Kinetics study of the disproportionation of the iodous acid in aqueous sulfuric acid solution
UNIVERSITY THOUGHT
Publication in Natural Sciences, Vol. 6, No 1, 2016, pp. 27-31.
doi:10.5937/univtho6-10474
Original Scientific Paper
KINETICS STUDY OF THE DISPROPORTIONATION OF THE IODOUS ACID IN
AQUEOUS SULFURIC ACID SOLUTION
Smiljana Marković1, Biljana Petrović2
1
Faculty of Technological Sciences, University of Priština, Kosovska Mitrovica, Serbia.
Faculty of Science, University of Kragujevac, Kragujevac, Serbia.
2
ABSTRACT
In this paper we reported the kinetics of the dm3mol-1s-1, k298 = (1.30 0.07) dm3mol-1s-1and k303
disproportionation reaction of iodous acid (HOIO) = (1.50 0.10) dm3mol-1s-1, respectively. The
in aqueous sulfuric acid solutions (0.18 mol/dm3) corresponding activation energy was determined,
studied by spectrophotometrical measurements of for the chosen temperature interval, by a graphical
the absorbance at suitable wavelength. The method. In addition, obtained value of activation
changes of the absorbance were caused because the energy is Ea = 38 5 kJ/mol. The negative value of
absorbing 2 molecule species were generated Gibbs energy change and other thermodinamical
during the reaction. The disproportionation rate parameters show that is the disproportionation
constants are calculated at the temperature range reaction thermodynamically feasible.
between 285 and 303 K with average values: k285 =
(0.90 0.08) dm3mol-1s-1, k291 = (1.10 0.10)
Key words: iodous acid, disproportionation reaction, rate constants, activation energy.
1. INTRODUCTION
The homogeneous disproportionation reaction of
HOIO in aqueous solutions of H2SO4, can be
considered as bimolecular chemical reaction:
results. As the consequence, it is rather difficult to
obtain reproducible experimental measurement. The
limitations in mechanism discussion are referred to the
obtained results.
The values of some kinetic parameters, such as
reaction rate constant, activation energy and Arrhenius
constant were relatively precisely and accurately
calculated on the base of limited experimental data.
More precisely in the literature are given some data for
the rate constant at 298 K. However, data differs from
author to author. For example, Noszticzius
(Noszticzius et all., 1983) obtained value of the rate
constant of 5.4 dm3mol-1s-1 in H2SO4 concentration
range between 0.05 and 0.15 moldm-3 at 298 K, while
Lengyel (Lengyel et all., 1996) obtained value for the
rate constant of 25 dm3mol-1s-1 in the similar
experimental conditions at 298 K.
Taking into account that the rate constants are
significantly different, we investigated and reported
the results for disproportionation reaction and its
temperature and acidity dependence under different
experimental conditions (Markovic et al., 2002, 2015),
(Markovic & Rakicevic, 2006), (Markovic &
Cekerevac, 2009), (Markovic & Petrovic, 2010).
In this paper, since our reaction system operates at
relatively high acid concentration, we reinvestigated
2 HOIO IO-3 + 2 H+ + HOI
Namely, the iodous acid simultaneously oxidized
to iodate and reduced to hypoiodous acid. According
to the published results, this process is proved to be
very complex. (Noszticzius et al., 1983), (Lengyel et
al., 1996), (Hegedűs et al., 2001). Also, rate constant
for the disproportionation reaction was determined in
strong acidic water solutions and it was found that the
process is slow and autocatalytic (Lengyel et al.,
1996).
High acidity of solution, from sulfuric acid
addition, is one of the major factor that cause the
complexity of the mechanism of studied process.
Because of the high sulfuric acid concentration
used in the preparation of HOIO, the experimental
conditions for studying the disproportionation are very
limited. Reaction mechanism models of this complex
system include large number of elementary steps, or
those, which can be presented as elementary steps with
undetermined reaction rate constants. This is likely
cause of lack of compliance between calculated value
from the mathematical model data and experimental
Chemistry
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the reaction close to our experimental conditions. The
Hindmarsh version of the Gear’s integrator (Markovic
et al., 2002) is used for the numerical simulation.
Otherwise, experimental relevant data for the
kinetics of iodous acid disproportionation, based on
the values obtained in our experiments and data
obtained in our numerical calculations, are compared
with results obtained in some other investigations
found in literature.
and experiments are performed in semidarkness
conditions and absence of direct overhead lights.
3. RESULTS AND DISCUSSION
The disproportionation reaction of iodous acid
(HOIO) was studied at different experimental
conditions by different experimental method.
According to the previously published results,
(Noszticzius et al., 1983), (Lengyel et al., 1996) the
values of the obtained rate constants for
disproportionation reaction are very different. Results
evaluated from our spectrophotometric measurements
in temperature range from 285-303 K enable the
calculation of the rate constants for the reaction of
disproportionation of HOIO. All values are given in
Table 1.
The light absorbance is caused exclusively by I2
molecule absorption (Awtrey & Connik, 1951).
The increase of the concentration of originated I2
during the reaction was followed at 469 nm
(absorbance maximum). The other part on the
spectrum was unchanged during the reaction. The
molar absorption coefficient value for I2 is determined
from the calibration diagram and it is equal to ε =746
m2mol-1.
Determination of the concentration other relevant
species, such as H+, H2IO+, and IO3- in the pH range
between 1 and 2, formed during the disproportionation
reaction, was discussed in detail in the paper
(Markovic et al., 2015).
The reactions were followed under the different
initial concentration of reactant (for the iodous acid
from 1.00 x 10-4 to 1.40 x 10-4 moldm-3 and iodate
from 1.00 x 10-4 to 4.80 x 10-4 moldm-3) at different
values temperature (285, 291, 298 and 303 K) with
constant solution acidity (0.18 moldm-3 H2SO4).
The numerical simulation of obtained experimental
data was performed in a manner similar to one
described in previous paper (Markovic et al., 2002).
The method consists of converting the chemical
reactions by equations (denoted by (R1) to (R4) for
direct reactions and (R-1) to (R-4) for a reverse
reactions) given mechanism in the model. Accepting a
model of the mechanism and using the experimentally
measured value for the rate constants, could be carried
out successful simulation.
2. EXPERIMENTAL
The experiments are performed in a similar way as
described in our previous papers (Markovic et al.,
2002), (Markovic & Petrovic, 2010) where the process
is studied at isothermal conditions.
Experimental solution is prepared with double
distilled water with conductivity meter tested purity.
The stock solutions are prepared using pro analysis
chemicals produced by “Merck”.
To obtain HOIO, it is necessary to prepare the
solution tha (...truncated)
