Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 617
10.4.1 ACID-BASE DEFINITIONS USED FOR THE DESCRIPTION OF
DONOR-ACCEPTOR INTERACTIONS IN IONIC MEDIA
Donor-acceptor interactions in ionic media are often described as acid-base interactions according
to Lewis 1 and Lux-Flood 2-5 definitions. The classic variant of the former definition
considers acids as acceptors of electron pairs and bases as their donors. In modern variant of
this definition, acids are the electron pair (or anion) acceptors or cation (proton) donors,
bases are the electron pair (anion) donors or cation (proton) acceptors.
Lux-Flood definition considers bases as donors of oxide-ion, O 2- , its acceptors are Lux
acids.
10.4.1.1 The Lewis definition
The Lewis acid-base process may be described by the following scheme:
A+ : B = AB : , K
[10.4.1]
where:
A an acid,
B a base,
K the equilibrium constant (used below pK ≡ -log K).
This definition can be used for the description of interactions in ionic melts, containing
complex anions undergoing heterolytic dissociation. Processes reverse to [10.4.1], i.e.,
the acid-base dissociation of A:B adducts (ions) in some solvents are considered as
acid-base equilibria of these solvents.
Alkaline chloroaluminate melts containing the excess of AlCl3 vs. the stoichiometry
are promising solvents for preparing sub-ions, e.g., Cd + 2+ + 3+ 6,7
(Cd 2 ), Bi , Bi 5 , the excess of
acid (AlCl3) favors their formation. Tremillon and Letisse, 8 Torsi and Mamantov 9,10 studied
the acid-base properties of molten mixtures AlCl3-MCl (M=Li, Na, K, Cs) with AlCl3 concentration
exceeding 50 mol% in the temperature range 175-400 o C. Equilibrium molarities
of Cl - were determined by a potentiometric method with the use of a chloride-reversible
electrode. The solvents undergo the acid-base dissociation according to the following equation:
( )
− − − −
2 4 2 7 3 4
AlCl = Al Cl AlCl ⋅ AlCl + Cl , pK
[10.4.2]
In the melts, chloride-ion donors were bases and substances increasing AlCl 3 concentration
were acids. The pK values decrease with the temperature (from 7.1 to 5.0 at temperatures
175 and 400 o C, respectively, Na-based melt 9 ) elevation and from Cs to Li (at 400 o C
pK were 3.8, 5.0, 5.8, 7.4 for Li-, Na-, K- and Cs-based melts, respectively 10 ). The latter effect
may be explained from the point of view of “hard” and “soft” acids and bases 11-13 - Li-Cl
complexes should be more stable than Cs-Cl ones as formed by “hard” base (Cl - ) and “more
hard” acid (Li + ).
Dioum, Vedel and Tremillon 14 investigated molten KGaX 4 (X=Cl, I) as background
for acid-base processes. The following acid-base equilibria exist in the pure solvents:
( )
− − − −
2GaCl 4 Ga2Cl 7 GaCl 3 GaCl 4 Cl
= ⋅ + , pK=4.25±0.05 [10.4.3]
− −
GaI = GaI + I , pK=2.6±0.05 [10.4.4]
4 3