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the standard potential for reduction to pure metal.
Thus, is the potential corresponding to the extraction
process and does not require the simultaneous
use of metal activity coefficients that are
far from 1 at infinite dilution.
It. should perhaps be noted that the electromotive
series is thermodynamically equivalent to other
means of expressing the free energy of the extraction
process, such as equilibrium constants or decontaminaiion
factors. It is used because it permits
ranking the metals in an order which djrectly
indicates their relative ease of extraction regardless
of their ionic valence and because it has
proven useful in the experiments where a beryllium
reference electrode is used to monitor the reduction
process.
From data previously reported the series has
been constructed as shown in Table 12.2. The
numbers in parentheses after the rare earths are
the potentials calculated by using the experimental
C'
fractional valences. For beryllium, (.- o, the potential
for reduction to the pure metal, is shown. All
of these except ~e'+ are calculated relative to the
Lit value.
Although better measurements may change these
values somewhat, they indicate the relative ease
of extraction. 'The numbers in parentheses are
probably a better approximation to the true values
despite the peculiar valences. All elements up
through europium can be extracted completely k-
fore metallic beryllium begins to form; this forma-tion
of He" represents the ultimate limit of the
process. An order of magnitude change in the ratio
[(mole fraction in metal)/(moIa fraction in saltjl
corresponds to 0.057 v for divalent and 0.058 v ior
trivalent species.
Table 12.2. -?; at 600°C (volts vs H2-HF : 0.00)
t
Li
BaZ+
EU 2 '-
Nd3+
+ SmZ
ce3+
La3i
1.92
1,81 CF o)
1.79
1.62 (1 61)
1.58 (1,511
1.58 (1.49)
1.57 (1.45)
1.52 (1.47)
~
155
We have begun a reinvestigation of fission
product extraction. In this study we will measure
fission product distribution as a ftirlction both of
lithium concentration nnd of temperature with
(hopefully) improved accuracy. Preliminary results
of the first of this series, using cerium, are shown
in Fig. 12.5. A failure ended the experiment before
good concentration data could be gotten, but those
wc: did get: indicate a valence close to 3, not 2.3 as
before. The apparent minimum of ?; (Ce) at 700°C
is not explained arid is rather hard to believe. For
lithium, f''; was calculated from the measured
lithium concentration and the potential between the
bismuth pool and beryllium metal electrode, for
which the temperature coefficient is known. The
"MSR Program Semianri. Progr. Kept. Feh 28. 1966,
ORNL-3936, p. 141.
11
C;. F, Haes, Jr~, Keactor Chern. Div. Atin. F'rogr.
Rept. Dec. 31, 196.5, ORNL-391.3, p, 20.
2.4
2.0
1.9
i .a
-
>
-
. 4.7
Q
4.6
4.5
.n,
ORNL. DWG 67-14825
__ . . ._
7.-
+.3
400 500 600 700 800 900
T ("GI
Fig. 12.5. b:i for Lithium and Cerium.