Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 631
Equilibrium
m
NaI
-pK
N mol%
− 2−
4−
2VO3 + O = V2O7 5.40±0.3 7.10 4.10
4− 2−
3−
VO 2 7 + O = 2VO4
1.68±0.3 1.68 1.68
2− 2−
−
BO + O = 4BO
5.02±0.8 3.37 5.37
4 7
2
The results included in the Table 10.4.1 are in good agreement with the similar studies,
performed using a membrane oxygen electrode Ni,NiO|ZrO2. The use of the gas platinum-oxygen
electrode resulted in significant underrating of constant values (of order of 3-4
pK units).
10.4.4.5 Other alkaline halide melts
Some studies of acid-base equilibria in molten alkaline halides have been performed by
Rybkin et al. 17-20 The empirical acidity scales have been constructed on the base of
potentiometric studies in molten KCl 17,18 and CsI. 19 The practical significance of the men-
2- 2-
tioned works consists in the proposition to use some buffer solutions - SO4 /S2O8 ,
2- 2- 3- 4-
WO4 /W2O7 ,PO4 /P2O7 as reference standards for the indicator electrode calibration and
determinations of pO in molten alkaline halides.
Rybkin and Banik 20 obtained the empirical acidity scale in molten NaI at 700 o C. The
2- -
basicities of CO3 and OH were close which is incorrect since 0.01 mole/kg solutions were
used for the scale constructing while equations
1 1
OH = H2O+ O
2 2
− 2−
[10.4.45]
2−
2−
CO = CO ↑ + O
[10.4.46]
3
2
showed that these additions were not equivalent. CO 3 2- concentration recalculated to oxide
ions was twice as high as OH concentration. Indeed, in the later work these authors 100,101
showed that hydroxide ion was stronger base than carbonate ion.
We have performed a study of acidic properties of boron (III) and vanadium (V)
oxo-compounds in molten NaI at 700 o C 98 (see Table 10.4.1). A comparative study of
strength of Lux bases OH - ,CO 3 2- ,SO4 2- was determined 100,101 by potentiometric titration using
sodium pyrophosphate as acid. Two moles of the first base may be neutralized by 1 mole
of P 4O 7 2- , while two other moles react with the acid in ratio 1:1. On the base of e.m.f. drop
magnitude at the equivalence point, the bases have been arranged in sequence
OH - >CO 3 2- >SO4 2- of basicity decrease. The equilibrium constants were not estimated. 100,101
There is no correlation 17,19 between substances entered into melt and those really existing
in it. E.g., potassium nitrate and nitrite 17,18 are decomposed to K 2O at temperatures considerably
lower than the temperature of the experiment (700 o C). Ditto 17,18 referred to K 2S 2O 7
and Na 2S 2O 8 as acidic phosphates. 19 In the latter work it has been found that pyrophosphate
acidity is larger than that for metaphosphate. However, titration of phosphorus (V)
oxo-compounds in melts runs according to the scheme [10.4.39]. The acidity decreases,