Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 627
Hence, the use of a majority of proposed bases for quantitative studies of oxoacidity is not
possible, especially for acids having apparent oxidizing properties (chromates, vanadates,
etc.).
Oxide ions in molten nitrates exist as the so-called pyronitrate ions N 2O 7 4- which was a
strong base with the basicity approximately equal to those for CO 3 2- . 55 The modern point of
view excludes the existence of nitrogen compounds with the coordination number exceeding
3 by oxygen, hence pyronitrate was hardly existing in the mentioned conditions. Indeed,
following oxoanion studies 51 have demonstrated the absence of the mentioned ion in basic
nitrate solutions. A possible way for the stabilization of O 2- in nitrate melts is the formation
of silicates due to reactions with walls of the container (usually it is pyrex).
The anomalous behavior of the electrode Pt(O 2) has been noted and explained by “peroxide”
function of the gas oxygen electrodes (of the first type). 57,58 On the contrary to gas
electrodes, metal-oxide electrodes (of the second type) were reversible with the slope corresponding
to the reaction:
1/ 2O + 2e
= O
2
2−
[10.4.36]
The acid-base equilibria in complex solutions based on nitrates containing phosphates
and molybdates were attributed 59,60 to the formation of heteropolyacids in molten nitrates.
This is confirmed by cryoscopic studies. 61
Some features of potentiometric titration techniques in molten salts have been discussed.
62,63 The automatic potentiometric titration described in details 62 consists of the following.
A rod, prepared as frozen K2Cr2O7 solution in the melt, was immersed into the melt
containing a base with the constant rate. e.m.f. data were recorded by the recording potentiometer.
Such a routine may be used only for the detection of equivalence points in melts
(with accuracy of ~3.5 %).
The effect of acidic oxides on the thermal stability of molten NaNO3 has been investigated.
64 The increase of oxide acidity results in the decrease of the melt decomposition temperature.
This is explained by the reduction of the activation energy of the decomposition
owing to fixing oxide ions by acids. But there was also another explanation: decreasing temperature
of melt decomposition is caused by the equilibrium shift towards the oxide-ions
and nitrogen oxide formation under the action of the acid. Acidic properties of oxides decrease
in the sequence SiO2>TiO2>ZrO2>Al2O3>MgO. Potentiometric method 65-67 used metal-oxide electrodes (Nb|Nb2O5, Ta|Ta2O5, Zr|ZrO2) for oxoacidity studies in the nitrate melt. The empirical acidity scale in molten potassium
nitrate at 350 o C was constructed: NH4VO3>NaPO3>NaH2PO4>K2Cr2O7>K2HPO4> Na4P2O7>NaHAsO4>K2CO3>Na2O2. This sequence includes some acids seldom existing in
molten salts. NH4VO3 at 350 o C should be completely transformed into V2O5 with a partial
reduction of the latter by ammonia. Acidic salts mentioned were decomposed to corresponding
pyro- and meta-salts. Therefore the scale obtained contains a number of errors.
10.4.4.2 Sulphate melts
Sulphate melts are referred to melts with the own acid-base dissociation equilibrium:
2
SO = SO ↑ + O
− 2−
4
3,
l l
[10.4.37]