Page 186 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN008H-970 June 29, 2001 16:46

666 Liquid Alkali Metals

these three metals are more readily purified by filtration carbon (CO and CO 2 ) that are swept away by a flow of
and gettering. argon gas. The rate at which carbon diffuses through the
membrane is proportional to the activity of carbon in the
sodium.
D. Continuous Monitoring of Impurities
Methods based on electrochemical cells come closest to
When a liquid metal is used as a coolant fluid, it is pumped satisfying the demands for continuous monitoring of spe-
aroundacircuitthatcarriesawayheatfromtheheatsource. cific impurities, and such meters for hydrogen, oxygen,
In the case of liquid sodium in the nuclear reactor, for in- and carbon in solution in sodium have been the subject of
stance, it is essential that information of any change in extensive research. They not only provide the basis for re-
impurity levels should be available immediately, as would actor instrumentation but are also finding increasing use in
be the case if leakage of water, for example, into the cir- the research laboratory. The meters are, in effect, concen-
cuit was to occur. For this reason, sampling and chemical tration cells and thus incorporate two electrodes and a con-
analysis by traditional methods are no longer acceptable, ducting electrolyte; one electrode is the liquid metal. In the
and the withdrawal of liquid metal samples could itself case of the hydrogen meter, a mixture of calcium chloride
introduce impurities. A simple device that has been in use with calcium hydride forms the electrolyte, and the refer-
for some years is the plugging meter. Part of the sodium ence electrode is a lithium metal–lithium hydride mixture
flow is diverted through a bypass that carries an orifice, the that provides a fixed hydrogen activity at a fixed tem-
temperature of which can be controlled. As the tempera- perature. The electrochemical oxygen meter differs from
ture of the bypass is decreased, the saturation temperature the hydrogen meter in that a solid electrolyte is available
of the impurity (e.g., oxygen) is eventually reached, and that is compatible with liquid sodium. The electrolyte in
precipitation of the impurity (e.g., sodium oxide) restricts common use is a solid solution of yttrium oxide (Y 2 O 3 ) in
the flow of the liquid sodium. This occurs at the plugging thoriumoxide(ThO 2 )throughwhichelectricalconduction
temperature, and with suitable calibration the plugging takes place by migration of oxygen ions. A platinum–air
temperature can be related directly to the impurity con- reference electrode system has been widely used both in
centration. For example, if the impurity were oxide and the United States and the United Kingdom, and reference
the plugging temperature 200 C the impurity level would electrodes consisting of metal–metal oxide couples (par-
◦
be about 10 parts per million by weight of oxygen. The ticularly the indium couple In–In 2 O 3 ) are also suitable. In
plugging meter is a rather crude instrument since it can- thecarbonmeter,attentionhasbeencenteredaroundliquid
not be specific for any one impurity; nevertheless, it is still electrolytes. During the 1970s, cells were described that
regarded as a most reliable and rugged indicator of total used solutions of acetylides (e.g., CaC 2 in molten CaCl 2 or
impurity levels. Li 2 C 2 in molten LiCl–KCl mixture) as electrolytes. These
Methods are now available that provide a continuous appear to be unsuitable, partly because of their chemi-
record of specific impurities. These are mostly electro- cal instability, and in 1982 a satisfactory meter was de-
chemical in nature, but an interesting alternative technique scribed that used a molten mixture of anhydrous sodium
has been developed in the United Kingdom, the United carbonate and lithium carbonate. This electrolyte is held
States, France, The Netherlands, and Russia, for metering between electrodes of thin iron, and cementite (Fe 3 C) pro-
hydrogen in solution. This technique is based on the ability vides the reference electrode. The meter responds rapidly
of hydrogen to diffuse, from solution in sodium, through to changes in carbon content of the sodium, but the use
a thin nickel membrane. The methods differ in detail, but of the carbon meter to determine actual carbon content
all employ the same principle. A tube carrying a nickel remains a major problem. Unlike hydrogen and oxygen,
membrane is inserted into the sodium stream. Hydrogen carboninsolutioninsodiumispresentbothasdimeric(C 2 )
diffusing from solution through the membrane is swept and monomeric (C) units, and the electromotive force of
away in a stream of argon, and its concentration mea- the concentration cell cannot be converted immediately
sured by ion pump or Katharometer. The rate and extent into carbon concentration. Furthermore, the solubility of
of this diffusion can give a direct measure of the hydrogen carbon in sodium is extremely small (Table V).
in solution. A technique employing a similar principle is The fact that is has been possible to develop meters that
in use for the determination of carbon in liquid sodium. can record concentrations in liquid sodium as low as a few
The instrument makes use of a tube incorporating an α-Fe parts per million represents a remarkable achievement.
membrane, the inner surface of which has been oxidized Some of the permeation and electrochemical meters can,
to form a film of iron oxide. When immersed in liquid in principle, be used for liquid lithium, but in practice
sodium (at 500–750 C) carbon diffuses through the iron problems arise due to corrosion and the different solubility
◦
membrane. Upon reaching the inner surface, the carbon of nonmetals in liquid lithium (Tables III and V). Lithium
reacts with the iron oxide film to form gaseous oxides of meters are in an early stage of development.