion dependent in each of the diluents: benzene, Fig. 2; n-heptane, Fig. 6. In each case, the numerical value of the inverse hydrogen ion dependency is equal to the numerical value of the charge on the cation extracted. Table I. Degree of association, "0, in dry diluent and pKa in 75% ethanol of H D O P and of selected reference H D G P compounds Compound Formula Representation (n-CaHlrO)2PO(OH) (2-C~Hs • C~H120)2PO(OH) (n-CsHIT)2PO(OH)

HDOP HDEHP H[DOP]

Benzene 2"0" 2"0t 2' 1§

n-Heptane 2' 2* 2"2*

pKa 75% Ethanol 3"30* 3-49:~ 5"28t

*New data. tRef. [14]. ~tRef. [15]. §Ref. [10]. 10, t I. 12. 13. 14. 15.

D. F. Peppard, G. W. Mason and S. Lewey, J. inorg, nucl. Chem. 27, 2065 (1965). D. F. Peppard, G. W. Mason and I. Hucher, J. inorg, nucl. Chem. 18, 245 (1961). Z, Kolarik, J. Hejna and H. Pankova, J. inorg, nucl. Chem. 30, 253 (I 968). D. F. Peppard, J. R. Ferraro and G. W. Mason, J. inorg, nucl. Chem. 7, 231 (1958). D . F . Peppard, G. W. Mason and C. Andrejasich, J. inorg, nucl. Chem. 28, 2347 (1966). G . W . Mason, N. L. Schofer and D. F. Peppard, J. inorg, nucl. Chem. 32, 3911 (1970).

3902

G, W. MASON, N. L. S C H O F E R and D. F. P E P P A R D

I

i

J

I

-

5

Tm

4

3 E I--

Z Q_ Eu

I

o~ J

i 0

-I

-I

-2 Lines of slope: - 3 . 0 -3

-I

-2

0

Log F H +

Fig. 1. Hydrogen ion dependency of the extraction of M 3+ from 1.0 F (NaCI + HCI) into HDOP, benzene diluent. F of HDOP: Tm, 0.075; Eu, 0.15; Ce and Pm, 0.30.

I

u°;~~ x I

I

I

I

3

2 Y

2

I

I

>.

Cm

E~ <~

~ o

Am o I

-i

~;

~; o~

~o

-2

-3

-3 Lines of slop

J -2

t

-4 I -I

I

I 0

Log F H +

Fig. 2. Hydrogen ion dependency of the extraction of M 3+ and UO22+ from 1.0 F (NaCI + HCI) into HDOP, benzene diluent. F of HDOP: UO22+, 0.027; Y, 0.075; Am, 0-15; Cm, 0.30.

The extraction of U(VI) and selected M(III) cations

3903

The extraction of UO~ z÷ is directly second-power extractant dependent in each of the diluents; benzene, Fig. 4; n-heptane, Fig. 8. So the measured distribution ratio, K, may be expressed as: U(VI): K = KsF2I[H+] 2

(1)

where F is the concentration in formality units of H D O P in the organic phase, [H +] is the concentration of hydrogen ion in the aqueous phase and Ks is a constant, characteristic of this system at 22 + 2°C, which is hypothetically numerically equal to K in a system 1.0 F in H D O P and 1.0 F in H +. (In many such systems departures from a straight line in the log K vs. log F of extractant plot is noted in the region 0.5-1-0 F.) Correspondingly, the stoichiometry of extraction of UOz z+ from a chloride phase, in systems employing either benzene or n-heptane as diluent, may be represented as: U O 2+2A+ 2(HY)2o ~ UO~(HY2)2o + 2Ha +

(2)

where the subscripts A and O refer respectively to the mutually equilibrated aqueous and organic phases. (Throughout, the representation of an extracted entity is formulated in terms of material balance only and is not in any sense meant to indicate an established structure. For example, UOz(HY2)2 may be re-formulated as UO2Y2(HY)2). In contrast with U(VI), the M(III) cations are extracted with different extractant dependencies in the two different diluent systems. The extraction of each of the M a+ cations is directly third-power dependent in benzene, Figs. 3 and 4, but I

]

I

1

I

4

5 mm

Ce

2

Eu

2, g

I

E ~-

0

~2

Pm

8' .J -I I

-2

-I e: 5"0

-5

-2 J -2

i

t h -I Log F HDOP

I 0

Fig. 3. Extractant dependency of the extraction of M ~÷ into HDOP, benzene diluent, from HCI. F HCI: Ce, 0.050; Pm and Eu, 0.10; Tm, 0.50.

3904

M A S O N , N. L. S C H O F E R and D. F. PEPPARD

G.W.

J

I

F

'

+2

U02

I

4

Arn

3

0 ~O

2

>(.3

I-

g

..J

-2

0

-~

-3

s: 2.01:5"0

-2 r

~

J -I

-2

Log F

J

-4

I 0

HDOP

Fig. 4. Extractant dependency of the extraction of M 3+ and U O 2 2+ into HDOP, benzene diluent, from HCI. F of r i C h A m and Cm, 0" 10; Y, 0"50; UO22÷, 1"0.

directly 2.5-power dependent in n-heptane, Figs. 7 and 8. So the measured distribution ratio, K, is expressed as: M(III), benzene:

K = KsFa/[H+] ~

(3)

M(III), n-heptane:

K = KsF2"5/[H+] 3

(4)

Correspondingly, the stoichiometries of extraction of M a+ from a chloride phase in systems employing benzene and n-heptane as diluents are represented as: benzene: n-heptane:

MA 3+ + 3(HY)z o ~ M(HY2)3o + 3HA +

(5)

MA3+ + 2.5(HY)2 o ~ MY(HYz)2 o + 3Ha+.

(6)

Using values of K taken from the straight lines as drawn on the hydrogen ion dependency curves of Figs. 1, 2, 5 and 6 and the accompanying extractant concentration data in Expressions (1), (3) and (4), values of K, were calculated for U(VI) and for each of the seven M(III) elements studied. In Table 2, the Ks values in H D O P (diluent) vs. 1.0 F (NaCI + HC1) systems at 22 + 2°C are presented side by side for the two diluents, benzene and n-heptane. For comparison, Ks values in the H D E H P (toluene) vs. 1.0 F (NaCI + HC1) system and the H[DOP] (benzene) vs. 1.0 F (NaCI + HCI) system derived from or taken from published data are included. In comparing the data of Table 2 it should be recalled that: (1) H D O P and H D E H P are representatives of the same general type of extractant, (GO)2PO(OH), a di-ester of orthophosphoric acid, (2) H[DOP] is a representative of the general type of extractant (G)2PO(OH), a phosphinic acid, (3) the G groups are the same

T h e extraction of U ( V I ) and selected M(I 11) cations

3905

Table 2. K, values for U ( V I ) and selected M(II1) elements in H Y (diluent) vs. 1.0 F (NaCI + HCI) s y s t e m s at 22 + 2°C, H Y being H D O P , H D E H P or H [ D O P ] *

Ks M n+ UO22+ C e 3+ P m 3+ Eu ~+ T m 3+ y3+ A m 3+ C m 3+

HDOPf Benzene n - H e p t a n e

HDEHP:~ Toluene

H[DOP]§ Benzene

2 X 103 1 x 10-1 3 × 10-1 2 1 x 103 2 x 10z 1 x 10-1 1 × 10 -1

5 X 102

5X 6x 9x 7x 2x 6x 1x 2x

2X 2x 4x 1x 5x 1x 2x 2x

104 10 10 10z 104 104 10 10

1 x 10-z 1 × 10 3 2 x 10-a

105 10-6 10-5 10 -4 10-1 10-2 10-5 10-.~

* T h e K, values from 1-0 F (NaC1 + HCI) were calculated by the u s e o f the following expressions. All values have been rounded. U(VI), all s y s t e m s : K = K,F~/[H+] z M(III), H D O P (benzene) and H D E H P (toluene): K = KsF3/[H+] a M(III), H D O P (n-heptane) and H [ D O P ] (benzene): K = KsFZ'5/[H +I a. t T h i s work. ~Ref. [16] for U(VI). Ref. [171 for M(III), the calculated Ks values from 1"0 F (NaCIO4 + HCIOa) being converted to approximate Ks values from 1"0 F (NaCI + HC1) by division by 2, based u p o n the chloride complex stability c o n s t a n t data o f Ref. [9] and the established ratio o f Ks values in chloride and perchlorate media of Ref. [ 111. §Ref. [10] for U ( V I ) a n d M(III).

octyl isomer, n-octyl, in H D O P and H[DOP], (4) the G group in H D E H P is an octyl group also but one which is branched, being C4HgCH(C~Hs)CH2-, (5) the stoichiometry of extraction of U(VI) is the same in all systems corresponding to Expression (2), (6) there are two different stoichiometries of extraction of M(III) corresponding to Expressions (5) and (6). Considering only the H D O P data of Table 2 the Ks value for each M p+ is higher in n-heptane than in benzene. But the ratio of Ks values in the two diluents is only 10 for UO22+ while it ranges from 50 to 200 for the M a+ elements. In comparing the H D E H P (toluene) data with the H D O P (benzene) data of Table 2 it should be noted that Ks values in (GO)2PO(OH) in diluent vs. 1-0 F (NaCI + HCI) systems are expected to be slightly higher in systems employing toluene than in those employing benzene, since in otherwise identical systems, Ks values tend to decrease with increasing aromatic character of the diluent [7]. It will be noted that the Ks for each M p÷ included is smaller in the H D E H P system than in the H D O P system. The ratio of Ks ( H D O P ) to Ks ( H D E H P ) is only 4 for UOz 2+ but ranges from 30 for Pm 3+ to 100 for Tm 3+ for the four M 3+ elements 16. D. F. Peppard, G. W. M a s o n , I. H u c h e r and F. A. J. A. Brandao, J. inorg, nucl. Chem. 24, 1387 (1962).

3906

G . W . MASON, N. L. S C H O F E R and D. F. P E P P A R D

i

6

I

Tm

4

3

5

Pm 4 Eu Q;

3 a

t~

g,

Ce

2

o

_J

o~ ..J

-I

I

0

-2 Lines of slope:-~rO

-I

I

J

-3

I

-2

-I

0

Log F H+

Fig. 5. Hydrogen ion dependency of the extraction of M a÷ from 1-0 F (NaCI + HCI) into HDOP, n-heptane diluent. F HDOP: Tm, 0.019; Pm, 0.075; Ce, 0.11 ; Eu, 0.15.

I

r

I

i

r

uo~ 2

Y Am

:5

2

).-

<~

Cm

2

-

I ~"

.J

-I

-2 Lines of slopes: -2.O,

•

- 2 --

-- - 3 J -2

I

I -I Log F H +

I

I O

Fig. 6. H y d r o g e n ion d e p e n d e n c y o f the e x t r a c t i o n o f M s+ and U Oz 2+ f r o m 1.0 F ( N a C I + H C I ) i n t o H D O P , n - h e p t a n e d i l u e n t , F H D O P : UO22÷ and Y , 0-019; C m , 0.075; A m ,

0"15.

The extraction of U(V1) and selected M(IlI) cations 4

i

l

i

I -J Z uo~2

_I >-

--I

2

E

tD

,<

-2

I

o~ .J

o

--5

-I

--4

~

-2

2

I

.

J

-2

0

,

2-5

I

1

-I

--5

I

0

Log F HOOP

Fig. 7. Extractant dependency of the extraction of M 3+ into HDOP, n-heptane diluent, from HCL. F HCt: Ce, 0.25; Eu and Pm, 0-50; Tm, 1.0.

[

I

I

F

r

mm

Eu

3 --

-

2

E z -

(3-

Ce

hi

Pm o=

..J

g -- 0 ...I

I

---I

-I

_

'

I

-2

x

-2

2.5

I

-I

t

I

-3

0

Log F HOOP Fig. 8. Extractant dependency of the extraction of M 3+ and UO22+ into HDOP, n-heptane diluent, from HCL F HCI: Cm and Am, 0-50; UO2z+ and Y, 1.0.

3907

3908

G.W.

MASON, N. L. S C H O F E R and D. F. P E P P A R D

included. Although H D E H P is a somewhat weaker acid than is H D O P (Table l) and would therefore be expected to be associated with lower Ks values for M p+, especially M 3+, probably most of the lowering of Ks values is due to steric factors. If the H D E H P molecule is represented as [(n-C4Hg)(C2Hs)CH-CH2-O-]2PO(OH) it is realized that between them the butyl and ethyl groups on the C atom which is number three atom from the P atom introduce a moderate amount of shielding about the PO(OH) function. The intercomparison of the H D O P (benzene) and H[DOP] (benzene) data, Table 2, shows a drastically lower Ks for each M 3+ in H[DOP] than in H D O P , the ratio of Ks in H D O P to Ks in H[DOP] varying from approximately 3 × l03 for Pm 3+, Eu a÷ and y3+ to approximately 2 × 104 for Ce 3+. Conversely, the Ks for U(VI) is slightly higher in H[DOP] than in H D O P . Since H[DOP] is a weaker acid than H D O P by 2 pK units, Table l, it seems logical to attribute the lower Ks values for M z+ elements primarily to an acidity effect. That is, at a given hydrogen ion concentration in the aqueous phase, for a given concentration of extractant the more weakly acidic the extractant (HY)z the smaller the concentration of HY2- (or equivalent ions). DISCUSSION

In the H D O P (diluent) vs. 1.0 F (NaCI + HCI) systems the diluent effect, as exemplified by n-heptane and benzene, is quite large for the M(III) examples but moderate for U(VI). The ratio of Ks (n-heptane) to Ks (benzene) is in the 50-100 range for M(III) but only 10 for U(VI). In studies of H D H o E P (diluent) vs. 1.0 F (NaCI + HCI) systems ( H D H o E P is bis hexoxyethyl phosphoric acid, (n-C6H13OC2H40)2PO(OH)) the ratio of Ks (cyclohexane) to Ks (benzene) was reported as approximately 500 for Eu(ll I) and Am(III) [6] while the ratio of Ks (n-heptane) to Ks (benzene) for U(VI) was reported as approximately 8 [7]. It is tentatively concluded that extraction of U(VI) by a (GO)2PO(OH) extractant in a diluent is only moderately affected by the aromatic character of a hydrocarbon diluent whereas the extraction of M(III) actinides, lanthanides and yttrium is strongly affected, the extraction in each case being the lower in the more aromatic diluent. This conclusion is presumably applicable to systems involving any aqueous phase containing a strong inorganic acid and its simple salt if none of the anion of the inorganic acid is incorporated in the extracted entity. The stoichiometry of extraction of UO~ 2+ is the same for all four systems of Table 2 and for the two H D H o E P (benzene, or n-heptane) vs. 1.0 F (NaCl + HC1) systems [7] being represented by Expression (2). But the M 3+ cations are extracted according to two different stoichiometries, Expression (5) being applicable to the H D O P (benzene) and H D E H P (toluene)[ 17] systems and Expression (6) to the H D O P (n-heptane), H[DOP] (benzene) and the two H D H o E P (benzene or n-heptane) [6] systems. Although H D O P and H[DOP] contain the same G groups, n-octyl, the Ks values for M(III) are greatly different, the ratio of Ks ( H D O P ) to Ks (H[DOP]) for M(III) exceeding l03 for yttrium, the four lanthanides and the two actinides 17. D. F. Peppard, G . W . Mason, W. J. Driscoll and R. J. Sironen, J. inorg, nucl. Chem. 7, 276 ( 1958).

The extraction of U(VI) and selected M(III) cations

3909

studied. Presumably, a large part of this effect may be attributed to the difference in acid strength of the two compounds, the PKA values in 75 per cent ethanol differing by two pK units, Table 1. (In their study of the variation of pK,4 of (X)(Y)PO(OH) with X and Y, Peppard e t al.[8] reported the PKA values of (cyclo-C6HllO)2PO(OH) and (cyclo-C6H102PO(OH) in 75 per cent ethanol at 22 _+2°C as 3.81 and 5.92 respectively, the ApK being 2.11 +_0.10. It was suggested [8] that the effect of interposing an oxygen atom between G and P to transform a G - P acidic compound to a G - O - P acidic compound is to lower the pK by 1. l0 +_ 0. l0 pK units.) However, innate structural effects must also be important, at least for U(VI), since the Ks for U(VI) is somewhat higher in the system embodying the more weakly acidic extractant, H[DOP]. A study of such structural effects is in progress. In acidity, H D E H P is nearly equivalent to H D O P so the large difference in K, values for M(III) in the two systems embodying H D E H P (toluene) and H D O P (benzene), Table 2, must be largely due to an isomer effect, the two different octyl groups being respectively C4HgCH(C2Hs)CH2- and n-CsHlr-. Presumably this isomer effect is specifically a steric one. If the effect noted is primarily a steric one then double substitution at the number two C position from the P atom should enhance it. For example, still working with G as an octyl group, (GO)2PO(OH) in which G is C 4 H g C ( C H 3 ) 2 C H 2 - should exhibit smaller K~ values for M(III) than does H D E H P . A report of a study of this extractant, bis neo-octyl phosphoric acid, [n-C4HgC(CHz)2CH20]2PO(OH), symbolized as H D N O P , is in preparation. In an extension of the steric study, bis(2,6-dimethyl-4-heptyl) phosphoric acid, [(i-C4Hg)2CHO]~PO(OH), in which G is a highly hindered secondary nonyl group, has been studied as an extractant. A detailed study will be reported shortly. A procedure for separating oxidized americium, presumably Am(VI), from actinides(III) and lanthanides(III) based upon the observation that this extractant discriminates strongly in favor of U(VI) (and, therefore, presumably other actinide(VI) cations) with respect to M(III) lanthanides and actinides has been published [ 18]. 18. G.W. Mason, A. F. Bollmeier and D. F. Peppard, J. inorg, nucl. Chem. 32, 1011 (1970).