Induction period in the BrO3 -, Ce(III), H2SO4, oxalic acid and ketone oscillating reaction

Article abstract of DOI:10.1039/a904609g

Contrary to what has been thought, systems classified as bromine- hydrolysis-controlled (BHC) show an induction period. In studies on the effect of a family of ketones upon this type of oscillators, it was found that an induction period appears in an interval of concentrations of the ketone, or in an interval of the value of the enolization constant. Simulations of the processes involved and a corresponding explanation are shown in this paper.

Full text of DOI:10.1039/a904609g

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Induction period in the BrO —, Ce(III), H SO , oxalic acid and
3
2
4
ketone oscillating reaction
Igal Berenstein, Jesus Agreda and Daniel Barragan*
Universidad Nacional de Colombia, Facultad de Ciencias, Departamento de Qu•mica, Bogota,
Colombia. E-mail: dbarrag=ciencias.ciencias.unal.edu.co
Received 9th June 1999, Accepted 17th August 1999
Contrary to what has been thought, systems classiÐed as bromine-hydrolysis-controlled (BHC) show an
induction period. In studies on the e†ect of a family of ketones upon this type of oscillators, it was found that
an induction period appears in an interval of concentrations of the ketone, or in an interval of the value of the
enolization constant. Simulations of the processes involved and a corresponding explanation are shown in this
paper.
lowed with a platinum electrode with an internal reference,
Cole-Parmer 27006-21, connected to Hewlett-Packard
1. Introduction
a
The classical BelousovÈZhabotinsky (BZ) reaction is the bro-
mination and oxidation of an organic compound, catalyzed by
a metallic ion in an acidic medium.1 The organic compound
that is traditionally used is malonic acid, with the Ce(III)È
Ce(IV) redox pair as the metallic ion.2 Variation of the organic
compound establishes a distinction between families of BZ
type oscillators. Oscillators can be classiÐed, with some
Model 3478A multimeter with sensitivity of 0.1 lV. Data
acquisition was made through a GPIB IEEE-488 interface.
The reagents used were sulfuric acid (AnalytiCals, Carlo
Erba, 96%), oxalic acid dihydrate (Merck, f.a), cerium(III)
nitrate hexahydrate (Merck, ultrapure), potassium bromate
(Mallinckrodt, recrystallized from sulfuric acid), acetone
(Merck, f.s), methyl ethyl ketone (Merck, f.s), methyl propyl
ketone (Merck, f.s) and methyl isobutyl ketone (Merck, f.s). All
solutions were prepared in 0.8 M sulfuric acid. The reaction
common characteristics (such as BrO ~ in an acidic medium),
3
(a) bromine-hydrolysis-controlled (BHC), (b) non-bromide-
ion-controlled and (c) uncatalyzed-bromate oscillators.3
started after the addition of 10 cm3 of 0.182 M KBrO to a
3
solution containing 20 cm3 of 2.17 ] 10~3 M (COOH) , 27
2
Corresponding to current experimental evidence and in
comparison with the classical BZ reaction, the main charac-
teristics of the BHC oscillators are (a) the absence of an
organic compound easily brominated under the reaction con-
ditions and the lack of a brominated organic compound that
can be readily oxidized by the metallic ion, (b) the control of
bromide ion concentration by the hydrolysis of molecular
cm3 of 0.8 M H SO , 1 cm3 of 0.029 M Ce(NO ) and 0.2
2
4
3 3
cm3 of ketone. This volume of ketone is typical for observing
the induction period using methyl isobutyl ketone.
3. Results and discussion
Fig. 1 shows the e†ect on the global dynamics of the BHC
type oscillator of acetone, methyl ethyl ketone, methyl propyl
ketone and methyl isobutyl ketone.
The progressive growth of the period of oscillation caused
by methyl ethyl ketone and methyl propyl ketone with respect
to that observed for acetone and the appearance of the induc-
tion period when methyl isobutyl ketone was used are due to
the following reactions, as the initial concentrations of each of
the species are the same in the four experiments:4
Br , not by the oxidation of organic compounds, and (c) the
2
absence of an induction period, as the most important
feature.3,4
In the classical BZ reaction [BrO ~, Ce(III), H SO ,
3
2
4
malonic acid], the existence of an induction period is
explained by the time required for accumulation of a critical
concentration of one of the brominated organic products, e.g.,
bromomalonic acid,5 while in the BHC type oscillators,
BrO ~ÈCe(III)ÈH SO Èoxalic acidÈ(inert gas Ñow or acetone),
3
2
4
this compound is not present.
For the BHCÈBrO ~ÈCe(III)ÈH SO Èoxalic acidÈacetone
CH COR ] H` H CH 2COHR ] H`
(R20)
3
2
4
3
2
oscillator, a detailed mechanism has been proposed that
explains very satisfactorily all of the characteristics observed
so far.4
For acetone: K \ 8.3 ] 10~5 l mol~1 s~1, K
\ 21.3 l
20
~20
mol~1 s~1.
In this paper, we show that the observed induction period
can also be explained and simulated in the conceptual frame
o†ered by this mechanism.
CH 2COHR ] Br ] BrCH COR ] Br~ ] H` (R21)
2
2
2
K
21.03 ] 107 l mol~1 s~1.
21
CH \ COHR ] Br ~ ] BrCH COR ] 2Br~ ] H`
2
3
2
2. Experimental
(R32)
The BHC oscillator was studied under batch conditions in a
cylindrical double-walled glass cell of 80 cm3 capacity, ther-
mostated to 25.00 ^ 0.01 ¡C by circulating water through the
internal wall of the cell. The solution was continuously mag-
netically stirred from the bottom, at a frequency about 250
r.p.m, taking care not to form a vortex. The reaction was fol-
K
\ 2.8 ] 106 l mol~1 s~1.
These three reactions are part of the 24 that compose the
mechanism proposed by Field and Boyd4 to explain the prin-
cipal features of the BHC type oscillator with acetone. Reac-
tions (R20), (R21) and (R32) control the concentration of
32
Phys. Chem. Chem. Phys., 1999, 1, 4601È4603
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by the following reaction:3
Br ] H O H Br~ ] HOBr ] H`
(R3)
2
2
The critical role that reaction (R3) plays in the control of the
molecular bromine and bromide ion concentrations is evident.
If the Br concentration is very low, so will be the Br~ con-
2
centration, and the reaction will remain in the global process
B. On the other hand, if the concentration of Br is high, the
2
reaction will remain in the global process A.
This also o†ers an explanation for the appearence of an
induction period in BHC type oscillators. If the concentration
of ketone is low enough, reaction (R20) at the beginning does
not control e†ectively the concentration of Br , so the global
2
process A takes control of the reaction until the concentration
of Br is low enough for the process B to begin.
2
This same explanation is valid when we analyze reaction
(R20) as a function of the enolization constant of the ketone.
Hence in this type of oscillators we can Ðnd induction periods
with ketones, by changing the initial concentrations leaving
the enolization constant invariant (using only one kind of
ketone), or keeping the initial concentrations at a constant
value and varying the enolization constant (using di†erent
ketones).
This result is veriÐed using numerical simulations. First, the
modiÐed Oregonator of Field and Boyd4 was studied for their
generalized model, but it was not possible to reproduce the
experimental behavior, as this model is too small, and con-
siders the concentration of oxalic acid and ketone to be con-
stant, which is not correct for batch experiments. The model
of Gaspar and Galambosi6 for BHC type oscillators, in which
the ketone is replaced by a Ñow to remove molecular bromine,
also does not reproduce the experimentally observed induc-
tion period.
Fig. 1 BHC oscillator BrO ~ÈCe(III)ÈH SO È(COOH) È with: (a)
3
2
4
2
acetone, (b) methyl ethyl ketone, (c) methyl propyl ketone and (d)
methyl isobutyl ketone.
molecular bromine, so the three global processes to be pre-
sented can alternate and, thus, allow the ocurrence of oscil-
lations. These three global processes are as follows:
(A) 3H` ] 2Br~ ] BrO ~ ] 3CH COR ]
3
3
3BrCH COR ] 3H O
2
2
(B) 5H` ] 4Ce(III) ] BrO ~ ] 2H O ] HOBr ] 4Ce(IV)
(C) 2Ce(IV) ] HOBr ] 2(COOH) ] 2Ce(III) ] 4CO ]
3
2
Hence it is necessary to make the simulations with the com-
plete mechanism proposed by Field and Boyd4 of 21 variable
species and 24 elementary steps. The integration was carried
out using a suitable subroutine for sti† non-linear ordinary
di†erential equation systems.7
2
2
H O ] 3H` ] Br~
2
Processes A and B alternate control of the reaction, depending
on the concentration of Br~, as is well explained by the FKN
mechanism. As in the BHC type of oscillators, there is no oxi-
dation of brominated organic compounds, the bromide ion is
produced by the hydrolysis of molecular bromine, as is shown
Fig. 2(a) shows the simulation of oscillations when the
initial concentration of acetone is 0.0047 M and Fig. 2(b)
Fig. 2 (a) Simulation of oscillations under the experimental conditions: [BrO ] \ 0.03 M, [H`] \ 1.1 M, [Ce(III)] \ 0.000 57 M,
3 0
[(COOH) ] \ 0.008 M, [acetone] \ 0.047 M. (b) Simulation of the induction period by varying the initial concentration of acetone:
2 0
[BrO ] \ 0.03 M, [H`] \ 1.1 M, [Ce(III)] \ 0.000 57 M, [(COOH) ] \ 0.008 M, [acetone] \ 0.025 M. (c) Simulated curves for oxalic acid
3 0 2 0
0
0
0
0
0
0
and enol ketone when an induction period appears: [BrO ] \ 0.03 M, [H`] \ 1.1 M, [Ce(III)] \ 0.000 57 M, [(COOH) ] \ 0.008 M,
3 0
0
0
2 0
[acetone] \ 0.025 M.
0
4602
Phys. Chem. Chem. Phys., 1999, 1, 4601È4603

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shows the induction period in Br , Br~ and Ce(IV) when the
Recently, it has been shown for BHC type oscillators that
the use of the pair Mn(II)ÈMn(III) as a metallic ion catalyst10
also leads to an induction period, but the authors placed no
emphasis on this feature. The presence of an induction period
must be characteristic of the family of BHC type oscillators,
the bromine being removed chemically using acetone or physi-
cally using an inert gas stream,11 stirring or by CSTR
methods. We are currently working on all the family to verify
the presence of an induction period in each of them, with the
aim of constructing a generalized model that can predict its
presence, no matter which method is used to remove bromine.
2
initial concentration of acetone is 0.0025 M.
Similar results were obtained when the initial concentration
of ketone was 0.047 M and the constant of enolization was
varied from 8.3 ] 10~5 to 3.3 ] 10~5 l mol~1 s~1.
We do not know the real values of the enolization constants
for methyl ethyl ketone, methyl propyl ketone and methyl iso-
butyl ketone, but in another studies we estimated these values
from a correlation between experimental information and
simulations of the induction period in the classical BZ reac-
tion.8
Data for the Br~ concentration show that its initial concen-
tration is high, by reaction (R3), and that Br was not con-
2
References
sumed rapidly.
We also analyzed simulated concentration curves of oxalic
acid, ketone, enol ketone and both forms of the brominated
ketone (bromoketone and dibromoketone), looking for a
relationship between these species and the induction period.
Curves for ketone and both forms of the brominated ketone
show monotonic behavior, no matter whether there is an
induction period or not. On the other hand, curves corre-
sponding to oxalic acid and enol ketone show agreement with
the induction period observed for BHC oscillators [Fig. 2(c)].
From Fig. 2(c), the induction period could be associated
with a critical concentration of enol ketone, which is in con-
cordance with our explanation because if the concentration of
1
B. P. Belousov, in Oscillations and T ravelling W aves in Chemical
Systems, ed. R. J. Field and M. Burger, Wiley, New York, 1985, p.
605.
2
3
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7
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accepted.
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Bogota, 1999.
Br is high, enol ketone does not reach the critical concentra-
2
tion.
8
9
This work shows that the BHC type oscillators,
BrO ~ÈCe(IV)ÈH SO Èoxalic acidÈketone, do show an induc-
3
2
4
tion period, and that the explanation for this feature is given
by the kinetic control of the global processes that characterize
the reaction.9 This explanation allows the generalization of
the deÐnition of an induction period because it is also valid
for the classical BZ reaction.
10 M. C. Guedes and R. B. Faria, J. Phys. Chem., 1998, 102, 1973.
11 Z. Noszticzius and J. Bodiss, J. Am. Chem. Soc., 1979, 101, 3177.
Paper 9/04609G
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