Thermolysis of disubstituted lithium and sodium orthoperiodates

Article abstract of DOI:10.1134/S0036023607040031

Na₂H₃IO₆ decomposes with water and oxygen evolution in the range of 180-250°C under the atmospheric pressure or in a dynamic vacuum. The solid residue is an equimolar mixture of NaIO₃ and Na₃IO₅. Na₃IO₅ likely occurs in the form of the dimer Na₆I₂O₁₀. An intermediate thermolysis stage is the elimination of a water molecule from the salt molecule with simultaneous disproportionation of the salt to trisubstituted orthoperiodate and metaperiodate: Na₂H₃IO₆ = 0.5Na₃H₂IO₆ + 0.5NaIO₄ + H ₂O. Li₂H₃IO₆ thermolysis under the atmospheric pressure proceeds in the same way; however in vacuum, the thermolysis yields LiIO₃ and Li₅IO₆ in the ratio of 3: 1 (mol/mol). The reason for the different thermolysis routes lies in the different stability of the intermediates M₃H₂IO ₆ and MIO₄. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036023607040031

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 4, pp. 479–485. © Pleiades Publishing, Inc., 2007.
Original Russian Text © N.I. Nikitina, Z.K. Nikitina, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 4, pp. 535–541.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Thermolysis of Disubstituted Lithium
and Sodium Orthoperiodates
N. I. Nikitina and Z. K. Nikitina
Institute of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
Received November 10, 2005
Abstract—Na2H3IO6 decomposes with water and oxygen evolution in the range of 180–250°C under the atmo-
spheric pressure or in a dynamic vacuum. The solid residue is an equimolar mixture of NaIO3 and Na3IO5.
Na3IO5 likely occurs in the form of the dimer Na6I2O10. An intermediate thermolysis stage is the elimination of
a water molecule from the salt molecule with simultaneous disproportionation of the salt to trisubstituted ortho-
periodate and metaperiodate: Na2H3IO6 = 0.5Na3H2IO6 + 0.5NaIO4 + H2O. Li2H3IO6 thermolysis under the
atmospheric pressure proceeds in the same way; however in vacuum, the thermolysis yields LiIO3 and Li5IO6
in the ratio of 3 : 1 (mol/mol). The reason for the different thermolysis routes lies in the different stability of the
intermediates M3H2IO6 and MIO4.
DOI: 10.1134/S0036023607040031
Lithium periodate Li2H3IO6 and sodium periodate of the formation of hexavalent iodine compounds upon
the thermolysis of disubstituted potassium and rubid-
ium orthoperiodates.
Na2H3IO6, poorly soluble in water, were among the first
salts of orthoiodic acid prepared more than 100 years
ago. In 1960, Siebert [1, 2] studied the preparation con-
ditions of these salts and characterized them structur-
ally on the basis of IR spectra. Later, an X-ray structure
experiment was performed for both salts [3, 4] and
H3IO26– anions were identified in them. According to
Dratovsky [5, 6], the thermolysis of the specified salts
occurs at about 200°C with the elimination of water and
oxygen; the solid thermolysis products are formulated
as M2IO4. Proceeding from rather unconvincing mag-
netic measurements, it was claimed that the products
Later [10], Dratovsky and colleagues themselves
cast doubt on their own interpretation of the thermoly-
sis of the lithium and sodium salts. We decided to study
in more details the thermolysis of Li2H3IO6 and
Na2H3IO6 with the goal of elucidating whether this pro-
cess is radically different in periodates of light and
heavy alkali metals. Another goal was to understand the
nature of the solid thermolysis products.
EXPERIMENTAL
were salts of the hexavalent iodine anion IO24– .
Periodates Li2H3IO6 and Na2H3IO6 were prepared
by the procedure described in work [1]. The reagents
and their thermolysis products were characterized by
chemical analysis, powder X-ray diffraction, IR spec-
troscopy, and thermogravimetry. For chemical analysis,
The ordinary thermolysis route for acid salts of
polybasic acids is the elimination of water and the pro-
duction of a polynuclear anion, in particular a binuclear
anion; examples are hydrogensulfate–pyrosulfate and
hydrogenchromate–bichromate transformations. In
some cases, water can split from the acid orthoanion to an aliquot of the substance was dissolved in water or
hydrochloric acid. Ions I7+ and I5+ were determined by
iodometric titration in the presence of mannitol. Pow-
der X-ray diffraction patterns were recorded don an
ADP-2-01 diffractometer using CuKα radiation. A sam-
ple (a fluorinated oil mull) was placed in a fused silica
cell. IR spectra were recorded as Nujol mulls between
KBr plates interlaid with a polyethylene film on a UR20
spectrophotometer. Thermogravimetric experiments
were performed on a Paulik-Paulik-Erdey Q-1500 deri-
vatograph from 20 to 500°C at a heating rate of
5 K/min; sample sizes were 0.2–0.3 g. Thermolysis at
a fixed temperature was carried out in vacuum system
yield a metaanion, e.g., H2PO–4 = H2O + PO3– .
It was only recently that we managed to develop
processes for the preparation of disubstituted orthope-
riodates of heavy alkali metals (potassium [7] and
rubidium [8]). We found that K2H3IO6 and Rb2H3IO6,
unlike their lithium and sodium analogues, split water
even at ~100°ë:
2K2H3IO6 = K4I2O9 + 3H2O;
they convert to the salts of the binuclear heptavalent
iodine anion I2O49– , which is built of two face-sharing
iodine–oxygen octahedra [9]. We did not find any sign at p ~ 133 Pa with continuous removal of volatiles.
479

480
N.I. NIKITINA, Z.K. NIKITINA
330°ë (Fig. 1) does not show the absorption bands
ν(éç) or δ(IOH). The X-ray diffraction pattern of this
residue (Table 1) shows NaIO3 reflections and far
weaker reflections from one more phase or phases. In
the residue, there are 25.4% I7+ and 27.2% I5+; the
molar proportion is I7+ : I5+ = 0.48 : 0.52.
Disubstituted orthoperiodate can be represented as
a complex of metaperiodate, sodium oxide, and water:
NaIO4 · 0.5Na2O · 1.5H2O. After removal of all water
and conversion of one-half periodate to iodate, the
composition of the residue (0.48NaIO4 · 0.50Na2O or
NaIO4 · 1.04Na2O) approximately corresponds to salt
Na3IO5. We prepared this compound in the individual
state through dehydrating trisubstituted sodium ortho-
periodate Na3H2IO6 [1]. The process is smooth at
270°ë and the atmospheric pressure. The IR spectrum
of the resulting Na3IO5 is displayed in Fig. 1; its X-ray
diffraction pattern is described in Table 1.
e
d
c
The isothermal thermolysis of Na2H3IO6 was stud-
ied in a vacuum at 180°ë. We chose the lowest temper-
ature at which the thermolysis still occurs at a notice-
able rate in hopes of resolving process stages. Weight
loss from an Na2H3IO6 sample (1–2 g) under these con-
ditions occurs for 15–18 h; its value is 12.5–13.0%. The
composition of the solid product (I7+, 26.0%; I5+,
26.9%), its IR spectrum, and X-ray diffraction pattern
coincide with those of the aforementioned DTG prod-
uct. This means that the IR spectrum of the product cor-
responds to the superposition of the NaIO3 and Na3IO5
spectra and the X-ray diffraction pattern contains weak
Na3IO5 reflections in addition to NaIO3 reflections.
Thus, the complete thermolysis in a vacuum at 18°C, as
during DTG, can be described by
b
‡
Na2H3IO6 = 0.5NaIO3 + 0.5Na3IO5 + 1.5H2O + 0.25O2. (1)
We also studied solid products at intermediate
stages of Na2H3IO6 thermolysis after 6.9 and 11.5% of
the weight was lost. In the first case, we found 41.1%
I7+ and 9.0% I5+ in the product; the molar proportion
was I7+ : I5+ = 0.82 : 0.18, which corresponds to the ther-
molysis of 0.36 mol of the starting periodate by Eq. (1).
However, the calculated weight loss should be as low as
12.87 · 0.36 = 4.63%; this means that 6.9 – 4.63 =
2.27% ç2é is evolved as a result of dehydration only:
400
500 600 700
800 900 1100 1300
–1
ν, cm
Fig. 1. IR absorption spectra of sodium iodates:
(a) Na H IO , (b) its thermolysis product in a vacuumat
180°C until weight loss is 6.9%, (c) a DTG product,
(d) Na IO , and (e) NaIO .
2
3
6
3
5
3
Salt samples were 1–2 g. The weighing accuracy
was 10–4 g.
Na2H3IO6 = 0.5NaIO4 + 0.5Na3IO5 + 1.5H2O. (2)
In the IR spectrum of the product, indeed, there is
a narrow absorption band of the IO–4 ion at 850 cm–1; in
the X-ray diffraction pattern, there is the strongest
reflection of NaIO4 (d = 4.901 Å). The weight loss in
reaction (2) is 9.93%; from this, the degree of decom-
position by this equation is 2.27/9.93 = 0.23.
RESULTS AND DISCUSSION
Na2H3IO6. During a thermogravimetric experiment,
weight loss starts at 205–210°ë and proceeds in the
range of 205–270°ë in two poorly resolved stages at
205–230 and 240−270°ë. Both stages are accompanied
by endotherms. The second endotherm is about one-
tenth as strong as the first one. Weight loss at the first
In the product of the experiment with 11.5% weight
loss, we found 31.6% I7+ and 20.32% I5+; the mole propor-
stage is ~8% and at the second, ~5%; the overall weight tion is I7+ : I5+ = 0.61 : 0.39, which corresponds to the of
loss is ~13%. The IR spectrum of the solid residue after decomposition 0.78 mol of the starting periodate by
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THERMOLYSIS OF DISUBSTITUTED LITHIUM AND SODIUM ORTHOPERIODATES
481
Table 1. X-ray diffraction data for the thermolysis products of sodium orthoperiodate Na2H3IO6
Thermolysis products
NaIO3
Na3IO5
DTG
180°C; weight loss, 6.9%
d, Å
Irel
d, Å
Irel
d, Å
Irel
d, Å
Irel
5.007
60
4.942
4.770
30
17
4.901
100
4.683
4.450
4.270
4.058
69
26
59
16
4.281
4.080
3.968
67
15
10
4.270
4.058
70
20
3.978
2.683
80
85
3.507
3.198
2.945
2.876
2.827
54
22
50
42
42
3.203
2.954
2.876
2.829
2.679
2.141
2.108
2.679
32
100
41
16
16
28
8
3.198
2.945
2.876
32
100
23
2.784
2.726
28
67
16
2.644
2.512
19
9
2.518
2.484
22
10
2.512
22
2.417
2.338
11
15
2.347
2.284
2.141
2.108
2.038
1.893
1.860
1.836
1.796
1.717
1.677
1.648
1.592
16
10
28
8
2.347
2.139
2.034
14
29
10
2.287
30
2.139
1.653
2.042
1.899
16
15
20
28
2.108
2.038
25
10
18
10
21
30
10
36
10
10
100
1.868
48
1.836
1.794
1.716
1.674
18
28
11
29
1.807
1.716
1.675
1.653
34
12
27
15
1.647
1.591
43
36
1.512
17
1.475
1.429
14
11
1.474
1.426
11
11
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482
N.I. NIKITINA, Z.K. NIKITINA
Li2H3IO6. Weight loss during thermogravimetric
analysis starts at 190°ë and ends about 250°ë. The pro-
cess is accompanied by a strong endotherm at
190−225°ë, which transforms to a small exotherm at its
end. The relevant weight loss is 14.1%. In the IR spec-
trum of the solid residue (Fig. 2b), there are no absorp-
tion bands ν(éç) or δ(IOH); in the X-ray diffraction
pattern (Table 2, column T), there are reflections of
LiIO3 and far weaker reflections from one more phase
or phases. In the solid thermolysis product, we found
33.08 I7+ and 28.08% I5+ 28.08; i.e., the molar propor-
tion is I7+ : I5+ = 0.54 : 0.46. These data can be ascribed
to the occurrence of the reaction
d
c
b
Li2H3IO6 = 0.5LiIO3 + 0.5Li3IO5 + 1.5H2O + 0.25O2 (4)
by analogy with the sodium salt (Eq. (1)), but only by
92%. The remaining 8% Li2H3IO6 are likely dehy-
drated:
Li2H3IO6 = 0.5Li4I2O9 + 1.5H2O.
(5)
‡
The isothermal thermolysis of Li2H3IO6 was studied
in a vacuum at 180°ë. Under these conditions, an
Li2H3IO6 sample weighing 1.55 g lost weight over the
course of about 25 h (stage F); the loss was 18.5%. We
studied the final product (F) and the products at inter-
mediate thermolysis stages after the weight loss was
10.2% (stageA), 11.9% (stage B), and 15.3% (stage C).
The results of analysis of the thermolysis products are
listed in Table 3. Li2H3IO6, unlike the sodium salt, loses
some iodine during thermolysis; the loss is small but
noticeable. In the solid products we found 0.98–0.95 of
the iodine contained in the starting disubstituted lith-
ium orthoperiodate. It is known that lithium salts of
hydrohalic acids decompose in two routes: one route
involves the production of halogen and the evolution of
only oxygen, and the other involves the production of
oxide and the evolution of oxygen and elementary halo-
gen [11, 12]. The lithium oxide produced by the ther-
molysis reaction
400 500
600
700 800 900 1100 1300
ν, cm–1
Fig. 2. IR absorption spectra of lithium periodate and its
thermolysis products: (a) Li H IO , (b) a DTG product,
(c) the final thermolysis product (stage F) at 180°C in vac-
2
3
6
uum, and (d) an intermediate (stage B) at 180°C in vacuum.
Eq. (1). The calculated weight loss is 12.87 · 0.78 =
10.04%. This means that 11.5 – 10.04 = 1.46% ç2é
is evolved in reaction (2); the degree of decomposition
of the start ing periodate is 1.46/9.93 = 0.15.
Li2H3IO6 = Li2O + I + 1.5H2O + 1.75O2
(6)
does not remain free; rather, it can participate in the for-
mation of Li2H3IO6, Li3IO5, or Li5IO6. Of these com-
pounds, Li5IO6 is the only known and characterized
compound [6, 13]. However, we will demonstrate that
there are reasons to think that all these compounds are
formed during the process in question.
Probably, as the sodium salt, Li2H3IO6 starts its ther-
molysis with the abstraction of a water molecule and
disproportionation to Li3H2IO6 and LiIO4. The distinc-
tion is that lithium metaperiodate at 180−200°ë decom-
The analysis of the solid products of incomplete
thermolysis of Na2H3IO6 proves the complexity of the
process. Likely, the process starts with the elimination
of one water molecule from the salt molecule and
simultaneous disproportionation of disubstituted ortho-
periodate to metaperiodate and trisubstituted orthoperi-
odate:
Na2H3IO6 = 0.5Na3H2IO6 + 0.5NaIO4 + ç2é. (3)
Convincing evidence in favor of intermediate NaIO4
formation was presented above. A provisional presence poses to iodate, and it is not found in the thermolysis
of Na3H2IO6 in the solid product is proven by a weak products. There are strong reflections of LiIO3 in the
reflection with d = 4.44 Å on the X-ray diffraction pat- X-ray diffraction patterns of the solid thermolysis prod-
terns of the products of incomplete thermolysis: this ucts at all stages (Table 2). Apart from these reflections,
reflection is not observed when the thermolysis is over. two sets of differing reflections are distinguished.
This is the strongest reflection in our X-ray diffraction Referring to the IR spectra of the products (Fig. 2), we
pattern of Na3H2IO6.
assign these sets of reflections to Li3H2IO6 and Li3IO5,
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THERMOLYSIS OF DISUBSTITUTED LITHIUM AND SODIUM ORTHOPERIODATES
483
Table 2. X-ray diffraction data for the thermolysis products of lithium orthoperiodate Li2H3IO6
T
B
F
LiIO3
d, Å
Irel, %
d, Å
Irel, %
d, Å
Irel, %
d, Å
Irel, %
4.732
4.599
15
19
4.732
13
4.75
23
4.529
4.077
100
17
4.364
59
4.013
3.626
3.493
11
23
100
3.493
3.215
74
3.493
100
3.50
100
136
3.018
2.739
2.691
2.585
11
24
15
18
2.739
17
2.739
2.585
23
27
2.741
2.587
27
8
2.582
2.518
31
11
2.501
2.420
2.371
2.270
2.186
2.156
2.049
6
8
7
2.368
2.270
2.206
2.154
2.032
1.989
1.879
1.821
1.792
6
16
5
2.371
2.270
5
2.374
2.272
8
16
10
17
8
16
10
14
7
2.156
2.076
21
6
2.158
1.882
18
23
8
1.879
27
24
7
1.881
36
3
1.794
1.748
4
4
1.795
1.750
3
3
1.755
1.735
10
12
1.717
1.695
1.620
1.581
1.512
1.497
1.473
1.395
1.350
1.276
1.245
4
16
17
4
1.695
1.621
1.584
22
12
7
1.695
1.621
1.581
26
21
8
1.696
1.621
1.583
19
7
6
9
8
1.473
1.394
1.350
1.277
1.244
6
9
7
8
9
4
1.473
1.395
1.349
1.277
1.244
7
12
6
1.473
1.395
1.349
1.276
1.243
8
5
5
5
3
8
4
5
10
10
7
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

484
N.I. NIKITINA, Z.K. NIKITINA
Table 3. Parameters of Li2H3IO6 thermolysis at 180°C in
Different thermolysis products are obtained in the
TGA experiment under the atmospheric pressure and
after the complete thermolysis in a vacuum at 180°ë
(stage F). In the TGA experiment, the major products
are approximately equimolar amounts of LiIO3 and
Li3IO5; the complete thermolysis in a vacuumyields
LiIO3 and Li5IO6 in a molar proportion of about 3 : 1.
The X-ray diffraction pattern of the product of stage F
(Table 2) is a set of reflections from LiIO3 and contains
one weak reflection with d = 2.076 Å. This reflection is
the strongest in the X-ray diffraction pattern of Li5IO6
[6, 13]. In the X-ray diffraction pattern of the solid res-
idue from TGA (Table 2), there are strong reflections
from lithium iodate and, in addition, reflections that can
be assigned to previously unknown Li3IO5.
vacuum
Concentration
in the solid
product, %
Proportion
Time, Weight
Stage
I7+ : I5+ : I0
h
loss, %
I7+ I5+
A
B
C
F
8
10.2 35.18 22.80 0.59 : 0.39 : 0.02
11.9 30.17 28.30 0.50 : 0.48 : 0.02
15.3 25.32 34.32 0.41 : 0.55 : 0.04
18.5 19.31 42.21 0.30 : 0.65 : 0.05
13.6 33.08 28.08 0.54 : 0.46 : 0
14
20
25
DTG, 5 K/min,
30 min
The solid products of stages A, B, and C show the
same strong reflections of lithium iodate and 12 weaker
reflections from an unknown phase; these 12 reflections
are the strongest in the product of stage B, weaker in the
product of stage A, and the weakest in the product of
stage C. The X-ray diffraction pattern of the product of
stage B is described in Table 2. It is logical to assign
these reflections to trisubstituted lithium orthoperiodate
for the following two reasons. First, their intensity cor-
relates with the suggested Li3H2IO6 proportion in the
solid thermolysis products at these stages. Second, the
IR spectrum does not contradict this assignment. The
region of 900–1300 cm–1, in which librations and bend-
ing vibrations of I–O–H groups appear, in product B
differs from that in Li2H3IO6: instead of a single band
respectively. Analyzing the data obtained by chemical
analysis, IR spectra, and X-ray powder diffraction and
in view of the above reasoning, we can identify the ther-
molysis products as follows (referred to 1 mol of the
starting Li2H3IO6):
Stage A:
0.16Li2H3IO6 + 0.43Li3H2IO6
+ 0.39LiIO3(solid residue)
+ 0.02[I] + 0.83H2O + 0.46[O]
(volatiles, 10.4%),
at 1270 cm–1, there are three narrow medium-intensity
absorption bands at 915, 1200, and 1280 cm–1 (Fig. 2).
Stage B:
0.47Li3H2IO6 + 0.03Li5IO6
+ 0.48LiIO3(solid residue)
+ 0.02[I] + 1.03H2O + 0.53[O]
(volatiles, 12.3%),
Thus, a product of composition å2IO4 can be
obtained as a result of the thermolysis of Na2H3IO6 or
Li2H3IO6, but this is an approximate equimolar mixture
of MIO3 and M3IO5 rather than an individual compound
of hexavalent iodine as believed in works [5, 6]. The
question is whether M3IO5 is a salt of the anion contain-
ing pentacoordinate iodine or this is dimer M6I2O10.
Unambiguous structural evidence in favor of the exist-
Stage C:
0.29Li3H2IO6 + 0.12Li5IO6
+ 0.55LiIO3(solid residue)
+ 0.04[I] + 0.21H2O + 0.68[O]
(volatiles, 15.7%),
ence of the IO35– ion, in which the iodine is surrounded
by five oxygen atoms, has not been obtained. The con-
clusions made in works [14, 15] with the help of anal-
ogy with rhenium salts of similar composition do not
seem convincing. The IR spectrum of the salt with com-
position Na3IO5 we obtained provides evidence in favor
of the dimer. The salt exhibits absorption bands at 500–
550 cm–1 associated with I–O bridging bonds, as the
Stage F:
0.08Li3IO5 + 0.23Li5IO6
+ 0.65LiIO3(solid residue)
+ 0.05[I] + 1.5H2O + 0.77[O]
(volatiles, 19.0%).
I4O4– anion, while monomeric anions, e.g., IO56– , do
9
not demonstrate this band.
There is some analogy with Ag2H3IO6 [16, 17].
The thermolysis products in works [16, 17] were a mix-
ture of AgIO3 and Ag3IO5 or Ag5IO6, and an intermedi-
ate (Ag2HIO5) was isolated; actually (as shown by
X-ray diffraction [18]), this intermediate contained the
The calculated weight of the volatiles satisfactorily
matches the measured weight loss at each stage
(Table 3).
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THERMOLYSIS OF DISUBSTITUTED LITHIUM AND SODIUM ORTHOPERIODATES
485
8. G. V. Shilov, Z. K. Nikitina, Yu. A. Dobrovol’skii, et al.,
4–
10
dimeric anion H2I2O . Salts of heavy alkali metals
Koord. Khim. 30 (7), 483 (2004).
K2H3IO6 and Rb2H3IO6 reversibly lose their water at
~100°ë [7, 8]; their lithium, sodium, and silver ana-
logues, with an essential covalence of the interaction
between the metal and anion, split water at higher tem-
peratures. As a result, the anion degrades, and some
iodine is reduced to the pentavalent state.
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3169 (1965).
12. D. G. Lemesheva and V. Ya. Rosolovskii, Zh. Neorg.
Khim. 23 (8), 2055 (1978).
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007