THERMODYNAMIC STABILITY OF SAMARIUM AND EUROPIUM TRICHLORIDES IN MOLTEN ALKALI CHLORIDES
A. Novosielova, V.
Shishkin, V. Khokhlov
Institute
of High-Temperature Electrochemistry
20
S. Kovalevskaya Str., 620219 Ekaterinburg, RUSSIA
The dissociation of samarium or europium
trichloride according to the reaction:
LnCl3(l) Û LnCl2(l) + ½ Cl2(g)
has been studied by means of measuring redox
potentials of ESm3+/Sm2+ and EEu3+/Eu2+
in electrochemical cells:
С(s)|[LnCl3, LnCl2, MCl](l)||MCl(l)|Cl2(g), С(s)
where MCl = NaCl-KCl, KCl and CsCl.
It is possible to estimate the chlorine equilibrium
pressure (
, Torr) over alkali chloride melts containing samarium or europium tri-
and dichlorides by the equation:
using the formal standard redox potentials of E*Sm3+/Sm2+
and E*Eu3+/Eu2+ measured
relative to the reference chlorine electrode.
The chlorine equilibrium pressure calculated at
molar ratio of oxidized and reduced ions of rare-earth metal equal to unit is
presented below.
Table. The chlorine equilibrium pressure
over alkali chloride melts containing samarium or europium tri- and dichlorides
|
|
SmCl3(l) Û SmCl2(l) + ½ Cl2(g) , Torr |
EuCl3(l) Û EuCl2(l) + ½ Cl2(g) , Torr |
||||
|
1073 К |
1123 К |
1173 К |
1073 К |
1123 К |
1173 К |
|
|
NaCl-KCl |
1.65·10-15 |
2.22·10-14 |
2.39·10-13 |
8.09·10-5 |
3.17·10-4 |
1.11·10-3 |
|
KCl |
2.47·10-16 |
3.50·10-15 |
3.95·10-14 |
9.18·10-6 |
4.11·10-5 |
1.62·10-4 |
|
CsCl |
1.79·10-17 |
3.05·10-16 |
4.08·10-15 |
4.58·10-7 |
2.50·10-6 |
1.18·10-5 |
From the data listed in the Table it is seen
that the stability of both SmCl3 and EuCl3 rises with the increasing cation radius of salt-solvent and with decreasing temperature.
These regularities can be explained by the complexing process. It is obvious
that relative stability of lanthanide (III) complexes is more than that of
lanthanide (II) ones. The largest stability of lanthanide trichlorides is
observed in CsCl melts. Moreover the SmCl3 stability is higher than
the EuCl3 one in all the salt-solvents studied. Taking into account the literature
data on the dissociation pressure over pure samarium and europium trichlorides
[1] one can point out that the SmCl3 and EuCl3 stability
in molten alkali chlorides grows considerably.
—————————————
[1]. O.G. Polyachenok, G.I. Novikov: Zh. Neorgan. Khim., Vol. 9 (1964),
p. 773-777 [in Russian].