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to the end of the nickel anode. A mixture of H2
and HF that passed through this electrode estah-
lished the reaction at the anode as
1/2 H, t F- : HF i e- .
The cathode reaction was the deposition of any of
the cations present, particularly thorium at above its
deposition potential, and hopefully protactinium after
it vias added to the melt.
For the preliminary study of the decomposition po-
tential of ThF4, the vessel was maintained at 6SO’C
while the cathode compartment was swept for 3 hr
with HF and then overnight with H2. The cathode
coinpartrrient was capped both at the inlet and the
outlet, and a stream of HF-i12 was introduced
through the anode tube. The gas flow was adjusted
to approximately 100 cm3/rnin. The mixture was ,
analyzed at the exit tube and was found to contain
3.8 mole “0 HF.
Electrolysis was performed with dry cells as the
source of power. The voltage was tapped off a
slide-wire voltage divider, and simultaneous read-
ings were taken of the voltage and the current.
Four runs were made, starting each time at zero
voltage and measuring the current at 0.2-v inter-
vals. The plot of current vs voltage given in Fig.
12.4 shows a decomposition potent-ial in this cell
of -0.86 V.
The mole fractions of each of the reactants are re-
lated by the equation
-0.86 = Eo
100 7
152
(0.038)
x log t (1)
(0.962)’ ’’ (0.25)’ ’4
from which E, : - 1.09 is calculated for the reaction
2H, i ThF, = 4IIF t Th .
After completing these measurements the flow of
HF was cut off, while 112 flow continued. After 1 hr,
both electrodes were raised out of the melt, and the
H2 flow was allowed to continue until the pot was
cold.
For the study of protactinium removal, the cathode
compartment was opened with as milch. care as pos-
sible to avoid the entrance of moisture, and to that
compartment was added 5 g of LiF-ThF4 which had
been irradiated in a reactor to give a 233Pa activity
4.59 (923)
of about 1 mc. The salt also contained approximately
27 mg of 231Pa . ‘The vessel was closed and installed
in the furnace in a glove box in the High-Alpha Molten-
Salt Laboratory. After HF treatment for 4 hr, followed
by H2 overnight, a filtered salt sample was counted for
233Pa.
The anode gas mixture was adjusted to be approximately
the same as that used in the previous tun except
that a slightly higher HF concentration, 7.5
mole %, was used. The decomposition potential for
ThF4 at this concentration of IIF from Eq. (1) should
be .--0.92 v, so electrolysis was carried out at .--0.90 v.
The cijiient averaged about 10 ma; although it was less
for the first 20 min, after which it rose iather rapidly
to the 10-ma value expected from the earlier experiment.
Assuming 100% Faraday efficiency, the deposition
of 27 mg of protactinium would require 75 rnin
at 10 ma. Electrolysis was carried out for 165 min,
more than twice the minimuiii tiil1e for complete
2.3 104
deposition.
The gas at the end of the rxpeiiment contained only
~
4.1 molc “7, kIF, although the H2 rate wa? unrhanged.
-- OWL 0~F67 448~ There is no indication at what time during the elec-
I
trolysis the decrease in HF occurred. At this lower
Fig. 12.4. Current-Voltage Curve.
concentration of HF the decomposition pot~ntial 15
calculated to be -0.87 v, so that if the decrease in
rat? took place early in the electrolysis period, the
applied potential was too high.
At the conclusion of the run the cathode was raised
out of the melt, thus interrupting electrolysis. A
sample of melt was then taken, and buth cathode and
sample were counted with a multichannel analyzer.
The sample of melt gave essentially the same count
as before electrolysis. A number of gamma peaks were
observed in the cathode spectrum, but none corresponded
to the ‘Pa peaks, which were strong in the