Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 623
MeBr 4 2- in the case of chloride and bromide melts. Therefore, regardless of the cation composition,
the oxide solubilities are essentially affected by anion composition of a melt. 27
Equations [10.4.14], [10.4.18] and [10.4.22] estimate pI KCl-LiCl vs. KCl-NaCl as 7-8, 25
3.7 24 and 3.4 24 , respectively. This estimation is considerably larger than the estimation
based on eq. [10.4.18] and [10.4.22]. For Ca 2+ based chloride melts, the similar estimation
made on the basis of [10.4.14] gave pI~10 25 vs. ~4 determined on the basis of [10.4.22]. 28
So, the oxobasicity indices calculated according to HCl/H 2O equilibrium constant are essentially
different from those obtained using reactions without reactive gas atmosphere.
Probably, this discrepancy may be explained by the features of the water behavior in
aprotic ionic melts. It is known that, similar to basic properties, water possesses oxoacidic
properties according to the reaction:
2
( ( ) 2)
2
HO+ O ↔2OHorOH
2
− − −
[10.4.23]
Such polynuclear complexes are stable according to spectral data 29 because of the
H-bonds formation. Three acids (HCl + Me + +H2O) exist in ionic melts saturated with water
and HCl and consequently reaction [10.4.23] depends on the partial pressure of water. In Liand
Ca-based chloride melts, retention of water 30 and the solubility of H2O is appreciably
higher than in e.g. KCl-NaCl.
Apparently, even the simplest reaction [10.4.13] is actually complicated by additional
interactions favoring the fixation of “free” oxide ions. Values of oxobasicity indices calculated
from eq. [10.4.13] are thus somewhat overestimated.
The oxobasicity index for KCl-LiCl (0.4:0.6) compared with KCl-NaCl at 700 o C lies
within the range 3.4-3.7 log units.
10.4.3 METHODS FOR ESTIMATIONS OF ACIDITIES OF SOLUTIONS BASED
ON IONIC MELTS
The literature data show that many methods were used for oxoacidity studies and estimation
of the acidic properties of melts. One simplest method 31 involves indicator. Acid-base indicators
usually employed in aqueous solutions for protic acidity measurements have been
used for acidity studies in molten KNO3-LiNO3 at 210 o C and KSCN at 200 o C. The color of
indicator solution, relative to acidity, changes during titration of bases (sodium hydroxide
or peroxide) by potassium pyrosulfate, K2S2O7: 2−
−
2HInd + O = 2Ind
+ H O
2
[10.4.24]
where:
HInd the protonized form of the acid-base indicator
Ind- the anionic form of the said indicator
In molten nitrates such transition was observed only for phenolphthalein (yellow-purple),
other indicators seemingly were oxidized by the melt (the conclusion made 31 was “insoluble
in the melt”). In molten KSCN not possessing oxidizing properties color transitions
were observed for all indicators used (methyl red, thymolphthalein, etc.). The employment
of indicators for acidity estimation is limited mainly by their thermal instability and tendency
to oxidize at high temperatures. Since ionic melts, as a rule, have no constitutional
water, the reverse transition of indicator into the protonized form [10.4.24] is hardly possible
(the solution of phenolphthalein became yellow at the reverse transition “base-acid” 31 ).