Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 619
10.4.2.1 Oxygen-less media
There are no oxide ions in the composition of these melts, therefore the pure melts cannot
possess oxide-donor properties. Real oxygen-less melts contain small amounts of O 2- owing
to inevitable ingress of oxygen-containing impurities into the melt, but oxide ion concentration
in the “pure” melts is variable, depending on the concentration and acid-base character
of impurities. For example, even considerable amounts of sulfates in melts do not create appreciable
O 2- concentration, carbonate ion dissociation is substantially stronger, hydroxide
ions may be referred to most strongly dissociated Lux bases. In “pure” oxygen-less melts
oxygen index pO is usually in the range 3 to 4.5. The employment of strongest purifying
agents (HCl, CCl4, etc.) does not allow to decrease this concentration essentially, the latter
being in “an unavoidable harm” causing errors at the quantitative investigations, especially
when small amounts of acid or bases are studied. But, in some cases, oxygen index for such
a “pure” melt has been used as the internal standard for construction of acidity scales. 17-20
Quantitative studies of different Lux-Flood acids and bases in ionic melts were performed
by two main ways: the construction of empirical acidity scales to estimate relative
acidic strength of the substances and the determination of acid-base equilibria constants using
potentiometric titration techniques. The first approach has been proposed in the classic
work of Lux 2 who obtained the acidity scale for the equimolar mixture of potassium and sodium
sulfates. Although this work was the basis for series of later studies, the results cannot
be considered as undoubted ones. Addition of acids (bases) to the mentioned melt led to increasing
(decreasing) e.m.f. of cell with the oxide-selective electrode vs. the corresponding
magnitude in the neutral melt. Then, the e.m.f. value shifted progressively to that for the
neutral melt because of SO3 (Na2O) evaporation from the acidic (basic) melt. Therefore,
Lux extrapolated values of e.m.f. to the point of acid (base) addition to the melt. This resulted
in the decrease of the data 2 accuracy.
The empirical acidity scales 17-20 give some information about strength of the acids and
bases in melts studied. The principal error related to the term “acidity scale” in the case of
oxygen-less melts is because it is not connected with the melt properties. Therefore, the values
obtained could not be considered as the quantitative characteristics of the oxygen-less
melts. The acidity scale length in a solvent is believed to be the interval (measured in acidity
index units) between standard solutions of strong acid and base. If the solvent possesses its
own acid-base autodissociation equilibrium then a substance creating unit concentration of
acid (base) of solvent in the standard solution should be considered as the strongest acid
(base). Addition of stronger acids (bases) should not result in extension of the acidity scale
because of the known phenomenon of leveling acidic and basic properties by the solvent. 21
Oxygen-less melts do not possess acid-base equilibrium, therefore, values of the basicity index,
in the “pure” melts may have any reasonable value.
10.4.2.2 Oxygen-containing melts
Acid-base processes in oxygen-containing melts are more complex than those in oxygen-less
ones, since they are accompanied by competitive equilibria of own acid-base
autodissociation of the melt-solvent. The coexistence of acidity and basicity “carriers” into
melt is the characteristic feature of oxygen-containing melts making them similar to
low-temperature molecular solvents with own acid-base equilibrium.
But, there exist some principal features due to relatively high temperatures of the liquid
state. Own acids of the melts are often unstable or volatile, 2,22 therefore, acidic solutions,