Thermal decomposition kinetics of potassium iodate

Article abstract of DOI:10.1007/s10973-010-1162-5

The thermal decomposition of potassium iodate (KIO₃) has been studied by both non-isothermal and isothermal thermogravimetry (TG). The non-isothermal simultaneous TG-differential thermal analysis (DTA) of the thermal decomposition of KIO_3<

Full text of DOI:10.1007/s10973-010-1162-5

J Therm Anal Calorim (2011) 103:943–955
DOI 10.1007/s10973-010-1162-5
Thermal decomposition kinetics of potassium iodate
•
K. Muraleedharan M. P. Kannan
•
T. Ganga Devi
Received: 20 September 2010 / Accepted: 3 November 2010 / Published online: 8 December 2010
´
´
ꢀ Akademiai Kiado, Budapest, Hungary 2010
Abstract The thermal decomposition of potassium iodate
(KIO3) has been studied by both non-isothermal and iso-
thermal thermogravimetry (TG). The non-isothermal
simultaneous TG–differential thermal analysis (DTA) of
the thermal decomposition of KIO3 was carried out in
nitrogen atmosphere at different heating rates. The iso-
thermal decomposition of KIO3 was studied using TG at
different temperatures in the range 790–805 K in nitrogen
atmosphere. The theoretical and experimental mass loss
data are in good agreement for the thermal decomposition
of KIO3. The non-isothermal decomposition of KIO3 was
subjected to kinetic analyses by model-free approach,
which is based on the isoconversional principle. The iso-
thermal decomposition of KIO3 was subjected to both
conventional (model fitting) and model-free (isoconver-
sional) methods. It has been observed that the activation
energy values obtained from all these methods agree well.
Isothermal model fitting analysis shows that the thermal
decomposition kinetics of KIO3 can be best described by
the contracting cube equation.
has been used to obtain thermal stability parameters of
solids [1–4]. The thermal decomposition data generated
from TG can be analyzed and manipulated to obtain kinetic
parameters such as activation energy (E) and pre-exponen-
tial factor (A) [5, 6]. Solid state kinetic data are of practical
interest for the large and growing number of technologically
important processes. Kinetic studies predict how quickly a
system approaches equilibrium and also help to understand
the mechanism of the process [7]. A number of reviews are
available in the literature on these processes [8–14]. Several
authors have emphasized the practical and theoretical
importance of information on the kinetics and mechanism of
solid state decompositions [15–20].
KIO3 is a white crystalline powder having a molar mass
of 214.001 g which decomposes around 800 K forming
potassium iodide (KI) and oxygen. It is sometimes used in
radiation treatment, as it can replace radioactive iodine
from the thyroid. KIO3 is used for iodination of table salt
and also as an ingredient in baby formula milk. Like
potassium bromate, KIO3 is occasionally used as a
maturing agent in baking.
Keywords Contracting cube equation ꢀ Isothermal
decomposition ꢀ Model-free methods ꢀ Potassium iodate ꢀ
Thermal decomposition kinetics
KIO3 may be used to protect against accumulation of
radioactive iodine in the thyroid by saturating the body
with a stable source of iodine prior to exposure. KIO3,
when administered at high dosage for extended periods of
time, increases the occurrence of tumors in lab rats and has
poor shelf life in hot and humid climates. Single-dose,
acute toxicity experiments, using KIO3 showed that iodates
cause intoxication and death when administered in suffi-
cient quantities; in some instances, death is attributed to
renal damage with retention of non-protein nitrogen [21]. It
has been reported that the crystal structure of the I-phase in
KIO3 is rhombohedral perovskite and the shorter I–O bond
exhibits a covalent bonding character and others (I–K,
K–O, and longer I–O bonds) an ionic [22].
Introduction
Thermal decomposition of solids is an important field of
solid state chemistry with wide technical applications and
K. Muraleedharan (&) ꢀ M. P. Kannan ꢀ T. Ganga Devi
Department of Chemistry, University of Calicut, Calicut,
Kerala 673 635, India
e-mail: kmuralika@gmail.com
123

944
K. Muraleedharan et al.
Polyaniline was selectively formed on wool during the
method agree very well with the results obtained from the
reference method, flame atomic absorption spectrometry
(FAAS). Suba and Udupa [32] studied the reaction between
potassium iodate and molybdenum (VI) oxide in mixtures
of different mole ratios employing TG, derivative ther-
mogravimetry (DTG) and DTA techniques in static air
atmosphere; followed the kinetics of the reaction and
computed the values of E. Results of radiosensitization
studies by KIO3 were also reported in the literature [33].
Xiao et al. [34] studied the optical absorption properties of
KIO3 single crystals, calculated the dependence of the
absorption coefficient and discussed the characteristics of
the absorption edge.
polymerization of aniline, using KIO3 and the infrared (IR)
spectrum of polyaniline–wool composite formed shows the
presence of cysteic acid units (Cy–SO3H) resulted from the
oxidation of cystine bonds (Cy–S–S–Cy) in wool [23]. The
oxidized layer of the wool textiles became thicker and
cystine bonds in wool were converted into more cysteic
acids by oxidation, using KIO3 than using (NH4)2S2O8 and
K2Cr2O7. The formation of polyaniline on wool occurs
remarkably in the case of polymerizing aniline, using
KIO3. It has been reported that the addition of KIO3 to both
crude and refined salt resulted in significant darkening and
discoloration of most pickled vegetables regardless of the
salt source [24].
Hegde et al. [35] used slurries containing molybdenum
oxide abrasives with KIO3 as the oxidizing agent to polish
copper disks and films, and yielded relatively high removal
rates. They reported that the relatively high rates for the
removal of copper observed were due to the in situ gener-
ation of I2 by the reaction between KIO3 and MoO2. The
surface quality of the polished Cu films, however, was very
poor with surface roughness values as high as 140 nm. A
second polishing using a dilute colloidal silica suspension
containing H2O2, benzotriazole, and glycine improved the
post-polish surface quality, to surface roughness values as
low as 0.35 nm [35]. Cherian and Narayana [36] developed
a simple and selective spectrophotometric method for the
determination of trace amounts of arsenic using azure B as a
chromogenic reagent. They observed that the liberated
iodine, by the reaction of arsenic (III) with KIO3, bleaches
the violet color of azure B and is measured at 644 nm. This
decrease in absorbance is directly proportional to the con-
centration of As(III), and Beer’s law is obeyed in the range
0.2–10 lg mL-1 of As(III). They also observed that this
method can be successfully applied for the determination of
arsenic in various environmental and biological samples.
Henson et al. [25] employed the modified potassium
iodate method for the determination of hydrolysable tan-
nins for determining the protein-binding capacity of plant
polyphenolics (tannins). Shibli and Saji [26] studied the co-
inhibition characteristics of sodium tungstate along with
KIO3 for using it as a corrosion inhibitor in industrial
cooling water systems. They carried out detailed studies to
investigate the effect of oxygen in the inhibition process
and explained the nature and strength of the passive film,
and the mechanism of its formation based on the studies
conducted under different static and dynamic conditions.
The infant mortality in Xinjiang province, People’s
Republic of China, which is an area of severe iodine
deficiency and has a high infant mortality rate, has been
studied by replacing iodine through iodination of the irri-
gation water [27]. The iodine supplementation of irrigation
water in areas of severe iodine deficiency decreases neo-
natal and infant mortality and observed that iodine
replacement has probably been an important factor in the
national decrease in infant mortality. Wang et al. [28]
studied on the influence of KIO3 on the metabolism of
Escherichia coli by intrinsic fluorescence and reported that
KIO3 has great inhibiting effects on the growth of E. coli
through the pathway of protein synthesis and respiratory
chain. As arsenic reacts with KIO3 (in acidic conditions) to
liberate iodine, Revanasiddappa et al. [29] developed a
cost-effective and sensitive spectrophotometric method for
the determination of arsenic at trace level in environmental
samples. Kargosha et al. [30] reported a novel and selective
procedure (by injecting KIO3 solution) for the determina-
tion of L-cysteine and L-cystine, in pharmaceutical and
urine samples, based on vapor-generation Fourier trans-
form infrared spectrometry (FTIR).
Objectives of the present investigation
KIO3 and similar type of compounds have attracted atten-
tion due to their technological importance. The thermal
decomposition of KIO3 occurs in the temperature range
795–815 K forming the stable KI [37]. KIO3 shows an
overall decomposition of the type A ? B ? C, where A
and B are solid phases, and C is a gas. Such reactions,
typically resulting in highly reactive solid products have
attracted a big deal of research due to their theoretical and
technical relevance. Even though a vast number of publi-
cations are available in the literature on KIO3 [21–36],
studies on the thermal decomposition and kinetics are not
available. Industry needs measurements of kinetic parame-
ter for the accurate design of installation and treatment
conditions, because augmentation of temperature or elon-
gation of reaction time means more cost. The results of the
Themelis et al. [31] reported a highly selective and
simple flow injection method for the determination of
Au(III) in jewel samples, based on the catalytic effect of
Au(III) on the oxidation of 4-amino-40-methoxydiphenyl-
amine hydrochloride (Variamine Blue B base, VB) by
KIO3. They observed that the results obtained using this
123

Thermal decomposition kinetics of potassium iodate
945
kinetic investigations can also be applied to problems as
useful lifetime of certain components, oxidative and thermal
stability and quality control [38]. The objective of this work
is to investigate the thermal decomposition and kinetics of
KIO3 by both isothermal and non-isothermal TG.
Table 1 Different reaction models used to describe the reaction
kinetics
SI. no.
Reaction model
Function, g(a)
1
Power law
a1/4
2
Power law
a1/3
3
Power law
a1/2
a3/2
Reaction kinetics by TG
4
Power law
5
Exponential law
One-dimensional diffusion
Mampel (first order)
Avrami–Erofeev
Avrami–Erofeev
Avrami–Erofeev
Avrami–Erofeev
Three-dimensional diffusion
Contracting sphere
Contracting area (cylinder)
Second order
ln a
a2
The most common experimental technique employed to
study kinetics of thermally activated reactions is TG, under
the conditions of isothermal and/or non-isothermal condi-
tions. Kinetic studies of thermal decomposition of solids
constitute one of the most important applications of ther-
mal analysis. In non-isothermal TG two methods, model
fitting and model-free analyses, have been used for the
evaluation of kinetic parameters. Model-fitting methods
were among the first and most popular methods for kinetic
description, because it requires only a single heating rate
experiment to calculate the kinetic parameters. However,
the popularity of this method has been declined in favor of
isoconversional method of model-free approach [39–41].
All kinetic methods assume that the isothermal rate of
conversion, da/dt, is a linear function of the temperature
dependent rate constant, k(T), and a temperature-indepen-
dent function of conversion, f(a), which depends on the
mechanism of the reaction as:
6
7
-ln (1 - a)
[- ln (1 - a)]1/4
[- ln (1 - a)]1/3
[- ln (1 - a)]1/2
[- ln (1 - a)]2/3
8
9
10
11
12
13
14
15
16
[1 - (1 - a)1/3 2
]
1 - (1 - a)1/3
1 - (1 - a)1/2
(1 - a)-1 - 1
ln [a/(1 - a)]
Prout–Tompkins
For kinetic analyses, the main task is to get the solution
of the above temperature integral. Several methods are
available under different approaches, viz., integral, differ-
ential and approximation, for the evaluation of the tem-
perature integral. However, most of the approximation
methods neglect the low temperature end of the tempera-
ture integral. It has been reported that two-dimensional
quantities significantly influence the approximation meth-
ods [42], and the solution of the temperature integral is
achieved by numerical integration with respect to a
dimensionless activation energy variable [43].
da=dt ¼ kðTÞfðaÞ
ð1Þ
The temperature dependent function k(T) is of the
Arrhenius type and can be written as (under isothermal
conditions):
gðaÞ ¼ kt þ C
ð2Þ
where g (a) is a function, which depends on the mechanism
of the reaction. Several kinetic models are available in the
literature to describe the mechanism of solid state reactions
(see Table 1). The mechanism of the decomposition reac-
tion is usually found out by fitting the TG data into these
kinetic models/equations and choosing the model which
gives the best fit. The slope of the best fitted line will
give k. The kinetic parameters are evaluated by plotting ln
k vs. reciprocal temperature (in K).
The model-free method
Measuring the evolution of overall physical properties of a
system, by thermal methods of analysis, provides infor-
mation on macroscopic kinetics. The macroscopic kinetics
is inherently complex, because they include information
about multiple steps that occur simultaneously. Unscram-
bling complex kinetics presents a serious challenge that can
only be met by kinetic methods that provide means of
detecting and treating multistep processes. Isoconversional
methods, based on multiple heating programmes, are the
most popular methods that can meet this challenge [44].
Isoconversional kinetics rests upon evaluating a depen-
dence of the effective activation energy on conversion or
temperature and using this dependence for making kinetic
predictions and for exploring the mechanism of thermal
processes. These methods are based on the single-step
kinetic equation, Eq. 3, of non-isothermal decomposition
and are the quickest way to derive kinetic parameters for
Under non-isothermal conditions, Eq. 1 becomes:
da=fðaÞ ¼ ðA=bÞeꢁE=RT dT
ð3Þ
where b = dT/dt, the heating rate. Upon integration and
taking logarithms yields
ln gðaÞ ¼ ln½AE=bRꢂ þ ln pðxÞ
ð4Þ
R1
where pðxÞ ¼
ðeꢁx=x2Þdx and x = E/RT
x
This is the basic form of equation used for analysis of
non-isothermal data. This equation can be readily applied
once the form of the function p(x) is established.
123

946
K. Muraleedharan et al.
complex reactions involving multiple processes [45].
According to isoconversional principle, at a constant extent
of conversion, the reaction rate is a function only of tem-
perature so that:
sample mass: 10 mg, sample pan: silica. Duplicate
run’s were made under similar conditions and found that
the data overlap with each other, indicating satisfactory
reproducibility.
d½lnðda= dtÞꢂa= dTꢁ1 ¼ Ea=RT
ð5Þ
the subscript a designates the value related to a given value
of conversion
Results and discussion
The isoconversional method suggested by Flynn–Wall–
Ozawa [46] uses approximation of the integral equation,
which leads to simple linear equation for evaluation
of E. This method assumes that the conversion function
does not change with the alteration of heating rate for all
values of a, i.e., measurement of temperature correspond-
ing to fixed values of a at different heating rate is required.
Under these conditions Eq. 3 becomes:
Preview of earlier work
Solymosi reported that the thermal decomposition reaction
of KIO3, at one atmosphere pressure and in the temperature
range 485–520 ꢁC, is autocatalytic and observed rapid
reaction up to 25–30% conversion; beyond which the
reaction suddenly slows down and shows a maximum rate
around 26% conversion [50]. The Prout–Tompkins [51]
equation proved to be the most suitable for kinetic analysis
of the decomposition curve; E values of 258 and
224 kJ mol-1 were reported for the acceleratory and the
ln b ¼ ln½AfðaÞꢂ=½da=dtꢂ ꢁ E=RT
ð6Þ
and a plot of ln b vs. 1/T should give a straight line with a
slope of (-E/R).
If the values of E determined for the various values of a,
are almost constant, then certainly the reaction involves
only a single step. On the contrary, a change in E with
increasing degree of conversion is an indication of a
complex reaction mechanism that invalidates the separa-
tion of variables involved in the Ozawa, Flynn, and Wall
analysis [47]. These complications are serious, especially
in the case where the total reaction involves competitive
reaction mechanisms [48].
–1
DTG
DTA
= 10 K min
10
9
0
–5
–10
–15
–20
–25
–30
8
TG
7
Model-free kinetics rests on evaluating the Ea depen-
dence, which is adequate for both theoretical and practical
purposes of kinetic predictions [49]. Normally, model-free
kinetics does not concern with evaluating A and g(a) or f(a)
because they are not needed for performing kinetic pre-
dictions. Also, these values are hardly suitable for theo-
retical interpretation because of the strong ambiguity
associated with them. However, these values can be
determined in the frameworks of model-free kinetics.
6
780
800
820
840
860
880
900
T/K
Fig. 1 Simultaneous TG–DTG–DTA curve for the thermal decom-
position of KIO3 at 10 K min-1
1.0
0.8
–1
Experimental
3 K min
–1
5 K min
–1
7 K min
–1
0.6
AnalaR grade KIO3 of E merck is dissolved in water, re-
crystalized, dried, powdered in an agate mortar, fixed the
particle size in the ranges, 90–106 lm and kept in a vac-
uum desiccator. Non-isothermal TG–DTA analyses of
KIO3 samples were carried out on a SETARAM made
Labsys TG-DTA-1600 at four different heating rates, viz,
3, 5, 7, and 10 K min-1. The isothermal TG measurements
were carried out using the same instrument at different
temperatures, viz., 790, 795, 800, and 805 K. The opera-
tional characteristics of the TG–DTA system are; atmo-
10 K min
α
0.4
0.2
0.0
795
810
825
840
855
870
T/K
Fig. 2 a–T curve for the thermal decomposition of KIO3 at different
temperatures
sphere: flowing nitrogen, at a flow rate of 60 mL min-1
,
123

Thermal decomposition kinetics of potassium iodate
947
Fig. 3 Isothermal
decomposition of KIO3 at
different temperatures
795 K
10.0
9.5
9.0
8.5
8.0
7.5
7.0
790 K
20
40
60
80
100
20
40
60
805 K
10.0
9.5
9.0
8.5
8.0
7.5
7.0
800 K
20
40
60
10
20
30
40
t/min
t/min
decay period, respectively. Chloride, bromide, and iodide
ions exerted considerable acceleratory effects on the
reaction [50]. At higher temperatures the decomposition
was found to be of deceleratory in nature; at lower tem-
peratures the rate maximum occurred at smaller conver-
sions. The presence of halides decreased the activation
energy of the thermal decomposition reaction. Nickel oxide
effectively catalyzed the decomposition. In this case too
the decomposition started with the highest rate. The first
order equation was found to be the most suitable for the
calculation of rate constant of the catalytic reaction. The
1.0
0.8
0.6
0.4
0.2
0.0
790 K
795 K
800 K
805 K
α
value of the E observed was 216 kJ mol-1
.
0
15
30
45
60
75
90
Nuclear quadrupole resonance of KIO3 indicated that
KIO3 behaves quite differently from the other alkali metal
iodates. It is ferroelectric below 212 ꢁC and undergoes four
phase transitions, at -190, -10, 75, and 220 ꢁC [50, 52].
Above 220 ꢁC, the0 symmetry is rhombohedral (a =
t/min
Fig. 4 a–t curves for the thermal decomposition of KIO3 at different
temperatures
˚
9.012 A, a = 89ꢁ 14 ). Below this temperature, some tri-
occurred at -163, -15, 72.5, and 214 ꢁC. Pure quadrupole
resonance of 127I has been detected in all the alkali metal
iodates, showing that none of them has a perovskite-like
structure [50]. Based on this and additional DTA
gonal lines show further splitting; caused by deviation from
the trigonal symmetry of the high temperature triclinic
form. Salje [53] found that phase transitions of first order
123

948
K. Muraleedharan et al.
(a) 2 IOꢁ3 ! 2 Iꢁ þ 3O2; iodates of Cu, Cd, Zn, Ni and
probably Mg, Ti(I), and Pb go to the oxide through,
(b) 4IO3 ! 2O2ꢁ þ I2 þ 5O2; iodates of Na and K also
go partly with this route, and iodates of Li, Ca, Ba, Sr
and rare earth form orthoperiodate as,
investigations, it is concluded that KIO3 has at least two
monoclinic, two rhombohedral and one triclinic modifica-
tion, and it is very likely that several modifications of KIO3
exist together at room temperature, readily transforming
from one to another [54]. This could be the reason why the
lattice of KIO3 contains many faults and the diffractograms
of different preparations are slightly different.
(c) 5 IOꢁ3 ! IO5ꢁ þ 5O2 þ 2 I2
Reactions of the type (a) for Na, K, Rb, and Cs attain
equilibrium and these constants were calculated, no cal-
culations were carried out for type (b) reactions but for
It has been reported [55] that iodates of alkali metals
(except Li), Ag, Co and probably Hg(I) and Hg(II)
decompose according to:
Fig. 5 Typical model fits for
the isothermal decomposition of
KIO3 at 795 K
Model No. 1
Model No. 2
1.0
0.8
0.6
0.4
1.0
0.8
0.6
r = 0.8947
r = 0.9075
α
0.4
1.0
Model No. 4
Model No. 3
0.9
0.6
0.3
0.0
r = 0.9919
r = 0.9298
0.8
0.6
0.4
0.2
α
8
16
24
32
40
8
16
24
32
40
t/min
t/min
Fig. 6 Typical model fits for
the isothermal decomposition of
KIO3 at 795 K
Model No. 5
r = 0.8493
Model No. 6
r = 0.9960
0
–1
–2
–3
3
0.8
0.4
0.0
α
Model No. 8
r = 0.9708
Model No. 7
r = 0.9901
1.2
0.9
0.6
2
α
1
0
8
16
24
t/min
32
40
8
16
24
t/min
32
40
123

Thermal decomposition kinetics of potassium iodate
949
reactions of type (c) calculations were made only for Ca
since no S0 (298 K) values for the periodates of Li, Ba, and
Sr are available [55].
It has been observed that KIO3 decomposes around 800 K
forming KI by releasing oxygen. The energetics and
kinetics, of the thermal decomposition of KIO3, have been
studied by applying model-free method of kinetic analysis.
The a–T curves for the non-isothermal decomposition of
KIO3 at different heating rates are shown in Fig. 2. The
observed mass changes for the decomposition agree very
well with the theoretical values for all samples of KIO3 at
all heating rates. The isothermal TG (at different temper-
atures) and a–t curves for the isothermal decomposition of
KIO3 are shown, respectively, in Figs. 3 and 4.
Thermal decomposition of KIO3
Figure 1 shows the TG–DTG–DTA curves for KIO3 at a
heating rate of 10 K min-1 in nitrogen atmosphere. Similar
curves were obtained for the thermal decomposition of
KIO3 at heating rates of 3, 5, and 7 K min-1 (not shown).
Fig. 7 Typical model fits for
the isothermal decomposition of
KIO3 at 795 K
1.6
1.2
0.8
0.4
6
Model No. 9
Model No. 10
r = 0.9809
1.6
1.2
0.8
0.4
r = 0.9942
α
Model No. 12
0.4
0.2
0.0
Model No. 11
r = 0.9592
r = 0.9498
4
2
0
α
8
16
24
32
40
8
16
24
32
40
t/min
t/min
Model No. 13
Fig. 8 Typical model fits for
the isothermal decomposition of
KIO3 at 795 K
Model No. 14
0.8
0.6
0.4
0.2
0.0
r = 0.9999
r = 0.9979
0.6
0.4
0.2
0.0
21
α
Model No. 16
Model No. 15
r = 0.9737
r = 0.8448
2
0
14
7
α
–2
0
8
16
24
32
40
8
16
24
32
40
t/min
t/min
123

950
K. Muraleedharan et al.
Table 2 Values of slope, correlation coefficient (r) obtained from isothermal model fitting to different equations at different temperatures
Model no.
Temperature/K
790
795
800
805
Slope/min-1
r
Slope/min-1
r
Slope/min-1
r
Slope/min-1
r
1
2
0.0070
0.0087
0.0111
0.0156
0.0368
0.0155
0.0429
0.0119
0.0154
0.0225
0.0701
0.0057
0.0095
0.0119
0.2062
0.0796
0.8946
0.9074
0.9298
0.9919
0.8492
0.9959
0.9902
0.9707
0.9811
0.9942
0.9501
0.9593
0.9999
0.9979
0.8455
0.9737
0.0106
0.0130
0.0167
0.0234
0.055
0.8946
0.9075
0.9298
0.9918
0.8493
0.9959
0.9902
0.9708
0.9809
0.9942
0.9502
0.9594
0.9999
0.9979
0.8457
0.9738
0.0142
0.0174
0.0223
0.0314
0.0740
0.0311
0.0862
0.0240
0.0311
0.0446
0.1410
0.0115
0.0192
0.0240
0.4146
0.1602
0.8947
0.9075
0.9298
0.9920
0.8493
0.9960
0.9901
0.9708
0.9809
0.9942
0.9498
0.9590
0.9999
0.9979
0.8448
0.9737
0.0185
0.0227
0.0291
0.0408
0.0963
0.0405
0.1122
0.0312
0.0404
0.0580
0.1834
0.0149
0.0250
0.0312
0.5395
0.2085
0.8950
0.9079
0.9302
0.9920
0.8497
0.9959
0.9900
0.9710
0.9811
0.9943
0.9497
0.9590
0.9999
0.9980
0.8448
0.9739
3
4
5
6
0.0232
0.0644
0.0179
0.0232
0.0333
0.1052
0.0086
0.0143
0.0179
0.3097
0.1196
7
8
9
10
11
12
13
14
15
16
Bold values indicate the maximum correlation coefficient
Table 3 Values of slope and correlation coefficient (r) obtained from isothermal model fitting to different equations for the thermal decom-
position region, a = 0.05–0.5
Reaction model
790 K
795 K
800 K
805 K
Slope/min-1
r
Slope/min-1
r
Slope/min-1
r
Slope/min-1
r
1
2
0.0171
0.0200
0.0233
0.0175
0.1026
0.0126
0.0322
0.0206
0.0245
0.0294
0.0318
0.0021
0.0095
0.0135
0.0469
0.1347
0.9598
0.9671
0.9791
0.9978
0.9334
0.9862
0.9996
0.9745
0.9810
0.9909
0.9970
0.9721
0.9999
0.9999
0.9928
0.9614
0.0256
0.0300
0.0349
0.0263
0.1539
0.0189
0.0483
0.0308
0.0367
0.0441
0.0477
0.0032
0.0143
0.0202
0.0703
0.2021
0.9600
0.9672
0.9792
0.9978
0.9337
0.9862
0.9996
0.9746
0.9811
0.9910
0.9970
0.9721
0.9999
0.9999
0.9928
0.9616
0.0345
0.0404
0.0469
0.0354
0.2071
0.0255
0.0649
0.0415
0.0494
0.0594
0.0642
0.0042
0.0192
0.0271
0.0945
0.2720
0.9610
0.9681
0.9800
0.9975
0.9349
0.9855
0.9994
0.9754
0.9818
0.9914
0.9973
0.9713
0.9999
0.9999
0.9924
0.9625
0.0446
0.0522
0.0607
0.0458
0.2677
0.0330
0.0840
0.0537
0.0638
0.0768
0.0830
0.0055
0.0248
0.0351
0.1224
0.3517
0.9597
0.9669
0.9790
0.9979
0.9331
0.9863
0.9995
0.9743
0.9808
0.9908
0.9970
0.9721
0.9999
0.9999
0.9927
0.9612
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Bold values indicate the maximum correlation coefficient
Kinetics of thermal decomposition
squares analysis using various kinetic reaction models
given in Table 1. Typical model fitting plots, for various
models given in Table 1, for the thermal decomposition of
KIO3 in the range a = 0.05–0.95 at 795 K are shown in
Figs. 5, 6, 7, and 8. Similar types of curves were obtained
at all other temperatures studied (not shown). The values of
The method of model fitting
The a–t data in the range a = 0.05–0.95 of the isothermal
decomposition of KIO3 were subjected to weighted least
123

Thermal decomposition kinetics of potassium iodate
951
Table 4 Values of slope and correlation coefficient (r) obtained from isothermal model fitting to different equations for the thermal decom-
position region, a = 0.5–0.95
Reaction model
790 K
795 K
800 K
805 K
Slope/min-1
r
Slope/min-1
r
Slope/min-1
r
Slope/min-1
r
1
2
0.0033
0.0042
0.0060
0.0132
0.0142
0.0151
0.0503
0.0090
0.0124
0.0199
0.0286
0.0078
0.0095
0.0109
0.3469
0.0645
0.9653
0.9671
0.9706
0.9871
0.9595
0.9926
0.9937
0.9987
0.9994
0.9999
0.9991
0.9887
0.9999
0.9985
0.9058
0.9994
0.0049
0.0064
0.0090
0.0198
0.0214
0.0227
0.0756
0.0134
0.0186
0.0299
0.0429
0.0118
0.0143
0.0164
0.5212
0.0969
0.9648
0.9666
0.9702
0.9869
0.9590
0.9924
0.9939
0.9986
0.9994
0.9999
0.9991
0.9890
0.9999
0.9984
0.9063
0.9995
0.0066
0.0085
0.0121
0.0265
0.0287
0.0304
0.1012
0.0180
0.0249
0.0400
0.0575
0.0158
0.0192
0.0220
0.6969
0.1299
0.9659
0.9677
0.9712
0.9876
0.9601
0.9930
0.9933
0.9988
0.9995
0.9998
0.9989
0.9883
0.9999
0.9987
0.9042
0.9993
0.0086
0.0111
0.0158
0.0345
0.0373
0.0396
0.1319
0.0235
0.0324
0.0522
0.0749
0.0205
0.0250
0.0287
0.9090
0.1692
0.9654
0.9672
0.9707
0.9873
0.9596
0.9927
0.9936
0.9987
0.9994
0.9998
0.9990
0.9886
0.9999
0.9986
0.9052
0.9994
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Bold values indicate the maximum correlation coefficient
Fig. 9 Arrhenius plot for the
thermal decomposition of KIO3.
Model no. 13:
a (a = 0.05–0.95),
b (a = 0.05–0.5),
a
c
b
–3.6
–1
E = 335.8 kJ mol
ln A = 46.50
–1
E = 338.3 kJ mol
ln A = 46.89
r = –0.9951
r = –0.9950
–4.0
–4.4
d (a = 0.5–0.95) and Model no.
14: c (a = 0.05–0.5)
–4.8
–3.2
d
–1
E = 335.9 kJ mol
ln A = 46.86
–1
E = 337 kJ mol
–3.6
–4.0
–4.4
–4.8
ln A = 46.69
r = –0.9953
r = –0.9950
1.240
1.248
1.256
1.264
1.240
1.248
1.256
1.264
103 T –1/K–1
103 T –1/K–1
slope and correlation coefficient (r) obtained by weighted
least squares plot at 790, 795, 800, and 805 K for all
kinetic models are given in Table 2. Perusal of Table 2 and
Figs. 5, 6, 7, and 8 shows that the contracting cube equa-
tion, 1 - (1 - a)1/3 = kt, gave the best fits (r = 0.9999) at
all temperatures studied. Separate kinetic analysis of the
a–t values corresponding to the ranges a = 0.05–0.5 and
a = 0.5–0.95 showed that the former range gave the best
fits to both contracting cube and contracting area models
whereas the latter range gave best fits to the contracting
cube model. Description of reaction kinetics using different
rate laws for different ranges of a is not unusual in solid-
state reactions. For instance Philips and Taylor used Prout–
Tompkins equation to describe the acceleratory region of
the decomposition of KIO4 and the contracting cube
equation for the decay stage [56]. It has also been reported
that under isothermal conditions KIO4 decomposes via two
stages; the Prout–Tompkins equation best describes the
acceleratory stage and the deceleratory stage proceeds
according to contracting area law [57–59]. The
123

952
K. Muraleedharan et al.
Tompkins relation with separate rate constants [50]. It has
also been reported that KBrO3 decomposes in two stages,
both stages following the contracting area equation but
with different rate constants [61]. Kim et al. [62] has
reported that the reaction model varies with reaction tem-
perature in isothermal pyrolysis of polypropylene and they
observed that the Arrhenius parameters derived from the
assumptions of nth order model would be improper.
Table 5 Values of slope, E, r, and % deviation (in E) obtained from
model-free analysis at different conversions
Conversion/% Slope/K-1 E/kJ mol-1 -r
% Deviation in Ea
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
41.37
41.78
41.70
41.82
41.97
42.01
42.15
42.09
42.35
42.05
42.24
42.31
42.22
42.18
42.14
42.22
42.21
42.14
344.0
347.3
346.7
347.7
348.9
349.3
350.4
349.9
352.1
349.6
351.1
351.8
351.0
350.7
350.3
351.0
350.9
350.4
a
0.9834
0.9839
0.9847
0.9846
0.9844
0.9841
1.62
0.65
0.84
0.55
0.19
0.10
The values of slope and r obtained from least squares fits
for various reaction models given in Table 1 for both
ranges of the isothermal decomposition of KIO3 are given
in Tables 3 and 4. Values of E, A, and r are shown in the
Arrhenius plots, for the ranges a = 0.05–0.95, a =
0.05–0.5 and a = 0.5–0.95, of the isothermal decomposi-
tion of KIO3 (Fig. 9). It has been found that the values of
E obtained for the different ranges of a and for the two
models, contracting cube and contracting square, remain
0.9847 -0.23
0.9844 -0.09
0.9840 -0.71
0.9833
0.00
0.9830 -0.44
0.9832 -0.62
0.9828 -0.41
0.9820 -0.31
0.9811 -0.21
0.9810 -0.40
0.9799 -0.37
0.9791 -0.22
within 337.2 1.8 kJ mol-1
.
The model-free approach
The apparent activation energy values were also estimated
by the isoconversional method suggested by Ozawa, Flynn,
and Wall [47] for the thermal decomposition of KIO3 at
different percentage of conversion by fitting the plots of
ln b versus 1/T and are given in Table 5 along with the values
of slope, r and percentage deviation in E. Table 5 shows that
the isoconversional plots for the thermal decomposition of
KIO3 at all percentages of conversions give high values of
r (*-0.9999) with E values of 348 4 kJ mol-1. Typical
isoconversional plots for the thermal decomposition of
Average value of E = 349.6. From average value of E
acceleratory stage in the decomposition of Lithium per-
chlorate followed Prout–Tompkins rates law whereas the
decay stage followed the monomolecular model [60].
Similarly both the acceleratory and decay regions of the
thermal decomposition of sodium perchlorate and of
potassium bromate were well described by the Prout-
Fig. 10 Typical
2.8
2.4
2.0
a
b
3.2
isoconversional plots for the
thermal decomposition of KIO3
at different conversions
(a = 20%, b = 40%, c = 60%,
d = 80%)
r = 0.9996
r = 0.9999
2.8
2.4
1.6
3.6
c
d
4.0
r = 0.9976
r = 0.9984
3.6
3.2
2.8
3.2
2.8
1.240
1.248
1.256
1.264
1.240
1.248
1.256
1.264
103 T–1/K–1
103 T–1/K–1
123

Thermal decomposition kinetics of potassium iodate
953
Fig. 11 Typical
2.4
2.0
1.6
1.2
isoconversional plots for the
thermal decomposition of KIO3
at different conversions
(a a = 0.2, b a = 0.4,
c a = 0.6, d a = 0.8)
a
b
r = –0.9847
r = –0.9847
β
1.200
1.208
1.216
1.224
1.184
1.192
1.200
1.208
1.216
2.4
2.0
1.6
1.2
c
d
r = –0.9811
r = –0.9830
β
1.176
1.184
1.192
1.200
1.208
1.168
1.176
1.184
1.192
1.200
103 T–1/K–1
103 T–1/K–1
KIO3 are shown in Fig. 10. The apparent activation energy
values determined for the various values of conversion show
little deviation indicating that the thermal decomposition
of KIO3 proceeds through a single step.
Table 6 Values of slope, E and correlation coefficient (r) obtained
from isothermal isoconversional analysis at different conversions
a
Slope/K-1
E/kJ mol-1
r
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
40.41
40.46
40.76
40.30
40.30
40.55
40.47
40.37
40.42
40.48
40.40
40.36
40.63
40.35
40.55
40.37
40.47
40.43
40.55
336.0
336.3
338.9
335.1
335.1
337.1
336.5
335.7
336.1
336.6
335.9
335.6
337.8
335.5
337.1
335.7
336.4
336.1
337.1
0.9981
0.9989
0.9997
0.9999
0.9999
0.9999
0.9996
0.9996
0.9992
0.9991
0.9989
0.9984
0.9986
0.9981
0.9981
0.9976
0.9976
0.9974
0.9971
The a–t data, in the range of a = 0.05–0.95, of the
isothermal decomposition of KIO3 were also subjected to
isoconversional studies for the determination of apparent
activation energy as a function of a from the sets of iso-
thermals obtained. A plot of ln t (t being the time required
for reaching a given value of a at a constant temperature
T) versus the corresponding reciprocal of the temperature
(1/T) would lead to E for the given value of a. Typical
isothermal isoconversional plots are shown in Fig. 11. The
results are given in Table 6. A perusal of Table 6 reveals
that the values E obtained for different values a lies in the
range of 335–339 kJ mol-1, which is in good agreement
with those obtained from conventional method.
Mechanism of thermal decomposition
This study revealed that the thermal decomposition of
KIO3 undergoes through contracting cube equation with an
E value of *340 kJ mol-1. Separate kinetic analysis of the
a–t values corresponding to the range a = 0.05–0.5
showed that this range gave the best fits to both contracting
cube and contracting area models. Both the models, con-
tracting cube and contracting area, are deceleratory in
nature. According to contracting cube model, the initial
nucleation occurs rapidly over all surfaces for a single cube
of the reactant and the interface established progresses
thereafter in the direction of the centre of the crystal,
reaction is deceleratory throughout since the reaction
interface progressively decreases. But in the case of con-
tracting area model, the interface established progresses in
the direction of the centre of the crystal with a progressive
decrease in contact area. Initially nucleation and growth of
the reactant, KIO3, take place in two/three-dimensional
123

954
K. Muraleedharan et al.
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Earlier workers observed that both Prout–Tompkins and
first-order equation describes the thermal decomposition of
KIO3 [50]. Prout-Tompkins equation is autocatalytic, while
first-order equation is deceleratory in nature. The present
investigation revealed that the thermal decomposition of
KIO3 proceeds according to contracting cube equation,
which is deceleratory in nature. Model-fitting analysis
shows that both Prout–Tompkins and first-order models
gave poor correlation for the thermal decomposition of
KIO3 (see Tables 2, 3, 4; Figs. 5, 6, 7, 8). The rate of solid
state reactions is usually controlled either by electron
transfer, or diffusion (of ions in the lattice) mechanism.
Electron transfer mechanism involves the transfer of an
electron from the iodate anion to potassium cation to form
the free radicals K• and IO3•. As IO•3 involves a one-electron
bond, it is very unstable and readily decomposes to give O•
and relatively stable I•. Two O• species combine to give one
oxygen molecule. I• is stabilized by receiving an electron
from K• forming KI. In the case of diffusion mechanism, the
diffusion of the similarly sized cation and/or anion [63]
toward potential sites is the rate-determining step of the
decomposition; at the potential sites, these ions undergo
spontaneous reaction producing highly reactive radicals,
which break down to form solid KI and gaseous oxygen.
Further investigations, such as pretreatment studies, are
required to clearly establish the mechanism, the rate deter-
mining step, of the thermal decomposition of KIO3.
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