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H. Sekimoto et al. / Journal of Alloys and Compounds 509 (2011) 5477–5482
105
104
103
102
101
100
10-1
10-2
60 mol% NaCl-40 mol% SrCl
T
= 700 °C
Vycor glass crucible [11]
NaCl-KCl equimolar
T
= 700 °C
Vycor glass crucible [12]
LiCl-KCl eutectic
T
= 500 °C
Pyrex glass crucible [13]
NaCl-KCl equimolar
T
= 740 °C
Ti crucible [14]
NaCl-KCl equimolar
T
= 740
°C
Steinless-steel [14]
analytical
Ti
x
2+
Fig. 1. Relation between the concentration quotient and the concentration of Ti2+
in the literatures [11–14].
of the concentration of Ti3+. This could be caused by some system-
atic errors or by the effect of some impurities in molten salt. In
previous paper, we reported that the systematic error in the chemi-
cal analysis was negligible [14]. Therefore, some impurities initially
contained in molten salt could affect equilibrium (1). A considerable
Fig. 2. Experimental apparatus for absorbance measurement of molten salt at high
temperature.
setup for the spectroscopic experiments are attached on the bot-
tom of the glove box, and the spectroscopic experiments were
conducted in the Ar filled glove box, where the concentration of
oxygen and water were maintained under 1 ppm. The weighted
sample of NaCl–KCl equimolar mixture, NK-c, was inserted in the
optical quartz cell. Then, the sample was heated to melt at 700 ◦C
in the electric furnace. After the intensity of the transmitted light
became stable, the spectroscopic measurement to the molten salt
was carried out. Then, TiCl3, which is purified by distillation from
the mixture of TiCl3 and AlCl3 (76–78.5% as TiCl3, Alfa Aesar, cat.
#36729), was added into the molten salt and the intensity of the
transmitted light was measured. This procedure was repeated sev-
eral times for the measurement with various concentration of TiCl3.
passed through NaCl–KCl equimolar molten salt and that through
TiCl3–NaCl–KCl molten salt, the absorbance was calculated. Finally,
a pellet of 2 mol% Na2O–NaCl–KCl molten salt, which was pre-
pared using NK-b and Na2O synthesized from metallic sodium and
NaOH [16], was introduced to the molten salt as a source of O2−, and
the spectroscopic measurement on the molten salt was carried out
toinvestigate the solubleion ofTi3+ andO2−. Afterthespectroscopic
experiments, the molten salt was quenched into room tempera-
ture and analyzed by X-ray diffractmeter (X’Pert PRO, PANalytical,
Almelo).
impurity is oxide ion, O2−
.
In this study, we thus determined the concentration of O2− in
NaCl–KCl molten salt. Then, it was investigated that the effect of
oxide ion on the equilibrium between titanium ions and metallic
titanium in NaCl–KCl equimolar molten salt by absorption spec-
troscopy. Finally, the concentration quotient of titanium ions and
metallic titanium was revaluated and discussed.
2. Concentration of oxide ion in NaCl–KCl equimolar
The concentration of oxygen in the mixture of NaCl and KCl
was measured with nitrogen/oxygen combustion analyzer (EMGA-
923, Horiba ltd., Kyoto). Samples were prepared by five procedures.
Table 1 shows the procedures of the samples and the concentra-
tion of oxide ion in their samples. In this study, it is considered that
all amount of oxygen species exists as oxide ion, O2−, in molten
salt. It was confirmed that the concentration of O2− remarkably
decreased by drying at 200 ◦C in vacuo as shown in NK-b. The con-
centration of O2− in NK-d is higher than that in NK-c. This implies
that the contamination of O2− comes from the oxide film on the
surface of stainless-steel. Actually, the color of the sample NK-d
was faint pink, while the color of the other samples was transpar-
ent or white. This suggests that NK-d contains something cation of
the elements consisting of stainless-steel by the dissolution of the
oxide film. Consequently, it is concluded that the concentration of
oxide ion in NaCl–KCl molten salt is about 800 and 1500 ppm in the
case using titanium container and using stainless-steel container,
respectively.
3.2. Results
nominal concentration of Ti3+ in mol L−1 is represented as Cnominal
.
3+
The spectra have an absorption peak at 786 nm whose absoTribance
depends on Cnominal. This peak attributes to the chloro complex
3+
of trivalent titTainium ion [17]. Fig. 4 shows the absorption spectra
of the molten salt before and after addition of O2−. No additional
absorbance at 786 nm decreased. This indicates that the concen-
tration of the chloro complex decreases. Additionally, there could
be no soluble species except for the chloro complex in the molten
salt. Fig. 5 shows the photographs of the TiCl3–NaCl–KCl molten
salts before and after addition of O2−. TiCl3–NaCl–KCl molten salt
was a clear green liquid. When O2− was introduced into the molten
3.1. Procedure
Fig. 2 shows the schematic illustration of the experimental appa-
ratus for absorbance measurement. The detail of the optical system
is described in the literature [15]. The electric furnace and the