Is Iodine Oxidation With Hydrogen Peroxide Coupled With Nucleation

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Cite This: J. Phys. Chem. C 2019, 123, 16671−16680

Is Iodine Oxidation with Hydrogen Peroxide Coupled with Nucleation Processes? Kristina Z. Stevanovic,́ Itana Nuša M. Bubanja, and Dragomir R. Stanisavljev*

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Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia ABSTRACT: An intriguing step of the Bray−Liebhafsky oscillatory reaction, that is, iodine oxidation with hydrogen peroxide, was examined. This process is characterized by enormous stochasticity of reaction induction times in the presence of mild mixing. In the present investigations, it was found that even in the presence of mixing, stochasticity is seriously reduced with simple glass powder addition. Using transparent glass, standard deviations changed from 184.9 to 10.6 min for 19 °C, and from 128.2 to 1.9 min for 27 °C. Repeating experiments with amber glass further confirmed those results as standard deviations changed from the mentioned ones to 8.7 min for 19 °C and to 2.5 min for 27 °C. Inert glass particles enhanced heterogeneous nucleation of oxygen formed in chemical reactions. Together with the previous analysis of the involved kinetic barrier of the whole reaction, it is an additional strong evidence of a possible energetic coupling between nucleation of glass cavities and chemical reactions. Such processes are usually neglected and may considerably change the investigations of the reaction mechanisms in other complex systems. Reaction was followed by the potentiometric method at 19 and 27 °C. The adjusted stopped-flow titration method was used in order to confirm the high degree of iodine to iodate conversion in the presence of nucleation centers.

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Liebhafsky7 was the first who investigated this reaction and reported that reaction under certain conditions may appear only after hours-long induction period. From then, it was examined mostly in the presence of auxiliary components like iodate and silver ions to avoid long induction period.8,9 As it may mask important reaction properties, Olexová and coworkers were among the first who studied it without any additional substances.10 They carried out a large study by conducting reactions in different experimental conditions. From their experiments, the importance of gaseous oxygen becomes evident as the reaction induction period is considerably diminished in the absence of mixing or controlling oxygen escape from the mixture. Various parameters affecting the (short) induction period without mechanical mixing are examined. Examined separately and without any auxiliary components, the induction period preceding the reaction can be described through a set of three reactions7,8

INTRODUCTION Chemical oscillatory systems are phenomena known for almost 100 years, but despite this, there are still doubts about their full mechanism and dynamics. One of the simplest chemical oscillators, according to its initial chemical composition, is the Bray−Liebhafsky (BL) oscillatory reaction. It represents very specific decomposition of hydrogen peroxide on water and oxygen, in the presence of hydrogen and iodate ions1,2 H+, IO3− 2H 2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2H 2O + O2

(1)

This decomposition proceeds through two periodically dominating reactions 2IO3− + 2H+ + 5H 2O2 → I 2 + 5O2 + 6H 2O

(2)

I 2 + 5H 2O2 → 2IO3− + 2H+ + 4H 2O

(3)

An important property of the BL reaction is that reactants of the reduction branch (2) and oxidation branch (3) are stable components and can be investigated alone, beyond the BL system. The question that still remains is how the second one, the slow iodine oxidation reaction, can periodically be a dominant process because oxidation of I(+1) components (without additives) is related to a high energetic barrier and induction period.3 According to this, a reaction may be initiated by some (specific) energy redistribution in the system, which is tentatively assumed with experiments in which periodic changes in H NMR spectra4 and periodic excitation of hydrogen peroxide are observed.5 Also, it was found that just before the oxidation branch, production of highly energetic free radicals occurs.6 © 2019 American Chemical Society

I 2 + H 2O ↔ HIO + I− + H+

(4)

HIO + H 2O2 → O2 + I− + H 2O + H+

(5)

I− + H+ + H 2O2 → HIO + H 2O

(6)

Balanced reactions (5) and (6) control concentrations of I− and HIO keeping the iodine concentration constant and accompanied by only steady degradation of peroxide to oxygen. Existence of the induction period is related to high Received: March 18, 2019 Revised: June 12, 2019 Published: June 14, 2019 16671

DOI: 10.1021/acs.jpcc.9b02563 J. Phys. Chem. C 2019, 123, 16671−16680

Article

The Journal of Physical Chemistry C

before every experiment its concentration was verified spectrophotometrically. In order to obtain good repeatability of experiments and to avoid work with saturated iodine solutions (which may change with ambient temperature), the concentration of working iodine solution was 9.6 × 10−4 M, which is about 10% below the saturation limit. As such, a slightly lower initial iodine concentration diminishes its evaporation from the mixture, also. Experiments are performed with two types of glass powder made from transparent glass (used usually for making laboratory vessels) and amber glass (used as containers for chemicals sensitive to light). Both types of glass are inert for most chemicals and different glass colors come from suitably adjusted ratio of ferrous/ferric ions to the ratio of sulfide/sulfate sulfur states in the final glass composition. The dry grinding ceramic ball mill is used for preparation of both glass powders. The average size of the glass particles was determined by a Mastersizer 2000 (Malvern Instruments) and it was in a range from 0.8 to 10 μm, peaked to 1 μm (Figure 1) for transparent glass and in a range from 0.4 to 10 μm, peaked at 0.7 μm for amber glass (Figure 1).

thermodynamic barrier for one electron oxidation of HIO (or some other I(+1) component).3 This step limits transfer of electron from HIO to other components and contributes the most in forming the activation energy (kinetic barrier) to the whole multielectron oxidation of HIO to higher oxidation states. After the induction period, almost all iodine ends up, in a step-like dynamics, as iodate.3 Therefore, after a period of spontaneous catalytic decomposition of hydrogen peroxide, the system creates conditions for the kinetically demanding oxidation of HIO to prevail. Our previous findings showed that iodine oxidation reaction is also accompanied with large stochasticity of data, especially in the conditions where slow mixing of the reaction mixture was included.11 Those results strongly indicate that overcoming the reaction barrier is connected with some stochastic process. In such a chemically simple system, the source of stochasticity and the energetic coupling mechanism may be a nucleation of oxygen cavities. As well understood in sonochemical experiments, energetic coupling between chemical reactions with nucleation may be reasonably described by releasing energy from fast collapsing of the unstable critical gaseous nucleuses. Such analysis assumes the essential role of heterogeneous effects in the reaction mechanism. As this introduces a new approach in understanding conversion of iodine to iodate, further investigations are necessary for its affirmation. In this article, we presented results showing the large influence of inert glass powder on the stochastic reaction dynamic supporting previous results. Thus, stochasticity of induction times is used as a source of information about the reaction mechanism. Large stochasticity of macroscopic chemical systems is not a common phenomenon in chemistry and only few redox systems with well-confirmed stochastic behavior are presented in literature: thiosulfate−chlorite reaction,12 iodide−chlorite system,13 iodate−arsenous system,14 periodate−arsenous,15 iodine formation after oscillatory Briggs-Rauscher reaction.16 An interesting classification of the mentioned systems, in relation with the present results, is discussed.

Figure 1. Glass particles’ size distribution. It represents average number of particles obtained from three different measurements.

Powdered glass is treated with chromosulfuric acid as a standard procedure for removing organic and inorganic impurities and thoroughly washed with deionized water. In order to confirm the absence of adsorbed chromate on glass particles, infrared (IR) spectra of treated (with chromosulfuric acid) and untreated powders are compared (Figure 2). IR spectra are recorded with an FTIR Avatar370 Thermo-Nicolet spectrophotometer.

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EXPERIMENTAL SECTION All experiments were performed in a batch reactor, protected from light. Temperature was kept constant by a PolyScience thermostat (accuracy ± 0.1 °C) connected to a reaction vessel. Iodine oxidation was followed by the potentiometric method, where platinum (Metrohm, 6.0301.100), as a working electrode, and Ag/AgCl (Metrohm, 6.0726.100), as a reference electrode, were used. A data acquisition homemade voltmeter (PC-MultiLab EH4 16-bit ADC), coupled with a personal computer, was used for electrode potential recording in time. An Agilent 8453 diode array spectrophotometer was used for iodine concentration determination before every experiment and together with stopped-flow device (Applied Photophysics ProK2000 Stopped-Flow) for examination of degree of conversion of iodine to iodate . For initial homogenization of the reaction mixture or to apply stirring during the whole experiment, a magnetic stirrer was used (stirring rate was 120 rpm). Initial concentrations of the reactants were [H2SO4]0 = 8.316 × 10−1 M, [I2]0 = 8.0 × 10−4 M, [H2O2]0 = 1.98 × 10−2 M, and the adding order was sulfuric acid, iodine, and hydrogen peroxide at the end. All stock solutions were prepared with deionized water (18 MΩ cm electrical resistivity) and all chemicals were from Merck of p.a. grade. Iodine stock solution was kept at room temperature in a wellstopped bottle, because of its relatively high vapor pressure and

Figure 2. IR spectra of treated and untreated glass powders confirmed the absence of characteristic bands of dichromate at 950 and 760 cm−117 in glass particles after the cleaning treatment. 16672


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decreased standard deviation from 184.9 to 8.7 min for 19 °C, and from 128.2 to 2.5 min for 27 °C (Table 2). Therefore, the striking effect of inert glass addition is greatly reduced stochasticity of results even in the presence of mixing. As can be seen from Tables 1 and 2, amber glass reduced average values of induction times more because of somewhat greater number of particles (at the same weighted mass) according to the slightly shifted size distribution presented at Figure 1. In Figures 3 and 4, typical potential evolutions with time for three dynamic regimes can be seen also. A steep rise in the signal of the platinum electrode corresponds to the I2 oxidation in all the experiments. It could be ascribed to increased HIO in the system during oxidation.19 Figures 3 and 4 illustrate that the presence of both types of glass particles at given stirring (and for both temperatures) significantly reduced induction periods and their stochasticity despite the presence of mixing. To check the degree of iodine conversion to iodate in the presence of glass particles and mixing, stopped-flow titration of formed iodate is performed as described in our previous paper.11 As seen in Figure 5 conversion degrees remained at high values in the presence of glass as it is found in experiments without mixing. As in previous experiments, a slight decrease of iodine conversion for longer experiments (with mixing and without glass particles) may be ascribed to iodine evaporation from the mixture. The smooth curves in Figure 5 show that conversion degree decreases in a smooth way with increasing induction times, independently of the way the experiments were conducted. It indicates that lower conversion degrees may be ascribed to smooth evaporation (loss) of iodine during reaction induction time. The smoothness of conversion curves and constant initial iodine concentration indicate, also, that evaporation of iodine is not responsible for the stochasticity of induction times. Evaporation of iodine is somewhat diminished by adjusting its initial concentration below the saturation limit and working at relatively low temperatures. In order to compare the possible effects of glass particles on convection movement of the fluid, densities of the reaction solution and the same solution with dispersed glass particles is determined to be 1.0379 g cm−3 for the solution and 1.0621 g cm−3 for the dispersion with transparent glass (and 1.0590 g cm−3 with amber glass). Density of the dispersion is only 2.3% (2.0%) smaller whereas effects on average induction period are decreased 96.1% (97.4%) and standard deviation 98.5% (98.0%), at 27 °C [at 19 °C effects are something smaller, so average induction period is decreased 88.0% (93.3%) and standard deviation 89.2% (95.3%)].

Before stopped-flow titration of formed iodate, in the experiments with glass addition, the reaction mixture was filtrated through a Teflon filter in order to avoid interference with the spectrophotometer reading.

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RESULTS As well as in our previous studies,3,11 iodine oxidation reaction was monitored by the potentiometric method, whereas the adjusted titration method by stopped-flow technique18 was used in order to determine the degree of iodine to iodate conversion at the end of the reaction. As we suggested in our recently published paper11 that nucleation processes are important parts of the reaction mechanism, here we tried to examine the influence of inert glass particles on induction times. Experiments are performed at 27 and 19 °C. First of all, we repeated some experiments with and without mixing for both temperatures to confirm previously obtained trends and experimental procedure. It total, the experiments were repeated seven times at each temperature. Data including the new results are presented in Table 1. As in the previous group Table 1. Average Value of Induction Period (ti̅ nd) and Standard Deviation (σm) for Seven Repeated Experiments without and with Mixing at Two Different Temperatures without mixing

with mixing

temperature (T/°C)

ti̅ nd/min

σm/min

ti̅ nd/min

σm/min

19 27

37.9 10.8

19.9 2.5

230.4 259.3

184.9 128.2

of experiments, mechanical mixing is related with enormous stochasticity at both temperatures relatively to the experiments without mixing. Also, greater stochasticity is preserved at lower temperatures. Standard deviations of experiments including new results are slightly different from old results because of the stochastic nature of the induction times. Now, for the same initial composition and for both temperatures we have done experiments in the presence of the glass particles, under mixing conditions (Table 2). The Table 2. Average Value of Induction Period (ti̅ nd) and Standard Deviation (σm) for Five Repeated Experiments with Mixing in the Presence of Transparent and Amber Glass Particles at Two Different Temperatures transparent glass + mixing

amber glass + mixing

temperature (T/°C)

ti̅ nd/min

σm/min

ti̅ nd/min

σm/min

19 27

27.7 10.2

10.6 1.9

15.4 6.8

8.7 2.5

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DISCUSSION We examined one important subsystem of the BL oscillatory reaction, that is oxidation of iodine to iodate by hydrogen peroxide. This oxidation reaction is very specific as it, despite a large thermodynamic barrier,3 periodically dominates the whole BL reaction. In addition to this, the reaction showed enormous stochasticity under very mild mixing conditions and increase of reaction induction times.11 Needless to say, stochasticity cannot be explained by impurities in reactant components as we always used chemicals from the same supplier. Also, their initial concentration must be constant by keeping constant the initial concentrations of reactants in all experiments. Increase of induction time and stochasticity with mixing shows undoubtedly that initiation of iodine oxidation is

stirring rate was the same, 120 rpm, and the mass of the added glass was 0.2 g. As it can be seen in Figures 3 and 4, at both temperatures addition of glass particles (transparent or amber) significantly shortened the induction times and decreased scattering of data. In the presence of glass powder, all points are now distributed in a narrow range, like the ones in the simple experiments without mixing. The number of repeated experiments with added glass powder was five at each investigated temperature. Transparent glass powder addition (at constant mixing) decreased standard deviation from 184.9 to 10.6 min for 19 °C, and from 128.2 to 1.9 min for 27 °C (Table 2). Addition of amber glass at the same conditions 16673


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Figure 3. Induction period without mixing (square), with mixing (circle), and with mixing and glass particles (triangle) at 19 °C, as well as potential of Pt electrode during iodine oxidation in three representative experiments (corresponding points are marked with arrows) for all these three different conditions. In all experiments, the initial concentrations of reactants were the same: [H2SO4]0 = 8.316 × 10−1 M, [I2]0 = 8.0 × 10−4 M, [H2O2]0 = 1.98 × 10−2 M.

Figure 4. Induction period without mixing (square), with mixing (circle), and with mixing and glass particles (triangle) at 27 °C, as well as potential of Pt electrode during iodine oxidation in three representative experiments (corresponding points are marked with arrows) for all these three different conditions. In all experiments, the initial concentrations of reactants were the same: [H2SO4]0 = 8.316 × 10−1 M, [I2]0 = 8.0 × 10−4 M, [H2O2]0 = 1.98 × 10−2 M.

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should give reproductive reaction times in repeated experiments; 3 .with deterministic chemical reactions at constant mixing and temperature, even the diffusion of oxygen and iodine through the interphase surface should be deterministic (although with complicate dynamics), that is it cannot introduce stochasticity of the results. In such a simple chemical system (containing initially only iodine and hydrogen peroxide in acidic water solution) the source of stochasticity was assumed to be the nucleation of oxygen in small cavities (usually but unjustifiably called bubbles), preceding formation of a new (gas) phase in the system.11 This process has inherent stochasticity as formation of critical cavities is related to random local fluctuations in the concentration of dissolved oxygen. The process of nucleation is strongly dependent on mixing because of the decrease of oxygen content by processes of convection and diffusion through the interphase surface. At higher oxygen concentrations (in the absence of mixing) critical nucleuses are smaller (more easily formed by spontaneous fluctuations) and the process is characterized by shorter induction times and low stochasticity. The reverse is true for lower oxygen concentrations, that is the presence of mixing provokes greater stochasticity and longer induction times.11 The analysis given in our previous work may offer, also, a reasonable explanation for overcoming the energetic barrier for iodine oxidation based on well-known outcomes from ultrasound-chemistry22−24 or laser-induced cavities.25,26 Collapse of unstable oxygen cavities may locally release energy and significantly facilitate reactions of iodine oxidation.11 What is nice in all the approaches is that almost adiabatic bubble collapse is a local phenomenon of energy redistribution, leaving the bulk temperature unchanged. This collapse may affirm (reasonably) the unwelcomed idea of “sudden” initiating of radical reactions at ordinary temperatures. To make cavity collapse less mysterious, evolution of ultrasound-induced formation and collapse of cavities is presented at Figure 6 obtained from Blake et al26 Collapse of the cavities is a consequence of lower vapor pressure inside (curved) the cavities relatively to flat surfaces. In externally created cavities by ultrasound or lasers, rapid growth of cavities is not accompanied by adequate increase of vapor pressure and cavities reach unstable states whose collapse is accompanied by large release of energy in the

Figure 5. Degree of iodine conversion to iodate at the end of the reaction vs reaction induction period for the experiments without and with mixing, as well as with mixing and glass particles (transparent and amber) at two different temperatures.

connected with amount of dissolved oxygen: without mixing, more oxygen is present in the solution and reaction is initiated after a shorter induction time. Evaporation of iodine may only decrease the degree of conversion (Figure 5). The most interesting effect of mixing is stochasticity of the results. This aspect of iodine oxidation was intentionally (or unintentionally) suppressed in previous investigations of iodine oxidation (by addition of Ag + , Hg 2+ Tl + , iodate, absence of mixing...),8,10,20,21 in order to “simplify” interpretation of results. Our attempt to look at the process without those simplifications revealed very specific properties of the reaction mechanism.3,11 Instead of better reproduction of results, mixing introduced extremely pronounced stochasticity. This effect is interesting from various physicochemical points of view: 1. mixing itself could introduce only negligible energy in the system and should not influence activation energies of chemical reactions; 2. under the constant initial composition, temperature, and mixing rate, chemical reactions are deterministic (in the reasonably small limits of experimental error) and

Figure 6. Photographic series of a trapped sonoluminescing bubble driven at 21.4 kHz. The top row presents the bubble growth and collapse dynamics at an interframe time of ca. 2.5 ms. The bottom row shows the bubble collapse with a fivefold temporal resolution (500 ns interframe time). The scale of the image is indicated by the ruler (100 μm).26 16675


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process, we compared IR spectra of treated and untreated glass (Figure 2) showing the absence of characteristic dichromate peaks in the cleaned glass used in the experiments. Taking into account that the densities of the reaction solution and the same solution with dispersed glass particles differ by only about 2%, the influence of glass particles on fluid convection should be negligible. All this additionally supported the validity of the obtained results and introduced explanations. Previously obtained results can significantly modify understanding the reaction mechanism of the investigated iodine oxidation but also of the BL oscillator as well. Instead of considering a chemical oscillator as a “homogeneous” system, inhomogeneity created by formation of another phase in the system seems to be an important part of the mechanism. Noyes28 was among the first who mentioned the importance of a gaseous phase in a BL oscillator. It is further investigated by the Slovak group, who tried to simulate effects of transport of oxygen out from the BL system.29,30,30,31 Besides that, this group also pointed to the strange effects of oxygen in iodine oxidation reaction by changing the interphase surface area and mixing rate.10 Despite the articles noticing the unusual role of the gaseous phase in the reaction mechanism, simple bubbling of the reaction mixture did not accelerate iodine to iodate conversion.9 Those results are now understandable as simple bubbling should only increase convection (mixing) effects without energetic coupling with chemical reactions. In the light of the presented ideas, fast oxidation of iodine to iodate with addition of specific components, which decrease the iodide concentration, may be given. It is known that in the presence of Ag+or Hg2+8 or Tl+20 iodine oxidation by H2O2 may be fast. These ions rapidly react with iodide, forming either insoluble salts AgI or TlI or soluble HgI+. Such reactions rapidly shift the iodine hydrolysis to the right, producing a “large” amount of HIO, for example in the presence of Ag+

form of shock waves, creation of very reactive chemical radicals, and sometimes even of light emission. Those effects are important in studying corrosion and damage of fast-moving parts in water. Collapse of nanometer-sized unstable critical cavities during nucleation (preceding creation of a new phase in the system) is caused by their extremely small curvature. As it is well known from the Kelvin equation,27 vapor pressure over small convex surfaces (of surrounding water) may be considerably lower relatively to the flat surfaces and consequently very small cavities tend to collapse. Although cavities of this size cannot be visible with the present experimental techniques, their intensified collapse has very observable physicochemical consequences. The collapse of small cavities is one of the main reasons for overheating the very clean liquids beyond their boiling points in order to create a new phase. Prevention of overheating is usually accomplished by the presence of inert particles acting as heterogeneous nucleation centers which facilitate formation of nucleuses. (The same effects, i.e., large vapor pressure inside droplets, is responsible for the bursting of small critical droplets hindering formation of the liquid phase. To prevent large overcooling of vapors in clouds and formation of ice particles, clouds are artificially seeded by nucleation centers.) Great stochasticity of results, fortunately emphasized in our experiments,11 as well as basic physicochemical aspects of the process of nucleation, strongly suggests the possibility of coupling of nucleation and chemical reactions in iodine oxidation by hydrogen peroxide. Aware that this new possibility may influence understanding of many other processes (with formation of a new phase) we tried to additionally emphasize potential validity of introduced concepts by simply “seeding” the reaction solution with nucleation centers made from powdered transparent glass. As shown in Figures 3 and 4 the presence of glass particles has tremendous effects on otherwise very stochastic experiments. Even at the same mixing (at unchanged initial conditions of course) all reaction induction times fall into the rather narrow interval, corresponding to experiments without mixing. Regarding the given explanations, it is an encouraging result as glass particles act as nucleation centers facilitating formation of critical nucleuses and initiation of iodine oxidation. As glass powder is obtained from the same type of glass from which reaction vessels are made, we considered them chemically inert particles influencing only the process of nucleation. According to a suggestion of reviewers we tried the same experiments with amber glass (brown colored glass for making containers for storing chemicals) differing in chemical composition from the transparent glass. All results obtained with transparent glass are repeated with amber glass having the same effect of great reduction of induction times and stochasticity (Figures 3 and 4). It is more quantitatively presented in Table 2 where standard deviations decreased from 184.9 to 10.6 min for 19 °C, and from 128.2 to 1.9 min for 27 °C using transparent glass and to 8.7 min for 19 °C, and to 2.5 min for 27 °C using amber glass. The obtained results in those simple experiments confirmed that despite different chemical compositions, both types of glass particles acted as inert material influencing only the nucleation processes. A similar effect of transparent and amber glass on reaction dynamics is the result of similar wetting angles (interfacial surface tensions) of both materials despite their different chemical compositions. As the results of glass addition may be interfered with by adsorbed chromosulfuric acid during the standard cleaning

I 2 + H 2O + Ag + ⇔ (AgI)s + HIO + H+

(7)

It is in agreement with the finding of Holló and co-workers19 who correlated Pt electrode potential “jumps” (in iodine oscillators) to the increase of HIO in the system. Such a high HIO concentration produces a large amount of O2 in reaction with hydrogen peroxide (whose rate constant is estimated by Liebhafsky as high as 104 M−1 min−1) HIO + H 2O2 → I− + H+ + O2 + H 2O

(8)

Thus, in the presence of Ag+ ions, in the system containing initially only I2 and H2O2, formation of the gas phase is greatly facilitated and supports nucleation-induced oxidation under those conditions. At a large concentration of HIO (effected in the presence of Ag+, I2 and H2O2 only), alternative oxidation of HIO may be assumed, also, through disproportionationinduced transformation: 3HIO → IO3− + 2I− + 3H+. Its first step is a rate-determining one-electron disproportionation process (exchange of more electrons cannot be regarded as elementary at least from electrostatic reasons and large reorganizing energy of the surrounding medium) 2HIO → IO• + I• + H 2O

G 0 = +160 kJ/mol (9)

As the activation energy of this endergonic elementary reaction (whose thermodynamic may be suitably followed by a Latimer diagram3) may be only higher than the estimated 16676


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processes as kinetically fast is very deceiving especially when some constituting one electron steps indicate large energetic barriers. It seems obvious that either adopted sets of rate constants should be significantly changed or either a new conceptual approaches allowed for further model development. Despite the mentioned problems, the postulated model points to the most interesting processes, which represent core reactions in the reaction mechanism open for future development. According to our results11 and Slovak group,10 kinetically demanding3 oxidation of iodine by hydrogen peroxide is greatly stochastic. It is seriously influenced by mixing and the presence of inert glass particles. Taking into account that both types of perturbations introduce only negligible energy into the system some very specific energetic events can be involved. Large effects of mixing and inert particles on the dynamics on any other system, producing a new phase, may be regarded as a “sign mark” of nucleation... It is possible that despite the large contribution to affirm the existence of oscillatory phenomena in chemistry, Bray himself1 introduced the earliest misbelief about their “homogeneity”. It is related with his well-known slow experiments at 25 °C with a period between oscillations of a few days. At those conditions he concluded: “oxygen is evolved so slowly that it diffuses out of solution without the formation of bubbles”. Although we did not investigate the full BL oscillator (only the oxidation branch) our results suggest that oscillations (periodic iodine oxidation) may never happen without nucleation, that is heterogeneous effects... At such slow O2 production it is quite possible that even micron-sized “bubbles” (much larger than the critical ones) are not observed, especially during very short periods of triggering oscillations at about every 3rd day. This possibility is even more pronounced taking into account that there is no convenient “electrode” for continuous “monitoring” nucleation as it is available for recording redox properties of the system. The fact is that nobody exactly reproduced Bray’s experiments probably because of very large stochasticity of critical “bubble” formations at low oxygen supersaturations as we explained in a previous paper.11 Despite that at other conditions evolution of bubbles during oscillations is observable even with the naked eye, investigators are inclined to treat the system as homogeneous because of the much easier description (modeling). Our nucleation-induced approach may nicely explain the recently published results38 of stochastic behavior of the BR reaction after oscillatory evolution. The authors found that after regular and very deterministic oscillatory evolution (accompanied by production of O2 and CO2), the reaction may end in state I with low iodine content or state II with high iodine content easily observed by colored or discolored reaction solution. At high mixing rates (900 rpm), the system as a rule finishes only in state I (low iodine) but a smaller (“imperfect”) mixing (100 rpm) system finishes in state II (high iodine) with a very pronounced stochasticity (ranging from few minutes to 45 min) of times for I → II transition. As almost all iodate is consumed at the end of reaction, most of the initially present iodine ends in iodomalonic acid CHI(COOH)2 (or diiodo malonic acid CI2(COOH)2). It is reasonably concluded that I2 formation at the end of reaction (state II) must proceed through iodomalonic acid decomposition mediated by I• formation. This decomposition (i.e., transition I → II) is energetically demanding and may be facilitated by light.39 Besides a very puzzling stochasticity, the

thermodynamic barrier, this process itself should be too slow to explain fast iodine oxidation by H2O2 in the presence of Ag+. The whole analysis favors nucleation-induced oxidation, strongly supported by the present data showing a large increase of iodine oxidation in the presence of small inert glass particles. Although our experiments with iodine oxidation show considerable stochasticity, it is not so pronounced in the oscillatory BL reaction where repeatability of induction time is in the range of 10 min. Less-pronounced stochasticity of the BL oscillator may result from much larger O2 production in the presence of iodate with reactions conducted at higher working temperatures. As it is suggested we discussed, also, widely considered pathway of oxidizing iodine21,29,32−38 effected through two consecutive steps SS1 and SS2. Their incorporation into the reaction model gives good simulations of the dynamics of some intermediates and produced oxygen in the oscillatory BL reaction as it is demonstrated in the mentioned references. As HIO cannot be oxidized directly by peroxide, noticed even by Liebhafsky, spontaneous formation of another iodine (1+) component, I2O, from HIO is assumed 3 × 108 M−1 min−1

2HIO Xoooooooooooooooo oY I 2O + H 2O 3 −1 5 × 10 min

(S1)

It is followed by fast two-electron oxidation of the produced I2O 5 × 105 M−1 min−1 I 2O + H 2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ HIO2 + HIO

(S2)

A closer look at SS1 shows that it is not a redox process and represents simple formation of HIO anhydride. Although it is expectable in concentrated sulfuric acid, acting as a strong dehydration agent, it is quite improbable in water solution. Water is not known as a very “drying” component! It seems very interesting that such unrealistic assumption gives good numerical simulations presented in the cited articles. The most probable explanation is that the overestimated I2O production in SS1 is “balanced” with its overrated removal in S2. Formally looking, this fast two-electron oxidation itself is in contradiction with the high spontaneity of process SS1 because of the more stable components I2O + H2O relative to HIO (products of SS1 should contain less free energy because of assumed SS1 spontaneity). In other words, oxidation of I2O should be even more difficult than HIO whose direct oxidation is a very slow process (Figure 7). Although the existence of I2O cannot be denied, we consider that taking thermodynamically favorable multielectron redox

Figure 7. Thermodynamic relations between the proposed oxidation pathway through steps SS1 and SS2. 16677


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The Journal of Physical Chemistry C Table 3. Characteristic Reaction Parameters for Repeated Experiments in Various Redox Systems redox system 1. 2. 3. 4. 5. 6.

iodine-peroxide BR I → II transition thiosulfate−chlorite chlorite−iodide arsenous acid−iodate arsenous acid−periodate

τyx1 x (rpm or d) y (ind or 50)

τyx2

tr = τyx2/τyx1

number of points with increased stochasticity

τ120rpm ≈ 259 min ind τ900rpm > 24 h ind τ760rpm ≈ 180 s 50 τ750rpm ≈ 150 s 50 τ700rpm ≈ 280 s 50 τ20mm ≈ 900 s 50

τ0rpm ind ≈10, 8 min τ100rpm ≈ 17 min ind τ620rpm ≈ 30 s 50 τ570rpm ≈ 125 s 50 τ500rpm ≈ 260 s 50 τ9.9mm ≈ 900 s 50

24 >85 6 1.2 1.1 1

Large increase Large increase Increase Slight decrease ≈constant ≈constant

interesting question is why this transition occurs at all, at the end of oscillatory evolution, taking into account that no new chemicals are added into the system. According to the mentioned Occam’s razor-attitude, the simplest explanation may be nucleation-coupled chemical reactions. As it is considered in this and previous papers, collapse of critical nucleation cavities may locally release energy and induce radical formation. On very fast mixing, all dissolved gas at the end of the reaction is expelled from the system (by increased convection and diffusion through an intermediate surface). At such conditions, nucleation and I → II transition are never observed. At lower mixing rates (“imperfect” mixing), the system is more supersaturated with gases and stochastic formation of critical bubbles may be qualitatively well correlated with stochasticity of I → II transition times. If our explanation is correct, transition (i.e., I2 formation) should start in the middle of the top surface of the solution. It is easy to understand as mixing effects at these points are the smallest because of the gradually diminishing viscous drag between solution layers more distant form the rotating stirring bar. Accordingly, the centers of layers at the top of the solution are more supersaturated with gasses and will be “propagators” of I → II transition. Exactly this phenomenon is observed in the BR system, where authors were so astonished with this spatial distribution of the transition that its initiation and development through the reaction volume is photographically recorded. Besides just analyzed stochastic effects in iodine oxidation by hydrogen peroxide and I → II transitions in the BR oscillator, it is interesting to make some comparisons with stochastic effects in other redox systems mentioned in the introduction of thiosulfate−chlorite reaction,12 iodide−chlorite system,13 iodate−arsenous system,14 periodate−arsenous.15 Stochastic effects in these systems are well established and connected with various experimental parameters such as rate of mixing, shape of the vessel, reaction volume, diameter of the magnetic stirrer, and reaction composition. In all stochastic systems it is common that some source of inhomogeneity must be present, which is further accelerated by chemical reactions and distributed throughout the whole system. A comparison between stochastic systems is a difficult task because of the different chemical mechanisms, used different mixing rates, different number of points representing stochasticity, and of course different sources of inhomogeneity. From published articles it seems that a rough, but interesting comparison between stochastic systems may be given by analyzing the effects of mixing on some characteristic reaction parameters. We find that one parameter, which correlates the effects of mixing in all systems, may be the average time of induction periods. Although this parameter is differently expressed in various systems, there are basically three ways of its formulation. In some systems it is expressed as average time of induction periods of all repeated experiments (under given

conditions and stirrer rotation in rpmrotations per minute) τ̅rpm ind . In the case of relatively fast processes, with a large number of repeated experiments, the corresponding parameter may be expressed as τrpm 50 representing time τ by which 50% of the experiments are finished (at given conditions and stirrer rotation rpm). In some systems, where mixing conditions are effected by using constant stirring rate but different stirrer diameters d, it is expressed as τ50d. Besides affecting the average time of experiments, in all investigated systems, mixing may influence stochasticity σ of data as well. Although very important, this type of parameter is not given for all systems and only a qualitative comparison is possible whether mixing increases the number of points with large dissipation (or not). Those data are presented in Table 3: It is interesting that the stochastic systems from Table 3 may be roughly divided into two groups. In one, mixing effects have a large influence on relative average time tr > 2 and generally significant increase of the stochasticity of data. The other subgroup is characterized with much smaller effects on relative average time tr < 2 and very slight influence on stochasticity. It is remarkable that only the first three systems are characteristic of the evolution of the gas phase. Stochastic effects in those systems may be well correlated with nucleation effects. Such a conclusion is further supported by the presented results in this work showing that inert glass particles may decrease average induction time from τind120rpm = 259 min to τind120rpm = 10.2 min (for transparent glass) or 6.8 min (for amber glass) (Tables 1 and 2). Stochasticity in the last three systems may be well described, as is suggested in the mentioned references, by the initial inhomogeneity of the reactants because of the imperfect mixing. Besides formal distinguishing of the two subgroups, the large impact of mixing in gas-producing systems may be understood on a more physicochemical basis. In those systems, formation of critical nucleuses requires energy11 provided by local fluctuations in the number of dissolved gas molecules. As shown previously, this energy (or probability of successful fluctuation) strongly depends on the concentration of dissolved gas and may explain the much larger influence of mixing in the mentioned systems. In short, spreading of inhomogeneous effects in the first subgroup is related with energetic factors whereas in the last three systems it is related more to the convection effects. Local redistribution of energy by collapse of critical cavities may be well correlated, also, with periodical changes of the shape of the H NMR water signal during oscillations.4 Those changes may be partially explained by periodical extension of hydrogen bonds near collapsing cavities. . Periodical excitation of hydrogen peroxide5 during oscillatory evolution also suggests the existence of specific redistribution of energy as well as formation of energy-rich chemical radicals preceding oscillations.6 Together with large effects of mixing, the newly obtained results showed an unexpectedly high effect of inert particles on 16678


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(2) Bray, W. C.; Liebhafsky, H. A. Reactions Involving Hydrogen Peroxide, Iodine and Iodate Ion. I. Introduction. J. Am. Chem. Soc. 1931, 53, 38−44. (3) Stevanović, K. Z.; Bubanja, I. N. M.; Stanisavljev, D. R. Domination of Thermodynamically Demanding Oxidative Processes in Reaction of Iodine with Hydrogen Peroxide. Chem. Phys. Lett. 2017, 684, 257−261. (4) Stanisavljev, D.; Begović, N.; Ž ujović, Z.; Vučelić, D.; Bačić, G. 1H NMR Monitoring of Water Behavior during the Bray−Liebhafsky Oscillatory Reaction. J. Phys. Chem. A 1998, 102, 6883−6886. (5) Stanisavljev, D. R. Energy Dynamics in the Bray−Liebhafsky Oscillatory Reaction. J. Phys. Chem. A 2010, 114, 725−729. (6) Stanisavljev, D. R.; Milenković, M. C.; Popović-Bijelić, A. D.; Mojović, M. D. Radicals in the Bray-Liebhafsky Oscillatory Reaction. J. Phys. Chem. A 2013, 117, 3292−3295. (7) Liebhafsky, H. A. The Catalytic Decomposition of Hydrogen Peroxide by the Iodine-iodide Couple at 25°. J. Am. Chem. Soc. 1932, 54, 1792−1806. (8) Furrow, S. Reactions of Iodine Intermediates in IodateHydrogen Peroxide Oscillators. J. Phys. Chem. 1987, 91, 2129−2135. (9) Schmitz, G. Iodine oxidation by hydrogen peroxide and BrayLiebhafsky oscillating reaction: effect of the temperature. Phys. Chem. Chem. Phys. 2011, 13, 7102−7111. (10) Olexová, A.; Mrákavová, M.; Melicherčík, M.; Treindl, L’. The Autocatalytic Oxidation of Iodine with Hydrogen Peroxide in Relation to the Bray-Liebhafsky Oscillatory Reaction. Collect. Czech. Chem. Commun. 2006, 71, 91−106. (11) Stanisavljev, D. R.; Stevanović, K. Z.; Bubanja, I. N. M. Outsized Stochasticity of Iodine Oxidation with Hydrogen Peroxide and Its Implications on the Reaction Mechanism. Chem. Phys. Lett. 2018, 706, 120−126. (12) Nagypal, I.; Epstein, I. R. Fluctuations and Stirring Rate Effects in the Chlorite-Thiosulfate Reaction. J. Phys. Chem. 1986, 90, 6285− 6292. (13) Nagypál, I.; Epstein, I. R. Stochastic behavior and stirring rate effects in the chlorite-iodide reaction. J. Chem. Phys. 1988, 89, 6925− 6928. (14) Valkai, L.; Csekő , G.; Horváth, A. K. Initial inhomogeneityinduced crazy-clock behavior in the iodate-arsenous acid reaction in a buffered medium under stirred batch conditions. Phys. Chem. Chem. Phys. 2015, 17, 22187−22194. (15) Valkai, L.; Horváth, A. K. Imperfect Mixing as a Dominant Factor Leading to Stochastic Behavior: A New System Exhibiting Crazy Clock Behavior. Phys. Chem. Chem. Phys. 2018, 20, 14145− 14154. (16) Pagnacco, M. C.; Maksimović, J. P.; Potkonjak, N. I.; Božić, B. D̵ .; Horváth, A. K. Transition from Low to High Iodide and Iodine Concentration States in the Briggs-Rauscher Reaction: Evidence on Crazy Clock Behavior. J. Phys. Chem. A 2018, 122, 482−491. (17) Nyquist, R.; Putzig, C. Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salts; Elsevier: Amsterdam, Netherlands, 1971. (18) Stanisavljev, D.; Bubanja, I. N.; Stevanović, K. Determination of Iodate Ion in the Presence of Hydrogen Peroxide with the StoppedFlow Technique. React. Kinet., Mech. Catal. 2016, 118, 143−151. (19) Holló, G.; Kály-Kullai, K.; Lawson, T. B.; Noszticzius, Z.; Wittmann, M.; Muntean, N.; Furrow, S. D.; Schmitz, G. Platinum as a HOI/I2 Redox Electrode and Its Mixed Potential in the Oscillatory Briggs-Rauscher Reaction. J. Phys. Chem. A 2017, 121, 429−439. (20) Liebhafsky, H. A. The Catalytic Decomposition of Hydrogen Peroxide by the Iodine-Iodide Couple. II and III. The Rate of Oxidation in Neutral, and in Acid, Solution of Hydrogen Peroxide by Iodine. J. Am. Chem. Soc. 1932, 54, 3499−3508. (21) Schmitz, G. The Oxidation of Iodine to Iodate by Hydrogen Peroxide. Phys. Chem. Chem. Phys. 2001, 3, 4741−4746. (22) Xu, H.; Zeiger, B. W.; Suslick, K. S. Sonochemical Synthesis of Nanomaterials. Chem. Soc. Rev. 2013, 42, 2555−2567. (23) Leong, T.; Ashokkumar, M.; Kentish, S. The Fundamentals of Power Ultrasound - A Review. Acoust Aust. 2011, 39, 54−63.

the kinetically demanding iodine oxidation. Processes accompanying formation and collapse of critical gas cavities are discussed and related with a number of well-known physicochemical experiments. Also, many facts related to iodine oxidation and oscillatory reactions are explainable with an alternative mechanism of coupling nucleation processes with chemical reactions. Our results and discussion of results from other authors may strongly suggest that chemical oscillators may be seriously influenced by heterogeneous effects. This approach introduces new concepts in modeling and deserves further (as simple as possible) confirmations and investigations. The obtained results may have a significant influence on understanding other chemical systems with production of the gaseous phase. They may be important, as well, for biological systems whose metabolism is related with the formation of new phases.

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CONCLUSIONS We examined the influence of inert glass particles’ addition on the dynamics of iodine oxidation with hydrogen peroxide reaction, as a part of the BL oscillatory system. Experiments were first conducted in the presence of mixing (without glass particles), which showed enormous stochasticity of induction times. The presence of inert glass particles has surprisingly large effects on reducing stochasticity in the presence of mixing (standard deviation for 19 °C decreased from 184.9 to 10.6 min, and for 27 °C from 128.2 to 1.9 min for transparent glass. Standard deviation with amber glass is reduced at 19 °C to 8.7 min, and at 27 °C to 2.5 min), resembling experiments without mixing. Both effects, mixing and inert glass addition, indicate the possible significant role of nucleation on the kinetically demanding iodine oxidation. The obtained results showed that formation of the gaseous phase in iodine oxidation by hydrogen peroxide has essential positive feedback on reaction dynamics and may conceptually improve understanding of the BL mechanism, which is unjustifiably treated as homogeneous for a long time. Carefully designed experiments are needed to investigate those effects in other oscillators producing a new phase.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: dragisa@ffh.bg.ac.rs. ORCID

Dragomir R. Stanisavljev: 0000-0003-1361-7977 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry for Science of the Republic of Serbia (grants no. 172015). We offer special gratitude to Danica Bajuk (Faculty of Physical Chemistry, University of Belgrade) for IR measurement and Smilja Marković (Institute of Tehnical Sciences of the Serbian Academy of Sciences and Arts) for obtaining information of particle size distribution.

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REFERENCES

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Is Iodine Oxidation With Hydrogen Peroxide Coupled With Nucleation

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