Studies of the Catalyzed Bromate Ion-Benzoquinone- Sulfuric Acid Chemical Oscillator

Transcription

1 Studies of the Catalyzed Bromate Ion-Benzoquinone- Sulfuric Acid Chemical Oscillator Ariel S. Hyre Senior Honors Thesis: May 2013 Advisor: Professor Irving R. Epstein Presented to the Department of Chemistry Brandeis University, Waltham, MA, 02454

2 Dedicated to Otto Hadac for his incredible strength and spirit in times of adversity 2

3 Table of Contents 1. Abstract Introduction A Brief History of Nonlinear Chemical Dynamics The Belousov-Zhabotinsky Reaction The Field-Koros-Noyes Mechanism Motivation General Experimental Reagents Preparation of Reaction Mixture Reactor and Data Acquisition Results and Discussion Reproduction of Literature Results Effects of Light Intensity on the Cerium (IV)-Catalyzed Oscillatory System Other Catalysts Tris-2,2'-bypiridyl-ruthenium(II) Manganese(II) Ferroin Demonstrating the BZ-Type Nature of the System Oscillations in the Concentration of Bromide Ions Oscillations in the Concentration of Cerium(IV) Ions Creating a Modified FKN Mechanism Roles of Malonic Acid Induction Period and Photoreduction of Q Identification of Reducing Agent Identification of the Brominated Species Ongoing Work Conclusions Acknowledgments References

4 1. Abstract The behavior of the photosensitive catalyzed bromate ion-benzoquinone-sulfuric acid oscillator was studied in order to understand the mechanism of its oscillations. In the first series of experiments, the composition and light sensitivity of the system were examined. It was found that the purity of the organic compound and the intensity of irradiation have significant effects on the behavior of the system, and that several commonly used metal catalysts are able to catalyze the oscillatory reaction. A second series of experiments were conducted to determine whether or not the system is of the family of chemical oscillators known as Belousov-Zhabotinsky (BZ) reactions. It was shown that the catalyzed bromate ion-benzoquinone-sulfuric acid system exhibits oscillations in the concentration of bromide ions and oxidized catalyst, implying that it is indeed a BZ-type oscillator. A mechanism for the oscillatory reactions in the system was investigated. Experiments were performed to identify the reducing agent and brominated species. Candidates have been selected to fill in these roles, but further tests are needed in order to verify the hypotheses developed over the course of this research. 2. Introduction 2.1 A Brief History of Nonlinear Chemical Dynamics Oscillations of chemical origin are present everywhere in nature, but the first report of oscillations in a chemical system under laboratory conditions was published by Gustav Fechner in He described an oscillating current produced by an electrochemical cell. Wilhelm Ostwald, in 1899, observed that the rate at which metallic chromium dissolved in acid increased and decreased in a periodic fashion. Both of these systems were inhomogeneous, and because of this, it was long held that that inhomogeneity was the source of the observed oscillations. 1 The first homogeneous oscillatory chemical system was described by William C. Bray in Iodine and hydrogen peroxide were combined to produce iodic acid, and the rates of oxygen and iodine production varied periodically according to the following reactions: 5H 2 O 2 + I 2 2HIO 3 + 4H 2 O 5H 2 O 2 + 2HIO 3 5O 2 + I 2 + 6H 2 O Although the Bray Reaction is now recognized as the first homogeneous chemical oscillator, the paper 4

5 was greeted with skepticism at the time of its publication. It was believed that a homogeneous system behaved like a pendulum that swings through its equilibrium point. However, an oscillatory chemical reaction that passes through its equilibrium state with every cycle would violate the Second Law of Thermodynamics. 1 Many years after Bray's reaction was published, it was realized that chemical oscillators do not pass through equilibrium states at all rather, they exist far from equilibrium, and obey the Second Law differently from the way a pendulum does. In the pendulum analogy, products of low Gibbs free energy would have to be converted back into high free energy reactants, which violates the Law. In reality, the free energy of the system declines monotonically, while the concentration of certain intermediates increases and decreases periodically. The requirement that entropy always increase is fulfilled by the realization that an open system continuously releases energy into the surroundings. Although the actual behavior of the entropy is unknown, this treatment allows for an explanation of how a homogeneous oscillatory chemical system can exist while still obeying the laws of thermodynamics. 1 The new theory which articulated this concept was published by Ilya Prigogine and Gregoire Nicolis in The most famous oscillator was developed in the USSR by Boris Belousov, who studied a solution comprising sodium bromate, citric acid, and ceric ions (Ce 4+ ). He observed that the reaction did not monotonically reduce the yellow Ce 4+ into colorless Ce 3+, as was expected. Rather, the color of the solution oscillated between the two states when stirred in bulk, and traveling waves of yellow were produced when the mixture was left unstirred in a graduated cylinder. The first two attempts to publish his findings, in 1951 and 1957, were rejected. The system's only publicity came in the form of an abstract in the proceedings of a 1958 conference, until a graduate student named Anatol Zhabotinsky began investigating Belousov's original recipe in Zhabotinsky exchanged malonic acid for the original citric acid, penned a manuscript of his investigations, and mailed it to Belousov in Due to Cold War tensions, interest in the oscillator remained primarily localized in the Soviet Union 1 before the first paper in English was published in It was not until Zhabotinsky gave a presentation at a 1968 conference in Prague that the topic garnered much attention west of the Berlin Wall. Before Belousov's death in 1970, Zhabotinsky and his colleague A. N. Zaikin demonstrated that a ferroin-catalyzed variant of the reaction displayed spatial self-organization as well as temporal. 5 These were the first of a large family of systems, known collectively as Belousov-Zhabotinsky reactions, 6 which will be discussed in more detail in the 5

6 following section. More recently, the study of oscillatory chemical systems has expanded into new areas of biology, chemistry, and physics. The Nonlinear Chemical Dynamics group at Brandeis University has developed and used algorithms to design new chemical oscillators, and modified existing ones to display behaviors like directed spatial pattern formation 7,8 and chemical chaos. 9,10 They also use chemical networks to mimic biological ones, like neurons. 11 BZ-type mixtures are used by the group of Seth Fraden to create and study biomimetic active matter systems, as well as the collective dynamics of coupled aqueous BZ droplets in oil. 12, The Belousov-Zhabotinsky Reaction BZ is one of the best understood chemical oscillators in the field of nonlinear chemical dynamics. The classical reaction proceeds through oxidation of malonic acid by bromate ions in an acidic medium, in the presence of Ce(SO 4 ) 2 in a closed, or batch, reactor. 14 As with many far-fromequilibrium chemical systems, oscillations in the BZ reaction can only proceed when the concentrations of reagents are within a certain range. The concentration of intermediates (Br -, HOBr, HBrO 2 ) oscillates as the reaction proceeds, giving rise to changes that can be monitored in various ways. The presence of brightly colored catalysts allows for visual tracking of their oxidation states with UV-Vis spectrometry or cameras. The electrochemical reduction-oxidation potential of the reaction mixture also shows oscillations, which can be followed by a platinum-reference electrode pair; oscillations in the concentration of bromide ions can be monitored independently with a Br-ISE. 15 BZ can display many types of self-organization depending on experimental conditions. Thin films of the solution are capable of forming target patterns, 5 spirals, and wave fronts 7,8,16 ; macroemulsions of coupled droplets can display complex synchronization patterns 13 ; and a specific formulation called BZ-AOT can form Turing patterns. 17 6

7 Figure 1: Target patterns formed over time by propagation of concentration waves in BZ solution The Field-Koros-Noyes Mechanism 18 The mechanism which describes the detailed interaction of the components of BZ was published in 1972 by Richard J. Field, Endre Koros, and Richard M. Noyes. This mechanism, named FKN after the scientists who proposed it, explains the oscillatory process using the reactions of the oxybromine chemistry known at the time. The FKN mechanism of the BZ reaction consists of three main processes, designated A, B, and C, which are controlled by the concentrations of common intermediates: namely, Br, HBrO 2, Ce 4+ /Ce 3+. During the reduced state, when both [Ce 3+ ] and [Br ] are high, Process A is running: PROCESS A R3 Br BrO 3 + 2H + HBrO 2 + HOBr R2 Br - + HBrO 2 + H + 2HOBr R1 (Br - + HOBr + H + Br 2 + H 2 O) x3 R8 (Br 2 + CH 2 (COOH) 2 BrCH(COOH) 2 + Br - + H + ) x3 2Br BrO 3 + 3H + + 3CH 2 (COOH) 2 3BrCH(COOH) H 2 O In Process A, the reaction chain of R3 R2 R1 produces HOBr and Br 2. These species can 7

8 react with the organic substrate, malonic acid (MA), as seen in reaction R8, producing bromo-malonic acid (BrMA). During this process, concentrations of HBrO 2, HOBr, and Br 2 are very low, because their production is considerably slower than their consumption. When Br concentration falls below a critical value, due to its consumption by reactions R1, R2, and R3, process A stops. The critical [Br ] is determined by [BrO 3 ] and the ratio of the reaction rates of R2 and R5 as follows: [Br ] crit = (k 5 /k 2 )[BrO 3 ] = 5*10-6 [BrO 3 ] When Br is below the critical value, which means that Br is virtually absent, autocatalytic production of HBrO 2 takes place in Process B: PROCESS B R5 R6 BrO HBrO 2 + H + 2BrO 2 + H 2 O (Ce 3+ + BrO 2 + H + Ce 4+ + H 2 O + HBrO 2 ) x2 2Ce 3+ + BrO HBrO 2 + 3H + 2Ce 4+ + H 2 O + 2HBrO 2 During Process B, concentration of HBrO 2 reaches a value approximately 6 orders of magnitude higher than it does during Process A. Eventually, when the oxidation of the reduced catalyst (Ce 3+ ) by R6 is complete, the autocatalytic cycle in Process B stops, and reactions of Process C take over: PROCESS C - R4 2HBrO 2 BrO 3 + HOBr + H + R9 6Ce 4+ + CH 2 (COOH) 2 + 2H 2 O 6Ce 3+ + HCOOH + 2CO 2 + 6H + R10 4Ce 4+ + BrCH(COOH) 2 + 2H 2 O Br - + 4Ce 3+ + HCOOH + 2CO 2 + 5H + When there is no more Br or Ce 3+ present, the production of HBrO 2 stops. Its rapid disproportionation in R4 then occurs: [HBrO 2 ] decreases, and temporal increase of [HOBr] and [Br 2 ] can be observed due to R2 and R1, respectively. The oxidized catalyst, Ce 4+, gets reduced in reactions R9 and R10. High [Br ] is restored by R10, which enables Process A to resume. The oscillatory cycle then begins again. 8

9 Although the oxybromine species present in Process A (BrO 3, HBrO 2, HOBr) can be reduced by Ce 3+ in one-electron steps, these reactions are significantly slower than the two-electron reduction by Br. In Process A, therefore, when [Br ] is high, one-electron step processes are not considered. In Process B, however, when [Br ] is low, the one-electron step reaction (R6) between the intermediate BrO 2 and Ce 3+ gains significance. 2.4 Motivation Today, many scientists working in the field of nonlinear chemical dynamics study the effect of external feedback on chemical systems. The most common chemical oscillators used to study this interaction are modified versions of the classical BZ reaction. Feedback can be applied by chemical perturbations, 19 or by adding photosensitive components. The most widely used technique to interact with an oscillating system is irradiation by light. 20 Inhibition can be carried out in a BZ-type oscillator if tris-2,2'-bypiridyl-ruthenium(ii) 2+ (Ru(bpy) 3 ) is used as catalyst. 21,22 In the presence of this catalyst, oscillations can be stopped by irradiating the reaction mixture with 450nm light, and spontaneously resumed by turning the light source off. This technique provides a simple, non-invasive way of inhibiting oscillations. Figure 2: A ruthenium-catalyzed system exposed to 450nm light between 650 and 900sec. Oscillations are inhibited under illumination, and response of the system to the perturbation is almost immediate. 23 Until very recently, chemical oscillators were activated exclusively by chemical perturbation. In 2006 and 2007, the research group of J. Wang published papers describing a chemical oscillator upon which light has a constructive effect. 24,25 The composition of this system is very similar to the classical BZ, but malonic acid was replaced by the photosensitive compound 1,4-benzoquinone (Q). In the paper, this compound was described as the organic substrate. 26 Many research applications, like that of the Fraden lab, would benefit from a system which can 9

10 be both activated and inhibited by shining light on it at two different wavelengths. By combining the classical BZ with the Q containing system, we intended to create a BZ-type oscillator that could be both activated and inhibited using light. Our initial experiments uncovered discrepancies in the Wang group's experimental findings, and the mechanism proposed in their work 24,26 was criticized during a Gordon Research Conference in Therefore, we set out to revisit the catalyzed bromate ion benzoquinone sulfuric acid system, in order to better characterize it and to provide a more feasible explanation for the oscillatory behavior. Our final goal is to produce a system which will enable researchers to use activation along with inhibition to provide external feedback to these systems. 3. General Experimental 3.1 Reagents Cerium(IV) sulfate (Aldrich, 97%), tris-2,2'-bypiridyl-ruthenium(ii) chloride hexahydrate (Aldrich, analytical grade), sodium bromate (Alfa Aesar, 99.5%), 2-hydroxy-hydroquinone (Alpha Aesar, 99%), potassium bromide (Janssen Chimicha, 99%), potassium chloride (Fisher, analytical grade), potassium sulfate (Acros Oganics, 99+% anhydrous), 0.025M ferroin solution (Ricca Chemical Co.), manganese(ii) sulfate monohydrate (Sigma, 98%), malonic acid (Acros Organics, 99%), and 10N sulfuric acid solution (Fisher Scientific) were used without further purification. Dilute bromine water was made dissolution of elementary bromine (Fisher, laboratory grade) in deionized water. Concentration of the solution was confirmed via spectrophotometry prior to use. Stock solutions were prepared in volumetric flasks with ultrafiltered water (18.2 megaohms resistance) purified with an Aries Filterworks Gemini Q was purchased from Sigma-Aldrich (99%) and Acros Organics (98%). For most of the experiments, the raw greenish-brown powder was sublimed into pure yellow-gold crystals using an aluminum-foil-wrapped 800mL Pyrex beaker, covered with a watch glass and heated with a sand bath (65 C, 8h., ~50% yield). Because Q reacts readily with water in the presence of light, the pure substance was stored in an aluminum-foil-wrapped glass vial at -24 C to prevent decomposition. One batch could be used for several experiments over the course of a month, as the method of storage was sufficient to maintain reagent purity. 10

11 3.2 Preparation of Reaction Mixture Standard concentrations for components of the experimental system were chosen from those originally specified in J. Wang's published works. Reagent Stock (M) Volume (ml) Mixture (M) H2SO NaBrO Ce(SO4)2 5x x10-4 Q -- (0.113g) 0.35 Table 1: Concentrations of stock solution, volume added, and concentration of reagents in experimental systems. For all experiments, the above reagents will be present in the specified concentrations unless otherwise noted. A balance (Fisher Scientific Accu-124) was used to weigh solid Q (0.113g, 45mmol) into a 50mL Pyrex beaker and a teflon-coated stir bar (Fisher, 1cm x 0.5cm) was added. Eppendorf micropipettes were used to add water and sulfuric acid. The beaker was covered in aluminum foil and its contents were stirred until no solid Q remained, at which point aliquots of sodium bromate solution and catalyst solutions were added. The total volume was 30mL. 3.3 Reactor and Data Acquisition For all experiments, the reaction vessel was held at 22 C in a water-circulating thermal jacket (NESLAB RT3-111) and placed on a stir plate (Ica Color Squid) to stir at a constant 600rpm. The reaction mixture was irradiated with white light provided by a Dolan-Jenner Industries Fiber-Lite PL- 800 with a 150W Osram halogen bulb. Spectral signature of the light was regulated by control of voltage to the bulb, and an iris diaphragm was used to limit the intensity of the light. Intensities were measured prior to each experiment with a Newport 181-C power meter featuring an 883-UV sensor. The experimental setup was placed in a blackout box that isolated it from light while measurements were taken. A platinum working electrode (Radiometer Analytical M241Pt) and Cole-Parmer Ag AgCl KCl reference electrode (with a saturated K2SO4 junction) were immersed in the reaction mixture. These electrodes were connected to an EXTECH Instruments Multilog 720 multimeter, which was connected to a PC via an RS232 port. Data were acquired at 2Hz and recorded with a homebrew application developed in the LabView 2012 SDK. A Thermo-Orion 9435 half-cell bromide-selective electrode was also used with the same reference-junction pair and data acquisition setup. UV-Vis spectrophotochemical studies were 11

12 conducted on a Shimadzu UV-1650PC spectrophotometer, using quartz cells. These data were collected using UV Probe All collected data were analyzed using MatLab R2012b and graphed with OriginPro 8.0 and 9.0 software. 4. Results and Discussion 4.1 Reproduction of Literature Results The first batch of Q that was ordered did not look the way the substance is described in the specifications: reagent from Sigma-Aldrich was a greenish-brown powder, and Acros Organics delivered a visibly different reddish-black product. Another inconsistency was seen when the raw Q was dissolved in water: some part of the powder, probably impurities, remained in the solid state. The papers by J. Wang's group did not mention purification of the reagent 26,24,25,27 ; however, the literature revealed that most other references specify purification of Q by sublimation prior to use. 28,31-35 This departure from what appeared to be standard operating procedure in the scientific community suggested that a test of the differences between purities would be prudent. Experiments were carried out according to the methods outlined in section 3. Three solutions were prepared and isolated in the blackout box. Each contained 1*10-4 M Ce(SO4)2, 0.05M NaBrO 3, 0.15M H2SO4, and 0.035M Q, but the quality of Q varied between solutions. One contained raw reagent from Sigma-Aldrich (99% purity); a second contained raw reagent from Acros Organics (98% purity); and the third was Sigma-Aldrich sourced Q that had been freshly sublimed. When used in otherwise identical experimental conditions, the different samples of aspurchased Q displayed markedly different behaviors. 12

13 a E (V) 0.8 b 0.8 c Time (s) Figure 3: Redox potential vs. time data for the Ce(SO4)2 catalyzed system allowed to react in the dark. [Q] 0.035M; other reagents are present in amounts specified in Table 1. Quality and source of Q: a) Raw, Acros Organics, 98%; b) Raw, Sigma-Aldrich, 99%; c) Sublimed, Sigma-Aldrich, with [Q] = 0.035M. The impurities in the raw reagent have not been characterized, meaning that there were unidentified species present in the aqueous mixture. These unknown compounds turned out to have significant effects on the behavior of the system. These results indicate that the behavior of as-received Q is largely dependent on whence it came. It is also not universally true that Q will display oscillatory behavior when in acidic sodium bromate solution in the presence of catalyst and absence of light. Since the purified sample of Q did not oscillate at all, it can be inferred that there is some species present in the raw reagent which is responsible for the observed oscillatory behavior. At this point, it was decided that consistent results could only be achieved by using pure, sublimed reagent for all future experiments. 4.2 Effects of Light Intensity on the Cerium (IV)-Catalyzed Oscillatory System Because the system made with pure Q would not oscillate without light, the next step was to determine the minimum threshold of light under which oscillations can be observed, and what effects the intensity of incident light has on the behavior of the system. Experiments were carried out according to the method described in section 3. Solutions were prepared with reagent concentrations described in Table 1. Each was placed in the reactor and 13

14 irradiated with light of a different intensity, ranging from 0.45mW/cm 2 to 11mW/cm 2. E (V) a 0.8 b 0.8 c 0.8 d 0.8 e 0.8 f 0.8 g Time (s) Figure 4: Redox potential vs. time data for the cerium-catalyzed system, comprising reagents in the concentrations specified in Table 1. Light intensities in mw/cm 2 : a) 0.45; b) 1; c) 2; d) 3; e) 4.5; f) 6; g) 11. Oscillations occur under illumination. Comparing these results with those from Figure 3c, it is evident that the catalyzed bromate ion-benzoquinone-sulfuric acid system requires light in order to begin oscillating; that is, is light-activated. The response is not as immediate as that seen in the 2+ Ru(bpy) 3 -catalyzed BZ oscillator described in section 2.5 (Figure 2). For all cases examined, oscillations start at a high redox potential and end at a low one, and a higher intensity of incident light causes oscillations to begin after a shorter induction period. A MatLab script was used to analyze the data shown in parts b, c, d, e, and f of Figure 4. The percentage of time per cycle spent in the oxidized state (Θ) was plotted versus cycle number, and these data were then fitted to an exponential regression. All plots corresponded well to the general formula Θ = A*exp(-x/t) + y 0 where x is the cycle number, and the values for A, t, y 0, and the correlation coefficient R 2 are as follows: 14

15 Light Intensity (mw/cm 2 ) A t y 0 R Table 2: Variables and coefficients of determination for exponential fits of Θ for five light intensities Experimental Θ for 4.5mW/cm 2 Exponential fit: Θ = e (-x/20.318) R 2 = Θ cycle Figure 5: Θ for the data shown in Figure 4e, along with exponential fit. This analysis shows that as the intensity of irradiation increases, Θ shows faster decay. The general decay to the reduced state within each system can be explained by the fact that these are closed batch systems, without replenishment of reagents. Over time, the oxidizer (NaBrO 3 ) is consumed, and the reaction mixture slowly loses its oxidizing power. The changes in oscillatory behavior seen over a range of irradiation intensities indicates a correlation between the intensity of the light and the rate of bromate ion consumption. By taking x = 0, it also becomes evident that Θ 0 is smaller when light intensity is higher. In addition, as light intensity increases, y 0 decreases; this means that as x goes to infinity, the amount of time that the system spends in the oxidized state goes to 0. This agrees with experimental observations. Because the system takes a very long time to begin oscillating, an additional goal of these 15

16 experiments was to find the conditions under which the induction period was shortest. This factor was balanced by the need for an oscillatory period of long duration and oscillatory features that remain regular over time. At higher intensities of light, the irradiated solution would begin oscillating sooner, but display fewer cycles this can also be attributed to accelerated consumption of bromate ions. Because the system displayed a longer oscillatory period under 3mW/cm 2 of light and the characteristics of the cycles at this intensity were the most stable over time, it was chosen for use in other experiments. 4.3 Other Catalysts While Ce 4+ is used in the classical BZ system as a catalyst, other catalysts are also used in the literature. 23,29 Mn 2+ 2+, Ru(bpy) 3, and ferroin were substituted for Ce(SO 4 ) 2 to see whether they too can be used as catalysts in the bromate ion-benzoquinone-sulfuric acid chemical oscillator Tris-2,2'-bypiridyl-ruthenium(II) Mixtures were prepared for the ruthenium-catalyzed group according to the method described in section 3, containing 1*10-5 M Ru(bpy)3 2+, no Ce(SO 4 ) 2, and other reagents as specified in Table 1. Each was placed in the reactor and irradiated with light of a different intensity, ranging from 0mW/cm 2 to 6mW/cm 2. 16

17 a E (V) b c 0.8 d Time (s) 2+ Figure 6: Redox potential vs. time data for the Ru(bpy) 3 -catalyzed system under different light intensities in mw/cm 2 2+ : a) 0.5; b) 3; c) 4.5; d) 6. [Ru(bpy) 3 ] = 1*10-5 M; [Ce(SO 4 ) 2 ] = 0M; other reagents present in the concentrations specified in Table 1. Just like in the Ce(SO 4 ) 2 -catalyzed system, light is required for oscillations to occur when 2+ Ru(bpy) 3 was used. Similarly, higher light intensity results in a greater portion of time spent in the reduced state, although a detailed MatLab analysis of oscillatory cycles was not performed in this case. The intensity range in which oscillations occur is narrower than that of the Ce(SO 4 ) 2 -catalyzed system, 2+ which may be attributed to the light sensitivity of Ru(bpy) Manganese(II) A reaction mixture containing 1*10-4 M MnSO 4, no Ce(SO 4 ) 2, and other reagents according to section 3 was placed in the reactor and irradiated with 3mW/cm 2 of white light. Redox potential vs. time data were recorded. 17

18 1.2 E (V) Time (s) Figure 7: Redox potential vs. time data for the reaction mixture comprising the reaction mixture comprising [Mn 2+ ] = 1*10-4 M, no Ce(SO 4 ) 2, and other reagents according to Table 1, irradiated with 3mW/cm 2 light. A light intensity dependency study was not carried out for Mn 2+. The system displays oscillations of similar shape and amplitude to those seen in the presence of Ru(bpy) 3 2+ and Ce(SO 4 ) 2, suggesting that the underlying mechanism of the oscillations has similar features Ferroin A reaction mixture containing 1*10-4 M ferroin, no Ce(SO 4 ) 2, and other reagents according to section 3 was placed in the reactor and irradiated with 0.2mW/cm 2 of white light according to the literature procedure. 30 Redox potential vs. time data were recorded E (V) Time (s) Figure 8: Redox potential vs. time data for the reaction mixture comprising [ferroin] = 1*10-4 M, no Ce(SO 4 ) 2, and other reagents according to Table 1, irradiated with 0.2mW/cm 2 light. 18

19 The system containing ferroin did not display oscillations. Rather, the redox potential gradually declined over a very long period of time. A possible explanation for this is the relatively weak oxidizing power of ferroin compared to Ce(SO 4 ) 2, Mn 2+ 2+, and Ru(bpy) 3, which can be seen in the potential at which the system began the other reaction mixtures have a starting potential over 1V, whereas the ferroin-catalyzed system has a maximum of 0.82V. In addition, ferroin tends to hydrolyze when present in acidic medium for a long time, which also may explain these observations. 4.4 Demonstrating the BZ-Type Nature of the System Although the J. Wang group proposed a mechanism which is based upon FKN, with some modifications (Table 3), no experimental data were collected to demonstrate the existence of oscillations in the concentrations of bromide ions and oxidized catalyst. 26 Table 3: Model proposed by the J. Wang group. 26 In order to determine whether experimental data supports the notion that this system is a BZtype oscillator, experiments were performed to show the oscillations of intermediates, as well as to identify the organic substrate and brominated species. 19

20 4.4.1 Oscillations in the Concentration of Bromide Ions A standard experimental solution was prepared according to the protocol of section 3 and irradiated with 3mW/cm 2 of light. A bromide-selective electrode was used to monitor the bromide ion potential of the mixture. The electrode was calibrated using standard solutions of KBr. [KBr] (M) Potential (V) Table 4: Calibration series of the bromide-selective electrode. According to the Nernstian response, a change of 0.059V in the signal indicates a tenfold change in [Br - ]. The deviation seen in Table 4 is due to the use of the junction to isolate the chloridecontaining reference electrode from the reaction mixture E(Br-ISE) (V) Time (s) Figure 9: Bromide potential vs. time data collected with a bromide-selective electrode from a standard solution under 3mW/cm 2 light. [Br - ] can be calculated from Figure 9 by using the calibration in Table 4. In this case, [Br - ] oscillates somewhere between the orders of 10-5 M and 10-4 M. The actual values may depend on the other components that are present in the reaction mixture. Even with this uncertainty, these observations support the notion that the bromate ion-benzoquinone-sulfuric acid system is BZ-type, because oscillations in [Br - ] are characteristic of all members of the BZ family of oscillators. 20

21 4.4.2 Oscillations in the Concentration of Cerium(IV) Ions The other characteristic intermediate that displays oscillations in BZ-type reactions is the oxidized form of the catalyst. Ce 4+ has a broad absorption peak at 317nm with a high extinction coefficient 6100M -1 cm -1 that can be followed by UV-Vis spectrophotometry, while its redox pair Ce 3+ does not have significant absorption at that wavelength. This makes it convenient to track oscillations in [Ce 4+ ] without interference from the reduced form Ce 4+ Ce 3+ Absorbance d Wavelength (nm) Figure 10: UV-Vis spectra of 2.5*10-4 M solutions of Ce(SO 4 ) 2 (yellow) and Ce 2 (SO 4 ) 3 (blue). However, following the Ce 4+ concentration was not a simple task, and required a different experimental setup to accomplish, because the reaction mixture required constant and controlled illumination. The main reaction vessel was placed under light according to the standard protocol in section 3. While the reaction progressed, the reaction mixture was circulated between the beaker and a QS flow-through cell with a 1cm light path using a Rainin Dynamax RP-1 peristaltic pump at a flow rate of 2mL/min. The cell was placed in the spectrophotometer and spectra were taken between 340nm and 600nm every 100 seconds. The redox potential of the system was monitored simultaneously, also according to section 3.3 protocol. The absorbance at the absorption maximum of Ce 4+ could not be used for monitoring oscillations, as it is at a wavelength which is overlapped by quinoid compounds. This results in the absorbance being too high for the instrument to measure, as can be seen in the example spectrum shown in Figure 11. Therefore, the absorbance at 365nm was tracked over time (Figure 12a). 21

22 Absorbance Wavelength (nm) Absorbance at 365nm a E (Pt) (V) b Time (s) Figure 12: Data collected from a standard oscillatory reaction mixture under 3mW/cm 2 light. a) The UV-Vis absorbance at 365nm over time; b) the composite redox potential collected with the platinum electrode. In Figure 12a, the monotonic increase in absolute value of the trace is due to production of some other species that also absorbs in that range. The oscillations in absorbance on the shoulder of the Ce 4+ peak indicate a change in concentration of about 10-5 M, which is about 10% of the initial concentration of the catalyst. These results, combined with those in section 4.4.1, indicate that the catalyzed bromate ionbenzoquinone-sulfuric acid system has the characteristics of a BZ-type oscillator. Thus, its oscillations can be described by a modified version of the existing FKN mechanism. 22

23 4.5 Creating a Modified FKN Mechanism Roles of Malonic Acid The only difference between the composition of this system and that of the classical BZ oscillator is that the organic substrate, malonic acid, was replaced with Q. Therefore, in order to propose a modified mechanism, it is necessary to identify the compounds that take over the roles of malonic acid and its reaction products. In the classical BZ, malonic acid is oxidized and brominated, and its brominated derivative (bromomalonic acid) is oxidized to release bromide ions. Scheme 1: The three primary reactions in which malonic acid and its derivatives participate: bromination, oxidation, and oxidation of the brominated moiety to release bromide ions. One or more compounds in the bromate ion-benzoquinone-sulfuric acid system may fill these roles, and identification of these active species is key to understanding the mechanism of the oscillator Induction Period and Photoreduction of Q It is known that Q participates in photoreduction processes when it is dissolved, and is particularly active in aqueous solution. 31 In the absence of oxygen, Q is reduced to hydroquinone and 1,2,4-trihydroxybenzene (hydroxyhydroquinone, H 2 QOH); in an aerobic environment, both Q and H 2 QOH are then oxidized to the intermediate species 2-hydroxy-1,4-benzoquinone (QOH). QOH is 23

24 highly reactive, and readily forms dimers 32 that can continue to polymerize under certain conditions. Evidence of at this process has been seen by UV-VIS spectrophotometry. The polymer has humic acidlike properties and a reddish color. 33 The rate of the reaction is ph-dependent, progressing much more slowly between ph 4 and 6 than it does above and below these values. Addition of oxidizers like bromate ions, as are present under the BZ conditions, favors formation of QOH and also allows hydroquinone to be oxidized back into Q. 33 Dimer Scheme 2: Simplified mechanism of the photoreduction of Q in aqueous medium Identification of Reducing Agent Because the reaction proceeds so slowly, finding a way to shorten the induction period was a high priority. To that end, the acidic Q solution was allowed to react under illumination for different lengths of time prior to the addition of the oxidizers. Three different reaction mixtures were prepared by dissolving Q in acidic solution and irradiating with 3mW/cm 2 of white light. Appropriate amounts of NaBrO 3 and Ce(SO 4 ) 2 were added to each solution after intervals of 8000, 12000, and seconds, respectively. The final composition of each reaction mixture was the standard described in Table 1. The solutions continued to be irradiated for the entire reaction period. 24

25 a E (V) b c Time (s) Figure 13: Redox potential vs. time data for three systems of [Q] = 0.035M and [H 2 SO 4 ] = 0.15M exposed to 3mW/cm 2 light, with [NaBrO 3 ] = 0.05M and [Ce(SO 4 ) 2 ] = 1*10-4 M added at different times: a) 8000sec; b) 12000sec, c) 16000sec. Initially, the system was in the reduced state; once NaBrO 3 and Ce(SO 4 ) 2 were added, the redox potential immediately jumped. When the addition took place relatively early, at 8000 seconds, the observed dynamics resembled those of standard experiments. Just as when all reagents were combined at the beginning, oscillations started after a long period of time in the oxidized state. However, when the oxidizers were added later in the other two experiments, different dynamics were observed. First the systems spent some time in an intermediate state, and then and oscillations began almost immediately. If the acidic Q solution was allowed to photoreduce for several thousand seconds prior to addition of the oxidizers, the reaction mixture slowly changed from the bright yellow of dissolved Q to an intense red-orange color. This can be explained by the appearance of the dimers and polymers that are products of the photoreduction of Q (see section 4.6.2). Following the addition of the oxidizers, while the system's potential is increasing, the color of the solution transitions rapidly back to yellow and a white precipitate forms. Since reaction mixtures containing pure Q did not display oscillations in the dark (see section 4.1), it was hypothesized that some photoreduction product, generated during the induction period when the oxidizers were present, was participating in oscillations. The length of the induction period 25

26 after the addition of NaBrO 3 and Ce(SO 4 ) 2 at 8000 seconds indicates that the amount of this active compound was not yet sufficient for oscillations to begin. Addition at seconds results in a medium-length induction period, but oscillations begin sooner overall than when the oxidizers are added at seconds, or when everything is combined at once. The differences in oscillation start time indicate that there is an optimal concentration of the photoreduction product at which oscillations will begin. The photoreduction of Q in acidic solution has been explored in the literature, 33,34 and its mechanism has been well-characterized (see section 2.4). However, given the results above, it can be assumed that some other process takes over when oxidizers are present. It has been shown that H 2 QOH can be oxidized into 2,5-dihydroxybenzoquinone (Q(OH) 2 ) 35. A new hypothesis states that production of Q(OH) 2 becomes favorable when oxidizers are added to the system. This reaction pathway takes over because the oxidation of QOH is faster than its dimerization and subsequent polymerization to a humic acid-like polymer. 33 Scheme 3: Simplified oxidation pathway of hydroxyhydroquinone, H 2 QOH. If it is true that the presence of H 2 QOH and its oxidation products is necessary for the system to begin oscillating, then addition or continuous inflow of H 2 QOH to the reaction mixture should make oscillations begin without a long induction period. Bulk Addition of H 2 QOH In an effort to make the system start oscillating immediately, a small amount of H 2 QOH was added to the mixture at the beginning of the experiment. A reaction mixture comprising [NaBrO 3 ] = 0.05M, [Ce(SO 4 ) 2 ] = 1*10-4 M, and [H 2 SO 4 ] = 0.15M were prepared. A g sample of solid H 2 QOH was added and the system was illuminated at 3mW/cm 2. 26

27 E (V) Time (s) Figure 14: Redox potential vs. time data for a standard reaction mixture with [H 2 QOH] = M added. The system was illuminated at 3mW/cm 2. This initial attempt to shorten the induction period by adding H 2 QOH in bulk at the beginning of the reaction also proved unsuccessful when different amounts of reagent were used, as the system did not begin to oscillate (Figure 14). A possible explanation is that the oscillatory system is analogous to a semi-batch type, wherein H 2 QOH is produced continuously by photoreduction instead of being added by hand during the experiment. To examine this, a semi-batch system was constructed. Continuous Inflow of H 2 QOH In order to mimic the effects of photoreduction of Q, another experiment was performed in which both Q and light were replaced by a slow, continuous inflow of H 2 QOH. A reaction mixture comprising [NaBrO 3 ] = 0.05M, [Ce(SO 4 ) 2 ] = 1*10-4 M, and [H 2 SO 4 ] = 0.15M was prepared and placed in the reactor. A 5*10-3 M H 2 QOH solution was prepared in 0.15M H 2 SO 4 and loaded into a Hamilton GASTIGHT #1010 syringe. The syringe was mounted on a Harvard Apparatus PHD2000 syringe pump. The H 2 QOH solution was pumped into the reaction vessel through a PTFE tube at 0.4mL/hour. No Q was added and the reaction vessel was kept in the dark. 27

28 1.2 E (V) Time (s) Figure 15: Redox potential vs. time data for a reaction mixture containing [Q] = 0M and other reagents as specified in Table 1, while 5*10-3 M H 2 QOH was added at 0.4mL/hour. Light = 0mW/cm 2. Oscillations began relatively quickly, at ~6000sec. A small induction period was observed, suggesting that some further chemical transformations are still required for oscillations to occur. The reactions proposed in Scheme 3 might be what accounts for this behavior. The difference between a continuous slow flow of reagent and a bulk addition confirms that, while under illumination, this oscillator is analogous to a semi-batch system because the reducing agent, H 2 QOH, is continuously replenished over time by the photoreduction of Q. The fast-starting oscillations seen upon slow addition of H 2 QOH indicate that it or, more likely, its oxidation products according to Scheme 3 might behave as the substrate during the oscillatory period Identification of the Brominated Species J. Wang et. al.'s proposed mechanism claimed that the replacement for malonic acid was Q itself, which would be brominated according to reaction R9 in Table 3. To test this idea, purified Q was treated with bromine in acidic solution and monitored to see whether a reaction would proceed. A quartz cuvette was filled with 3mL of solution comprised of M Q, M Br 2, and 0.135M H2SO4 and covered with a glass cap. The cuvette was placed in the spectrophotometer and a spectrum was taken, then the solution was left in the dark. Spectra were taken at 0, 10, 30, and 60 minutes of reaction time. After 1 hour the cuvette was irradiated with 4.5mW/cm 2 light. Fresh spectra were taken periodically. Between times of data collection, the cuvette was returned to the illumination chamber. 28

29 Absorbance Br 2 alone, dark Q alone, dark Br 2 +Q, dark, 0min Br 2 +Q, dark, 60min Br 2 +Q, light, 0min Br 2 +Q, light, 5min Br 2 +Q, light, 15min Br 2 +Q, light, 55min Br 2 +Q, light, 75min Br 2 +Q, light, 87min Wavelength (nm) Figure 16: UV-Vis spectra of a solution comprising [Q] = M, [Br 2 ] = M, and [H2SO4] = 0.135M over time Light Exposure Start 0.85 Absorbance at 400nm Time (min) Figure 17: Absorbance at 400nm over time of a solution comprising [Q] = M, [Br 2 ] = M, and [H2SO4] = 0.135M. 29

30 It was found that in the dark, pure Q will not interact with elemental bromine. Tracking the UV- Vis spectra over time at 400nm shows that no reaction occurs until the mixture is exposed to light (Figure 17), at which point photoreduction products of Q are produced. The bromine concentration only changes under illumination, suggesting that it is consumed by one or more of the photoreduction products. In fact, the reactions of H 2 Q and H 2 QOH with oxidizers are known 32,34. Since bromine is an oxidizer, it is likely that it is being reduced. Combined, these observations reinforce the notion that the reaction does not take place according to R9 in Table 3. As such, if the system is to be described by a mechanism based on FKN, a different bromination product must be present. Since Q cannot be brominated, and its photoreduction products are the only other organic molecules present, then one of them must fill this role. As H 2 QOH is more likely to be oxidized than brominated, and its oxidation product QOH is not commercially available to be tested, the species Q(OH) 2 is the most promising candidate for bromination. 36 Scheme 4: Simplified oxidation pathway of H 2 QOH, including proposed bromination step. If these compounds take over the roles of malonic acid and its derivatives, then bromanilic acid should be oxidized to release bromide ions and some other, currently unknown organic species. This latter step is required in order to restore the reduced state of the system with high bromide concentration. 5. Ongoing Work In order to complete the modified FKN mechanism that will describe the oscillations of the catalyzed bromate ion-benzoquinone-sulfuric acid system, the proposed oxidation steps of H 2 QOH must be experimentally investigated and the final products must be identified. Key questions to address include whether Q(OH) 2 can be brominated under oscillatory conditions, and whether bromanilic acid will release bromide ions. Kinetics of each reaction step must be studied in order to form a complete 30

31 mechanism of the oscillatory system and run simulations. A better understanding of the oscillatory behavior may lead to identification of conditions where the system shows a more immediate response to light. With the right initial concentration of reagents, it may be possible to use light to induce the optimal rate of production of the H 2 QOH such that oscillations stop when the system is not illuminated and begin again when illumination is resumed. In addition, by investigating the system's response to light of different wavelengths and using 2+ Ru(bpy) 3 as the catalyst, a system may be created in which two independent wavelengths can be used to induce or suppress oscillations. 6. Conclusions In this project, the catalyzed bromate ion-benzoquinone-sulfuric acid system was studied and characterized. It was shown that the behavior of the oscillator containing purified Q is different from the system explored in the literature, which contained the unpurified reagent. It was demonstrated that the characteristics of the oscillations are dependent upon the intensity of incident light. Oscillations were observed in the concentrations of both bromide ions and Ce 4+ under illumination, but not in the dark, showing that this system is both BZ-type and light-initiated. Other commonly used BZ catalysts were tested as replacements for Ce(SO 4 ) 2 in the system, and it was found that oscillations occur in the presence of Mn and Ru(bpy) 3 ; however, experiments are required to determine why no oscillations are observed in the presence of ferroin. Foundations of a new mechanism based on FKN have been established. H 2 QOH was proposed as the reducing agent, and it was shown that slow and continuous production of this compound by the photoreduction of Q is necessary for the oscillatory period to begin. The oxidized intermediates hydroxybenzoquinone and Q(OH) 2 may be brominated and further oxidized to release bromide ions. Further experimental work is required to verify the presence of these reaction pathways in the oscillatory system. 7. Acknowledgments My most heartfelt gratitude and admiration goes to Viktor Horváth, without whose tireless work, expertise, and guidance I would have crashed and burned long ago. He is the best mentor an undergraduate could hope for, and without him none of this research would have been possible. I would 31

32 like to thank Professor Irving R. Epstein for giving me the opportunity to carry out experimental work in his laboratory, as well as the other members of the Nonlinear Dynamics Group who welcomed me with open arms and provided invaluable support this year: Professor Milos Dolnik, Masahiro Toiya, and Delora Gaskins. I would also like to thank Professor Seth Fraden for providing the support to perform this research, and Nate Tompkins for giving me a place to start. In addition, Wes Napoline has always been there to guide me through the ups and downs of beginning a career in chemistry. His confidence and encouragement has been a great boon when the going got tough. Many people outside the laboratory have also helped to support me this year. I would like to extend great thanks to Jeremy Patton, Alexandra DeDenko, Elena Livak, Kalai Schneider, and Rebekka Steurs for reminding me how to smile. Finally, I thank and congratulate the rest of my classmates who have made this journey through the Chemistry department with me. Mazal tov, everyone we did it! 32

33 8. References 1 Epstein, I. R.; Pojman, J. A. An Introduction to Nonlinear Chemical Dynamics; Oxford University Press: New York, 1998, pp Bray, W. C. Periodic Reaction in Homogeneous Solution and its Relation to Catalysis. J. Am. Chem. Soc. 1921, 43, Prigogine, I.; Nicolis, G. Self-Organization in Nonequilibrium Systems: From Dissipative Structures to Order through Fluctuations; John Wiley & Sons: Vavilin, V. A.; Zhabotinsky, A. M.; and Yaguzhinsky, L. S. Dependence of the behaviour of an oscillating chemical reaction on the concentration of the initial reagents I. Oxidation of malonic acid, in Frank, G. M., Ed., Oscillating processes in biological and chemical systems, Science, Moscow, 1967a. 5 Zaikin, A. N.; Zhabotinsky, A. M. Concentration Wave Propagation in Two-Dimensional Liquid- Phase Self-Oscillating System. Nature, 1970, 225, Zhabotinsky, A. M. A History of Chemical Oscillations and Waves. Chaos, 1991, Vanag, V. K.; Epstein, I. R. Cross-Diffusion and Pattern Formation in Reaction-Diffusion Systems. Phys. Chem. Chem. Phys. 2009, 11, Yang, L.; Dolnik, M.; Zhabotinsky, A. M.; Epstein, I. R. Turing Patterns Beyond Hexagons and Stripes. Chaos 2006, 16, / /9. 9 Epstein, I. R.; Dolnik, M. Coupled Chaotic Chemical Oscillators. Phys. Rev. E 1996, 54, Epstein, I. R.; Sagues, F. Nonlinear Chemical Dynamics. Dalton Trans. 2003, 7, Bar-Yam, Y.; Epstein, I. R. Response of Complex Networks to Stimuli. Proc. Nat. Ac. Sci. USA 2004, 101, Delgado, J.; Li, N.; Leda, M.; Gonzalez-Ochoa, H. O.; Fraden, S.; Epstein, I. R. Coupled Oscillations in a 1D Emulsion of Belousov Zhabotinsky Droplets. Soft Matter 2011, 7, Toiya, M.; Gonzalez-Ochoa, H. O.; Vanag, V. K.; Fraden, S.; Epstein, I. R. Synchronization of Chemical Micro-Oscillators. J. Phys. Chem. Let. 2010, 1, Zhabotinsky, A. M. Periodic Process of the Oxidation of Malonic Acid in Solution (Study of Kinetics of Belousov's Reaction). Biofizika 1964, 9(3), Zong, C.; Gao, Q,; Wang, Y.; Feng, J.; Mao, S.; Zhang, L. Period-Doubling and Chaotic Oscillations in the Ferroin-Catalyzed Belousov-Zhabotinsky Reaction in a CSTR. Science in China B: Chem. 2007, 50, Vanag, V. K.; Epstein, I. R. Design and Control of Patterns in Reaction-Diffusion Systems. Chaos, 2008, 18, / / Epstein, I. R.; Vanag, V. K. Complex Patterns in Reactive Microemulstions: Self-Organized Nanostructures? Chaos 2005, 15, / /7. 18 Field, R. J.; Koros, E.; Noyes, R. M.; Oscillations in Chemical Systems. II. Thorough Analysis of Temporal Oscillation in the Bromate-Cerium-Malonic Acid System. J. Am. Chem. Soc. 1972, 94(25), Horvath, V.; Gentili, P. L.; Vanag, V. K.; Epstein, I. R. Pulse-Coupled Chemical Oscillators with Time Delay. Angew. Chem. Int. Ed. 2012, 51, Gaspar, V.; Bazsa, G.; Beck, M. T. The Influence of Visible Light on the Belousov-Zhabotinskii 33