Oscillations in the Permanganate Oxidation of Glycine in a Stirred

Article pubs.acs.org/JPCA

Oscillations in the Permanganate Oxidation of Glycine in a Stirred Flow Reactor Eszter Poros, Krisztina Kurin-Csörgei, István Szalai, and Miklós Orbán* Department of Analytical Chemistry, L. Eötvös University, P.O. Box 32, H-1518 Budapest 112, Hungary

ABSTRACT: Oscillatory behavior is reported in the permanganate oxidation of glycine in the presence of Na2HPO4 in a stirred flow reactor. In near-neutral solutions, long-period sustained oscillations were recorded in the potential of a Pt electrode and in the light absorbance measured at λ = 418 and 545 nm, characteristic wavelengths for following the evolution of the intermediate [Mn(IV)] and reagent [MnO4− ] during the course of the reaction. No evidence of bistability was found. The chemical and physical backgrounds of the oscillatory phenomenon are discussed. In the oscillatory cycle, the positive feedback is attributed to the autocatalytic formation of a soluble Mn(IV) species, whereas the negative feedback arises from its removal from the solution in the form of solid MnO2. A simple model is suggested that qualitatively simulates the experimental observations in batch runs and the dynamics that appears in the flow system.

1. INTRODUCTION The permanganate oxidation of amino acids has been the subject of many studies. Among these works, the greatest attention has been paid to the reaction that takes place between KMnO4 and the simplest amino acid, glycine. The kinetics and mechanism of the reaction were established in both acidic1,2 and neutral3,4 solutions, and autocatalysis was observed in each environment. It is generally accepted that, when autocatalysis is accompanied by a delayed negative feedback process, oscillations can appear in the system if it is kept far from equilibrium state. Oscillations have already been found in the permanganate oxidation of a great number of inorganic reductants.5,6 In our recent project, launched with the aim of finding chemical oscillations in the permanganate oxidation of organic compounds, amino acids, and simple peptides, we reinvestigated the MnO4−−glycine flow system that was reported by Li et al. to show oscillatory behavior in H3PO4 solutions.7 Despite our greatest efforts, we failed to reproduce their results, and we finally came to the conclusion that the oscillations shown in ref 7 are most probably an artifact, as they cannot be of chemical origin. In this work, we demonstrate that chemical oscillations in the MnO4−−glycine flow system can be observed under conditions that are significantly different from those described by Li et al.7 We present these conditions, show the measured © 2013 American Chemical Society

oscillatory traces, and suggest a model that accounts for the oscillations.

2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals involved in the system under study, the oxidant KMnO4 (Reanal), the reductant glycine (Aldrich), and Na2HPO4 (Reanal), were of analytical grade and were used without further purification. The experiments utilized 10−4−10−5 M KMnO4 solutions that were freshly diluted from a 0.02 M stock solution prior to use. Glycine is a mild reductant; therefore, it was used in high excess (0.2 M) with respect to the oxidant to enhance the rate of the KMnO4− glycine reaction. All runs were performed in the presence of a high concentration of Na2HPO4 (0.1 M). 2.2. Apparatus and Methods. The reaction between permanganate and glycine was studied under batch and CSTR (continuously stirred tank reactor) conditions. In both configurations, the course of the reaction was followed by potentiometric and spectrophotometric methods. The potential of a Pt electrode (vs Hg|Hg2SO4|K2SO4 reference electrode) was measured in the reaction mixtures with a Hanna pH-209 type pH meter, and the data were collected with a personal Received: July 18, 2013 Revised: August 23, 2013 Published: August 26, 2013 9023

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temperature (45 °C), low-frequency (period of about 45−60 min) and low-amplitude oscillations in the light absorbance measured at λ = 545 and 418 nm and in the potential of a Pt electrode could be recorded in a narrow ranges of the experimental constraints. Oscillations were obtained when a MnO4− concentration of (1−4) × 10−4 M, a glycine concentration of 0.1−0.2 M, a Na2HPO4 concentration of 0.075−0.2 M, and a flow rate of k0 = (1−2) × 10−3 s−1 were used. The oscillations in the reaction mixture started after an induction period. During this time, the solution was clear and pinkish-brown in color. The oscillations began when the solution turned brownish-yellow and a brown precipitate (solid MnO2) started to appear in the CSTR. The oscillatory responses are depicted in Figure 1. The shift in the oscillatory traces toward higher absorption values is probably due to light scattering caused by the MnO2 deposited on the wall of the reactor. We found no evidence of bistability (i.e., the existence of two stable steady states under the same experimental conditions) under any tested set of experimental conditions. The evolution of the title reaction in the mixture of oscillatory composition shown in Figure 1 was followed by recording the absorption versus time spectra at different instants during the time interval of 180 min. A sequence of such spectra is presented in Figure 2. The broad absorption at λ = 450−590 nm with maxima at λ = 525 and 545 nm in Figure 2a is characteristic of the light absorption of the reactant MnO4−. The product soluble Mn(IV) species absorb radiation through the entire wavelength regime, especially at λ < 450 nm. From the decrease of the peak at λ = 545 nm and the increase of the absorption at λ = 418 nm, it can be clearly seen that the MnO4− was completely consumed and the soluble Mn(IV) gradually increased during the first 10 min. Then, the absorption at λ = 418 nm started to decrease because of the conversion of soluble Mn(IV) to MnO2 precipitate. The absorbances measured at λ = 418 and 545 nm as functions of time were derived from the set of spectra in Figure 2a,b. These curves were also recorded in separate experiments in the presence of different amounts of Na2HPO4. The results are seen in Figure 3.

computer through an A/D converter (NI-6010 PCI). Light absorption spectra of MnO4−−glycine batch systems were recorded in time in the wavelength range of λ = 300−700 nm (path length = 1.00 cm) using a diode array spectrometer (Milton Roy Spectronic 3000). The decrease of the reagent MnO4− concentration and the increase of the product Mn(IV) concentration were followed in time by measuring the absorbances at λ = 545 nm (absorption maximum of MnO4−) and λ = 418 nm [where the absorption of the MnO4− is practically zero but Mn(IV) species absorb light]. The CSTR runs were performed in a 23.5 cm3 thermostatted glass reactor placed in the sample compartment of an Agilent 89090A stopped-flow spectrophotometer operated in kinetics mode, and the absorbances at λ = 545 and 418 nm were recorded as functions of time. The reactor in the spectrophotometer had a path length of 3.27 cm (equvivalent to the inner diameter of the reactor). The CSTR was fed by a Gilson Minipulse 3 peristaltic pump that transferred the input solutions into the reactor in low pulsation level through four channels: one channel carried KMnO4, the second transferred glycine, and the other two conveyed Na2HPO4 solution into the reactor. The excess of the reactor content was removed through a hole in the Teflon cap by another peristaltic pump that rotated in the opposite direction. The reaction mixture was stirred efficiently with an ultraflat IKA lab disc magnetic stirrer fitted in the compartment of the stopped-flow equipment. The flow rate in the reactor could be varied from zero to 9 × 10−3 s−1. All batch and flow runs were carried out at T = 45 °C.

3. RESULTS Prior to our systematic study on the dynamics of the MnO4−− glycine flow system, attempts were made to reproduce the highfrequency (period of about 90 s) and low-amplitude (approximately 30 mV) potential oscillations shown in the work of Li et al.7 using exactly the same experimental conditions (concentrations of the reagents [MnO4−] = 1 × 10−4 M, [glycine] = 2.5 × 10−3 M, and [H3PO4] = 2 × 10−3 M; flow rate k0 = 6 × 10−4 s−1; and temperature T = 303 K) as given in the caption of Figure 1 in ref 7. Additionally, several other compositions selected from the rather wide oscillatory domain of the MnO4− concentration versus glycine concentration phase diagram (shown in Figure 3 of ref 7) were tested without any success in observing oscillations in the system. The color of the reaction mixture in the CSTR remained clear and pink, and the potential of the Pt electrode was constant at all flow rates. To estimate the extension of the MnO4−−glycine−H3PO4 reaction at the composition that was claimed by Li et al.7 to show oscillations in a CSTR, the absorption spectra of the mixture were taken successively during the time interval of 140 min. There were no noticeable changes in these spectra during the first 10 min, and less than a 4% decrease in the absorbance at λ = 545 nm (i.e., in the concentration of the reagent MnO4−) was found at 140 min after the initiation of the reaction. During this time period, a small and continuous increase (about 0.05 absorption unit) was seen in the absorbance at λ = 418 nm. These results indicate an extremely slow reaction between MnO4− and glycine in the H3PO4 solution (the system pH was 2.42). However, we were successful in finding true chemical oscillations in the MnO4−−glycine CSTR system when we used conditions that were very different from those used by Li et al.7 In the basic pH range (pH 7.7−9) and at elevated

4. DISCUSSION Many permanganate oxidation reactions in which the substrate is an inorganic reductant have been shown to take place in an oscillatory manner in a flow reactor if PO43− ions are also present.5,6 The main role of the phosphate ions is to stabilize the product colloidal Mn(IV) species against flocculation by the adsorption of PO43− ions on the surface of the colloid. With the exception of the MnO4−−H2O2 reaction, which oscillates in solutions with H3PO4 concentrations in the range of (5−16) × 10−4 M (pH was not specified),8 all other systems exhibit periodic behavior only in the pH range of 6−8. From the realm of the known oscillatory substrates used so far in permanganate oscillators, amino acids are still missing. To date, only glycine has been tested and stated to be oxidized in an oscillatory fashion in the MnO4−−glycine−H3PO4 CSTR system.7 However, in light of our experimental findings described in the Results section, we question the reliability of the results and all observations reported in ref 7. The very slow reaction between MnO4− and glycine in dilute H3PO4 excludes the appearance of high-frequency oscillations in this system. We believe that the oscillations observed by Li et al.7 might be generated by the “peristaltic effect”, which can cause oscillations in a CSTR if, for example, the pulsation level of the peristaltic pump is high or the efficiency of the stirring is low, or if the 9024

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Figure 2. Light absorbance spectra for a reaction mixture with the same initial composition as in Figure 1 recorded at different times after initiation of the reaction. T = 45 °C, path length = 1.00 cm. Spectra recorded (a) during the first 10 min and (b) during the time interval between 12 and 180 min.

Figure 1. Oscillatory traces in the KMnO4−glycine−Na2HPO4 flow system. Concentrations in the reactor at the start (if no reaction were taking place): [MnO4−] = 2 × 10−4 M, [Gly] = 0.2 M, [HPO42−] = 0.1 M. Other conditions: pH in the reaction mixture, 7.70; flow rate, k0 = 2 × 10−3 s−1; temperature, T = 45 °C. Curves: (a) light absorbance at λ = 418 nm (brown), (b) light absorbance at λ = 545 nm (purple), (c) potential of Pt electrode (blue).

sensor (Pt electrode) and the input tubes in the reactor are not properly positioned, etc. No information was presented in ref 7 that would allow a possible source for the “artificial oscillations” reported in their work to be considered. We observed chemistry-based oscillations in the MnO4−− glycine−HPO42− CSTR system when we used conditions (high initial concentration of glycine, high pH, high temperature, low flow rate) such that sufficiently high conversions in the reaction were reached. To reveal the origin of these oscillations, the positive and negative feedback loops involved in the mechanism of the reaction between MnO4− and glycine were identified in

Figure 3. Light absorbance measured at (a−c) λ = 418 nm and (d) λ = 545 nm characteristic of soluble Mn(IV) and MnO4−, respectively, as a function of time. Initial concentrations of the reagents: [MnO4−] = 2 × 10−4 M, [glycine] = 0.20 M. Concentration of [HPO42−]: 0.05 M (blue), 0.1 M (green and purple), 0.2 M (red).

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v1 = (k n + ka[Mn(IV)])[MnO−4 ]

neutral or slightly basic solutions. In the plots of the absorbance at λ = 418 nm versus time (Figure 3a−c), an acceleration and a decay period are seen, which we believe to be associated with the positive and negative feedback processes. The stochiometry of the reaction is known to be as written in eq 1:

For removal of the autocatalyst, we applied a Langmuir− Hinshelwood-type rate expression (v2) that was suggested by Gray and Scott9 for use when the removal occurs on the surface of a precipitate

2MnO4 − + 3H 2N−CH 2−COOH + 2H+ → 2MnO2 + 3NH3 + 3HCHO + 3CO2 + H 2O

v2 = (1)

MnO4 + Mn(IV) → 2Mn(IV)

d[MnO4 −] = k nc + kac(c0 − c) dt

(7)

d[Mn(VII)] = −2v1 + k 0([Mn(VII)]0 − [Mn(VII)]) dt (8)

d[Mn(IV)] = 2v1 − v2 − k 0[Mn(IV)] dt

(9)

The simulations were performed with the program XPPAUT using the cvode integrator.10 In our calculations for kn and ka, values close to (within an order of magnitude of) those measured by Perez-Benito4 in a pH 6.8 mixture of KMnO4, high excess glycine, and K2HPO4/ KH2PO4 buffer (presented as k1 and k2 in Table 1 of ref 4) were used. The initial concentrations and flow rates (k0) were taken from our experiments. When these data and a high value for factor r were substituted into the model, low-frequency oscillations in the MnO4− and Mn(IV) concentrations could be simulated (Figure 4). The calculated time evolution of the Mn(IV) and MnO4− concentrations under batch conditions (k0 = 0) are shown in panels a−c and d, respectively, of Figure 5. The shapes of the simulated curves of the Mn(IV) and Mn(VII) concentrations versus time are similar to the experimentally measured ones, as seen in Figure 3. The curves shifted when r was varied, similarly

(2)

Process 2 represents the positive feedback loop in the permanganate oscillators. The negative feedbackthe other necessary requirement for the oscillations to occurarises from the removal of the autocatalyst by precipitation in the form of solid MnO2. Many physical processes, such as flocculation, aggregation, and particle growth, are involved in this part of the oscillatory reaction, most of them are affected by the presence of PO43− ions, one of the constituents of permanganate oscillators. With variation of the PO 4 3− concentration, the rate of the negative feedback step can be manipulated in the system. A qualitative model is proposed to simulate the oscillations observed under flow condition in a near-neutral mixture of KMnO4, glycine, and Na2HPO4 (+NaOH). In the model, the kinetic information reported in ref 4 was used. The following rate law was established for reaction 1 −

k 2[Mn(IV)] (1 + r[Mn(IV)])

where k2 is the rate constant for eq 5 and r is the saturation factor, which depends on the concentration of the stabilizing species PO43−. Gray and Scott showed theoretically that the combination of a second-order autocatalytic step and a removal step such as v2 leads to oscillations in the concentration of the autocatalyst if r exceeds a relatively high number. The time variations of the MnO4− and Mn(IV) concentrations in the flow system (where k0 is the flow rate) are expressed by the equations

The reaction was shown to be autocatalytic. 3 The autocatalysis was attributed to the soluble form of colloidal manganese dioxide, which formed as the end product in reaction 1. A detailed chemical mechanism was suggested for reaction 1 by Perez-Benito.4 In the mechanism, a noncatalytic pathway and an autocatalytic pathway were established, both of which were shown to be accelerated by increasing pH and temperature of the medium. In the noncatalytic pathway, the reaction between MnO4− and glycine produces colloidal Mn(IV) in a slow process that accelerates its own production in the catalytic pathway. In this part of the overall reaction, the excess glycine is readily adsorbed on the surface of colloidal Mn(IV) species in the form of Mn(IV)−glycine complex. The adsorbed glycine reacts faster with MnO4− than the glycine in the solution, leading to the formation of new colloidal Mn(IV) species. The autocatalysis is expressed schematically as −

(6)

(3)

where kn and ka are the pseudo-first-order and pseudo-secondorder rate constants, respectively, for the noncatalytic and autocatalytic (where the glycine is in high excess with respect to MnO4−) pathways; c0 is the initial MnO4− concentration; c represents the concentration of MnO4− at time t; and c0 − c represents the concentration of the autocatalyst at time t. Our simple model consists of two overall steps v1

2Mn(VII) → 2Mn(IV)coll v2

Mn(IV)coll → Mn(IV)solid

(4) Figure 4. Oscillations in the concentrations of Mn(IV) and MnO4− simulated with the model. Parameters: [MnO4−]0 = 5 × 10−4 M, [Gly]0 = 0.2 M, kn = 3 × 10−5 s−1, ka = 6 M−1 s−1, k0 = 5 × 10−4 s−1, r = 3 × 104 M−1 . Details of numerical integration: CVODE method;11 relative and absolute tolerances, 10−6 and 10−10, respectively; time step, 5 s.

(5)

In the first step, the autocatalyst Mn(IV)coll is formed at a rate of v1; in the second step, it is removed at a rate v2. The rate of formation of the autocatalyst can be approximated by the rate of disappearance of MnO4− 9026

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laboratory to produce such novel systems by testing a wide variation of MnO4−−amino acid reactions in a CSTR.

■

AUTHOR INFORMATION

Corresponding Author

* Tel.: 36-1-372 2542. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by grants from the Hungarian Academy of Sciences (OTKA K100891). REFERENCES

(1) Verma, S. R.; Reddy, M. J.; Shastry, V. Kinetic Study of Homogeneous Acid-Catalysed Oxidation of Certain Amino-Acids by Potassium Permanganate in Moderately Concentrated Acidic Media. J. Chem. Soc., Perkin Trans. 2. 1967, 469−473. (2) Insausti, M. J.; Mata-Perez, I.; Alvarez-Macho, M. P. Kinetic Study of the Oxidation of Glycine by Permanganate Ions in Acid Medium. Collect. Czech. Chem. Commun. 1996, 61, 232−241. (3) Perez-Benito, J. F.; Mata-Perez, F.; Brillas, E. Permanganate Oxidation of Glycine: Kinetics, Catalytic Effect, and Mechanism. Can. J. Chem. 1987, 65, 2329−2337. (4) Perez-Benito, J. F. Autocatalytic Reaction Pathway on Manganese Dioxide Colloidal Particles in the Permanganate Oxidation of Glycine. J. Phys. Chem. C 2009, 113, 15982−15991. (5) Orban, M.; Epstein, I. R. The Minimal Permanganate Oscillator and Some Derivatives: Oscillatory Oxidation of S2O32−, SO32− and S2− by Permanganate in a CSTR. J. Am. Chem. Soc. 1990, 112, 1812−1817. (6) Doona, C. J.; Kustin, K.; Orban, M.; Epstein, I. R. Newly Designed Permanganate-Reductant Chemical Oscillators. J. Am. Chem. Soc. 1991, 113, 7484−7489. (7) Li, H.; Huang, X.; Deng, J. Oscillations in the KMnO4− NH2CH2COOH−H3PO4 CSTR System. Chem. Phys. 1996, 208, 229− 232. (8) Nagy, A.; Treindl, L. Design of a Permanganate Chemical Oscillator with Hydrogen Peroxide. J. Phys. Chem. 1989, 93, 2807− 2810. (9) Gray, P.; Scott, S. K. Sustained Oscillations and Other Exotic Patterns of Behavior in Isothermal Reactions. J. Phys. Chem. 1985, 89, 22−32. (10) Ermentrout, B. Simulating, Analysing and Animating Dynamical Systems. A Guide to XPPAUT for Researchers and Students; SIAM: Philadelphia, PA, 2002. (11) Hindmarsh, A. C.; Brown, P. N.; Grant, K. E.; Lee, S. L.; Serban, R.; Shumaker, D. E.; Woodward, C. S. SUNDIALS, Suite of Nonlinear and Differential/Algebraic Equation Solvers. ACM Trans. Math. Software 2005, 31, 363−396. (12) Perez-Benito, J. F. Permanganate Oxidation of α-Amino Acids: Kinetic Correlations for the Nonautocatalyitc and Autocatalytic Reaction Pathways. J. Phys. Chem. C 2011, 115, 9876−9885. (13) Zahedi, M.; Bahrami, H. Kinetics and Mechanism of the Autocatalytic Oxidation of L-Asparagine in a Moderately Concentrated Sulfuric Acid Medium. Kinet. Catal. 2004, 45, 377−384.

Figure 5. Simulated curves of (a) [MnO4−] and (b) [Mn(IV)] vs time in the MnO4−−glycine batch reaction for r = 3 × 104 M−1 (red), 6 × 104 M−1 (green), and 9 × 104 M−1 (blue). Parameters as in Figure 4, except with [MnO4−]0 = 2.5 × 10−4 M and k0 = 0 s−1.

to how the absorbance versus time curves shifted when the Na2HPO4 concentration was changed in the batch experiments. The model did not predict bistability, in agreement with the experimental observations. Our model is far from the completeness needed for the full characterization of the dynamics observed in the neutral MnO4−−glycine−HPO42− flow system, but it can serve as a guideline for constructing such a mechanism that can describe more precisely the oscillations in permanganate−amino acid flow reactions.

5. CONCLUSIONS The observation of oscillations in the near-neutral MnO4−− glycine−HPO42− flow system demonstrates that beside inorganic reductants the organic compounds can also play the role of substrate in permanganate oscillators. Autocatalysis, the essential process giving rise to oscillations in liquid-phase chemical systems, has been shown to appear in the permanganate oxidation of many other amino acids, such as threonine, alanine, glutamic acid, leucine, and valine.12 The autocatalysis was shown to arise not only in neutral solutions but in strong acid environment as well. Here, the Mn(IV) species act only as intermediates, and Mn(II) ions are responsible for the autocatalytic effect. Zahedi and Bahrami13 established the same form of rate law as written in eq 6, with the modification that Mn(IV) is replaced by Mn(II), when Lasparagine was oxidized by MnO4− in a 3.15 M H2SO4 solution. We are convinced that the number of amino acids that are capable of participating in oscillatory permanganate oxidation reactions can be well extended. Work is in progress in our 9027

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