Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 621
n
0 ⎛
i
N 2 = N 2 ⎜1+
∑ K , N
⎝ i = 1
− −
O O li +
Me
⎞
⎟
⎠
[10.4.11]
It may be seen that the ratio “free oxide-ion/total oxide-ion” is the constant which may
be designated as I l: 24
( )
n
0 0 ⎛
i ⎞
Il = N 2− / N 2− = m 2− / m 2−
= 1/ ⎜1+
K N
O O O O ∑ li , + ⎟ [10.4.12]
Me
⎝ i = 1 ⎠
Now let us to estimate the equilibrium molarity of the constituent acidic cations in the
melt, e.g., the eutectic KCl-LiCl melt (0.4:0.6) contains ~8.5 mole of Li + per 1 kg. Usually
the ionic complexes in melts are characterized by the coordination number ~ 4-6. 21 For the
solution of O 2- of the 0.1 mole/kg concentration, the maximum possible quantity of fixed Li +
concentration may be estimated as 0.4-0.6 mole/kg, i.e., efficiently lower than 8.5. In this
case the change of actual Li + concentration is approximately equal to 5-7% and m Me+ in this
case may be suggested as constant. Therefore, for each melt the sum in the denominator of
[10.4.12] is the constant reflecting its acidic properties. So, pI l= -logI l is a measure of melt
acidities and may be denoted as the “oxobasicity index” of the melt. Since the determination
of the “absolute” concentration of free O 2- is practically impossible one should choose the
“standard melt”, for which I l is conditionally equal to 1 and pI l=0. It is reasonable to choose
the equimolar mixture KCl-NaCl as the “standard melt”, since this melt is most frequently
investigated. Further, one should choose “standard equilibria” and formulate the non-thermodynamic
assumptions which usually postulate that the constant of the “standard equilibrium”
calculated using “absolute” oxide ion concentrations remains the same for all other
melts.
Now let us consider possible variants of such “standard equilibria” and conditions of
their use.
One of the first attempts to define and estimate the oxoacidity parameters of ionic
(chloride-based) melts was connected with the studies of the equilibrium
H O + 2Cl ↔ 2HCl
+ O
2
− 2−
gas gas
[10.4.13]
in different chloride mixtures. 25,26 Here the partial pressures of H2O and HCl are known and
0 0
the oxide ion concentration is calculated on the base of the calibration data as m 2- ( N 2- ).
O O
The equilibrium constant of [10.4.13] may be represented in terms of the “absolute” mole
fractions of O 2- and Il by:
K
1
− ( 2−
)
2 1
pHCl N I O l
= = K I
2
−
p N
HO −
2 Cl
and, consequently:
pK = pK −pI
l KCl − NaCl l
KCl NaCl
−1
l
Combes et al. 25 introduced the oxo-acidity function, Ω:
[10.4.14]
[10.4.15]