Isotope Effects in Chemical Processes

Boris Kidrich Inst. Sci. 13, 29 (1962). (3) Glueckauf, E., J. Am. Chem. Soc. 81, 5262 (1959). (4) Glueckauf, E., Barker, K. H., Kitt, G. P., Disc. Far...
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4

The Enrichment of Lithium Isotopes by

Extraction Chromatography

D.

A . LEE

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Chemistry Division, O a k Ridge National Laboratory, O a k Ridge, T e n n .

Lithium

isotopes

tography.

were

fractionated

The chemical

ous lithium

hydroxide

exchange

equilibrated

supported

a dodecane

mobile

hydroxide.

concentrated

by

nature

magnitude

an

analysis

made

by

at

phase.

exchange

of the isotopic

species

of the separation

Granulated

solution factor

of

phase. lithium

was 1.003, as 6

"breakthrough" Separation

chromatography ion

aque-

dibenzoyl-

as the stationary

aqueous

in the aqueous

lithium

of d i b e n z o y l m e t h a n e -

stage separation

frontal

by extraction

separations The

was

The single

determined

isotopes

phase

chroma-

used was

in dodecane.

solution

-trioctylphosphine oxide in dodecane The

extraction

with

-methane-trioctylphosphine oxide Teflon

by

reaction

was

Li

lithium

compared

resin involved

of

with

chromatography. determined

the

factor.

' T ' h e r e are t w o isotopes of l i t h i u m ,

6

L i a n d L i , w h i c h occur i n nature 7

w i t h a L i / L i a b u n d a n c e r a t i o o f 0.080. S e v e r a l m e t h o d s h a v e b e e n 6

employed

7

f o r t h e f r a c t i o n a t i o n of these isotopes.

National Laboratory,

6

L i has been

A t the O a k Ridge

enriched to 99.999%

e l e c t r o m a g n e t i c separator ( C a l u t r o n )

purity i n a n

( 1 8 ) . T r a u g e r , et al. (27) h a v e

d e s c r i b e d t h e s e p a r a t i o n o f l i t h i u m isotopes

b y molecular distillation.

E l e c t r o m i g r a t i o n (8) has also b e e n u s e d to fractionate these

isotopes.

O k a m o t o a n d K a k i h a n a (20) h a v e e n r i c h e d l i t h i u m isotopes b y electrom i g r a t i o n i n a c a t i o n exchange m e m b r a n e . processes h a v e b e e n i n v e s t i g a t e d .

Several reversible chemical

L e w i s a n d M a c D o n a l d (17) s t u d i e d

the exchange o f l i t h i u m b e t w e e n l i t h i u m a m a l g a m a n d l i t h i u m c h l o r i d e i n absolute e t h y l a l c o h o l . L a t e r , L . P e r r e t , L . R o z a n d , a n d E . Saito (22) equilibrated lithium amalgam w i t h lithium bromide i n d i m e t h y l forrnam i d e a n d m e a s u r e d a s e p a r a t i o n factor of 1.05. E x t e n s i v e investigations h a v e b e e n m a d e o f i o n exchange t e c h n i q u e s (4, 10, 19, 23, 25, 26). I n 57

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

58

ISOTOPE E F F E C T S IN C H E M I C A L PROCESSES

these systems the m e a s u r e d s e p a r a t i o n factors w e r e g e n e r a l l y v e r y s m a l l (1.000-1.005). T h e influence of v a r i o u s p h y s i c o - c h e m i c a l parameters u p o n the s e p a r a t i o n f a c t o r f o r l i t h i u m isotopes

i n i o n exchange

systems has

been

s t u d i e d . I n c r e a s i n g the c r o s s l i n k i n g of the r e s i n f r o m 2x to 24x i n c r e a s e d the s e p a r a t i o n factor (11)

f r o m 1.0006 to 1.0038.

T h e c h a n g e i n the

s e p a r a t i o n factor w a s a t t r i b u t e d to changes i n h y d r a t i o n o f t h e l i t h i u m species i n the r e s i n phase. G l u e c k a u f (3) u s e d the s o l u t i o n m o l a l i t i e s of

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the r e s i n phase a n d the difference i n the c r y s t a l l o g r a p h i c r a d i i of and

7

L i to c a l c u l a t e the s e p a r a t i o n factors.

6

Li

H i s c a l c u l a t e d values agree

v e r y w e l l w i t h these o b s e r v e d s e p a r a t i o n factors. T h e effect of t e m p e r a t u r e u p o n the separation factor was s t u d i e d (12).

T h e s e p a r a t i o n factor decreased as t h e t e m p e r a t u r e i n c r e a s e d . T h e

exothermic 6

Li(aq.) +

e n t h a l p y of 7

exchange

(—AH°)

L i ( r e s . ) ^± L i ( a q . ) + 7

6

for

the isotopic

reaction

L i ( r e s . ) w a s 2.26 c a l . / m o l e , a n d

the e n t r o p y change w a s —1.81 X 10" c a l . / m o l e degree at 2 5 ° C . 3

T h e influence of the h y d r a t i n g t e n d e n c y of cations co-sorbed

with

l i t h i u m isotopes o n a n i o n exchange c o l u m n w a s i n v e s t i g a t e d i n a series of experiments s u m m a r i z e d i n T a b l e I. A s the heat of h y d r a t i o n of the co-sorbed c a t i o n i n c r e a s e d , the isotopic s e p a r a t i o n factor i n c r e a s e d

(13).

T h e n a t u r e of the a n i o n i n the s o l u t i o n phase h a d v e r y little effect u p o n t h e s e p a r a t i o n factor.

H o w e v e r , for

systems

i n v o l v i n g complexes

of

l i t h i u m w i t h e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d , there was a r e v e r s a l of the isotope effect.

T h a t is, L i c o n c e n t r a t e d i n the r e s i n phase i n s t e a d of 7

the aqueous phase as it u s u a l l y d i d i n i o n exchange r e s i n systems. Table I.

Variation of the Separation Factor with the N a t u r e of the Co-sorbed and E l u t i n g Cation (13) Cation NH K NH OH H Ca Cu Cr3 Al 4

Separation

+

+

3

+

+

2 +

2 + +

3 +

a

Factor

0

1.0023 1.0029 1.0033 1.0037 1.0037 1.0045 1.0053 1.0049

( L i / L i ) resin/( Li/ Li) aqueous. 6

7

6

7

K n y a z e v a n d S k l e n s k a y a ( 9 ) c a l c u l a t e d the i s o t o p i c s e p a r a t i o n f a c tors for exchange reactions b e t w e e n l i t h i u m complexes of n i t r i l o t r i a c e t i c acid, ethylenediaminetetraacetic acid a n d aminobarbituric-A^N'-diacetic a c i d , a n d aqueous l i t h i u m ions. T h e s e reactions w e r e p o s t u l a t e d for a single phase system; therefore, the separations cannot b e o b s e r v e d e x p e r i -

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

4.

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mentally.

Extraction

59

Chromatography

A s the p K of the l i t h i u m c o m p l e x i n c r e a s e d , the c a l c u l a t e d

separation factor increased. T h e influence of eluent c o n c e n t r a t i o n i n i o n exchange

systems o n

the s e p a r a t i o n factor for l i t h i u m isotopes w a s e x a m i n e d b y s e v e r a l w o r k ers (2, 6, 21).

L e e a n d D r u r y (14)

f o u n d t h a t the s e p a r a t i o n factors

decreased w i t h i n c r e a s i n g eluent c o n c e n t r a t i o n w h e n D o w e x - 5 0 a n d Zeo K a r b resins w e r e used.

C o m p a r a b l e results w e r e o b t a i n e d w i t h either

c h l o r i d e or acetate eluents. Downloaded by UNIV OF TEXAS AT DALLAS on July 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0089.ch004

T h e s e p a r a t i o n factors for h t h i u m isotopes i n i o n exchange

systems

w e r e too s m a l l to b e p r a c t i c a l . T o increase the s e p a r a t i o n f a c t o r signific a n t l y , l i t h i u m species i n v o l v i n g other types of l i t h i u m c o m p o u n d s w o u l d h a v e to b e c o n s i d e r e d for e q u i l i b r a t i o n . T h e s e p a r a t i o n of isotopes

by

c h e m i c a l exchange d e p e n d s u p o n the fact t h a t the isotopic species i n t h e t w o phases are d i s s i m i l a r w i t h respect to c h e m i c a l b o n d i n g . T h a t is, i n one phase the i s o t o p i c species s h o u l d be s t r o n g l y b o n d e d , a n d i n t h e other phase the species s h o u l d b e w e a k l y b o n d e d .

T h e c h e m i s t r y of

l i t h i u m l i m i t s the v a r i e t y of c h e m i c a l species a v a i l a b l e for isotope s e p a r a t i o n b y c h e m i c a l exchange.

I n aqueous s o l u t i o n , l i t h i u m exists almost

e x c l u s i v e l y as a h y d r a t e d i o n . T h e c o n c e n t r a t i o n of l i t h i u m i n the r e s i n phase of a n i o n exchange system is s o m e w h a t greater t h a n the c o n c e n t r a t i o n of l i t h i u m ions i n the exterior s o l u t i o n . A l t h o u g h there is association b e t w e e n the l i t h i u m ions a n d the f u n c t i o n a l groups of the r e s i n m a t r i x , s t i l l the l i t h i u m species i n v o l v e d is a h y d r a t e d h t h i u m i o n . T h i s s i m i l a r i t y of b o n d i n g o f the l i t h i u m species i n e a c h phase accounts for t h e s m a l l s e p a r a t i o n factors i n i o n exchange r e s i n systems.

Organolithium com-

p o u n d s , w h i c h are u s e d extensively i n c e r t a i n o r g a n i c syntheses,

are

b o n d e d differently. H o w e v e r , t h e y are v e r y r e a c t i v e c o m p o u n d s w h i c h are u n s t a b l e t o w a r d a i r a n d m o i s t u r e , a n d a n y use of o r g a n o l i t h i u m c o m p o u n d s i n isotope s e p a r a t i o n systems appears to b e i m p r a c t i c a l . R e c e n t l y , i t w a s f o u n d that l i t h i u m f o r m e d extractable

complexes

w i t h m i x t u r e s of d i b e n z o y l m e t h a n e ( H D B M )

and tri-n-octylphosphine

o x i d e ( T O P O ) or t r i b u t y l p h o s p h a t e

T h e /?-diketone a n d the

(TBP).

p h o s p h i n e oxide or p h o s p h a t e i n a h y d r o c a r b o n d i l u e n t w o r k e d synerg i s t i c a l l y to extract l i t h i u m f r o m b a s i c aqueous

solutions (15).

From

a l k a l i n e solutions of l i t h i u m salts, or f r o m m i x t u r e s of l i t h i u m a n d s o d i u m , or h t h i u m a n d a m m o n i u m salts, the extracted l i t h i u m LiDBM

complex

was

• 2 T O P O . F r o m a l k a l i n e solutions of l i t h i u m salt m i x e d w i t h

p o t a s s i u m , r u b i d i u m , or c e s i u m salts, the extractable l i t h i u m species w a s a dimer, L i ( D B M ) 2

2

• 2HDBM

• 4 T O P O (16).

T h e e n o l i c f o r m of

d i b e n z o y l m e t h a n e is a v e r y w e a k a c i d w h i c h , w h e n n e u t r a l i z e d , w i l l f o r m a chelate w i t h l i t h i u m i o n . T B P a n d T O P O are n e u t r a l a d d u c t - f o r m i n g donors w h i c h d i s p l a c e the w a t e r m o l e c u l e s a r o u n d the l i t h i u m i n the chelate, thus m a k i n g the c o m p l e x m o r e s o l u b l e i n the w a t e r - i m m i s c i b l e

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

60

ISOTOPE E F F E C T S IN C H E M I C A L PROCESSES

o r g a n i c phase.

I n the case of the d i m e r , excess H D B M

molecules

also

participate i n adduct formation. A l t h o u g h b o t h the l i t h i u m a t o m i n this n e w c o m p l e x a n d the l i t h i u m a t o m i n the h y d r a t e d l i t h i u m i o n are p r o b a b l y t e t r a h e d r a l l y c o o r d i n a t e d to o x y g e n atoms, s t i l l the force constants of the l i t h i u m - o x y g e n b o n d s are u n d o u b t e d l y i n f l u e n c e d differently i n the t w o species b e c a u s e of the structure of the l i g a n d s a n d the e n v i r o n m e n t of the p a r t i c u l a r solvent. T h e r e f o r e , i t w a s of interest to investigate the p o s s i b i l i t y of s e p a r a t i n g Downloaded by UNIV OF TEXAS AT DALLAS on July 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0089.ch004

l i t h i u m isotopes b y a n exchange r e a c t i o n i n w h i c h L i D B M • 2 T O P O i n a w a t e r - i m m i s c i b l e o r g a n i c solvent w a s e q u i l i b r a t e d w i t h h y d r a t e d l i t h i u m ions i n a basic aqueous

solution.

F r o m the s t a n d p o i n t of d i s s i m i l a r

species i n t h e t w o phases, this system appears to h a v e advantages the aqueous i o n exchange

r e s i n system for h t h i u m isotope

a n d a l a r g e r s e p a r a t i o n factor s h o u l d b e expected.

over

separation,

T o determine whether

or not this solvent e x t r a c t i o n process w a s feasible w i t h respect to exchange rates a n d m a g n i t u d e of the single stage s e p a r a t i o n factor, the process w a s evaluated

by

a

technique

new

to

isotope

separation,

extraction

chromatography. Extraction chromatography

( r e v e r s e d phase p a r t i t i o n c h r o m a t o g r a -

p h y ) has b e e n u s e d i n a n a l y t i c a l a n d b i o c h e m i s t r y to effect c h e m i c a l separations.

It is a m e t h o d w h i c h c o m b i n e s

the s i m p l i c i t y of i o n ex-

change a n d the s e l e c t i v i t y of solvent extraction.

I o n exchange

theory

m a y b e u s e d to c a l c u l a t e the n u m b e r of t h e o r e t i c a l plates i n the c o l u m n a n d the e n r i c h m e n t coefficient.

E x t r a c t i o n c h r o m a t o g r a p h y as a s e p a r a -

t i o n m e t h o d has b e e n r e c e n t l y r e v i e w e d b y C e r r a i (1)

and Katykhin

(7).

T h e p r o c e d u r e for the e n r i c h m e n t of l i t h i u m isotopes b y e x t r a c t i o n chromatography

w a s as f o l l o w s .

A d o d e c a n e or p-xylene

s o l u t i o n of

H D B M - 2 T O P O w a s a b s o r b e d onto a n i n e r t s u p p o r t of g r a n u l a t e d T e f l o n . T h i s w a s the stationary p h a s e i n a c o l u m n 120 c m . l o n g X 2.5 c m . I . D . A n aqueous l i t h i u m h y d r o x i d e s o l u t i o n c o n t a i n i n g the m i x t u r e of isotopes to b e separated was passed t h r o u g h the c o l u m n .

A t e a c h p l a t e i n the

c o l u m n , isotopic e q u i l i b r i u m was e s t a b l i s h e d b e t w e e n the l i t h i u m species i n the t w o phases.

M u l t i p l i c a t i o n of the e n r i c h m e n t o c c u r r e d as l i t h i u m

p r o c e e d e d d o w n the c o l u m n .

A t the " b r e a k t h r o u g h " the isotopes

were

p a r t i a l l y f r a c t i o n a t e d a l o n g the profile of the e l u t i o n c u r v e . T h e c o n c e n t r a t i o n of l i t h i u m i n the e l u t r i a n t samples w a s d e t e r m i n e d b y spectrophotometry.

t h e n c a l c u l a t e d f r o m a p l o t of the e l u t i o n c u r v e . w a s also e x a m i n e d .

flame

T h e n u m b e r of t h e o r e t i c a l plates i n the c o l u m n w a s T h e reverse s i t u a t i o n

T h a t is, a c o l u m n l o a d e d w i t h h t h i u m d i b e n z o y l -

m e t h a n e - t r i o c t y l p h o s p h i n e oxide c o m p l e x w a s s t r i p p e d f r o m the c o l u m n w i t h d i l u t e h y d r o c h l o r i c a c i d a n d the i s o t o p i c

s e p a r a t i o n factor

d e t e r m i n e d . I n this case, t h e isotopes w e r e e l u t e d i n reverse order.

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

was

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T h e c a l c u l a t i o n of the n u m b e r of plates i n the c o l u m n w a s a c c o m plished by

u s i n g the t h e o r e t i c a l treatment of

through" chromatography c o l u m n (N)

(5).

was given b y N =

W ' / ( V

v o l u m e at the p o i n t of i n f l e c t i o n a n d V c e n t r a t i o n c', defined b y c ' / c °

G l u e c k a u f for

"break-

T h e n u m b e r of t h e o r e t i c a l plates i n t h e

=

— V ' ) , w h e r e V is t h e e l u t i o n 2

is the e l u t i o n v o l u m e at t h e c o n -

0.1587. G l u e c k a u f gave a n a l t e r n a t i v e

e q u a t i o n for the n u m b e r of plates i n the c o l u m n ( F i g u r e 1 ) :

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N =

2TT(V/AV)2

A l o n g t h e g r a d i e n t of the " b r e a k t h r o u g h " c u r v e , the l i t h i u m samples w e r e i s o t o p i c a l l y assayed b y mass spectrometry. m e n t factor was d e t e r m i n e d as f o l l o w s

e

~

a

"

1

~~ *

i=l

T h e single stage e n r i c h -

(24):

cm

V^o

where a

=

Vi = d

the L i - L i s e p a r a t i o n f a c t o r 6

7

the v o l u m e of the i - t h f r a c t i o n c o l l e c t e d

(ml.)

— the m o l a r c o n c e n t r a t i o n of L i i n the i - t h f r a c t i o n

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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ISOTOPE E F F E C T S I N C H E M I C A L PROCESSES

Ri =

the r a t i o of L i t o L i i n the i - t h f r a c t i o n

R =

the r a t i o o f L i to L i i n the o r i g i n a l l i t h i u m c o m p o u n d

0

Q

=

6

7

6

7

the t o t a l c a p a c i t y of the exchange b e d i n m i l l i e q u i v a l e n t s .

It w a s f o u n d t h a t L i c o n c e n t r a t e d i n the aqueous phase, as s h o w n 6

i n F i g u r e 2. T h i s w a s a r e v e r s a l f r o m the u s u a l results o b t a i n e d i n i o n

1

1

r

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1—'—I—'—I— —I— —I— ~ 8.00 7.00 6.005.00-

5 o

4.003.00 2.001.00

10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

500 SAMPLE

Figure 2. Lithium chromatography.

10 2 0 3 0

600 NUMBER

isotope enrichment by extraction Li DBM • 2 TOPO vs. LiOH

exchange systems. I n the latter systems, L i u s u a l l y c o n c e n t r a t e d i n t h e 6

r e s i n phase. T h e single stage s e p a r a t i o n factor w a s 1.003, a v a l u e c o m p a r a b l e t o the s e p a r a t i o n f a c t o r t y p i c a l l y f o u n d f o r i o n exchange r e s i n systems.

I t w a s u n f o r t u n a t e that t h e significant c h a n g e i n t h e isotope

effect r e s u l t e d i n a n isotope r e v e r s a l i n s t e a d o f a n a d d i t i o n t o t h e effect f o u n d f o r aqueous i o n exchange systems. I f complexes of l i t h i u m c o u l d be f o u n d i n w h i c h l i t h i u m w a s c o o r d i n a t e d t o atoms other t h a n o x y g e n i n one phase, a l a r g e r isotope effect m i g h t b e expected.

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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Conclusion T h e present w o r k has b e e n the first a p p l i c a t i o n of e x t r a c t i o n c h r o m a t o g r a p h y to isotope separation. T h i s t e c h n i q u e p r o v e d t o b e a s i m p l e a n d convenient

laboratory-scale

method

for studying lithium

isotope

s e p a r a t i o n b y l i q u i d - l i q u i d extraction. T h e m e t h o d m a y h a v e e v e n m o r e i n t e r e s t i n g possibilities f o r isotopes of elements w h i c h f o r m a v a r i e t y of complexes w h i c h are s o l u b l e i n o r g a n i c solvents.

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T h e prospect o f separating l i t h i u m isotopes o n a l a r g e scale b y the e x t r a c t i o n o f l i t h i u m f r o m aqueous solutions is n o t v e r y p r o m i s i n g . I n the system w e h a v e s t u d i e d , reflux c o u l d b e a c c o m p l i s h e d b y a n acid-base m e c h a n i s m ; h o w e v e r , because o f the s m a l l s e p a r a t i o n factor, a n e x t r e m e l y large reflux r a t i o w o u l d b e r e q u i r e d . T h i s w o u l d necessitate a v e r y large p l a n t u s i n g enormous q u a n t i t i e s o f a c i d a n d base, a n d t h e cost w o u l d b e excessive. F r o m a n a c a d e m i c s t a n d p o i n t , the s e p a r a t i o n of isotopes b y extract i o n c h r o m a t o g r a p h y presents a u s e f u l t o o l f o r s t u d y i n g isotopic

species

i n s o l u t i o n . T h e n a t u r e of the species m a y sometimes b e e l u c i d a t e d b y d e t e r m i n i n g s m a l l v a r i a t i o n s i n the single stage s e p a r a t i o n factor i f isot o p e s e p a r a t i o n is p r o m o t e d

b y certain physico-chemical

parameters.

T h e s e parameters m a y i n c l u d e the p K o f t h e c o m p l e x , p H of t h e aqueous s o l u t i o n , t e m p e r a t u r e , c o n c e n t r a t i o n , n a t u r e o f t h e o r g a n i c solvent, a n i o n c o m p l e x i n g i n t h e aqueous phase, a n d other factors d e p e n d i n g o n t h e c h e m i s t r y o f the p a r t i c u l a r isotope.

Literature (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

Cited

Cerrai, E., Chromatog. Rev. 6, 129 (1964). Ciric, M . M . , Pupezin, J. D . , Bull. Boris Kidrich Inst. Sci. 13, 29 (1962). Glueckauf, E., J. Am. Chem. Soc. 81, 5262 (1959). Glueckauf, E . , Barker, K. H . , Kitt, G. P., Disc. Faraday Soc. 7, 199 (1949). Glueckauf, E . , "Isotope Separation by Chromatographic Methods," AERE-R2896, Atomic Energy Res. Estab., Harwell, Berkshire, 1959. Katal'nikov, S. G., Revin, V. A., Andreev, B. M . , Minev, V. A., Atomnaya Energiya 11, 528 (1961). Katykhin, G. S., Zh. Analit. Khim. 20, 615 (1965). Klemm, A., J. Naturforsch 6a, 512 (1951). Knyazev, D . A., Sklenskaya, E . V., Russ. J. Phys. Chem. 37, 1134 (1963). Lee, D . A., J. Chem. Eng. 6, 565 (1961). Lee, D. A., Begun, G. M . , J. Am. Chem. Soc. 81, 2332 (1959). Lee, D. A., J. Phys. Chem. 64, 187 (1960). Lee, D . A., J. Am. Chem. Soc. 83, 180 (1961). Lee, D . A., Drury, J. S., J. Inorg. Nucl. Chem. 27,1405 (1965). Lee, D . A., J. Chromatog. 26, 342 (1967). Lee, D . A., Taylor, W. L . (unpublished data, 1966). Lewis, G. N., MacDonald, R. T., J. Am. Chem. Soc. 58, 2519 (1936).

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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64

ISOTOPE E F F E C T S IN C H E M I C A L

PROCESSES

(1 8) Love, L . O., Bell, W . A., Jr., Prater, W . K., Banic, G . M . , Cameron, A. E . , "Proceedings of the International Symposium on Isotope Separation," J. Kistemaker, J. Bigeleisen, A. O. C . Nier, Eds., North-Holland Publishing Co., Amsterdam, 1958. (19) Menes, F . , Saito, E . , Roth, E . , "Proceedings of the International Symposium on Isotope Separation," J. Kistemaker, J. Bigeleisen, A. O. C . Nier, Eds., North-Holland Publishing Co., Amsterdam, 1958. (20) Okamoto, M . , Kakihana, H . , Nippon Kagaku Zasshi 88, 313 (1967). (21) Panchenkov, G . M . , Kuznetsova, E . M . , Kaznadzei, O. N . , Atomnaya Energiya 7, 556 (1959). (22) Perret, L., Rozand, L., Saito, E . , "Second International Conference on the Peaceful Uses of Atomic Energy," Vol. 4, 595, United Nations, New York, 1958. (23) Powell, J. E . , J. Inorg. Nucl. Chem. 24, 183 (1962). (24) Spedding, F . H . , Powell, J. E., Svec, H . J., J. Am. Chem. Soc. 77, 6125 (1955). (25) Taylor, T. I., Urey, H . C., J. Chem. Phys. 5, 597 (1937). (26) Ibid., 6, 429 (1938). (27) Trauger, D . B., Keyes, J. J., Jr., Kuipers, G. A., Lang, D . M . , "Proceedings of the International Symposium on Isotope Separation," J. Kistemaker, J. Bigeleisen, A. O. C. Nier, Eds., North-Holland Publishing Co., Amsterdam, 1958. RECEIVED August 28, 1967. Research sponsored by the U . S. Atomic Energy Commission under contract with the Union Carbide Corporation.

Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.