As a result of the wide range of oxidation states for some of the actinide ions, as shown in Table

9.4, the control of the oxidation state may play an important role in chemical separations, particularly if plutonium is involved. The oxidation-reduction chemistry is also important to the behavior of actinide elements and compounds if released to the environment.

The selective extraction of plutonium from uranium or fission products depends on proper adjustment of the valence state of plutonium relative to the other ions from which it is to be separated. For instance, in decontaminating plutonium by extraction with TBP, plutonium must be oxidized to the tetravalent state, without bringing cerium into the tetravalent, ceric state. Again, to separate plutonium from uranium and the fission products in the tributyl phosphate extraction process, plutonium must be trivalent and uranium hexavalent.

A typical oxidation-reduction reaction of the type met in processing plutonium is the reduction of Pu4+ to Pu3+ by ferrous ion:

FeJ+ + Pu4* ->■ Fe3+ + Pu3* (9.8)

In dealing with groups of such equilibria it is convenient to break them up into two half-reactions, or oxidation-reduction couples, which indicate the mechanism by which electrons are transferred from the reducing agent to the oxidizing agent.

The two couples for the above reaction are

Pu3+ -*• Pu4+ + є~ (9.9)

and Fe2+ -* Fe3+ + e~ (9.10)

The electrons from each couple may be thought of as exerting an electromotive force, which is the oxidation-reduction potential of the couple. When equilibrium is reached in a solution containing a number of oxidation-reduction couples, the potentials of all couples must be equal, otherwise electrons would be transferred from one couple to another and further reaction would take place. Thus, if we can evaluate the dependence of oxidation-reduction potentials on concentration, we can determine equilibrium concentrations in solution of ions of mixed valence.

The oxidation-reduction potential of a couple E, in volts, is related to the change in free energy AG when 1 g-equiv of electrons is produced:

AG = — SE (9.11)

where 7 is Faraday’s constant, 96,487 J/(Volt-g-equiv). The minus sign is used because of the negative charge on the electron. When the components of a couple are in their standard states of unit activity and electrons are in their standard state, the free-energy change is the standard free-energy change ДG°, and the potential is the standard oxidation potential E°. These are related by

AG°=-3E° (9.12)

The standard state for electrons is customarily taken to be that corresponding to equilibrium in the reaction

^ H2 (?, 1 atm) -*• H*(unit activity) + e ~ (9.13)

whose standard oxidation potential by definition is zero.

where Kjrp is the equilibrium constant for the overall iron-plutonium reaction (9.8). At 25°C, we have for A’:

„ о, RT. [Pu4*] — E?—E?+ — ln^j

Similarly, for the iron couple, _F rc RT. [Fe3*] £F — £F + ? In [pe2+] Because these two potentials must be equal at equilibrium,

RT [Pu*] [Fe3*] _RT

5 [Pu4+] [Fe2+] *

Afp = e3»-M(£F-£p)

For the plutonium-iron case, the values are [Al]

Pu3* -*■ Pu4+ + e~ Ep = —0.9819 V

Fe2+ ->■ Fe3+ + e~ Ep = — 0.7701 V

The equilibrium constant for the reaction (9.8) is

A:fp = e38.93(-0.7701+ 0.9819) = 3gl0

This shows that reduction of plutonium from tetravalent to trivalent can be made substantially complete with only a slight excess of ferrous ion.

A large negative oxidation-reduction potential means that the first member is a strong oxidizing agent. A positive potential means that the first member is a strong reducing agent (and could, in fact, reduce water) and the second member is a very weak oxidizing agent.

The general equation relating the equilibrium constant for a reaction involving the transfer of n electrons and standard oxidation-reduction potentials is

KAB = £ 38.93 (Яд £"1)7	(9.20)
For example, in the reaction
2PU4* + Sn2+ -* 2Pu3+ + Sn4+	(9.21)
the individual potentials are [Al ]
Pu3* -*• Pu4+ + e’ £p = —0.9819 V	(9.22)
Sn2+ -»■ Sn4* + 2e" Eg = —0.154 V	(9.23)

[Sn4*] [Pu*]:

Equation (9.21), as written, involves the transfer of 2 equivalents of electrons from tin to plutonium, so that the equilibrium constant is