THE RADIOCHEMISTRY OF PLUTONIUM - Sciencemadness.org

TABLE IY-5. Free Energy of Formation of Actinide Ions (kcal/mole)
M +++
M +t++
MO; MO~
u -124.4 -138.4 -237.6 -236,4
Np -128.4 -124.9 -221.0 -194,5
Pu -140.5 -118.2 -204.9 -183.5
Am -160.5 -110.2 (-194.5) -156.7
Thus the most stable +3 ion in this series is Am+ , wliie the most stable +6 ion is
uo~. Pu is the first member of this series in which the tripositive state is stable
enough in aqueous solution to be useful in separation chemistry.
This fact is of supreme importance in the radiochemistry of Pu, since Pu can be
selectively maintained in either the +3 or +4 oxidation states in separation schemes.
Advantage is taken of this ability in many of the collected procedures in section VI.
The oxidation-reduction behavior of Pu is complicated by several factors:
(1) f/0~ disproportionates into PU+4 and PuO~ and under certain conditions PU+3,
Pu , Puo;,
++
and PU02 can all exist in equilibrium;
+4
Pu. m a small, highly
charged ion and therefore undergoes extensive hydrcdysis at low acidity and forms
many stable complex ions. This tendency is a dominant feature of Pu(IV) chemistry.
Pu(IV) forms a long-chain compound or polymer by hydrolytic reactions. Pu(IV)
polymer is one of the more unpleasant aspects of Pu chemistry from the standpoint of
the radiochemist.
C. 2 Oxidation-Reduction Reactions
Table IV-6 lists reagents and conditions to effect changes in oxidation state for
Pu ions. It must be emphasized that a relatively small change in conditions may
produce a large change in the equilibrium, and that some of the oxidation-reduction
reactions may proceed beyond those listed. Therefore, the listed reactions should
be taken as a guide only.
In general, the change in oxidation state between +3 and +4 is rapid, since only
++
one electron is invalved in this change. Similarly PuO~ and PU02 changes are rapid.
Changes in the oxidation state of Pu which involve the making or breaking of a Pu -O
bond are usually slower. However, Hindman, 184 m a general review of the kinetics of
actinide oxidation-reduction reactions, gives examples of reactions involving M-O
bonds which proceed as rapi~y as some involving only electron transfer. Newton and
Rabideau302 have slso reviewed the kinetics of actinide oxidation-reduction reactions.
C. 3 Disproportionation Reactions
The potentials of the various Pu couples are such that appreciable quantities of
several oxidation states may exist in equilibrium. Pu(IV) and Pu(V) are both unstable
with respect to disproportionation into higher and lower oxidation states in weakly
acid, uncompleted media. There are several important eqtiibria which must be
considered for an understanding of Pu oxidation-reduction chemistry,