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COMMUNICATIONS TO THE EDITOR
Periodicity in the Rate of Heat Evolution during the Temporal Oscillation in the 2,4-Pentanedione-Bromate-Catalyst System
Sir: Recently Bowers, Caldwell, and Prendergastl claimed that during the oxidation of 2,4-pentanedione with bromate in the presence of manganese(I1) as a catalyst temporal chemical oscillation occurred which could be followed spectrophotometrically. In the course of our studies on the Belousov-Zhabotinsky type oscillating chemical r e a c t i ~ n we ~ , ~started to investigate the heat changQs accompanying the temporal concentration oscillation^.^ (Earlier observations in this field have been reported by Franck and Geiseler,s and by Busse.6) We have found that the rate of heat evolution is periodic in character; the high rate of heat evolution is synchronized with an abrupt increase in the redox potential and a sudden decrease in the bromide concentration. We regarded it of interest to find out what heat changes accompany the oscillating reaction described by Bowers, et ~ l . and , ~ to compare these findings with those obtained with the malonic acid-bromate-catalyst systems. In both cases manganese(I1) and cerium(II1) were used as catalysts. The calorimetric measurements were performed as described p r e v i ~ u s l y .The ~ redox potential was recorded against a smooth platinum electrode, and a double junction calomel reference electrode was used with a 10% potassium nitrate solution making contact with the reaction mixture. Unfortunately in the case of 2,4-pentanedione the bromide concentration could not be recorded with a bromide selective solid-state electrode which we had found to be the most suitable? and which was used throughout our studies on the Belousov-Zhabotinsky reaction.2.3 Namely, 2,4-pentanedione attacked the plastic body of the electrode making the function of it unreliable. Also an AglAgBr electrode could not be applied since its proper function was handicapped by the high sulfuric acid concentration (1M ) and the oxidizing nature of the oscillating system. The solutions were stirred. The heat and potential us. time curves for the.four systems are shown in Figures 1-4. The malonic acid-bromate-catalyst systems exhibit very similar curves (Figures 1 and 2) only the heat evolution and the potential change in a single step is greater with manganese as a catalyst; this means that the amount of reactants transformed in one period is greater. The 2,4-pentanedione-bromate-catalyst systems, however, differ considerably both from the other system and also from each other, i.e., the curves are considerably different when manganese is replaced by cerium. Figure 3 shows the manganese-catalyzed system. After mixing the reagents the temporal chemical oscillation starts immediately. In the early period of the reaction the heat output during one oscillation is large and it decreases The Journal of Physical Chemistry, Vol. 77, No. 26, 7973
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Typical potential ( I ) and temperature ( 1 1 ) against time curves for the chemical system containing malonic acid (0.4 M): K B r 0 3 (0.1M ) , MnS04 (0.0046 M ) , and H2S04 (0.5 M ) at Figure 1.
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Typical potential ( I ) and temperature ( 1 1 ) against time curves for the chemical system containing malonic acid (0.3 M ) , K B r 0 3 (0.1 M ) , Ce(N03)3 (0.0046 M ) , and HzS04 (0.5 M ) at 25'. Figure 2.
considerably in time; the reaction proceeds with a strongly damped character. The chemical oscillation is observable also by color change of the solution, as it has been reported by Bowers, et ul.? the colorless solution a t the sudden potential and temperature jumps turns yellow. The potential change of this system shows over- and under-shoots. This probably involves manganese(1V) formation. After a certain period of time especially above 30"
3123
Communications to the Editor I
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Typical potential ( I ) and temperature ( 1 1 ) against time curves for the chemical system containing 2,4-pentanedione (0.05 M ) , KBr03 (0.07 M ) , M n S 0 4 (0.046 M ) , and H 2 S 0 4 (1 M ) at 25". Figure 3.
a dark brown precipitate (hydrated manganese(1V) oxide) separated. The behavior of the cerium-catalyzed system is given in Figure 4. After an induction period of 1 hr, only a few (three-four) oscillations were observable. Small temperature jumps and small potential changes indicated the oscillating period of the reaction. This was followed by a great heat output; further heat evolution and potential oscillations, however, did not occur. The large heat output is not associated with a change in the redox potential; thus it can be assumed that a direct reaction occurs between the brominated dione and bromate. Comparing the 2,4-pentanedione-bromate-catalyst and the malonic acid-bromate-catalyst oscillating systems it can be established that there should be considerable difference in the mechanism of the reactions. In the malonic acid system the reaction between bromomalonic acid (which has formed in the course of the reaction) and the oxidized form of the catalyst generated the bromide ions, which later play an important role in switching on and off an autocatalytic reaction and thus controlling the steadystate concentration of HBr02 (for details see ref 3). The
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Typical potential ( I ) and temperature ( 1 1 ) against time curves for the chemical system containing 2,4-pentanedione (0.05 M ) , KBrO3 (0.07 M ) , Ce(N03)3(0.046 M ) , and H2S04 (1 M ) at 25'. Figure 4.
role of bromide ions can not be excluded even in the case of the 2,4-pentanedione-bromate-catalyst system; the mechanisms, however, are more involved owing to the separation of a precipitate in a later period of the reaction. Especially the peculiar behavior of the cerium-catalyzed system needs extensive investigations. References and Notes P. G. Bowers, K. E. Caldwell, and D. F. Prendergast, J. Phys. Chem., 76, 2185 (1972). R. M. Noyes, R. J. Field, and E. Koros, J. Amer. Chem. SOC., 94, 1395 (1972). R. J. 'Field,' E. Koros, and R. M. Noyes, J. Amer. Chem. SOC., 94, 8649 (1972). E. Koros, M. Orban, and 2s. Nagy, Nature (London), Phys. Sci., 242, 30 (1972). U. Franck and W. Geiseler, Naturwissenschaften, 58,52 (1970) H. G. Busse, Nature (London), Phys. Sci., 233, 137 (1971). E. Koros and M. Burger, Magy. Kem. Foiy., 79, 184 (1973); "Ion Selective Electrodes," Akademiai Kiado, Budapest, 1973, p 191.
lnstitute of lnorganic and Analytical Chemistry L. Eotvos University 1088 Budapest, Hungary
E. Koros* M. Orban 2s. Nagy
Received May 4, 1973
The Journal of Physical Chemistry, Vol. 77, No. 26, 1973
