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

668 Liquid Alkali Metals

VI. SPECIES FORMED BY the conduction band of the metal. Oxygen dissolves in
DISSOLVED ELEMENTS all the alkali metals. Cryoscopic studies on solutions of
oxygen in rubidium and cesium show that the oxygen
A. Types of Species is monatomic in solution. Hall coefficient values indi-
cate that two electrons are removed from the conduction
Various elements, both metallic and nonmetallic, will dis-
band per atom of oxygen, so that solution involves the
solve in the liquid alkali metals, and it is these dissolved
process
elements, although usually present only in small concen-
+4e
trations, that are responsible for many of the problems O 2 (gas) −−→ 2O 2− (in solution)
(e.g., corrosion) encountered in the use of the alkali met-
als on a large scale. In order to understand corrosion and There is no corresponding direct evidence as to the na-
other reactions, attempts have been made to define the ture of the species formed on solution of nitrogen, but
species in solution, but with only limited success. This is the extensive chemistry of nitrogen solutions is entirely
because a solute dissolved in a liquid metal is immersed in consistent with the process
a sea of electrons, and it is the number of these electrons, +6e
N 2 (gas) −−→ 2N 3− (in solution)
their energy, and the extent to which they are localized
on the metal medium that now determines the chemistry Metallic solutes fall into two classes. The first is repre-
of dissolved substances. For example, does oxygen dis- sented by the alkaline earth metals calcium, strontium, and
solved in cesium exist as O 2 ,O ,orO 2− species, and barium; these metals dissolve as monomeric species, and
−
will nitrogen give solutions consisting of N 2 ,N ,N , their solubility in the liquid alkali metals increases with
−
2−
or N 3− in liquid lithium? Spectroscopic methods, which atomic weight. The chemical reactivity of these metals is
are so useful with aqueous solutions, are not applicable to not inhibited by solution in the liquid metal, and a solu-
metallic-type solutions, and many of the conclusions on tion of barium in sodium, for example, behaves as though
the nature of dissolved species have to be arrived at by sodium were no more than an inert diluent for the barium.
indirect methods such as kinetics, phase equilibria, elec- From the chemical point of view, it is convenient to regard
trical conductivities, electrochemical measurements, and the valence electrons in the liquid alkali metal medium as
the like. A brief outline of some conclusions based on such being free from the atoms and existing in a conduction
measurements will now be given. band in the liquid medium. Liquid sodium, for instance,
The noble gases undergo negligible electronic interac- is then treated as an assembly of Na ions in a sea of free
+
tion with the liquid metals, and solubility is very small in- electrons. Solutions of the alkaline earth metals are then
deed (on the order of 1 × 10 −7 to 1 × 10 −8 M of solution regarded as containing M 2+ units, with two electrons from
◦
per atmosphere at 300 C). Solubility decreases with in- each atom added to the conduction band of the alkali metal
creasing size of the noble gas atom and increases only medium.
slightly with increasing temperature. All these properties The second class of metallic solutes is represented by
are consistent with a simple concept in which the solubil- the less electropositive metals. Here, the situation is the re-
ity represents the number of noble gas atoms that can be verse of that discussed above. Sodium amalgam is widely
accommodated in holes of appropriate size between the used in industry and in the laboratory and is a good ex-
metal atoms. ample of this class. Upon addition of mercury to liquid
With respect to diatomic molecules, a general guide to sodium, the reactivity of the sodium toward aqueous so-
solubility and dissolved species is provided by the sim- lutions is vastly reduced, and reaction with hydrogen is
ple chemistry of the binary systems. For example, lithium slower by an order of magnitude than that for pure sodium.
(solid or liquid) reacts readily with nitrogen to form the This fact is important in the operation of the Solvay cell for
stable product Li 3 N, and so nitrogen has appreciable sol- the industrial production of sodium hydroxide by electrol-
ubility in liquid lithium. In contrast, sodium and nitrogen ysis of brine, in which sodium amalgam forms one of the
do not yield a corresponding nitride, and so nitrogen is electrodes. In such amalgams, valency electrons from the
insoluble in liquid sodium. Some fairly firm conclusions conduction band of liquid sodium, which would normally
have been reached regarding the species formed by hy- be responsible for its chemical reactivity, are partially
drogen, oxygen, and nitrogen in solution. With hydrogen, localized on the mercury atoms, thus inhibiting the re-
both pressure measurements and cryoscopy indicate that activity of sodium.
solution involves the process
+2e B. Solvation

−
H 2 (gas) 2H (in solution)
+2e As indicated above, elements dissolved in the liquid
in which the hydrogen molecule dissociates into atoms, alkali metals form, in solution, species that are usu-
which are then converted to H ions by electrons from ally monomeric and vary in type from X n− (formed by
−