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Are Uncatalyzed Bromate Oscillators Truly Gas-Free? Erik Szabo, Lubica Adamcíkova, and Peter Sevcík* Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynska dolina, 842 15 Bratislava, Slovakia
bS Supporting Information ABSTRACT:
Gas evolution is a common byproduct of chemical oscillations but is seldom treated as anything but a complication. Its precise quantification is usually underestimated, and if not obvious enough, gaseous products are easily neglected completely. Reported herein is how evolution of gas from uncatalyzed bromate oscillators with pyrocatechol, pyrogallol, and 1,4-cyclohexanedione was measured by a high-precision batch-mode gasometric method, and the data obtained with unprecedented time resolution are presented. Even though within this completely closed arrangement of measurement no oscillations in rate of gas evolution were observed, it is verified that neither of the substrates can be considered totally free of gas production. Nevertheless, the amount of gas evolved from 1,4-cyclohexanedione was confirmed to be very small, and near negligible, if only oscillatory phase is of interest. Conversely, under identical conditions, the peak rates of gas evolution from the other substrates were reached much sooner, and even though the results in all three cases suggest some extent of suppression of gas production by oscillations, as much as 1 equiv of gas in total can be produced from the phenols, especially from pyrogallol, where the degradation was measured to be approximately twice as extensive. Greater caution is, therefore, recommended whenever the gas-free nature of these oscillators is considered.
1. INTRODUCTION A large part of the spirited interest in the field of oscillating chemical reactions1,2 is traditionally focused on the Belousov Zhabotinsky (BZ) reaction,3 6 arguably the most famous and best-understood example of a chemical oscillator. Accordingly, numerous variants were discovered to exhibit behavior as extraordinary as the original formula combining citric or malonic acid, bromate and a redox catalyst in acidic media, and the term is now closely related to a whole family of oscillatory reactions of bromate with organic substrates. One remarkable subclass of such modifications is represented by the so-called uncatalyzed bromate oscillators (UBOs).7 9 These systems maintain oscillations even without the redox catalyst, indispensable in the original BZ oscillator, because they employ specific organic substrates, mainly derivatives of aromatic phenols and amines.7 13 As revealed in the respective r 2011 American Chemical Society
mechanistic studies,14 17 this is so due to the fact that the substrates can be oxidized to stable semiquinone radical intermediates, capable of substituting for the catalyst. By far the most popular UBO, however, does not require any aromatic substrate, at least not directly, as it is based on alicyclic diketone 1,4-cyclohexanedione,18 20 which was proved to oxidize to aromatic species only in the course of the reaction.21 24 As a result of its ability to supply long-lived oscillations without formation of any bubbles of gaseous byproduct, it has become the substrate of choice, in both catalyzed and uncatalyzed variants, for various experimental applications of bromate oscillators25 38 but predominantly for examination of the spatiotemporal structure formation.28 38 Received: March 29, 2011 Revised: May 4, 2011 Published: May 26, 2011 6518
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Figure 1. Schematic representation of the apparatus employed for high-precision measurement of gas evolution in oscillating reactions by weighing of liquid displaced. All parts except for thermostat and PC are placed in a thermally insulating glovebox to prevent interferences of changes in external temperature due to air currents. For a detailed description of individual parts, please refer to section 2.
Nevertheless, spatiotemporal patterns have been reported successfully for many other UBO systems as well.39 44 Unfortunately, no special attention is usually paid to the many possibilities of why gas bubbles might not be observed, for example, very small concentrations of the organic species, very short reaction times, rapid escape of gas from the thin layer of solution, or even supersaturation due to little or no stirring, already demonstrated as possible in the BZ reaction.45 An incorrect perception that all UBOs are truly gas-free has become rather common. Only recently an example of UBO with pyrocatechol was explicitly doubted to be entirely free of gas production. The products of the reaction were isolated and investigated by NMR to prove that decarboxylation takes place and gas must be produced.46 Moreover, similar strategy, with added support of gravimetric data, was also employed with the reaction of bromate with 1,4-cyclohexanedione, and the results suggest that even this reaction, conventionally considered truly gas-free, does evolve some gaseous products, particularly at elevated temperatures.47 However, the information available still remained to contain only little time-resolved detail, especially for closed-system measurements without any additional flows of auxiliary gases, not mentioning the scarcity of measurements with common temperature and concentration conditions across several representative UBOs. We have recently demonstrated, that the role of varying amounts of gaseous products in oscillating reactions should not be underestimated.49 We also have good experience in highly precise closed-system real-time measurements of gases by simply weighing of liquid they displace,48 and we were convinced, that this method could be improved, so that even very small amounts of gases would be measurable precisely. Therefore, we decided to undertake the task of applying the method in quantifying gas evolution from some examples of UBOs, hoping to contribute to verifying their reputation as gas-free oscillators. Herein we wish to present and discuss our results, obtained employing pyrocatechol, pyrogallol, and 1,4-cyclohexanedione as the representative substrates.
2. EXPERIMENTAL SECTION Chemicals. Chemicals were used as supplied, all precautions taken to avoid any modification of the composition guaranteed. All solutions were prepared directly prior to measurements, and employed throughout was only water decontaminated of organic matter and deionized to specific conductivity e1 μS cm 1 by a Demiwa ROI unit. Oscillators were prepared using approximately 5 M stock solutions of H2SO4 (p.a., Merck) certified as 96.8% (acidimetry), approximately 0.05 M stock solutions of either pyrocatechol (g99%, Sigma-Aldrich), pyrogallol (puriss., Sigma), or 1,4-cyclohexanedione (g98%, Aldrich), and approximately 2 M stock solutions of NaBrO3 (puriss. p.a., Sigma-Aldrich). Should further details of composition of these chemicals be required, the certificates of analysis of the used batches are available as Supporting Information. The saturated mercurosulfate reference electrode was prepared from polarographic mercury (puriss. p.a., Fluka), Hg2SO4 (g98%, Aldrich) and K2SO4 (puriss. p.a., Sigma-Aldrich), and in the detection compartment of the apparatus, 5.0 5.5% solutions of nonionic neutral pH detergent AQUET (Bel-Art Products) were employed as the hydraulic liquid. Instrumentation. Measurements were conducted in the apparatus demonstrated in Figure 1, derived from the simpler version,49 but additionally implementing major improvements. Most notably, the system is now a whole glass ground-joint apparatus, and it is positioned in a thermally insulating glovebox. Reactions take place in a cylindrical round-bottom reaction vessel (Figure 1a), 45 mm in diameter and with 90 mL total volume, jacketed and connected to a powerful thermostat LAUDA E100. The vessel, usually charged with 30 mL of reaction mixture and a 10 10 5 mm cross-shaped magnetic stir-bar, is fixed on a digital magnetic stirrer IKAMAG RCT. The reaction vessel is fitted with a connector piece (Figure 1b) with four smaller joints, two of them occupied by electrodes 6519
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The Journal of Physical Chemistry A monitoring the reaction potentiometrically. The reference electrode (Figure 1c) is prepared from mercury and paste of Hg2SO4 in 0.01 M H2SO4 saturated with K2SO4, surrounded by crystals of K2SO4 in its saturated solution. The indicator electrode (Figure 1d) is made of Pt wire, 0.5 mm in diameter and 5 mm long. The remaining joints on the connector piece are fitted with elements that represent the other two most notable aspects of the construction’s functional redesign. The joint on the very top carries a pipettor-like construction (Figure 1e), which allows safe storage of a reactant solution inside the apparatus and its addition to the reaction vessel without opening the apparatus to the outside. It employs a pipet-like tube connected to a three-way stopcock with an L-bore, which either (1) opens the pipet so that suction can be applied to load in a solution, (2) blocks the pipet and holds the solution inside, or (3) discharges the element while maintaining full mechanical isolation of the apparatus. The construction fitted in the last joint merges two previously separate functionalities. Its central tube and the simple straight bore stopcock (Figure 1f) can serve to empty the vessel during cleaning or to supply inert atmosphere for the measurement. The outer jacket connects the apparatus to a three-way stopcock with a T-bore (Figure 1g), which is used (1) to open either of its sides to the outside, while isolating the other part or (2) to isolate the apparatus from the outside as a whole, leading gases from the reaction vessel to the compartment where detection takes place. The detection compartment (Figure 1h) is constructed as a bottomless graduated cylinder 38 mm in diameter and 75 mm long, immersed in detection hydraulic liquid in a 250 mL beaker. The liquid is charged into the cylinder by applying suction at the T-bore stopcock (Figure 1g), and by closing it, the liquid can be fixed in place. When the two sides of the apparatus are reconnected, the open bottom of the cylinder is the only point where the system is in mechanical contact with the exterior, and the equilibrium positions of the detection liquid in the detection compartment may be used as a signal for precise measurement of the amount of gas enclosed in the apparatus. Displacements of the liquid are monitored by a digital balance Axis AD200, and the pressure and temperature in the glovebox are measured by a digital thermohygrobarometer COMET D4130. Their signals, as well as the signal from digital multimeter METEX M-4660A, measuring the potential difference between the electrodes, are transmitted to a common desktop PC (Figure 1i), where they are recorded and evaluated. Calibration. The process of calibration of the apparatus consisted of two parts, determining the volume to mass conversion in the detection compartment and determining the volume signal dependence on temperature and pressure of the surroundings. In the first part, the calibration of detection with water at 25 °C demonstrated that the rate of change of mass readings with respect to volumes of gas in the cylinder represent 1.762 g mL 1 or, more generally, 1.767 times the density of an arbitrary detection liquid. This agrees well with approximate calculations of this factor from the dimensions of the cylinder and the beaker according to equations derived from Archimedes law. The second part of calibration assessed the influence of pressure and temperature on the volume signals. The results were shown to be in very good agreement with a simple calculation according to the ideal gas law and the hydrostatic equation, but this required determining the absolute volume of the apparatus, the density of the detection hydraulic liquid, and the dependence of the height of liquid in the cylinder on the
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reading of mass. While the volume of the apparatus was measured by completely flooding it with water, subsequently weighed to yield 278.7 mL, the latter parameters, entering the hydrostatic equation, remained to be determined before each experiment as an assembly specific part of the calibration. The effect of temperature was evaluated as the dependence of the volume of gas inside the charged apparatus on the temperature inside the glovebox, as indicated by the thermohygrobarometer. Even though no general relationship between the two was formulated, their dependence showed to be very well predictable as an empirical extrapolation of the data measured in the idle window before the reaction. Finally, blank experiments were performed using H2SO4 and NaBrO3, but no organic substrates, and this yielded to have a negligible effect on the data. Procedure. Measurements were performed for three UBO systems with pyrocatechol, pyrogallol, and 1,4-cyclohexanedione, in a total reaction volume of approximately 30 mL, at a temperature of 40 °C,44 and stirring of 750 rpm. Initial reaction concentrations were as close to 0.1 M NaBrO3, 0.9 M H2SO4, and 0.05 M organic substrate44 as possible with the given procedure, and bromate was employed as the initiating agent. Each measurement started with the preparation of fresh stock solutions, measuring the actual quantities of all substances employed by weighing, and approximately 100 mL of the detection hydraulic liquid was prepared in a similar fashion in the detection beaker from the detergent to prevent interfering fluctuations due to surface tension. The actual amounts of the stock solutions pipetted into the reaction vessel were also accurately recorded by weighing the pipettes, as well as the beakers with the stock solutions, both before and after transferring the solutions. The apparatus was assembled, the detection liquid was charged into the detection cylinder, and recording of data from the three measuring devices was initiated. Leak-tightness of the assembly was tested in a window of 900 s. With the detection liquid in the cylinder fixed in place by a stopcock, the pipettor was dismounted, filled with the correct amount of bromate solution, and its tip was dried of any solution to prevent its accidental escape into the reaction vessel. The bromate beaker, as well as the piece of absorbent paper, was weighed, both before and after charging. The glovebox with the apparatus was closed, and the thermostat was set to 40 °C. Having collected data for the baseline extrapolation for 3000 s after the target temperature was achieved, the stopcock of the pipettor was opened and the bromate solution charged into the reaction vessel, initiating the reaction. The system was then monitored undisturbed for 6 h. When the measurement was finished, the amount of any unreacted bromate solution, held in the tip of the pipettor due to surface tension, was also determined by weighing. The assembly specific part of calibration of the detection compartment was performed by taking readings of the detection liquid level from the millimeter graduation of the detection cylinder, and from the balance, for 5 10 different positions of the detection liquid, and by determining the density of the detection liquid pycnometrically. The volume signal was then corrected with respect to external pressure (according to ideal gas law and hydrostatic equation) and with respect to rising temperature inside the glovebox (by extrapolating the baseline), and finally, actual initial composition of the reaction mixture was calculated assuming negligible volume changes due to mixing. 6520
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Figure 2. Typical set of records of potentiometric signal (a) with a detail of oscillations, the signal of gas evolved (b) expressed at standard conditions, and its time derivative (c) measured in the systems of approximately 0.05 M pyrocatechol, 0.9 M H2SO4, and 0.1 M NaBrO3 at 40 °C and 750 rpm stirring. The actual initial concentrations in the measurement presented: 0.0500 M pyrocatechol, 0.904 M H2SO4, and 0.1041 M NaBrO3.
3. RESULTS Altogether, 20 experiments were performed with each substrate, pyrocatechol, pyrogallol, and 1,4-cyclohexanedione, and in our conditions, the results obtained following the above measurement procedure were, in general, well reproducible, as will be demonstrated by the intervals of the many characteristics recorded. As the aftermath has shown, in all experiments, we managed to keep the actual initial concentrations of the reactants within the limits of 0.895 0.915 M H2SO4, 0.0498 0.0504 M organic substrate, and 0.0994 0.1054 M NaBrO3. Pyrocatechol. Results of measurements with pyrocatechol are demonstrated on a typical set of records in Figure 2, acquired for actual initial concentrations of 0.904 M H2SO4, 0.0500 M pyrocatechol, and 0.1041 M NaBrO3. As the potential trace in Figure 1a shows, initiation of the reaction with bromate immediately caused a dramatic rise in the potential of the indicator electrode, followed by its more moderate decline. However, on its scale it is not visible very well that in the first 75 85 s the peak was preceded by a short stabilization of potential about 110 mV
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below and by its gradually faster decrease developing into a very rapid minimum about 500 mV below the peak, only then followed by an equally rapid rise to the peak values. The rate of the subsequent decline then gradually changed from 0.290 0.310 mV s 1 to 0.035 0.040 mV s 1. As expected, oscillations appeared around 2400 2700 s, approximately 90 mV in amplitude and 100 110 s in period. Within 9 12 cycles, the amplitude grew to approximately 110 mV and the period to 220 250 s, and when around 4300 4400 s, the oscillations vanished. A monotonous decrease of the potential was then restored, even though its rate soon fell practically to zero, and the potential remained virtually constant for most of the measurement. Volume of gas evolved by the reaction mixture and its time derivative are depicted in Figure 1b and c, respectively. It is evident that the production of gas recorded can be considered negligible for no more than first 200 s, whereupon its rate quickly reached its maximum of 3.2 3.5 10 3 mL s 1 and began to decrease, more or less linearly, at a rate of approximately 5 10 7 mL s 2. When the potential oscillations started, 6.41 6.49 mL of gas had already been evolved, and even in the oscillatory phase its production continued at a steadily decreasing rate, without any trace of oscillations present. A total of 9.70 9.95 mL of gas had been evolved when oscillations finished, and their termination was always accompanied by a transition of the otherwise linear decrease in the gas production rate from 5 10 7 to 5 10 8 mL s 2, whereupon linearity was restored and maintained almost perfectly until the end of measurement. Altogether, 26.0 26.5 mL of gas at standard conditions, in total, was produced during 21600 s of recording, corresponding to approximately 1.07 1.08 mmol or 0.71 0.72 equiv of the organic substrate, with around 25% of this amount present before and 37% after oscillations. Pyrogallol. A typical set of records obtained with pyrogallol is displayed in Figure 3, with results measured for the actual initial reaction mixture composition being 0.913 M H2SO4, 0.0502 M pyrogallol, and 0.1016 M NaBrO3. Initiation with bromate caused the potential signal, depicted in Figure 3a, to increase to only a little above zero. Nevertheless, as in the case of pyrocatechol, the signal stabilized and dropped to a rapid minimum, though this time even more gradually, and after only the first 40 45 s, before rising fast and even higher once again. Unlike the signal of pyrocatechol, the signal of pyrogallol then continued to increase, at 185 195 s, bringing in 2 3 oscillations. These increased in amplitude from approximately 80 to 220 mV, and if two periods could be determined, these also increased from 48 52 s to 90 98 s. In some cases, the measurement depicted in Figure 3 being one of them, one more oscillation appeared after another 630 710 s and approximately 150 mV of gradual increase, this time, however, only 60 150 mV in amplitude and with a much slower return to the original gradient. After the oscillations, the potential signal reached its maximum in about 2800 2900 s of measurement time and until its end decreased only a little. The signal of gas production in Figure 3b showed similar overall shape, as in the case of pyrocatechol, even though the time derivative in Figure 3c reveals several differences. There also was a period where almost no gas was evolved, this time representing approximately the first 80 s. However, when the rate of gas production by the reaction reached 3.6 3.8 mL 10 3 s 1, it remained more or less constant, and it only started to rise some more again at 1600 1700 s, approximately after where the last 6521
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Figure 3. Typical set of records of potentiometric signal (a) with a detail of oscillations, the signal of gas evolved (b) expressed at standard conditions, and its time derivative (c) measured in the systems of approximately 0.05 M pyrogallol, 0.9 M H2SO4, and 0.1 M NaBrO3 at 40 °C and 750 rpm stirring. The actual initial concentrations in the measurement presented: 0.0502 M pyrogallol, 0.913 M H2SO4, and 0.1016 M NaBrO3.
Figure 4. Typical set of records of potentiometric signal (a) with a detail of oscillations, the signal of gas evolved (b) expressed at standard conditions, and its time derivative (c) measured in the systems of approximately 0.05 M 1,4-cyclohexanedione, 0.9 M H2SO4, and 0.1 M NaBrO3 at 40 °C and 750 rpm stirring. The actual initial concentrations in the measurement presented: 0.0501 M 1,4-cyclohexanedione, 0.908 M H2SO4, and 0.0995 M NaBrO3.
extra oscillation appeared occasionally. A total of 8.78 8.85 mL of gas was evolved when the rate of its evolution reached its maximum of 4.0 4.4 10 3 mL s 1 at 2440 2510 s, and started to subside. In contrast to the record of pyrocatechol, the decline in the rate of gas production was much more gradual. This time, 39.3 40.1 mL of gas at standard conditions, in total, was produced in 21600 s of measurement, which corresponds to approximately 1.61 1.64 mmol or 1.07 1.09 equiv of the organic substrate, representing around 2.3 as much as is produced with pyrocatechol. 1,4-Cyclohexanedione. Finally, Figure 4 illustrates a typical set of records of the reaction of 1,4-cyclohexanedione, acquired at actual initial reactant concentrations of 0.908 M H2SO4, 0.0501 M 1,4-cyclohexanedione, and 0.0995 M NaBrO3. As in the case of pyrocatechol, the rise of the potential signal due to initiation of reaction was rather dramatic. However, no rapid minimum was observed this time, even though after a short plateau the potential started to decrease gradually, as on the potential record in Figure 4a. At 1620 1740 s, the oscillations
started with 85 105 mV amplitude and 20 23 s period. Whereas the baseline of the oscillations decreased almost linearly at a rate of approximately 0.12 mV s 1, the position of peaks shifted only a little higher and remained almost constant for most of the oscillatory phase. Only before the end of oscillations did their peaks start, more or less gradually, to decrease, with maximum amplitudes of 205 245 mV being reached at 2570 2620 s. The period, which to that point slowly shortened to 19 21 s, started to increase as well, and at 2910 2960 s, the oscillations disappeared, with amplitudes of 160 200 mV and periods 30 36 s long. In total, 63 69 oscillations were produced, and the potential signal following the last oscillation continued to decrease. However, it soon stabilized, and at 3350 3450 s, it grew back very fast to the peak values, where it remained until the end of measurement in 21600 s. As demonstrated in Figure 4b, the production of gas in the reaction with 1,4-cyclohexanedione is much smaller than in the previous two cases, and therefore, the trace of its time derivative in Figure 4c is also much more noisy. At the beginning of 6522
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The Journal of Physical Chemistry A oscillations, there was no more than 0.1 mL gas evolved, and when the oscillations disappeared, the rate of gas production was still no higher than 1.6 10 4 mL s 1, with the total amount of gas produced at only 0.25 0.26 mL. However, after the potential rose once again, the rate of gas production also increased significantly to reach its maximum of approximately 6.5 10 4 mL s 1 at 6300 6600 s, when 1.5 1.7 mL of gas was produced already. The evolution of gas then subsided almost as fast as it had built up, and at around 13500 s, its rate stabilized at approximately 7 8 10 5 mL s 1, making the total volume of gas recorded increase only very little by the end of measurement. Altogether, 4.0 4.2 mL of gas at standard conditions, in total, is produced during 21600 s of measurement, corresponding to approximately 0.16 0.17 mmol or 0.11 equiv of the organic substrate, with only around 2% of this amount present before and 6% after oscillations.
4. DISCUSSION Application of identical initial compositions of the reaction mixtures for all three substrates was successful, and periodic changes in the composition of the reaction were indicated in all experiments performed. Equally successful was also a practical application of the improved apparatus. Evolution of gas was also observed for all three substrates, and precise measurements of its dynamics, with a previously unavailable level of time-resolution detail, revealed many common features, even though the potentiometric signals appeared to exhibit fairly unique patterns. First, all experiment exhibited no oscillation in rate of gas evolution during the reaction. It was confirmed by many direct calibration experiments that once the gas separates as a distinct phase it is detected immediately and, therefore, it is most probably either the physics of gas evolution from solution in the closed-system arrangement or maybe even the chemistry of gas production itself that proceeds in nonoscillatory fashion. Even though at this stage it was not possible to determine which is the case, the value of the closed-system measurements (detecting gas without auxiliary gas flow) and the need for their further application is obvious. Regardless of the scales, overall shapes of the records were much more similar for the structurally closer pyrocatechol and pyrogallol, where evolution of gas starts relatively quickly at full power and for most of the time only subsides slowly. On the contrary, evolution of gas from 1,4-cyclohexanedione takes much longer to develop and then rather quickly recedes. As pyrogallol exhibited oscillations without an induction period, pyrocatechol was the only substrate where oscillations were observed while the rate of gas production was on the decrease. Yet, the decrease seemed to become more relaxed after oscillations finished, and some suppression of gas evolution by oscillations is, therefore, suggested, as in the other two cases, where oscillations seem to temporarily hold back the rising rate of gas production, leading either to a shaped rising edge of the record in the case of pyrogallol or almost totally suppressed evolution of gas in the case of 1,4-cyclohexanedione. As far as only the total amount of gas produced is considered, under the conditions employed, 1,4-cyclohexadiene confirmed its prime as a substrate for bubble-free bromate oscillators, with only 4.0 4.2 mL of gas at standard conditions produced during 21600 s, and as little as 0.25 0.26 mL of gas produced to the point when oscillations disappear. This corresponds fairly well with the recent results,47 where approximately half of the amount
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was determined at 0.60 the initial concentration of 1,4-cyclohexanedione, even though a much greater extent of degradation is suggested at higher temperatures. Both pyrocatechol and pyrogallol were demonstrated to produce significantly larger volumes in 21600 s of measurement at given conditions representing as much as 26.0 26.5 and 39.3 40.1 mL of gas at standard conditions or 0.71 0.72 and 1.07 1.09 equiv, respectively. In the case of pyrocatechol, this supports the recent findings, suggesting that 1 equiv of CO2 could be eliminated from the substrate,46 and on the other hand, in the case of pyrogallol, this result proves that much deeper degradation of the substrate must be occurring. Moreover, it should be taken into account that these amounts are already large enough to cause significant change in composition of the gas phase inside the apparatus, leading to partial dissolution of gas produced, and the gas production recorded could be even larger if the experiment were performed with more gas phase present or with an atmosphere identical to the gas produced. On the other hand, the evolution of gas could most probably be somehow diminished by supersaturation if stirring is reduced, as shown for BZ systems before.45 In the case of 1,4cyclohexanedione, under the conditions given, supersaturation alone could very probably lead to complete halting of gas evolution. Conversely, amounts of gas produced from pyrocatechol and pyrogallol suggest that, depending on concentrations and the thickness of the solution layer, escape of gas bubbles from the reaction solution is very likely to happen at some point, even though this might not have to occur before the time window of interest ends. The results, therefore, also suggest that it should always be taken into account that 1,4-cyclohexanedione might be one of a very few substrates, which at given conditions produce truly minimal amounts of gas. Others, as pyrocatechol and pyrogallol, may evolve significant amounts of gaseous products, even though larger parts of these amounts might be produced when the systems do not oscillate. Nevertheless, the production of gas should not be dismissed without measurement.
4. CONCLUSIONS It is demonstrated that, at given conditions, all substrates employed afford chemical oscillation, but neither pyrocatechol, pyrogallol, nor 1,4-cyclohexanedione can be considered totally free of gas production. An exceptional level of time resolution of the measured data revealed that the overall evolution of gas proceeds with no oscillation, even though it was not possible to determine if it is because of the process of physical release of gas from reactions in the closed-system measurement or because of the actual chemistry related to gas production. It was, however, observed that in all cases the oscillatory window shapes the production of gas to some extent, and it generally seems that oscillations hold the gas production back, if not almost completely suppress it. Whereas the phenols afford peak rates of gas production early after the onset of the reaction, in the case of 1,4cyclohexanedione, the peak starts to develop only after the oscillatory window is terminated. Moreover, the overall amount of gas evolved from 1,4-cyclohexanedione was found to be very small, and therefore, at given conditions, it was confirmed to be near negligible, if only the oscillatory phase is relevant. To the contrary, it was shown that, indeed, as much as 1 equiv of gas can be produced from the phenols, and especially pyrogallol, where the degradation was measured to be approximately twice as 6523
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The Journal of Physical Chemistry A extensive, and this should be taken into account whenever the gas-free character of these oscillators is considered.
’ ASSOCIATED CONTENT
bS Supporting Information. The certificates of analysis of the used batches of H2SO4 (p.a., Merck), pyrocatechol (SigmaAldrich), pyrogallol (puriss., Sigma), 1,4-cyclohexanedione (Aldrich), and NaBrO3 (puriss. p.a., Sigma-Aldrich). This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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