Handbook of Solvents - George Wypych - ChemTech - Ventech!

624 Victor Cherginets
Therefore, the use of indicator method is possible only at low-temperatures (250 o C) and in
non-oxidizing ionic melts.
Spectral methods were also used for the melt basicity estimation. 32,33 The scale of “optical”
basicity for metallurgical slugs with respect to CaO (standard) has been constructed. 32
The “optical” basicity, Λ, was estimated from spectral line shifts (the transition 1 S 0 → 3 P 1)
for Tl + ,Pb 2+ ,Bi 3+ cations in the basic medium. Ionic melts Na 2O-B 2O 3-Al 2O 3,Na 2O-B 2O 3-
SiO 2,Na 2O-SiO 2-Al 2O 3 have been investigated by X-ray fluorescent spectroscopy, 32 relative
oxide acidities decrease in the sequence B 2O 3>SiO 2>Al 2O 3.
The use of spectral methods is based on studies of cooled (quenched) samples, such a
routine may distort results because of inconsistency between the solution temperature and
that of acidity estimations.
Data on the melt basicity estimations by the determination of acidic gases (SO 3,CO 2,
H 2O) solubilities are presented elsewhere. 34,35 Sulphur trioxide solubility in molten sodium
phosphate was determined by thermogravimetric analysis, 34 the correlation between the
melt basicity and the SO 3 solubility was found. The use of CO 2 and H 2O for basicity estimation
was described elsewhere. 35 The similar methods may be used only for basic melts since
acids displace the acidic gas from the melt. Furthermore, the interaction products of the
acidic gas with the melt may be relatively stable, especially in basic solutions, and their formation
leads to irreversible changes of the melt properties.
A “kinetic method” was used for Lux acidity studies in molten nitrates. The interaction
between the nitrate melts KNO 3-NaNO 3 and potassium pyrosulfate was studied. 36,37
2−
− 2−
+
S O + NO = 2SO
+ NO
[10.4.25]
2 7
3 4
2
The formed nitronium cation, NO 2 + , reacted with NO3 - according to [10.4.9]. The latter
stage is considerably slower than [10.4.25], hence, its rate allowed to estimate the melt acidity,
which can be presented by the following sum: T A =[S 2O 7 2- ] + [NO2 + ]. 36
The acid-base interactions between Cr 2O 7 2- and chlorate ions, ClO3 - , in molten
KNO 3-NaNO 3 are described by the following equations. 38,39
2−
− 2−
+
Cr O + ClO = 2CrO
+ ClO
[10.4.26]
2 7
3 4
2
+ −
ClO + Cl →Cl ↑ + O ↑ [10.4.27]
2 2 2
From two above processes, reaction [10.4.27] is the limiting stage, the rate of the chlorine
evolution is proportional to the total acidity of the melt: T A = [Cr 2O 7 2- ] + [ClO2 + ].
The reaction of bromate ions, BrO 3 - , with potassium dichromate in the nitrate melt has
been studied. 38 Several studies 36-40 gave estimate of relative acidities of the oxo-cations. The
acidities increased in the sequence BrO 2 +