Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 629
creased with the reduction of the alkaline earth cation radius. The effect of the cation charge
and the radius on the basicity of lead meta-silicate melt at 800, 850, 900 o C by additions of
Tl2O, PbO, CdO, ZnO and Bi2O3 has been studied by potentiometry. 81
10.4.4.4 The equimolar mixture KCl-NaCl
This melt is the most investigated among chloride based melts. Some oxoacidity studies
were performed by Shapoval et al. 82-87 Main purposes of these works were to investigate oxygen
electrode reversibility and to obtain equilibria constants for acid-base reactions including
oxo-compounds of Cr VI ,Mo VI and W VI at 700 o C. Equilibrium constants of acid-base
- 82-84
interactions for CrO3 and PO3 have been determined. The titration of the former substance
proceeds in two stages:
3 1
28810 . 2 14 . 10
CrO ⎯⎯⎯⎯→CrO⎯⎯⎯→CrO −
3
× − × 2−
2 7
4
Sodium metaphosphate is two-basic acid too:
− ( 28812 . ± . ) × 10
4− ( 2. 5± 14 . ) × 10
PO ⎯⎯⎯⎯⎯→PO⎯⎯⎯⎯⎯→ PO4
3
3 1
2 7
3−
[10.4.38]
[10.4.39]
The excess of titrant, O 2- , has been found to cause the formation of basic phosphates
with assumed composition PO 3 - ⋅2O 2- . When Na2CO 3 was used as titrant, there was no formation
of the mentioned basic products. Ba 2+ and Li + cations have been shown to possess appreciable
acidic properties, corresponding constant values were estimated as 8.1×10 1 and
3.53×10 2 .
An investigation of acidic properties of MoO 3 was performed: 85
4
510 × 2− 880± 880
2−
MoO ⎯⎯⎯→MoO ⎯ ⎯ ⎯ → MoO
[10.4.40]
3
2 7
Acidic properties of oxo-compounds of W VI have been studied. 86,87 The titration of
WO 3 in molten chlorides was a two-stage process:
1460± 390
2− 880± 880
2−
WO ⎯⎯⎯⎯→WO⎯⎯⎯ →WO
[10.4.41]
3
2 7
Results formed the base for the following electrochemical studies of electroreduction
processes of Group VI metals. 88-90 These studies developed theoretical bases and principles
to control electrochemical processes of metals and their compounds (carbides, borides, silicides)
deposition from ionic melts. 81
The acid-base equilibria in the scheelite (CaWO 4) solutions in the said chloride mixture
at 1000K were investigated. 92 CaO solubility in molten KCl-NaCl has been determined
to be about 0.084 mol%, that of scheelite was 10 -3.5 mole/kg. Equilibrium constants for
acid-base reactions with WO 3 participation have been determined, too:
10
10
WO ⎯ ⎯→WO
3
2−
4
4
4
[10.4.42]
The reverse titration has been assumed by authors 92 to result in the formation of
polyanion W 3O 10 2- , pK=12.7. The titration curve was irreversible. This result can be easily
explained taking into account that WO 3 is unstable in strong acidic chloride melts and removed
from as WO 2Cl 2. Therefore, additions of WO 3 into the acidic melt should lead to un-