Vapor pressure over KI-CoI2 melts

Article abstract of DOI:10.1023/B:INMA.0000016099.26688.49

The vapor pressure in the KI-CoI₂ system is determined by isobaric boiling point measurements. Calculated vapor pressure isotherms of the KI-CoI₂ system show negative deviations from linearity. The vapor composition over pure CoI

Full text of DOI:10.1023/B:INMA.0000016099.26688.49

Inorganic Materials, Vol. 40, No. 2, 2004, pp. 202–206. Translated from Neorganicheskie Materialy, Vol. 40, No. 2, 2004, pp. 246–250.
Original Russian Text Copyright © 2004 by Kritskaya, Burylev, Moisov, Kostenko.
Vapor Pressure over KI–CoI2 Melts
E. B. Kritskaya, B. P. Burylev, L. P. Moisov, and N. B. Kostenko
OAO NIIMontazh, Rashpilevskaya ul. 148, Krasnodar, 350020 Russia
Received September 25, 2001; in final form, May 22, 2003
Abstract—The vapor pressure in the KI–CoI2 system is determined by isobaric boiling point measurements.
Calculated vapor pressure isotherms of the KI–CoI2 system show negative deviations from linearity. The vapor
composition over pure CoI2 is determined, and the temperature-dependent vapor pressure of CoI2 is assessed.
The boiling point of pure CoI2 is found to be 1160 K.
INTRODUCTION
The purpose of this work was to investigate the
vaporization of KI–CoI2 melts in broad pressure and
composition ranges and to assess the temperature-
dependent equilibrium vapor pressure of CoI2 with
allowance made for CoI2 dissociation.
Information about the physicochemical properties
of constituent components and reaction products is cru-
cial for designing new materials. Liquid-phase pro-
cesses have a number of advantages over solid-state
reactions. To assess possible vaporization losses in such
processes, it is desirable to know the solid–liquid and
liquid–vapor equilibria involved.
EXPERIMENTAL
There is considerable research interest in iodine-
containing compounds, which are used in various pro-
cesses [1, 2].
Vaporization processes in the KCl–CoCl2 and KBr–
CoBr2 systems were studied in [3, 4]. The vapor pres-
sure in the KI–CoI2 system has not yet been measured.
KI–CoI2 melts are difficult to study because they
decompose readily, especially at high CoI2 concentra-
tions.
In our studies, we used isobaric boiling point mea-
surements [5]: the temperature in the system was grad-
ually raised at constant pressure until the equilibrium
vapor pressure attained the external pressure.
Samples were heated in a silica tube (2) 20 mm in
diameter (Fig. 1), with a heating coil (5) of 1-mm
nichrome wire. The length of the isothermal zone in the
furnace was 100–140 mm. Heat insulation was made
from a mixture of kaolinite, alumina, and asbestos. The
1
6
2
15
7
12
3
17
16
5
11
4
9
13
14
8
19
10
18
Fig. 1. Schematic of the experimental setup (see text for explanations).
0020-1685/04/4002-0202 © 2004 MAIK “Nauka/Interperiodica”

VAPOR PRESSURE OVER KI–CoI2 MELTS
203
Table 1. Boiling point as a function of pressure in the KI–CoI2 system at mole fractions of CoI2 from 0.2 to 1.0
bp, K
p, Pa
bp, K
p, Pa
bp, K
p, Pa
bp, K
p, Pa
bp, K
p, Pa
1.0
0.8
0.6
0.4
0.2
1020
1048
1062
1080
10270
15070
22400
31730
1028
1076
1116
1184
1193
1225
9000
13270
21130
39400
50660
68000
1065
1088
1124
1146
1182
1203
1230
1269
7470
10470
14330
19400
25870
31730
40130
50660
1276
1319
1342
1369
1396
1419
1443
12130
17470
22270
27200
33330
40330
46660
1228
1284
1319
1359
1392
1419
1441
5330
10000
13930
21070
28260
34130
41930
Table 2. Coefficients A and B in Eq. (1)
Mole fraction of CoI2
0.20
0.40
0.60
0.80
1.00
A
B
7180
6340
5520
5920
8610
9.587
9.059
9.079
9.659
12.452
furnace was enclosed in another silica tube (3), which tube from above. The cold junctions were maintained at
was plugged by 45-mm-diameter rubber stoppers (1, 9) 273 K using a thermostat (15). The thermocouple was
with vacuum-tight seals. The current leads were intro- calibrated using K2Cr2O7, KCl, NaCl, and K2SO4 as
duced through the lower stopper, and the system was
evacuated through the upper stopper. The melt temper-
ature was measured by a thermocouple introduced
through a rubber seal in the upper part of the outer tube.
To eliminate radio noise, the thermocouple and con-
necting wires were shielded. The system was evacuated
by a VN-491M roughing pump (14). The apparatus
incorporated a 5-l buffer tank for preventing pressure
fluctuations in the case of unsteady pump operation.
The pressure in the vacuum system was controlled with
the use of a stopcock (13), which was placed down-
stream of the buffer tank and served to admit air with-
out turning off the pump. The pressure in the system
was varied from 6 × 103 to 9 × 104 Pa and was measured
by a U-tube vacuum gage (11) connected directly to the
vacuum system (Fig. 1). The accuracy in our pressure
measurements was ±70 Pa.
fixed-point materials. The thermocouple signal was
recorded using an I-37 dc amplifier (16) and N-37 chart
recorder (17) (100-mm wide chart paper).
After drying and weighing (with a relative accuracy
of 10–4), the starting reagents were mixed by grinding
in a mortar enclosed in a dry box, and the resultant mix-
ture was transferred to a silica tube (6), which was then
plugged and placed in the isothermal zone of the fur-
nace. Next, the upper part of the furnace was sealed,
and the system was pumped to a preset pressure and
then slowly heated. When the mixture began to boil, the
temperature stopped rising. The corresponding thermo-
couple signal was converted to temperature using the
calibration plot. For each composition, we carried out
six to ten repeated measurements.
As the starting chemicals, we used reagent-grade KI
and analytical-grade CoI2, which were additionally
degassed in vacuum and melted in a pure argon atmo-
sphere. The measurement results are summarized in
Table 1.
The ac power to the furnace was supplied through an
autotransformer (10) equipped with a multiple reduc-
tion gearbox (18) driven by an electric motor (19),
which ensured smooth heating at a controlled rate. The
setup proved convenient in operation. Owing to its
small volume, the vacuum system cooled down rapidly,
which allowed us to repeat measurements many times.
RESULTS AND DISCUSSION
The sample (7) was placed in a silica tube 8–10 mm
in diameter, which was then mounted in the furnace in
the center of the isothermal zone. The sample weight
was 3–5 g. The temperature was measured by a Pt/Pt–
Rh thermocouple (4), which was introduced into the
The data in Fig. 2 demonstrate that the temperature
dependence of pressure in the KI–CoI2 system follows
the relation
log p = B – A/T [Pa],
(1)
INORGANIC MATERIALS Vol. 40 No. 2 2004

204
logp [Pa]
KRITSKAYA et al.
p, Pa
30.0
4.5
4.0
3.5
3
2
22.5
5
15.0
6
4
2
3
5
7.5
0
1
1
4
7
8
9
10
104/T, K–1
KI
0.2
0.4
0.6
0.8
ëÓI2
Fig. 2. Temperature dependences of vapor pressure in the
KI–CoI system; mole fraction of CoI : (1) 0.2, (2) 0.4,
2
2
(3) 0.6, (4) 0.8, (5) 1.0. The solid lines represent least
Fig. 3. Vapor pressure isotherms in the KI–CoI system at
(1) 1020, (2) 1050, and (3) 1080 K; (4–6) Raoult’s law
behavior.
2
squares fits to the present data, and the dashed line repre-
sents data from [6].
where A and B are constants. The values of A and B show large negative deviations from linearity. Accord-
found by least squares fitting are listed in Table 2.
ing to earlier results [6], the equilibrium vapor pressure
of KI in the temperature range 958–1603 K is given by
The 95% confidence intervals for these coefficients
are ∆A = ±32–68 and ∆B = ±0.025–0.073. The uncer-
tainty in our temperature measurements was ±3 K and
that in our pressure measurements was ±70 Pa. The
accuracy in the measured and fitted values of vapor
pressure is determined by the confidence interval of
log p = –10050/í – 3.52logT + 22.535 [Pa]. (2)
The composition and temperature effects on the
total vapor pressure and vapor-phase decomposition
pressure are illustrated in Table 3. Above 1100 K, CoI2
may decompose.
log p ± ∆log p , which, in turn, depends on tempera-
ture. ∆log p varied from 0.02 to 0.04, depending on
composition, which corresponds to a relative uncer-
tainty from 2 to 9%.
The vapor phase in the KI–CoI2 system may contain
KI and CoI2 monomers, KCoI3, and molecular and
atomic iodine. The equilibrium vapor composition can
The calculated total vapor pressure and decomposi- be found by solving a number of equations (equal to the
tion isotherms are displayed in Fig. 3. The isotherms number of vapor species). The vapor species present
Table 3. Vapor pressure over KI–CoI2 melts as a function of temperature
p, Pa
Mole fraction of CoI2
1020 K
1050 K
1080 K
1110 K
0
121
353
213
561
356
868
577
1314
0.2
0.4
0.6
0.8
1.0
697
1049
1544
2224
4646
7161
10250
6634
9285
12760
21160
49560
10490
17870
15040
30170
INORGANIC MATERIALS Vol. 40 No. 2 2004

VAPOR PRESSURE OVER KI–CoI2 MELTS
205
Table 4. Equilibrium composition of the vapor phase result- ations from Raoult’s law behavior, in accordance with
ing from CoI2 vaporization and decomposition
our results (Fig. 3).
Thermal-emf measurements in the range 798–913 K
[15] also revealed appreciable negative deviations from
ideality at mole fractions of CoI2 from 0.3 to 0.9, which
lends further support to the present results.
T, K
1020
1048
1062
1080
pCoI , Pa
6346
1136
2784
9967
1619
3481
15439
2141
4821
22745
2806
6186
2
pI, Pa
CONCLUSIONS
pI , Pa
2
The vaporization of KI–CoI2 melts was studied in a
wide pressure range.
over pure CoI2 are I, I2, and, probably, Co2I4. Since
studies of cobalt halide dimerization [7–9] did not
reveal significant amounts of Co2I4, this dimer will be
left out of consideration in what follows.
To find the vapor composition over CoI2, we use the
following equations:
Calculations of vapor pressure isotherms in the KI–
CoI2 system revealed negative deviations from linear-
ity. The vapor composition over pure CoI2 was deter-
mined, and the temperature-dependent vapor pressure
of CoI2 was evaluated. The boiling point of pure CoI2
was found to be 1160 K.
ëÓ(s) + 2I(g) = ëÓI2 (g),
(3)
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K1 = pCoI /p21,
(4)
(5)
2
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(6)
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pI + pI + pCol = p .
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INORGANIC MATERIALS Vol. 40 No. 2 2004

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INORGANIC MATERIALS Vol. 40 No. 2 2004