496
J. Phys. Chem. 1980, 84,496-500
Conditions for the Onset of Chemical Oscillation Mlrla Burger* and Endre Koros Institute of Inorganic and Analytical Chemistry, L. Eotvos University, H- 1443 Budapest, Hungary (Received July 2, 1979)
The conditions for the onset of chemical oscillation were investigated for the malonic acid (MA) -Br03--HN03, MA-HC104-Mn(II), and MA-Br03--HN03 (5 M)-Ce(1V) reacting systems by following the accumulation of bromomalonic acid (BrMA). Chemical oscillation starts when BrMA reaches a crucial concentration value, [BrMAIcrucid, which depends on the initial concentration of bromate. Rate laws describing the rate of BrMA formation were also established. From the temperature dependence of the rate constants and the oscillation frequencies, respectively, the activation energies of both the preoscillatory and the oscillatory phases of the reactions were calculated. Still unsolved problems and contradicting views about the conditions of the transition of a chemical system from an unexcitable to an excitable state are also discussed.
Introduction The most extensively investigated oscillatory chemical reactions are those of the Belousov-Zhabotinsky type, Le., the catalytic oxidative bromination with acidic bromate of mostly aliphatic polycarboxylic acids or polyketones. Although at least 20 aliphatic compounds are known to react with bromate in an oscillatory manner, detailed information has been gathered primarily on the malonic acid (MA), bromate, sulfuric acid, and catalyst (mostly cerium) reacting systems. The generally accepted chemical mechanism is based on experimental investigations of the system mentioned ab0ve.l In spite of the fact that the Belousov-Zhabotinsky (BZ) systems have been studied under various conditions, some fundamental chemical problems have not yet been revealed; among these is the prerequisite of chemical oscillation. It has been known from previous publications that in the MA-Br0,--H2S04-catalyst system the chemical oscillation does not start immediately after mixing the reagents but after the elapse of a certain period of time. The length of this preoscillatory period varies from seconds to many tens of minutes. Zhabotinsky et aL2were the first to find that the addition of bromomalonic acid (BrMA) to a MA-Br0,-H2S04-Ce3+ system reduced the length of the preoscillatory period. Degn3 added four bromo compounds-bromomalonic acid, dibromomalonic acid, dibromoacetic acid, and tribromoacetic acid (assumed to form during the reaction)-separately to the BZ systems and observed the shortening of the preoscillatory period in each case. He concluded that dibromomalonic acid should reach a critical concentration to initiate the chemical oscillation. Later Field, Koros, and Noyesl established by means of paper chromatography that the main product early in the reaction was BrMA and that other organic bromo compounds accumulated only in negligible amounts. This observation was supported by the more thorough investigations of Hess et ala4+ They found that during the early phase of the reaction BrMA could be identified as the only brominated species and that dibromomalonic acid appeared only during a later phase. However, this latter species was present in the reaction mixture in only 10% yield, even after many hours of reaction time. Other analytical data (polarography and iodometry) are in agreement with those mentioned abovea7 In one of our earlier paperss we have dealt with the accumulation of BrMA in some reacting BZ systems, and the following has been established. BrMA starts to form immediately after mixing the reagents, and its rate of formation is nearly constant during the preoscillatory 0022-3654/80/2084-0496$0 1.oo/o
period. During the oscillatory phase of the reaction, however, the rate of formation of BrMA is periodic. Slow accumulation periods are followed by fast ones with period times equal to those observed for potential and bromide ion concentration oscillations. The fast accumulation periods coincide both with the sudden increase in potential and with the very pronounced change in bromide ion concentration (shown in Figure 1). The periodic nature of the accumulation of BrMA has been proved also by the spectrophotometric measurements of Busse and Hess et al.5 also revealed that in the BZ reaction, besides the bromo compounds, carbon dioxide is formed and formic acid is not. The papers cited above prove that the BZ reaction proceeds mainly according to the following stoichiometry:
-
2Br03- + 3CH2(COOH)2+ 2H+ 2BrCH(COOH)2 + 3 c 0 2 + 4H20 In the later phase of the reaction, an additional reaction should be accounted for:
-
2Br03- + 4BrCH(COOH)2 + 2H+ 3Br,CHCOOH
+ 6C02 + 4H20
Some carbon monoxide is also liberated.1° It is highly probable that bromide ion-the control intermediate of the oscillatory reaction-is generated by the oxidation of BrMA with cerium(1V):
-
BrCH(COOH)2 + 6Ce4+ + 2H20 6Ce3++ Br-
+ 3 c 0 2 + 7H+
Some bromide ion may also come from attack of HOBr or Br, on formic acid. Jwo and Noyesll claimed that bromide ion originated from the hydrolysis of the bromomalonyl radical in the following sequence of reactions:
- +- +
CH2(COOH)2+ Ce4+
Ce3+ H+ + -CH(COOH)Z
CH(COOH)2+ BrCH(COOH)2 CH2(COOH)2+ BrC(COOH)2 BrC(COOH),
+ H20
Br-
HOC(COOH)2 + H+
Undoubtedly BrMA plays an important role in MAcontaining BZ systems. For this reason our main concern was to clarify the following points regarding BrMA. (a) In different BZ systems, at what concentration of BrMA does chemical oscillation start, and how does this critical concentration depend on the chemical composition of the reaction mixture? (b) How does the rate of formation of 0 1980 American Chemical Society
The Journal of Physical Chemistty, Vol. 84, No. 5, 1980 ,497
Conditions for the Onset of Chemical Oscillation
TABLE I: Systems Investigated and Concentration Ranges
-
--.___-
___-.-
catalyst, M
system
malonic acid, M
NaBrO,, M
Mn( 11)
I I1
0.1-0.6 0.1-0.6
0.025-0.10 0.025-0.10
0.001-0.004
acid, M
-Ce(II1)
HNO,
HClO, 1.5
0.001-0.004
1.5 5.0
-
TABLE 11: Redox Potential of the Ce4+/Ce3+ Couple in Different Mineral Acids’ acid M redox potential, V 1.443
%SO,
1.0 4.0
HNC),
1.0-4.0 8.0 1.0
HC10,
2.4
8.0
Figure 1. Change with time of (a) BrMA concentration, (b) redox potential, and (c) bromide concentration during the BZ reaction.
BrMA dlependl on the concentration of the reactants and the temperature, and what is the activation energy of BrMA formation? The accumulation of BrMA has been studied earlier in the MA-BrO3--cata1yst-H2SO4 system, and quantitative data have been reported.8 Since we found that under appropriate conditions chemical oscillation is observable both in nitric acid and in perchloric acid solutions, the points raised above were considered relevant for the MABr03--catalyst-HN03 (or -HC104) systems. I t should be mentioned that Mishra and Singh12have reported on clhemical oscillation in citric acid, bromate, manganlese(II),and nitric acid, or phosphoric acid BZ-type reacting systems but have not performed detailed investigations. Prasad and Verma13 described chemical oscillations in manganese(I1)-catalyzed E3Z systems in phosphoric acid medium. Experimental Section The reagenk used were 2.0 M malonic acid, 0.25 M sodium bromate, 0.10 M cerium(1V) nitrate, 0.10 M manganese(T1) nitrate, concentrated nitric acid, A.R., and concentrated perchloric acid, A.R. Analytical Procedures. Polarographic Determination of BrMA. A plolarographic method has been developed for the determination of BrMA in the oscillatory reaction mixtures.8 BrMA shows a single cathodic wave on a dropping mercury electrode, the half-wave potential of which is concentration dependent. The logarithm of the limit current 1 plotted against the logarithm of the height of the mercury column gives a straight line with a slope of 0.5. This proves that the electrode process is diffusion controlled. There is a linear dependence of the limit current on the concentration of BrMA. Bromate, which is present in high concentrations in the reaction mixtures a t pH 2 3.5, gives a polarographic wave at more negative potentials. Procedure: Aliquoh of 5.00 cm3are withdrawn from the oscillatory mixture and added to an acetate buffer of pH 4.5. (At high acid concentrations it is necessary to check the pH, and the appropriate pH should be adjusted with
1.42 1.61 1.56 1.70 1.731 1.87
a 10% sodium hydroxide solution.) Then 0.2 cm3 of 0.1% Triton-X-100 is applied as a polarographic maximum suppressor. Before the polarograms are taken, nitrogen gas is bubbled through the solution for 5 min to remove dissolved oxygen. During the gas bubbling both the dropping mercury and the saturated calomel electrodes are immersed in the polarographic cell, and the electrode potential of the working electrode is set to +0.4 V against the reference electrode. If this oxidation is omitted, owing to the insoluble mercury(1) bromide coating the mercury electrode, the current drops abruptly near the limit-current range. The polarogram of the samples is taken with a polarization speed of 2 V/8 min. The lowest concentration of BrMA which can be measured is M; the accuracy is -+3.2%. Preparation of BrMA. BrMA was prepared as described by Conrad and Reinbach14 and its purity checked by d e mental analysis and iodometric titration. Measurement of the Accumulation of BrMA. The systems investigated and the concentration ranges are compiled in Table I. Procedure: Malonic acid solution, distilled water, and acid were pipetted into a reaction vessel thermostatted to the required temperature, and the reaction mixture was thermostatted for 20 min. A smooth Pt electrode and a Hg/Hg2S04 reference electrode were immersed into the solution, and the electrodes connected to a recorder (Radelkis OH-814/1). The reaction was initiated by the addition of bromate and catalyst. The total volume of the reaction mixture was 100 cm3. Aliquots of 5.00 cm3 were withdrawn from the reacting solution, and the BrMA content was determined as described above. Results and Discussion Chemical Oscillation i n Nonsulfuric Acid BZ Systems. Systems given in Table I were chosen on the basis of the following considerations. In the BZ systems the reaction between bromate and the reduced form of the catalyst is of special importance: Br03- + 4Mn+ + 5H+
-
HOBr
+ 4Mn+l+ 2H20
Redox potential datal5 (Table 11)yield information about conditions which allow the occurrence of the reaction. In the case of the Ce3+/Ce4+catalyzed systems, cerium(lI1) can reduce bromate already at low concentrations of oulfuric acid. Replacing sulfuric acid by nitric acid only at high nitric acid concentrations exceeds the actual redox potential of the Br03-/HOBr couple (1.58 V) vs. that of the Ce4+/Ce3+couple (1.56 V). In perchloric acid solution, however, under no circumstances can cerium(II1) reduce bromate, the redox potential of the Ce4+/Ce3+system being considerably higher than that of the Br03-/HOBr system.
498
The Journal of Physical Chemistry, Vol. 84, No. 5, 1980
TABLE 111: tp, and [ BrMA]
Burger and Koros
Values in Oscillatory Systems (temp, 1 5 C) catalyst,
l o 3M
system
MA, M
NaBrO,, M
I
0.2
0.05
[Mn"] = 2.2
I1 I
0.2 0.4
0.05 0.05
[Ce^] = 2.0 [Mnz+]= 2.2
I1
0.4
0.05
[Ce4t] = 2.0
These considerations have been verified experimentally. In a 3 M nitric acid solution chemical oscillation is observable; however, it is irreproducible. In a 5 M nitric acid solution well-reproducible and sustained oscillations could be recorded. In perchloric acid solution chemical oscillation could not be generated. For the Mn3+/Mn2+couple only one value is known (E, = 1.51 V), and this has been measured in sulfuric acid solution. Since neither manganese(I1) nor manganese(II1) forms sulfate complexes of considerable stability, the redox potential values in nitric acid or perchloric acid media may be close to this value. Our investigations were performed in 1.5 M acid, and in both nitric and perchloric acid easily reproducible oscillations could be realized. Preliminary experiments in 5 M nitric acid using cerium(II1) as a catalyst have shown that the parameters of oscillation (especially the length of the preoscillatory period and the frequency of oscillation) and the amount of bromate consumed during the preoscillatory period differ essentially from the manganese(I1)-catalyzed nitric acid and perchloric acid containing BZ systems. For this reason throughout this paper the MA-Br0
