APPENDIX A Status of Plutonium Chemistry—Round Table Discussion Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch028
R. A. PENNEMAN Los Alamos National Laboratory, University of California, Los Alamos, NM 87545
Remarks by R. A. Penneman, Round Table Discussion Leader: "This program on the 40th year of plutonium chemistry reminds us anew that plutonium has an exceedingly complicated chemistry. The near identity of its oxidation/reduction couples makes a l l oxidation states accessible and provides a wealth of chemistry not exceeded in any other element. "Indeed, plutonium marks an important turning point along the actinide series of elements, both in its chemistry and in the metallic state. The metallic state of plutonium is the most complicated known. Metals beyond plutonium are abruptly simpler. Also, the chemistry changes dramatically for elements beyond plutonium. For plutonium, valences 3 - 7 are known, with oxidation states of 3 through 6 being common. For the elements americium and curium, just beyond plutonium, their tetravalent states are such powerful oxidants [~1.5 to 2 volts higher than that for Pu(IV)] that their tetravalent ions are essentially missing from aqueous chemistry. Without powerful oxidative intervention accompanied by complexation, this barrier of the tetra-valent states beyond plutonium is not surpassed. This denies access to the pentavalent and hexavalent states of americium in ground waters, for example. In contrast, the higher valent states of plutonium can be prominent in aqueous solution, especially at ground water pH's frequently encountered. "In this symposium we have heard papers describing process chemistry, new complexing extractants, pyrochemical processing methods, spectroscopy, and photochemistry. The groundwork for these approaches is of earlier origin just now being pursued, however elegantly. Further, we saw several papers on plutonium hydrolytic behavior, leaching and speciation at midrange pH's. A pattern seems obvious and indicates to me the consequences of diminished funding for investigations of basic plutonium chemistry and funding focused on certain problem areas. "Where do we stand? Many needed complexity constants are unknown or poorly known. Questions of solubility under a variety of circumstances remain largely unanswered. Prof. Fuger summed 0097-6156/83/0216-0453$06.00/0 © 1983 American Chemical Society
Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch028
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PLUTONIUM CHEMISTRY
up h i s p r e s e n t a t i o n w i t h t h e c o n c l u s i o n t h a t p l u t o n i u m thermochem i s t r y i s n o t i n good o v e r a l l s h a p e . The f i e l d o f organometallic chemistry i s l a r g e l y untouched. I t i s o b v i o u s t h a t much r e m a i n s t o be done." S p e a k e r s and a u d i e n c e members were e n c o u r a g e d t o submit w r i t t e n v e r s i o n s of t h e i r p a r t i c i p a t i o n . E d i t e d v e r s i o n s f o l l o w . T h e r e was c o n s i d e r a b l e c o r r i d o r d i s c u s s i o n a f t e r a p r e s e n t a t i o n by Dr. G. L. S i l v e r , who " g o t t h e a t t e n t i o n o f t h e a u d i e n c e " by t a k i n g p l u t o n i u m c h e m i s t s t o t a s k c o n c e r n i n g ( a c c o r d i n g t o him) t h e i r e r r o n e o u s use o f a ( t o o ) s i m p l i f i e d summary e q u a t i o n i n v o l v i n g t h e d i s p r o p o r t i o n a t i o n o f P u ( I V ) and t h e i r l a c k o f a p p r e c i a t i o n of alpha c o e f f i c i e n t s . Dr. S i l v e r s t r e s s e d t h e use o f a l p h a coefficients and equations which e x p l i c i t l y involve acidity, h y d r o l y s i s o f P u ( I V ) , and e s p e c i a l l y t h e p r e s e n c e o f P u ( V ) , w h i c h i s too f r e q u e n t l y ignored. To illustrate the points of contention, accounts of Dr. S i l v e r ' s arguments and a r e p l y by Dr. B e l l (edited and softened) are given. Dr. G. L. S i l v e r , Monsanto R e s e a r c h C o r p . , Mound, M i a m i s b u r g , OH 45342: "In plutonium chemistry I f i n d c e r t a i n d e f i c i e n c i e s which impede t h e o r d e r l y d e v e l o p m e n t o f t h i s s c i e n c e . One of these d e f i c i e n c i e s i s c o n t i n u e d use o f t h e e q u a t i o n : 3Pu(IV) = 2 P u ( I I I )
+ Pu(VI)
,
(1)
which i s supposed to r e p r e s e n t the d i s p r o p o r t i o n a t i o n of i n i t i a l l y pure t e t r a v a l e n t plutonium. T h i s e q u a t i o n i s wrong f o r t h e p u r p o s e b e c a u s e i t v i o l a t e s mass and c h a r g e c o n s e r v a t i o n . I suggest t h e f o l l o w i n g e q u a t i o n as a r e p l a c e m e n t f o r Eq. ( 1 ) : (2+3M)Pu(IV) = ( l + 2 M ) P u ( I I I ) + Pu(V)
+ MPu(VI)
(2)
i n w h i c h M i s t h e e q u i l i b r i u m r a t i o [ P u ( V I ) ] / [ P u ( V ) ] , and i s f o u n d as t h e s o l e p o s i t i v e r o o t o f a c u b i c e q u a t i o n . T h i s c u b i c equa t i o n contains c o e f f i c i e n t s which are f u n c t i o n s of the a c i d i t y . The c o n s e q u e n c e i s t h a t Eq. (2) shows t h a t b o t h t h e e x t e n t o f d i s p r o p o r t i o n a t i o n and t h e s t o i c h i o m e t r y o f d i s p r o p o r t i o n a t i o n a r e d e p e n d e n t upon t h e a c i d i t y , s o m e t h i n g l a c k i n g i n Eq. ( 1 ) . But h a v i n g s a i d t h i s , I d i s a v o w Eq. (2) as i n a d e q u a t e f o r t h e g e n e r a l d e s c r i p t i o n of plutonium i n s o l u t i o n . B o t h Eqs. (1) and (2) p l a c e t o o much e m p h a s i s on o x i d a t i o n number Ν = 4.00. We need a method w h i c h e x p l i c i t l y d e f i n e s t h e o x i d a t i o n s t a t e d i s t r i b u t i o n f o r any v a l u e o f N, f o r any a c i d i t y , and f o r any d e g r e e o f s e q u e s t r a t i o n o f t h e f o u r o x i d a t i o n s t a t e s . These a d v a n t a g e s a r e accom p l i s h e d w i t h t h e a f o r e m e n t i o n e d c u b i c e q u a t i o n . I t i s f o u n d as Eq. (3) i n R a d i o c h i m i c a A c t a 21, 54 (1974) and i s u s e d w i t h Eqs. ( 7 ) - ( 1 0 ) t h e r e i n f o r d e t e r m i n i n g f r a c t i o n a l d i s t r i b u t i o n s o f o x i d a t i o n s t a t e s f o r s e l e c t e d v a l u e s o f Ν (3
