J . Phys. Chem. 1989, 93, 2801-2807
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Kinetic Study of the Autocatalytic Nitric Acid-Bromide Reaction and Its Reverse, the Nitrous Acid-Bromine Reaction Istvin Lengyel, Istvin Nagy, and Gyorgy Bazsa* Institute of Physical Chemistry, Kossuth Lajos University, Debrecen, 401 0, Hungary (Received: August 11, 1988; In Final Form: November 2, 1988)
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The kinetics of the forward and backward processes of the reaction NO3- + 2Br- + 3H' e Br, HNO, + H 2 0 have been studied. The overall process is a true dynamic equilibrium. The same equilibrium position can be reached from either side and can be shifted by changing any of the species involved. The equilibrium constant was found to be K , = (1.6 i 0.3) X IOd M4. The oxidation of bromide by moderately concentrated nitric acid in batch is autocatalytic, nitrous acid being the catalyst. The influence of initial concentration of nitric acid, bromide, and nitrous acid was determined. In a continuously stirred tank reactor the reaction exhibits bistability. The reaction between bromine and nitrous acid has regular kinetics. A seven-step mechanism with intermediates NO, NO2, BrN02, BrNO, and HOBr is proposed, which accounts for the stoichiometry, equilibrium nature, and kinetic behavior of the reaction. The agreement between experiments and calculations is generally satisfactory and in some cases excellent.
Introduction There are many autocatalytic reactions among nitric acid oxidations with both inorganic' and organic2 substrates. Recently attention has been focused on the autocatalytic processes taking place in batch and flow reactors, because some exotic kinetic phenomena were discovered during these investigation^.^ Bistability and hysteresis were found in the oxidation of i r ~ n ( I I and )~ thiocyanate oxidations in a continuously stirred tank reactor (CSTR), and similar phenomena are expected in other systems. In our laboratory systematic investigations are in progress to explore the kinetics and mechanism of nitric acid oxidations. This work is a part of these investigations. The stoichiometry of the title reactions is simple and well defined: NO3- 2Br- 3H' * Br2 H N 0 2 + H 2 0 (1) (Hereafter reaction (+1) will refer to the oxidation of bromide and reaction (-1) to the bromine-nitrite interaction.) The standard redox potentials of the two redox couples are close to each other:6 NO3- 3H' + 2e- = H N 0 2 H 2 0 E = 0.940 V (2)
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Br2 + 2e- = 2Br-
E = 1.087 V (3) Therefore this reaction is reversible: the same equilibrium position can be reached starting from either side of reaction 1, and the equilibrium reached can be shifted by adding or removing any of the compounds involved. The forward process (+1) is autocatalytic, as noted but not exhaustively studied by Longstaff7 and Feilchenfeld et a1.* The reverse reaction (-1) was investigated by Pendlebury and Smith9 in the pH range 0.8-5.8. In spite of these papers, it seemed necessary to reinvestigate the kinetics of reaction 1 and to extend the experiments to explain both the autocatalytic and reversible character in a batch reactor and the bistable behavior in a continuously stirred tank reactor. Experimental Section Materials. Highest grade commercially available HNO, (Carlo Erba), HC104 (Rudi Point, VEB Laborchemie, Apolda), Br2 (1) Bazsa, G.; Epstein, I. R. Comments Znorg. Chem. 1986, 5, 57. (2) Ogata, Z. Oxidations in Organic Chemistry, Part C; Trahanowsky, W. S.,Ed.; Academic Press: New York, 1978; p 296. (3) Epstein, I. R.; Kustin, K.; DeKepper, P.; Orbln, M. Sci. Am. 1983, 248, 1 12. (4) Orbln, M.; Epstein, 1. R. J . Am. Chem. SOC.1982, 104, 5918. (5) Bazsa, Gy.; Epstein, I. R. Znt. J . Chem. Kinet. 1985, 17, 601. (6) Standard Potentials in Aqueous Solution; Bard, A. J., Parsons, R., Jordan, J., Eds.; Marcel Dekker: New York, Basel, 1985. (7) Longstaff, J. V. L. J . Chem. SOC.1957, 3488. ( 8 ) Feilchenfeld, H.; Manor, S.; Epstein, J. A. J . Chem. Soc., Dalton Trans. 1972, 2675. (9) Pendlebury, J. N.; Smith, R. H. Ausr. J . Chem. 1973, 26, 1847.
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(Merck), and NaBr, N a N 0 2 , and N a N 0 3 (Reanal) were used without further purification. Nitric acid solutions were prepared by dilution and purged with nitrogen gas for 1 h to remove dissolved gases, particularly nitrogen oxides, because these compounds exhibit a catalytic effect and influence the kinetics of the reaction. To eliminate this problem in CSTR experiments, we used sodium nitrate perchloric acid and in other cases a mixture of nitric acid and perchloric acid. Saturated solutions of bromine were freshly diluted to the desired concentration before kinetic runs. The concentration of Br2 was determined iodometrically and photometrically.1° NaBr, N a N 0 2 , and NaNO, solutions were prepared by weighing the proper amount of the compounds. Kinetic Runs. Measurements were made on a Hitachi 150-20 spectrophotometer and on a Hitachi 100-60 spectrophotometer, equipped with a homemade rapid-mixing device based on the stopped-flow principle. The mixing time or dead time was 1 s, negligible even in the case of the shortest reaction times during the study of reaction 1. However, this delay was too long for some kinetic runs on the reverse reaction (-1). The flow-through cell had no empty gas space in order to eliminate evaporation of bromine or nitrogen oxides. The reaction was followed by continuous recording of the absorbance of bromine at 441 or 467 nm, where molar absorbances are 130 and 85 M-' cm-', respectively. At these wavelengths the absorption of nitrous acid is negligible. The path length of the cell was 0.5 cm. The cell was thermostated at 25 O C . At higher nitric acid concentrations the heat of dilution increased the temperature of the reaction mixture; therefore the reactant solutions were precooled to compensate for this effect. This certainly reduced the accuracy of measurements. In the case of the bromine + nitrite reaction, we were not able to follow the kinetics at acid concentrations higher than 2 M because the reaction is fairly fast and there is not enough time to establish thermal equilibrium, so the initial rate is very irreproducible. The CSTR apparatus consisted of a thermostated glass flow reactor with premixer and magnetic stirrer. It was connected to a DESAGA PLG 132100 peristaltic pump. Potentiometric measurements were performed by using a bright Pt electrode against SCE.
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Results Stoichiometric and Equilibrium Studies. On the basis of the stoichiometry of the title reaction given by eq 1 the equilibrium constant is defined as K1 =
[HN021 m 2 1 [NO3-][Br-I2h3
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(10) Grove, J. R.; Raphael, L. J. Inorg. Nucl. Chem. 1963, 25, 130.
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2802 The Journal of Physical Chemistry, Vol. 93, No. 7, 1989
Lengyel et al.
TABLE I: Equilibrium Colrrtants of Reaction 1 at Different Initial Reactant Concentrations expt no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
[H'], M 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 3.5 4.0 4.5 5.0 5.5 4.0 4.0 4.0 4.0
[NO2.5 X lo4 M. The calculated and experimental regions of bistability (Figure 3) agree excellently. If the system in a CSTR (conditions of Figure 3, ko = 0.007 s-l) being in SS2 stationary state was perturbed by added nitrite solution ([HNO,] = 0.01 mol dm-’), then a transition occurred to SS1 with a 4-min half-time. The calculated
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value of t,12 is the same. This is strong support of the validity of the model. (The opposite transition can be achieved by applying the hydrazine solution, but calculations would require the knowledge of kinetics of this interaction.)
Conclusions In this paper we presented a complete example where an autocatalytic redox reaction and its reverse were investigated and treated from the stoichiometric, equilibrium, and kinetic points of view. The overall process (1) is a true dynamic equilibrium reaction, and it was successfully described according to the mass action law. The oxidation of bromide in moderately concentrated nitric acid is an autocatalytic process, nitrous acid being the catalyst. In a continuously stirred tank reactor it shows bistability and hysteresis. Its reverse, the reduction of bromine by nitrous acid, has regular kinetics. On the basis of the experimental results, a seven-step mechanism was formulated together with rate equations for the steps. This mechanism describes all these behaviors, and the agreement between measured and calculated data in all cases is fairly good, in some respects excellent. The long-term aim of these studies is to test such autocatalytic reactions for bistable behavior in a CSTR and then to seek for a proper feedback reaction to produce oscillation. This last goal has not yet been reached, but such work is in progress in our laboratory. Acknowledgment. This work was supported by OTKA Grant 248 from the Hungarian Academy of Sciences. We thank Prof. M. T. Beck, Prof. I. R. Epstein, and Dr. Gy. PBta for helpful discussions and for a critical reading of the manuscript. Registry No. HNO,, 7697-37-2; Br-, 24959-67-9; HN02, 7782-77-6; Br,, 7726-95-6.
Design of a Permanganate Chemical Oscillator with Hydrogen Peroxide Arpid Nagy and iudovh Treindl* Department of Physical Chemistry, Comenius University, 842 15 Bratislava, Czechoslovakia (Received: August 15, 1988; In Final Form: November 22, 1988)
The permanganate chemical oscillator with hydrogen peroxide in the presence of phosphoric acid in a continuously stirred tank reactor is described in more detail. Three types of bistabilities and a tristability have been revealed on the basis of the phase diagrams in [H3P04]o-&, [H202]0-k,,,and [KMnO&-k0 planes. The reaction scheme of this permanganate chemical oscillator, stressing the crucial role of phosphoric acid in stabilization of colloidal MnO, species, is outlined.
Introduction In a homogeneous chemical reaction maintained far from equilibrium where the reaction mechanism contains coupled feedback steps, oscillations in the concentrations of certain intermediates are possible. At the beginning of the 1980s an efficient experimental method was proposed1” to elaborate and to design new homogeneous oscillators. Epstein et al.4 using this method based on a continuously stirred tank reactor (CSTR) technique succeeded in discovering several new oscillators, first of all the group of chlorite oscillators, and thus broadened the great family (1) De Kepper, P.; Epstein, I. R.; Kustin, K. J. Am. Chem. Soc. 1981,103, 2133. (2) Epstein, I. R. J . Phys. Chem. 1984, 88, 178. (3) Epstein, I. R.; Kustin, K. Structure and Bonding; Springer-Verlag: Berlin, 1984; Chapter 1 . (4) Epstein. I. R.; Orbln, M. In Oscillations and Traveling Waves in ChemicalSystems; Field, R. J., Burger, M., Eds.; Wiley: New York, 1985; Chapter 8.
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of the homogeneous oscillatory reactions based on the chemistry of oxyhalogen anions. Oscillating systems with no halogen compounds also have been o b ~ e r v e d . ~So far they form a relatively small group of homogeneous chemical oscillators. In 1986 we briefly described a new member of this group, the first permanganate chemical oscillator! It consists of an aqueous solution of permanganate ions, hydrogen peroxide, and phosphoric acid. A second permanganate chemical oscillator based on the reaction between permanganate ion and ninhydrin also has been described.’ Although a kinetic bistability in the permanganate oxidation of hydrogen peroxide or of oxalate in CSTR has been observedB-l0 and the reaction between per( 5 ) Pacault, A,; Ouyang, Q.; De Kepper, P. J. Stat. Phys. 1987,48, 1005. ( 6 ) Nagy, A.; Treindl, L. Nature (London) 1986, 320, 344. (7) Treindl, L.; Nagy, A. Chem. Phys. Letr. 1987, 138, 327. (8) De Kepper, P.; Ouyang, Q.;Dulos, E. In Non-Equilibrium Dynamics in Chemical Systems; Vidal, C., Pacault, A,, Eds.; Springer-Verlag: Berlin, 1984; Chapter 6.
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