Dehydration of lithium iodide crystal hydrate in vacuum

Article abstract of DOI:10.1134/S003602360711006X

Dehydration of the LiI ? 3H₂O crystal hydrate in vacuum has been investigated at 20-25°C. The decomposition of the LiI ? 3H ₂O crystal hydrate in vacuum proceeds to monohydrate. During heating of lithium iodide monohydrate, evolution

Full text of DOI:10.1134/S003602360711006X

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 11, pp. 1675–1678. © Pleiades Publishing, Inc., 2007.
Original Russian Text © D.S. Sofronov, A.Yu. Voloshko, O.V. Shishkin, K.A. Kudin, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 11, pp. 1783–1786.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Dehydration of Lithium Iodide Crystal Hydrate in Vacuum
D. S. Sofronov, A. Yu. Voloshko, O. V. Shishkin, and K. A. Kudin
GNU NTK Institute for Single Crystals, pr. Lenina 60, Kharkiv, 61004 Ukraine
e-mail: techno@isc.kharkov.com
Received November 30, 2006
Abstract—Dehydration of the LiI · 3H O crystal hydrate in vacuum has been investigated at 20–25°ë. The
2
decomposition of the LiI · 3H O crystal hydrate in vacuum proceeds to monohydrate. During heating of lithium
2
iodide monohydrate, evolution of water vapor is observed at 30–100 and 100–170°ë. Above 200°ë, evolution
of molecular oxygen is observed, possibly, because of the decomposition of lithium peroxide that is formed in
side reactions during the decomposition of lithium iodide crystal hydrate.
DOI: 10.1134/S003602360711006X
At present, lithium iodide based crystals are used as
Investigation of the dehydration of sodium iodide
neutron detectors [1]. A characteristic feature of crys- dihydrate in vacuum showed [6] that dehydration could
tals based on alkali metal iodides is the sensitivity of be carried out at 4.0–6.7 Pa at 20–30°ë with the forma-
their functional characteristics to oxygen-containing tion of anhydrous iodide. In this case, no hydrolysis of
impurities.
Contamination of crystals by oxygen-containing
impurities can proceed due to the following two factors:
lithium iodide or formation of impurity hydroxide
groups is observed. Therefore, there is a principal pos-
sibility to carry out the decomposition of crystal
hydrate in vacuum at temperatures no higher than 40°ë.
(
i) the starting salt used for growing the crystal In addition, on investigation of behavior of the starting
already contains oxygen-containing impurities, and salt of lithium iodide [7], which is used for growing sin-
their amount depends on the physicochemical parame- gle crystals, it is found that evolution of the water vapor
ters of salt synthesis and purification method;
is observed at 50–150°ë, and evolution of a component,
whose identification was not carried out is observed at
T > 280°C, According to spectrometric data, gas evo-
lution at T > 280°C is not associated with water evo-
lution [7].
(ii) oxygen-containing impurities can form during
crystal growth, and this process is determined by the
physicochemical growth conditions, as a rule, by the
composition of the gas phase.
The purpose of this work is to investigate the dehy-
dration of lithium iodide crystal hydrate in vacuum and
to identify the products of its thermodesorption.
The simplest and widely used methods for preparing
metal iodides are based on synthesis in aqueous solu-
tions. As a rule, treatment of pure lithium carbonate
solution by hydroiodic acid at T < 77°C or treatment of
a solution of lithium sulfate by barium iodide is used to
obtain lithium iodide [2].
EXPERIMENTAL
Test samples were lithium iodide crystal hydrate
Lithium iodide is isolated from aqueous solutions as
crystal hydrate, specifically lithium iodide trihydrate
LiI · 3H O obtained on vacuum evaporation of an aque-
2
ous solution acidified with hydroiodic acid.
LiI · 3H O. It is very difficult to dehydrate this com-
2
The dehydration of crystal hydrate was carried out
in two stages. At the first stage, dehydration was carried
out at room temperature (20−25°ë) under continuous
evacuation. For this purpose, a sample 10–50 g in weight
was placed into a silica cell. For investigation, we used a
setup, which is schematically drawn in Fig. 1. The cell
with the sample was connected to a vacuum system
and evacuated using forevacuum pump 3 at room
temperature through nitrogen trap 2 until acquiring
the limiting pressure (the pressure that was invari-
able for 30–40 min and was equal to 4.0–6.7 Pa).
pound and obtain an anhydrous high-purity product.
Removal of water molecules proceeds at elevated tem-
peratures and is hampered by hydrolysis. For example,
two water molecules are removed on heating to 300°ë,
and on the further heating, removal of remaining water
proceeds with an attendant hydrolysis of the salt to
yield lithium hydroxide or oxide [3–5]. Melting with an
excess of iodine does not prevent hydrolysis. Dehydra-
tion of LiI in a hydrofluoric acid flow for 1.5–2 h leads
to the formation of lithium iodide with a water content
of ~2–3%. Distillation in vacuum (1.33 Pa and
8
00−850°ë) allows decreasing the water content to
After carrying out the dehydration and achieving the
limiting pressure in the vacuum system, the thermode-
0
.01–0.02 wt % [3].
1
675

1
676
SOFRONOV et al.
1
2
1
1
5
4
8
9
2
1
0
1
6
7
3
Fig. 1. Schematic diagram of the setup for mass-spectrometry measurements: (1) furnace, (2) nitrogen trap, (3) forevacuum pump,
4) separation chamber with leak valve, (5) mass-spectrometer chamber, (6) pressure transducer, (7) temperature sensor, (8) nitrogen
trap, (9) diffusion pump, (10) forevacuum pump, (11) omegatronic partial pressure meter, (12) recorder.
(
sorption of the obtained samples was investigated (sec- ature, the product was obtained as a white finely dis-
ond stage). The cell with the sample dehydrated at persed easily flowing powder. According to the quanti-
room temperature was placed into a furnace and heated tative analysis (see table), the final product of dehydra-
to 400–440°ë at a rate of 100–150° K/h determining the tion of lithium iodide trihydrate 4.0–6.7 Pa was
pressure in the vacuum system according to readings of monohydrate.
PMT-2 transducer 6.
Since dehydration of lithium iodide trihydrate in
At both stages, the composition of the gas medium
vacuum 4.0–6.7 Pa proceeds to monohydrate, it is nat-
was determined by a mass spectrometer. The gaseous
ural that elevated temperatures will be necessary for the
compounds evolved from the sample entered the cham-
complete dehydration of lithium iodide monohydrate.
ber with leak valve 4 through the vacuum system and
Therefore, it is of interest to consider the dehydration of
lithium monohydrate on heating. For this purpose, the
then entered the chamber of an IPDO-2 mass analyzer.
The results of measurements were put out to detector
samples of lithium monohydrate obtained after dehy-
1
2. From the mass-analyzer chamber, the gas flow was
dration of trihydrate in vacuum at room temperature
were heated in vacuum to 400°ë. The results of mea-
surements are presented in Fig. 3.
pumped through nitrogen trap 8 using diffusion pump 9
and forevacuum pump 10.
The mass-spectrometry measurements of the com-
position of the gas phase on heating of lithium iodide
monohydrate show two temperature ranges of the elim-
ination of water molecules: 30–100°ë with a maximum
at 70°ë and 100–170°ë with a maximum at 145°ë.
Above 200°C, evolution of molecular oxygen is
observed. Above 280°ë, the intensity of its evolution
considerably increases. Therefore, the previously uni-
dentified change in pressure [7], which is observed on
heating the lithium iodide samples used for single crys-
tal grow above 280°ë in vacuum, is due to the evolution
of molecular oxygen into the gas phase.
RESULTS AND DISCUSSION
During continuous pumping out (Fig. 2, curve 1) at
0–25°ë, lithium iodide trihydrate decomposes at pres-
2
sures of 13.3–6.65 Pa. The mass-spectrometry mea-
surements of the composition of the gas phase during
dehydration showed water evolution to the gas phase.
No evolution of other gaseous compounds was found
during vacuum dehydration of crystal hydrates at room
temperature.
On attainment of the limiting pressure in the system,
pumping was stopped, and the product was analyzed
for water according to the Fischer method [8], iodine,
Oxygen evolution is unexpected, since impurity
and the metal cation. After dehydration at room temper- generation on the dehydration of lithium iodide trihy-
Composition of the starting lithium iodide crystal hydrate and the product of its dehydration in vacuum
Starting crystal hydrate, wt %
After dehydration in vacuum, wt %
Compound
M+
3.2
I
H O
M+
4.01
I
H O
2
2
LiI · 3H O
67.9
28.9
84.49
11.5*
2
*
The calculated water concentration in LiI · H O is 11.92 wt %.
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 11 2007

DEHYDRATION OF LITHIUM IODIDE CRYSTAL HYDRATE IN VACUUM
p, Pa
1677
7
6
5
3
2
1
9.8
6.5
3.2
9.9
6.6
3.3
0
300
600
900
1200
1500
τ, min
Fig. 2. Pressure in the vacuum system on dehydration of lithium iodide crystal hydrate at room temperature. Sample weight, 10 g.
p, Pa
1
1
1
6.00
3.30
0.64
1
2
7
5
2
.98
.32
.66
3
4
0
50
100
150
200
250
300
350
400
T, °C
Fig. 3. Total pressure (1) and partial pressures of H O (2), oxygen (3), and carbon dioxide (4) in the mass-spectrometer chamber
2
on heating the lithium iodide monohydrate obtained on dehydration of trihydrate in vacuum at room temperature.
drate, which decomposes with oxygen evolution, has oxygen evolution at T > 200°C. The qualitative test for
not been described. The analysis of the literature data peroxide ion with titanium(IV) ions [10] gave the posi-
on the properties of oxygen compounds of lithium tive result (the solution became yellowish-orange),
showed that this impurity can be lithium peroxide
2–
2
which confirms the presence of O .
Li O , which forms a crystal hydrate of the composition
2
2
Li O · H O [9]; the decomposition of this hydrate is
observed above T > 200°ë with oxygen evolution the carbon dioxide content in the gas mixture gradually
according to the equation
In addition, as lithium iodide is heated above 120°ë,
2
2
2
increases, which can be a consequence of the presence
of a carbonate impurity.
Li O + H O
2LiOH + 1/2O2.
2
2
2
In summary, the experimental data obtained show
At T > 300°C, along with decomposition of lithium per- that at 4.0–6.7 Pa at 20–25°ë, LiI · 3H O decomposes
2
oxide by the water vapor, its thermal decomposition is to monohydrate, which in turn decomposes on the fur-
also observed. It is possible that one side process on ther heating. In the lithium iodide samples that passed
dehydration of lithium iodide trihydrate is the forma- dehydration in two stages, namely, at room tempera-
tion of peroxide hydrate, whose decomposition leads to ture and on the further heating in vacuum with contin-
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 11 2007

1
678
SOFRONOV et al.
uous pumping out of evolving water, the content of
hydroxide groups in the dehydrated lithium iodide is
less than 0.1 wt %.
3. I. V. Tananaev, S. M. Petushkova, and G. V. Shpineva,
Zh. Neorg. Khim. 3 (5), 1071 (1958).
4. S. I. Bakum and S. F. Kuznetsova, Zh. Neorg. Khim. 39
(7), 1090 (1994).
5
6
7
8
. N. I. Pirogov and B. V. Ershler, Zh. Prikl. Khim. 29 (5),
071 (1956).
ACKNOWLEDGMENTS
1
We thank T.A. Blank for water determination in lith-
ium iodide samples.
. B. V. Grinyov, V. G. Gerasimov, A. J. Voloshko, et al.,
Func. Mater. 11 (3), 575 (2004).
. D. S. Sofrponov, B. V. Grinyov, A. Yr. Voloshko, et al.,
Func. Mater. 12 (3), 559 (2005).
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