ORNL-1816 - the Molten Salt Energy Technologies Web Site

solved in a formal way, but it is much more useful
to apply the following straightforward approach.
In Eq. 16 the greater oxide-ion affinity of carbon
dioxide compared with water determines the di-
rection of the reaction.
Lux’ and Flood and F&land’O have advanced
a definition of acids and bases which was de-
signed for treating the special problems en-
countered in nonprotonic fused oxide systems.
According to these authors, a base is an oxide-ion
donor and an acid is an oxide-ion acceptor; thus
(17) Base+ Acid + 0--
Under th i s definition the conventional class if i -
cation of slags as acidic or basic is preserved.
This acid-base concept has not been applied to
solvent systems, but, by a small extension, this
may be done.
Two competing equilibria of the above type can
be considered:
(18)
L A t t + 0--
A0 \-
d
0--+ Btf-BO
AO + B++F==-’.Bo + A++
Base I Acid I1 Base II Acid I
The acid-base equilibrium shown by Reaction 18
may be considered to be controlled by a balance
between the oxide-ion affinities of the reactant
acid and the product acid in a manner that would
be analogous to the balance between the proton
affinities of the bases in protonic solvents.
The self-dissociation of hydroxyl ions according
to Eq. 2 completely fulfills the requirements of
’OH. Flood
592, 781, 790
PERIOD ENDING DECEMBER 70,1954
Thus, water, H-0-H, may be regarded as a pyro-
analog compound of hydrogen and the hydroxyl
ion as the ortho-oxyanion of hydrogen. l3ecause
of acid-base relations involving pyroions, such
as shown in Eq. 19, two hydroxyl ions will always
be taken together to serve as a Lux base analog.
Oxidic-Solvolysis-Type Reactions. The solvo-
lytic reactions involving the oxides and oxysalt
solutes can be classified according to the ioni-
zation energy of the atom other than oxygen in
the oxide. This method of classification is
possible because the ionization energy of an atom
is related to its oxide-ion affinity. It is con-
venient to divide the elements into four groups
according to ionization energy.
First, oxides and oxysalts of atoms having high
ionization energy should react with th’e fused
hydroxides to give a change in coordination. For
example,
(20) SO, + 2CH-* SO,-- + HOH
(see also Eq. 16).
Second, oxides and oxysalts of atoms of medium
ionization energy should react with fused hy-
droxides either to form or to depolymeri1:e poly-
anionic acids. For example, the reaction
(21) P20,4’ + 20H-+2P0,3- + HClH
will probably take place. The possible reactions
of this type are numerous because of the sub-
stantial variety of polyanions. For example, there
could be the step-wise depolymerization of silicon-
oxygen compounds beginning with the three-di-
mensional network in silicon dioxide, passing
through the two-dimensional net polyanions,
through the one-dimensional chain (and ring) poly-
anions, the pyroanion, and finally ending with
the orthosilicate anion.
This second type of solvolytic r
ward depolymerization and, for some parent
go all the way to the ortho-oxyariion be-
of the relatively high basic character of
the hydroxyl ion. However, there is no assurance
that the ortho-oxyanion will be the most stable
and T. Fgrland, Acta Chem. Scand 1,
1947). (22) NiO + 20H-e Ni02-- + HOH
95