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ORNL P.
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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
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This paper was submitted for publication
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LEGAL NOTICE:
This report was prepared as an account of
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A. Makes any warranty or representa-
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B. Assumes any liabilities with respect
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ORNU-P-814
JANG. HOS CONF-650502-3
STIES
AN ALL-METAL CELL TECHNIQUE FOR THE MEASUREMENT OF THE Emf OF MOLATEN
METAL-METAL HALIDE SOLUTIONS CORROSIVE TO CERAMICS

un unintergrund
www101 MATRA
Harry R. Bronstein
Chemistry Division, Oak Ridge National Laboratoriostora
Oak Ridge, Tennessee
mote US There may be
DOLDURIETTEIT and
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TRARE
Electromotive force measurements of concentration cells with molten
metal-metal halide solutions can yield information as to the species pres-
ent in these systems. Its obvious value for these systems is that the
number of electrons involved in the potential-determining process is ob-
tained directly. For instance, for the rare earth systems, when corre-
lated with phase equilibria and electrical conductivity data, which have
shown the trend of increasing stability of the divalent species and de-
creasing electronic conductivity in going from La-Iax, to the Na-Nax,
systems, respectively, a better understanding of these solutions should
be possible. Also, electrochemical studies of the saturated molten metal-
metal halide electrode versus the appropriate halogen electrode as a
function of temperature permits the direct evaluation of the free energy,
heat, and entropy of formation of the species present. Of the few emf
measurements on the rare earth-rare earth halide solutions (Ce-Ceclz and
La-LaCly), the results have been invalidated by the reaction of these
solutions with the ceramic container material. There is no known ceramic
material which is immune to attack by these molten rare earth metal-rare
earth halide solutions. If emf measurements are to be made on these solu-
tions, it is quite apparent that a technique must be employed which elimi-
nates the use of ceramic containers and insulators. Such a technique will
be described which permits eaf measurements to be made in an all-metal
cell.

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APPROVED FOR PUBLIC RELEASE |
APPROVED FOR PUBLIC RELEASE
1
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First, let us consider the familiar saturated electrodes of aqueous
chemistry, the Ag-HgSO4 and Ag-Ag2SO4 half cells. The theoretical poten-
tial of these half cells when coupled together by means of a sulfuric acid
solution is +42.3 mv at 25°C and has been experimentally verified, +42.0
+ 0.5 mv (M. H. Lietzke and R. W. Stoughton, J. Am. Chem. Soc., 75, 5226
(1953) ): Normally, the saturated solutions with excess metal and salt are
contained in glass cells with the solutions of the cells in communication
with the sulfuric acid by means of a capillary or a sintered glass frit in
the cell wall. Electrode leads are platinum wire in contact with the mer-
cury and the silver metal of each cell. The open circuit potential of this
cell is measured in the normal manner. The current is comprised of elec-
tron transport from the Hg-HgSO4 electrode, Hg → Hg2+ + 2e", via the
platinum lead through the potential measuring instrument to the Ag-Ag2SO4
electrode, 2Ag+ + 2e + 2Agº with the sulfuric acid acting as the transfer
medium for the sulfate ion. The overall cell reaction being Hgº + Ag2504
→ HgSO4 + 2Agº.
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Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
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Now, let us place into separate tantalum metal containers the same
mixtures, (Hgº, Hg804 (solid, saturated solution) and (Agº, Ag2804 (solid),
saturated solution). The tantalum containers have now the same function
as the platinum wires of the glass cells. Tae two tubes are now placed in
the sulfuric acid solution. Communication of the solution in each tube
with the acid solution is provided by means of a sintered porous tantalum
· plug welded in the side of each tube. If there were no such communication, :
a current would not be expected to flow, since the potential difference
between the two electrodes 1.8 much less than the potential necessary for
decomposition of the sulfuric acid (H2O) and that of anodic dissolution of
tantalum metal. However, as the porous metal plug provides an ion path
for transport of current, the potential between these all metal cells should
be the same as that measured in the conventional glass insulator apparatus.
The potential of this cell measured with a Hewlett-Packard model 425 DC
microvolt-ammeter was +4.2.5 $ 0.mv at 25° in excellent agreement with the
theoretical value +42.3 mv and +42.0 $ 0.5 mv experimental.
For the high temperature test of this technique, the molten cadmium-
cadmium chloride system was used. Since these solutions are not corrosive
to ceramics, Meshovets ard Poddymov (Zh. Prikl. Khimi, 37, 813 (1964) )
have recently measured the emf of these solutions as a function of concen-
tration verous the saturated molten cadmium-cadmium chloride electrode in
a conventional quartz apperatus.
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For this system, three tantalum metal cells were employed, one the
saturated ca (excess) - cacia reference electrode and the other two con-
taining 2 and 10 mole percent cadmium dissolved in cadmium chloride, res-
pectively. The 15 mil wali tantalum tubes were 3/8" I.D. by 2-1/2" in
length. One-half inch from the bottom of each electrode capsule, a sintered
porous pellet of tantalum 3/8" diameter by 1/4" in length was welded into
the side of each capsule. The excess exposed length of the pellet was
welded closed so that access of liquid was only through the exposed end of
the pellet. After loading, the cells were capped and welded shut. Closure
of the cells was necessitated by the volatility of cadmium metal at the
temperature to be employed (above 600°c) and escape would be limited by
slow diffusion through the porous tantalum plug. All loading and welding
operations were carried out in an inert gas dry box. By means of a threaded
arrangement in the cell caps, 3/16" tantalum rods were attached to serve
both as electrode leads and as a means of lowering the cells into the melt.
The melt, a carefully dehydrated eutectic inixture of Lici and KCl, was con-
tained in a 3-1/2" high tantalum boat liner of a stainless steel tank of
5" ID by 14" in height. The tank cover had various entry ports for intro-
ducing the cells into the tank proper after appropriate evacuation and dry
inert gas conditioning. The tank was heated by means of a 6" ID x 24"
Marshall furnace and constant temperature of the melt was maintained by
conventional control devices. At temperature, the cells were lowered into
the melt by means of the tantalum rods which extended through pressure
seals in the top of each entry port. Care was taken that the cells did not
touch the metal bottom of the liner by first touching the bottom and then
raising the cell 3/8" from that point. Several hours at temperature (635°C)
were necessary for saturation o? the reference cell and complete solution
in the others. The potentials were +lll.o mv and +33.0 mv for the 2 and
.
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10 mole percent cadmium-cadmium chloride electrode ver sus the saturated
Cd-CaCl2 electrode. The stability of the system was demonstrated by the
constancy of the potential after a period of 16 hours at temperature. The
values obtained in this all-metal system are in excellent agreement with
the values of Mashovets and Poddymov, +113, and +35 mv for the 2 and 10 mole
percent solutions, respectively, obtained in an all glass conventional
apparatus. Since pure cadmium chloride was used as the communicating
liquid in the Mashovets-Poddymov cell and the eutectic Licl-KCl in this
experiment, the agreement in the experimental data indicates a negligible
or nonexistent junction potential in either system.
For the all-metal cell technique to function, the decomposition
potential of the communicating liquid must always be greater than the poten-
tial difference between the electrodes. .
It has been experimentally demonstrated, both in an aqueous system
and a molten metal-metal halide system, that electromotive force measure-
ments can be made in an all-metal apparatus by a technique which eliminates
the use of ceramics and insulators.
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