Handbook of Solvents - George Wypych - ChemTech - Ventech!

620 Victor Cherginets
as a rule, lose acids because of their evaporation 2 or decomposition, 22 the oxygen index of
such melts is shifted progressively to that of the neutral melt. Hence, in oxygen-containing
melts it is possible to observe not only the leveling of acidic properties but also an upper
limit of acidity.
CrO 3 and MoO 3 have been found 23 to breakdown the nitrate melt KNO 3 with the evaporation
of NO 2. It should be noted, that the temperature elevation leads to the efficient
shrinkage of the acidic region in the melt. The interactions between the oxides and the melt
have been assumed to result in the formation of nitrogen (V) oxide, N 2O 5, and corresponding
oxoanions. Since the acids studied (CrO 3, MoO 3) may be referred to as very strong
acids, it may be supposed that reaction:
CrO + NO = Cr O + NO
− 2−
+
2 3 3 2 7
2
[10.4.8]
should be completely shifted to the right and the strength of the acid is determined by acidic
properties of nitronium cation, NO 2 + . The stability of the latter is low and after achieving a
certain concentration its reaction with the melt anions becomes intensive enough:
+ −
1
NO2 + NO3 = 2NO2
↑ + O2
[10.4.9]
2
The so-called kinetic methods of melt acidity determination, which will be considered
below, are based just on this reaction. The reaction [10.4.9] leads to the decrease of
nitronium concentration and the rate of NO2 emission decreases until essentially constant
acid concentration is determined by a sequence of consecutive measurements. This concentration
is the “upper limit” of acidity of the melt. The thermal dependence of upper limit of
acidity 23 in nitrate melts can be easily explained on the basis of the increase of the melt temperature,
which leads not only to the reduction of nitronium stability but also to the elevation
of process [10.4.9] rate. Sulphate melts have the upper limit of acidity too, it seems
connected with limited and low solubility of SO3 at elevated temperatures, such assumption
may be confirmed by results. 2
10.4.2.3 The effect of the ionic solvent composition on acid-base equilibria
The equilibrium parameters of Lux acid-base reactions in ionic media (solubility products
of oxides and acid-base equilibrium constants) are essentially affected by the acidic properties
of the molten alkaline halide mixtures, i.e., they are dependent on the constituent cation
acidities. Therefore, one should consider the reverse problem - the estimation of the basicity
indices of ionic melts on the basis of the calculated equilibrium constants.
The default acid-base processes are actually a superimposed effect of interactions between
oxide-ions formed and the most acidic cations of the melt:
m+
2− im ⋅ −2
iMe + O ↔Me
O , Kl,i [10.4.10]
i
where:
Me m+
the most acidic cation of the solvent
The increase of melt acidity (Kl,i) leads the shift of interactions [10.4.10] to the right.
The distribution of oxide ions added to the melt between different complexes with the melt
cations may be presented by the following equation (N is mole fraction, m - molarity):