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

Liquid Alkali Metals 669

collecting electrons from the medium) to X n + (formed VII. ELECTRICAL RESISTIVITY
by donating electrons to the medium). The chemical be-
havior of these solutions cannot be fully explained if we For the study of the behavior of dissolved substances in
assume that these dissolved species have no further inter- the liquid alkali metals, and their interactions, the most
action with the medium. In fact, these charged ions collect useful technique has been found to be the measurement
around themselves atoms of the liquid metal concerned, of changes in electrical resistivity. This is understadable,
the number of such atoms being determined by such fac- since the electrons in the bulk metal that are responsible
tors as the charge and size of the dissolved ion. It has for electrical conductivity are also those that are involved
been calculated that in liquid sodium, the fluoride ion has in the solution process. All available evidence supports
six sodium atoms coordinated around it in an octahedral the belief that the introduction of a solute into the metal
arrangement, and the chloride ion is surrounded by a co- solvent leads to scattering of the conduction electrons, and
ordination shell of eight sodium atoms in a cubic arrange- the important practical result is that the resistivity of the
ment. This phenomenon is termed solvation, and since it pure alkali metal is invariably increased by the introduc-
occurs as a result of attraction between the solute ion and tion of any foreign element. The technique has been used
atoms of the medium, energy is released in the process. with success for following the rates of solution or precip-
The phenomenon is well established in aqueous solutions, itation, to determine solubilities, and to define phase di-
and it is of interest that the solvation energy of the chloride agrams. When two solutes are present, resistivity change
ion is similar in magnitude in water and in liquid sodium is additive, but if they interact, then this is revealed in the
(−381 and −305 kJ/g ion, respectively). Some values for resistivity value.
solvation energy are given in Table VI. As expected, the A remarkable feature about resistivity changes is that
values increase considerably with the charge on the dis- for dilute solutions, the resistivity (ρ) increases linearly
solved ion but are not influenced greatly by change in the with concentration (x) at a rate that is entirely character-
alkali metal that is used as solvent. istic of the particular solute in a given alkali metal. This
These energy values have practical significance. When can be defined in terms of a resistivity coefficient dρ/dx,
−8
water comes (by accident) into contact with one of the which expresses the increase in resistivity ( m × 10 )
−
liquid alkali metals, the hydroxyl ion OH is produced caused by solution of 1 mol% of the solute. Some values
within the metal, which may then dissociate as are given in Table VII, and a few broad generalizations
are possible. Thus, coefficients increase with increase in
−
−
OH + 2e H + O 2− ionic radius of the dissolved atom or ion, so that the size

and the change in solvation energy that is involved in the of a dissolved species is more important than its charge
dissociation is one of the factors that determines the extent in determining the resistivity coefficient. The coefficients
of the dissociation. In practice, it is found that dissociation are largely independent of the alkali metal used as solvent.
is complete in liquid lithium; no dissociation occurs in
liquid potassium, rubidium, or cesium, and there is an TABLE VII Resistivity Coefficients for Solutes in the Liquid
Alkali Metals dρ/dx a
equilibrium in liquid sodium, with all three species present
in the solution. Solvent Solute
metal Solute dρ/dx metal Solute dρ/dx
TABLE VI Solvation Energies in Liquid Alkali
Na H − 4.5 Li N 3− 7.0
Metals
O 2− 1.8 H − 4.9
Solvent Dissolved Solvation energy
Li 0.04 O 2− 2.1
metal anion (kJ/g ion)
K 1.14 Si 10.4
Li H − −425 Rb 3.4 Pb 9.0
Cl − −343 Cs 4.2 K F − 3.9
O 2− −1960 Sr 0.2 Cl − 5.7
N 3− −3473 Ba 2.5 Br − 6.9
Na H − −344 Pb 11.2 Na 1.2
Cl − −305 Ag 2.8 Rb 0.22
O 2− −1721 Au 5.0 Cs 1.61
K H − −362 Hg 4.1 Cs O 2− 3.2
Cl − −316 H − 4.7
O 2− −1642 Na 5.71
Rb Cl − −286 K 1.01
Cs Cl − −280
a m × 10 −8 per mol% solute.