Complexation Amplified pH Oscillation in Metal Involved Systems

Jun 26, 2012 - driving force at the molecular level, this work may help to broaden the driving range of pH oscillators. 1. INTRODUCTION. Chemical osci...
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Complexation Amplified pH Oscillation in Metal Involved Systems Lin Ji,* Haiyan Wang, and Xiangting Hou Department of Chemistry, Capital Normal University, Beijing 100048, China ABSTRACT: A novel mechanism of amplifying pH oscillations by pH-dependent EDTA−metal ion complexation is proposed. If there is a metal ion involved in the H+ consuming reactions in the pH oscillator and the metal's complex formation constant is big enough, the nonlinear coupling of the original oscillator with the complexation induced metal ion oscillation can finally amplify the pH oscillation. This effect is demonstrated in H2O2−S2O32−−Cu2+ system and further discussed in three other systems. Since pH oscillation is widely used in many areas as a spontaneous periodical driving force at the molecular level, this work may help to broaden the driving range of pH oscillators.

1. INTRODUCTION Chemical oscillations are nonlinear reactions that produce temporally periodic variation of the concentrations of one or more species.1 Most of the previous known chemical oscillators are derived from reactions based on oxyhalogen chemistry. In the 1980′s, a systematic design algorithm was proposed to increase the list of oscillating chemical reactions by using a slow feedback reaction periodically to drive a bistable system between its two steady states.2,3 Recently, inspired by the biological oscillation processes, Epstein′s group proposed a novel method to obtain oscillation of elements possessing only a single stable oxidation state.4 The central idea is to drive a reaction or process that evolves the target element by a core oscillator. They first couple a pH-dependent equilibrium with a pH oscillator to produce Ca2+, Al3+, and F− oscillation,4−7 then a redox core oscillating reaction is coupled with two consecutive reactions taking place between the components of the oscillator and the target element.8 pH oscillators are systems in which a hydrogen ion autocatalytic reaction and its consumption play the key role in the oscillation.9 It has been widely used in many areas, such as inducing the autonomous conformation change of gels10−12 and DNA13,14 to produce chemomechanical devices and light transmitting devices. 15 It is also reported to produce spontaneous drug delivery rhythm.16 In the recent oscillation design method,4 it is also often chosen as the core oscillator. To date, about 20 pH oscillators have been revealed in open systems.17−20 Poros et al.17 recently reported the method and applications of getting pH oscillations in a closed system. This kind of oscillation is the most promising molecular level autonomous periodical driven force in practical applications. However, the oscillation characters like the amplitude and frequency are highly dependent upon the reaction conditions, it cannot be easily adjusted to get the tailored driving force Since partially or completely unprotonated ethylenediaminetetraacetic acid (EDTA) can bind protons, its introduction has been reported to dampen or even totally suppress the core pH oscillations.4,5 However, EDTA is also well-known for its ability to bind metal ions, whose formation constant is highly dependent on pH value. If the metal ion is already part of a pH oscillator, the EDTA should induce an additional metal ion © 2012 American Chemical Society

oscillation. Since EDTA will generally bind more metal ions under high pH value environment and will continuously release them with the increase of H+ concentration, if the metal ion facilitates the negative feedback (H+ consuming process), it is possible to amplify the pH oscillation by introducing EDTA into the system. In this work, we demonstrate this idea in a metal ion involved pH oscillator. Control experiments in the corresponding metal-free system show no evidence of amplification, which further confirms that it is the complexation process that contributes to the amplification. These results might provide a promising method to enlarge the working scope of these pH oscillators as a driving force.

2. EXPERIMENTAL SECTION 2.1. System Selection. Two groups of pH oscillation systems are chosen here. Within each group, the two reactions share a similar mechanism, while one of the reactions involves a metal ion, and the other does not. The first group is the H2O2− S2O32−−Cu2+ (OSC)21 and H2O2−S2O32−−SO32− (OSS) systems.22 The reaction between H2O2 and S2O32− cannot support oscillation behavior under isothermal conditions because their original rate constants do not meet the necessary oscillation requirements. Orbán and Epstein21 found that trace amounts of Cu2+ ions can accelerate the H+ consumption processes in the reactions between H2O2 and S2O32− so that the parametric conditions for the oscillation are satisfied21,22 and that large-amplitude pH oscillations are observed in the OSC system. Rabai and Hanazaki′s further investigation22 shows that the addition of the SO32− in the initial feed may also force the system to an oscillatory state. Because SO32− is an important intermediate of the oxidation of S2O32−, its presence enhances the contribution of the key H+ producing autocatalytic reaction so that oscillation conditions are satisfied. The second group is the BrO3 −−SO32−−Mn2+/MnO4+ (BSM)23 and BrO3−−SO32−−Fe(CN)64− (BSF) systems.24 The BrO3−−SO32−−H+ (BSH) system is known to exhibit Received: March 13, 2012 Revised: June 26, 2012 Published: June 26, 2012 7462

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clock-reaction behavior in a batch reactor and bistability in a continuous-flow stirred tank reactor (CSTR) because of the autocatalytic multiplication of H+ involved in the reaction.25 The pH oscillations may be obtained when BSH is combined with an appropriate first-order H+-consuming process. Fe(CN)64− was first introduced into the system, and large amplitude pH oscillations were found.24 Later, Okazaki et al.23 prove Mn2+ or MnO4+ can also serve as the negative feedback species to produce large-amplitude pH oscillations. This is because their intermediate MnO(OH)+ may be reduced by HSO3− to give HS2O6− so that H+ is removed. 2.2. Materials and Methods. Analytical grade Na2S2O3, Na2SO3, H2O2 (Alfa Aesar), NaBrO3, MnSO4·H2O, KMnO4, HClO4, K4Fe(CN)6·3H2O (Tianjin Fuchen), CuSO4·5H2O, H2SO4 (Beijing Chemical Works), and EDTA (Tianjin Guangfu) were used without further purification. All solutions were made with distilled water. To ensure good reproducibility in the experiments, Na2SO3 and MnSO4 solutions in the BSM reaction were prepared daily with boiled distilled water to avoid possible air oxidation. In the OSC system, H2O2 solutions were prepared daily and kept in polyethylene bottles to retard decomposition; the CuSO4 solution was stored in dilute (pH ≈ 4) H2SO4 to prevent hydrolysis. In the OSS system, solutions were prepared daily and kept from air to avoid any effect of O2 and CO2. Boiled distilled water used in preparing solutions was first purged with N2 for elimination of O2 and CO2 impurities. Na2SO3, Na2S2O3, and H2SO4 were prepared in the same solution. In all the experiments, EDTA was introduced by replacing one of the reactant inflows by the mixture of it with EDTA of the required concentration. All the experiments were carried out in a water-jacketed glass CSTR. The reaction temperature was kept constant to ±0.l °C by circulating water between the water jacket and a thermostatted water bath (PolyScience 9006). Stock solutions were introduced into the reactor by a peristaltic pump (Watson-2058). The volume of the mixture in the reactor was kept at 48.0 mL by continuous aspiration of solution from the top of the reactor. The solution mixture in the CSTR was stirred magnetically by a Teflon-coated magnetic stir bar. Sufficient time was allowed at each flow rate for the system to reach the oscillatory state. The potentiometric progress of the reaction was monitored by a platinum redox electrode with an Hg|Hg2SO4|K2SO4 reference electrode through an advanced electrochemical system (PARSTAT 2273). The pH was followed with a combined glass electrode through a digital pH meter (ThermoScience 9157BNMD).

Figure 1. Schematic diagram of interaction between metal−EDTA equilibrium and pH oscillator. M+ stands for metal ions and M−EDTA stands for the complex of metal ion and EDTA. The arrow with the straight line (→) and filled circle with the dotted line (•) represents positive and negative feedback, respectively.

EDTA−metal ion−H+ effects in the pH oscillation system. It is clear that, if the metal doesn′t take part in the oscillation, like in previous works of Ca2+ oscillation,5 EDTA will simply dampen or even stop the oscillation. However, if the metal is involved in the reactions and facilitates the H+ consumption process, there will be a competition between H+ induced buffer effect and the metal ion induced amplification effect. Therefore, it is possible for the whole system to show amplification results 3.1. OSC and OSS Systems. The formation constant of Cu− EDTA is 6.31 × 1018. Its conditional formation constant in pH 3.5−6 (roughly the pHmin and pHmax in the OSC system) is 2.09 × 109 to 1.41 × 1014.26 From Figure 2, we can see that, with the increase of EDTA concentration, the oscillation in both pH and the redox potential are amplified significantly. When the EDTA concentration is even higher (Figure 2d), the oscillation stops after the EDTA is added. This is because the amount of EDTA ions in solution goes beyond the critical level so that the amount of H+/Cu2+ left in the solution is less than what is necessary to sustain the oscillation. Similar quenching phenomenon is also found in other systems in this work. Further experimental investigation shows that, with the increase of flow rate, the amplification phenomenon will appear when EDTA concentration is even lower, as shown in the phase diagram of Figure 2e. It is worthy to point out that, in the original nonoscillatory region when the flow rate is too high or too low, complexation-induced oscillations are found. This further demonstrates that the complexation-induced Cu2+ oscillation may help to strengthen the original feedback mechanism. In the low flow rate region, the induced oscillations can cover the whole original oscillation region, but when the flow rate is high, the induced oscillation only appears in quite a narrow scope when EDTA concentration is relatively high. This might be because the oscillation coupling based oscillation amplification/resume phenomenon is further reduced as the original oscillation scope is greatly reduced in the high flow rate region. In the corresponding OSS system, the oscillation amplitude in both pH and redox potential shrink upon the addition of EDTA, and higher EDTA concentration will completely inhibit the oscillation (Figure 3). 3.2. BSM and BSF Systems. In the BSM system, a different phenomenon is found when EDTA is introduced. From Figure 4, it is clear that the amplitude in redox potential may be significantly increased by the addition of EDTA with appropriate concentration. Similar as that in the OSC system case, the amplified redox potential oscillation here is mainly due to the

3. RESULTS AND DISCUSSION EDTA had been looked upon as a source of buffer to pH oscillators since it will release/bind H+ in high/low pH environment. However, EDTA may also release/bind metal ions when binding/release H+, which is essentially a competition process to the buffer effect. If the complex formation constant is big enough, it might be possible that the buffer effect be counteracted. Moreover, the metal−EDTA complex formation makes the free metal ion concentration also oscillate, with the maximum appearing in high pH environments and minimum in low pH. If the metal happens to take part in the H+ consuming process, the metal ion oscillation would just agree with the pH oscillation requirement on the amount of metal ion, providing an additional dynamical in-phase oscillation for the core pH oscillator, which can amplify it in the same way as pushing a baby on the swing. In Figure 1, we plot the scheme of the 7463

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Figure 3. Influence of [EDTA] on oscillation in the H2O2−Na2S2O3− Na2SO3−H2SO4 system. Input concentrations: [H2O2] = 0.0135 M; [Na2SO3] = 0.002 M; [Na2S2O3] = 0.006 M; [H2SO4] = 2.5 × 10−4 M; k0 = 3.6 × 10−3s−1, 25.5 °C. a a′, EDTA = 1 × 10−5; b, b′ EDTA = 2.5 × 10−5; c, c′ EDTA = 5 × 10−5; d, d′ EDTA = 2.5 × 10−4.

high, so that the amplitude of the redox potential oscillations is first increased then decreased. However, instead of being amplified, the corresponding pH oscillations are gradually reduced with an increase of EDTA concentration. This is mainly because, by our estimate, the complex formation constant here is limited. The formation constant of Mn−EDTA is 6.17 × 1013. Its conditional formation constant in pH 3−7 (roughly the pHmin and pHmax in the BSM system) is 1.56 × 103 to 2.97 × 1010, which are several orders lower than the corresponding ones in the Cu2+ case. This means both the metal ion oscillation and the antibuffer effect are weaker here than in the OSC system, so that the complexation induced metal ion oscillation is not sufficient to counteract the negative feedback coming from the buffer effect. Therefore, even though the redox potential oscillation can still be amplified, the pH oscillations are damped. In the corresponding metal-free system, the BSF oscillator shows significant buffer effect, as shown in Figure 5. Kurin-Csörgei et al. also report this kind of effect when employing this system in the Ca2+ oscillation system.5

Figure 2. Influence of [EDTA]c on oscillation in the H2O2−Na2S2O3− CuSO4−H2SO4 system. Input concentrations: [H2O2] = 0.1 M; [Na2S2O3] = 0.01 M; [CuSO4] = 2.5 × 10−5 M; [H2SO4] = 0.001 M; 700 rpm, 25.0 °C. The [EDTA] is a, a′ 5.0 × 10−6 M; b, b′ 1.0 × 10−5 M; c, c′ 1.5 × 10−5 M; d, d′ 2.0 × 10−5 M; (a−d) k0 = 3.0 × 10−3 s−1. The arrow indicates the addition time of EDTA. (e) Two-parameter phase diagram of oscillation: filled squares (■) indicate no oscillation situation, the circles (○) stand for no amplitude change, and stars (☆) represent apparent amplification.

complexation of Mn2+ with EDTA. The buffer effect of EDTA plays a role to inhibit amplification when EDTA concentration is 7464

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Figure 5. Influence of EDTA on the NaBrO3−Na2SO3−K4Fe(CN)6− H2SO4 system. Input concentrations: [H2SO4] = 0.01 M; [K4Fe(CN)6] = 0.02 M; [NaBrO3] = 0.065 M; [Na2SO3] = 0.075 M. [EDTA] = 2.5 × 10−3 M; k0 = 1.69 × 10−3, 500 rpm, 30 °C. (a) pH oscillation; (b) redox potential oscillation.

periodical binding provides not only a competition to the buffer process induced by the conditional EDTA−H+ binding but also an additional in-phase oscillation. As long as the complex formation constant of EDTA and the metal is big enough, it is possible to amplify the original pH oscillation by the dynamical nonlinear in-phase coupling. In this work, we demonstrate the amplification effect in the H2O2−S2O32−−Cu2+ system in both the redox potential and pH oscillation. In the BSM system, redox potential amplification is found after EDTA is introduced into the system, but the Mn−EDTA complex formation constant is not big enough so that the pH oscillation is still dampened by the buffer effect. Further control experiments in the metal-free pH oscillation systems with similar core kinetics reveals only dampened or even stopped oscillation, which also supports our assertation. pH oscillator is one of the most widely used nonlinear processes as spontaneous molecular level periodical driving forces. This work offers a promising way to strengthen them so that their application range can be broadened.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-10-6890-3086. Fax: 86-10-6890-2320. E-mail: jilin@ mail.cnu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21003090), Beijing Young Key Talents Culture Program (PHR201008076), and Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality (PHR20100718).

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Figure 4. Influence of [EDTA] on oscillation in the NaBrO3−Na2SO3− KMnO4−HClO4 system. Input concentrations: [NaBrO3] = 0.6 M; [Na2SO3] = 0.472 M; [KMnO4] = 0.006 M; [HClO4] = 0.066 M; k0 = 2.78 × 10−3, 600 rpm, 45.0 °C. [EDTA]: a, a′ 5.0 × 10−6 M; b, b′ 5.0 × 10−5 M; c, c′, 5.0 × 10−4 M; d, d′, 1.0 × 10−3 M; e, e′ 5.0 × 10−3 M.

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4. SUMMARY Inspired by the changing ability of EDTA binding/releasing both H+ and metal ions according to the solution pH value, it is proposed that, if the metal ion takes part in the negative feedback processes in the pH oscillation reaction, it is possible to amplify the oscillation by the EDTA-induced metal oscillation through nonlinear coupling of the two oscillators. The metal−EDTA 7465

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