Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 625
Me 2+
Cu 2+ or Co 2+
Metal cation acidities are comparable with acidic properties of dichromate-ion and
mentioned above 36-40 cations.
A potentiometric method, or more accurately, method of e.m.f. measurements with
liquid junction in cells with oxide-reversible electrodes was most advantageous in
oxoacidity studies of various melts. The principal construction of electrochemical cells for
oxoacidity studies may be presented by the following scheme:
Reference electrode || Melt+O 2- | Oxide-reversible electrode [10.4.29]
In all known studies, the potential of liquid junction was zero. The known reference
electrodes were metallic (i.e., metal immersed into the solution with the definite concentration
of the corresponding cation) or gas oxygen electrodes immersed into solutions with
definite oxide ion concentrations (concentration cells).
10.4.4 ON STUDIES OF THE HOMOGENEOUS ACID-BASE REACTIONS IN
IONIC MELTS
10.4.4.1 Nitrate melts
Traditionally, nitrate melts are considered among most comprehensively studied ones. Relatively
low melting temperatures both of individual nitrates and their mixtures allowed to
use simpler experimental techniques. The potentiometric studies gave the acid-base equilibrium
constants. Most studies were on oxoacidity reactions in molten KNO3 at 350 o C. 23,43-67
Constants of acid-base reactions of Group V highest oxides (P2O5,As2O5,V2O5) with Lux
bases were also investigated. 43-48 All oxides breakdown the melt, the first stage is the formation
of corresponding meta- acid salts and nitronium cation:
R O + NO = 2RO
+ NO
− − +
2 5 3 3 2
[10.4.30]
where:
R the designation of Group V element,
+ -
the second step is the redox interaction of NO2 with NO3 according to [10.4.9], which is
shifted to the right, NO2 evolution took place until e.m.f. values reached down to ~0.5 V,
this magnitude corresponded to the upper limit of oxo-acidity in molten KNO3. The oxygen
index value for this limit can be estimated as ~10 -16 . 43-48 Addition of Na2O2 as a Lux base led
to the following neutralization steps
RO + O = R O
[10.4.31]
− 2−
4−
2 3
2 7
4− 2−
3−
RO + O = 2RO
[10.4.32]
2 7
4
The acidic properties of oxides increased in the sequence V 2O 5