Handbook of Solvents - George Wypych - ChemTech - Ventech!

618 Victor Cherginets
Wagner 15 reported an electrochemical study of molten NaBF 4 at 420 o C. The range of
acid-base properties varying according to reaction:
− −
BF = BF + F , pK=1.8 [10.4.5]
4 3
has been estimated as ~2pF units. The acid-base ranges in ionic melts containing halide
complexes are relatively narrow, it means that varying their halide basicity (i.e., equilibrium
concentration of the halide ions) may be performed in 2-4 logarithmic units, although such
changes are enough to obtain some unstable compounds with the intermediate oxidation degrees.
6,7
10.4.1.2 The Lux-Flood definition
Oxygen-containing impurities in molten salts are most usual, their effect on technological
processes is mainly negative and consists of bonding acidic reagents with the formation of
insoluble (suspensions) or slightly dissociated products. Similar reactions result in retarding
the main processes caused by the decrease of the equilibrium concentrations of initial reagents,
forming oxide inclusions in metals obtained by the electrolysis of melts, inclusions
of insoluble particles in single crystals, considerable corrosion of container for crystal
growth, etc. Therefore, quantitative studies of reactions with participation of oxide ions in
ionic melts are of importance for scientific and engineering purposes.
Reactions with the transfer of oxide ions in ionic media are mainly considered as
oxoacidity or acid-base equilibria by Lux-Flood: 2-5
A+O -2 = B, K [10.4.6]
The addition of acids (bases) in a melt as it follows from [10.4.6] leads to changes of
the oxide ion activity, for the characterization of melt acidities (basicities) Lux 2 proposed
the oxygen index, or pO, which was similar to pOH (i.e., basicity index) in aqueous solutions:
( )
pO ≡−loga ≡−logm
2− 2 −
[10.4.7]
O O
where:
a O 2-,m 2- activity and molarity of oxide ions in the melt, respectively
O
The measurements of oxygen indices during various reactions in molten media define
their thermodynamic characteristics - dissociation constants and solubility products. Most
oxoacidity studies were made in nitrate melts, while there were much less communications
dealing with the similar studies in higher-melting alkaline chlorides, molten bromides and
iodides. There is a considerable scatter of obtained experimental results, which are often in
conflict one with another. There were no serious attempts to generalize these data or consider
them from the common point of view.
10.4.2 THE FEATURES OF IONIC MELTS AS MEDIA FOR ACID-BASE
INTERACTIONS
Ionic melts as media for Lux-Flood acid-base reactions may be divided into two types on
the base of constitutional (i.e., being a part of main components of the melt) oxygen ions:
oxygen-less and oxygen-containing ones. Let us consider some features of acid-base interactions
in the mentioned melts. 16