Bromide Ion Impurity-Induced Reaction between Selenium(IV) and Acidic Bromate: Prototype of a Cycle with Autocatalytic Behavior.

The selenium(IV)-bromate reaction in an acidic medium using phosphoric acid/phosphate buffer was investigated by UV-vis spectroscopy monitoring the formation of bromine. In an excess of bromate, the absorbance-time curves measured at 450 nm display a characteristic sigmoidal shape having a fairly long induction period, while in the opposite case, when selenium(IV) species is used in excess, the measured data follow the rise and fall behavior. Depending on the excess of Se(IV) the final bromine-containing product is either an elementary bromine or bromide ion. Simultaneous evaluation of the measured kinetic traces clearly indicated that, surprisingly, no direct reaction takes place between the reactants. Instead of that, a trace amount of bromide ion impurity in the stock bromate solution is sufficient to drive the system via the oxidation of the bromide ion by bromate producing elementary bromine followed by the subsequent selenite-bromine reaction reestablishing the bromide ion to open a new cycle. As a result, the concentration of bromide ions increases in a sigmoidal fashion during the course of the reaction unless enough selenium(IV) species is present; hence, the overall synergetic effect observed is the autocatalytic rise of bromide ions. Therefore, the cycle mentioned above may be considered as a prototype of autocatalytic cycles. This observation prompted us to clarify the explicit difference between an autocatalytic cycle and an autocatalytic reaction.

Kinetics and Mechanism of Selenium(IV) Oxidation by Aqueous Bromine Solution.

The bromine-selenite reaction at strongly acidic conditions was investigated by monitoring the absorbance-time traces at the isosbestic point of the bromine-tribromide system at a constant ionic strength (0.5 M adjusted by sodium perchlorate) and temperature. Despite the simplicity of the stoichiometry, the kinetics was found to be very complex. Although the formal kinetic orders of the reactants bromine and selenite are strictly 1, that of the hydrogen ion varies from -2 to less than -3 and notably depends on the initial bromide concentration as well. The bromide ion also inhibits the reaction, making the whole system as a sound example of efficient autoinhibition. We have clearly shown that the inhibitory effect of the bromide ion cannot be explained quantitatively by either exclusively considering the unreactivity of the tribromide ion over elemental bromine or driving the reaction via hypobromous acid formed from the well-known hydrolysis of bromine in aqueous solutions. Instead of that, bromonium ion transfer initiating equilibrium is suggested between the selenium(IV) and bromine species to produce bromide ion and SeO3Br- followed by the hydrolysis of this short-lived intermediate. This hydrolytic transformation was found to be catalytic with respect to hydroxide and bromide ions as well. We have also demonstrated that, among the wide variety of selenium species present in the acidic aqueous solution, the best result can be obtained by considering HSeO3 - as the kinetically active species toward bromine. The proposed mechanism containing 10 acid-base equilibria with known equilibrium constants, the above-mentioned initiating equilibrium, and the hydrolysis of SeO3Br- is able to fit all 49 kinetic absorbance-traces simultaneously, taking into account properly the most important characteristics of the measured data at strongly acidic conditions. Furthermore, this kinetic model was further extended by the direct reactions of hypobromous acid with selenium(IV) species suggested previously with reasonably modified rate coefficients to describe the pH dependence of the apparent second-order rate coefficients over the pH = 1-13 range, providing a useful tool to predict more accurately the kinetic behavior of selenium(IV) species in water treatment process conditions.

Compatible Kinetic Model for Quantitative Description of Dual-Clock Behavior of the Complex Thiourea-Iodate Reaction.

The thiourea-iodate reaction has been investigated simultaneously by ultraviolet-visible spectroscopy and high-performance liquid chromatography (HPLC). Absorbance-time traces measured at the isosbestic point of the iodine-triiodide system have revealed a special dual-clock behavior. During the first kinetic stage of the title reaction, iodine suddenly appears only after a well-defined time lag when thiourea is totally consumed due to the rapid thiourea-iodine system giving rise to a substrate-depletive clock reaction. After this delay, iodine in the system starts to build up suddenly to a certain level, where the system remains for quite a while. During this period, hydrolysis of formamidine disulfide as well as the formamidine disulfide-iodine system along with the Dushman reaction and subsequent reactions of the intermediates governs the parallel formation and disappearance of iodine, resulting in a fairly constant absorbance. The kinetic phase mentioned above is then followed by a more slowly increasing sigmoidally shaped profile that is characteristic of autocatalysis-driven clock reactions. HPLC studies have clearly shown that the thiourea dioxide-iodate system is responsible mainly for the latter characteristics. Of course, depending on the initial concentration ratio of the reactants, the absorbance-time curve may level off or reach a maximum followed by a declining phase. With an excess of thiourea, iodine may completely disappear from the solution as a result of the thiourea dioxide-iodine reaction. In the opposite case, with an excess of iodate, the final absorbance reaches a finite value, and at the same time, iodide ion will disappear completely from the solution due to the well-known Dushman (iodide-iodate) reaction. In addition, we have also shown that in the case of the formamidine disulfide-iodine reaction, unexpectedly the triiodide ion is more reactive toward formamidine disulfide than iodine. This feature can readily be interpreted by the enhancement of the rate of formation of the transition complex containing oppositely charged reactants. A 25-step kinetic model is proposed with just 10 fitted parameters to fit the 68 kinetic traces measured in the thiourea-iodate system and the second, but slower, kinetic phase of the thiourea-iodine reaction. The comprehensive kinetic model is constituted in such a way as to remain coherent in quantitatively describing all of the most important characteristics of the formamidine disulfide-iodine, thiourea dioxide-iodine, and thiourea dioxide-iodate systems.

Autoinhibition by Iodide Ion in the Methionine-Iodine Reaction.

The methionine-iodine reaction was reinvestigated spectrophotometrically in detail monitoring the absorbance belonging to the isosbestic point of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered acidic medium. The stoichiometric ratio of the reactants was determined to be 1:1 producing methionine sulfoxide as the lone sulfur-containing product. The direct reaction between methionine and iodine was found to be relatively rapid in the absence of initially added iodide ion, and it can conveniently be followed by the stopped-flow technique. Reduction of iodine eventually leads to the formation of iodide ion that inhibits the reaction making the whole system autoinhibitory with respect to the halide ion. We have also shown that this inhibitory effect appears quite prominently, and addition of iodide ion in the millimole concentration range may result in a rate law where the formal kinetic order of this species becomes -2. In contrast to this, hydrogen ion has just a mildly inhibitory effect giving rise to the fact that iodine is the kinetically active species in the system but not hypoiodous acid. The surprisingly complex kinetics of this simple reaction may readily be interpreted via the initiating rapidly established iodonium-transfer process between the reactants followed by the subsequent hydrolytic decomposition of the short-lived iodinated methionine. A seven-step kinetic model to be able to describe the most important characteristics of the measured kinetic curves is established and discussed in detail.

On the Kinetics and Mechanism of the Thiourea Dioxide-Periodate Autocatalysis-Driven Iodine-Clock Reaction.

The thiourea dioxide-periodate reaction has been investigated under acidic conditions using phosphate buffer within the pH range of 1.1-2.0 at 1.0 M ionic strength adjusted by sodium perchlorate. Absorbance-time series are monitored as a function of time at 468 nm, the isosbestic point of the I2-I3- system. The profile of these kinetic runs follows either sigmoidal-shaped or rise-and-fall traces depending on the initial concentration ratio of the reactants. The clock species iodine appears after a well-defined but reproducible time lag even in substrate excess, meaning that the system may be classified as an autocatalysis-driven clock reaction. It is also demonstrated that the age of the thiourea dioxide solution markedly shortens the Landolt time, suggesting that the original form of thiourea dioxide (TDO) rearranges into a more reactive form and reacts faster than the original one. The behavior found is consistent with that recently observed in other oxidation reactions of TDO. To characterize the system quantitatively, a 22-step kinetic model is constructed from adapting the kinetic model of the TDO-iodate reaction published recently by supplementing it with six different reactions of periodate. By the help of seven fitted rate coefficients a sound agreement between the measured and calculated absorbance-time traces is obtained.

Exact Concentration Dependence of the Landolt Time in the Thiourea Dioxide-Bromate Substrate-Depletive Clock Reaction.

The thiourea dioxide (TDO)-bromate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen-phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The title system shows a remarkable resemblance to the classical Landolt reaction, namely, the clock species (bromine) may only appear after the substrate TDO is completely consumed. Thus, the title system can be classified as substrate-depletive clock reaction. Despite the well-known slow rearrangement characteristic of TDO in acidic solution, it is surprisingly found that the Landolt time of the title reaction does not depend at all on the age of TDO solution applied. It is, however, shown experimentally that the inverse of Landolt time linearly depends on the initial bromate concentration as well as on the square of the hydrogen ion concentration. In addition to this, it is also noticed that dihydrogen phosphate markedly affects the Landolt time as well, and this feature may easily be taken into consideration by the H2PO4- dependence of the rate of bromate-bromide reaction quantitatively. Based on the experiments, a simple three-step kinetic model is proposed from which a complex formula is derived to indicate the exact concentration dependence of the Landolt time.

Autocatalysis-Driven Clock Reaction III: Clarifying the Kinetics and Mechanism of the Thiourea Dioxide-Iodate Reaction in an Acidic Medium.

The thiourea dioxide-iodate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The system was found to exhibit clock behavior, having a well-defined and reproducible time lag called Landolt time, though elementary iodine may even be detected in substrate excess; hence, under these conditions, the reaction can be classified as an autocatalysis-driven clock reaction. It is clearly demonstrated that the previously proposed kinetic model suffers from serious drawbacks from both theoretical and experimental points of view. The reaction may be characterized by either sigmoidal-shaped or rise-and-fall kinetic traces, depending on the initial concentration ratio of the reactants. Iodide significantly accelerates the appearance of the clock species iodine acting therefore as an autocatalyst. The age of stock TDO solution also has a great, so far completely overlooked impact on the Landolt time. On the basis of evaluating simultaneously the kinetic curves, a 16 step kinetic model including 5 well-known rapidly established equilibria is proposed with 7 fitted rate coefficients in which the rate coefficients of both forms of TDO were determined.

Kinetics of the Two-Stage Oxidation of Sulfide by Chlorine Dioxide.

The sulfide-chlorine dioxide reaction was found to have two distinct kinetic stages at alkaline conditions. The first stage proceeds so rapidly that it can only be measured by a stopped-flow technique at low temperature and leads to the parallel formation of polysulfide and sulfate as sulfur-containing products. At the same time, chlorite, chlorate, and chloride are produced from chlorine dioxide in detectable amounts, suggesting a complex stoichiometry. A nine-step kinetic model including short-lived intermediates like sulfide radical and •HSClO2- is proposed to describe the kinetic data in this rapid stage. In an excess of chlorine dioxide, the first stage is followed by a significantly slower one to be measured by conventional UV-vis spectroscopy at room temperature. Considering that tetrasulfide is formed during the first rapid course of the reaction, the subsequent slow kinetic stage can only be described by the direct oxidation of tetrasulfide by chlorine dioxide and, surprisingly, the tetrasulfide-catalyzed disproportionation of chlorine dioxide.

Autocatalytic Oxidation of Trithionate by Iodate in a Strongly Acidic Medium.

The trithionate-iodate reaction has been studied spectrophotometrically in an acidic medium at 25.0 ± 0.1 °C in phosphoric acid/dihydrogen phosphate buffer, monitoring the absorbance at 468 nm at the isosbestic point of the iodine-triiodide ion system and at I = 0.5 M ionic strength adjusted by sodium perchlorate. The main characteristics of the title system are very reminiscent of those found recently in the pentathionate-iodate and the pentathionate-periodate reactions, the systems paving the way for classifying clock reactions. Thorough analysis revealed that the direct trithionate-iodate reaction plays a subtle role only to produce a trace amount of iodide ion via a finite sequence of reactions, and once its concentration reaches a certain level, then the reaction is almost exclusively governed by the trithionate-iodine and iodide-iodate reactions. The title reaction, as expected, was experimentally proven to be autocatalytic with respect to iodide ion. A simple three-step Landolt-type kinetic model is proposed to describe adequately the most important kinetic features of the title system that can easily be extended to a feasible sequence of elementary and quasi-elementary reactions.

Kinetics and Mechanism of the Chlorite-Periodate System: Formation of a Short-Lived Key Intermediate OClOIO3 and Its Subsequent Reactions.

The chlorite-periodate reaction has been studied spectrophotometrically in acidic medium at 25.0 ± 0.1 °C, monitoring the absorbance at 400 nm in acetate/acetic acid buffer at constant ionic strength (I = 0.5 M). We have shown that periodate was exclusively reduced to iodate, but chlorite ion was oxidized to chlorate and chlorine dioxide via branching pathways. The stoichiometry of the reaction can be described as a linear combination of two limiting stoichiometries under our experimental conditions. Detailed initial rate studies have clearly revealed that the formal kinetic orders of hydrogen ion, chlorite ion, and periodate ion are all strictly one, establishing an empirical rate law to be d[ClO2]/dt = kobs[ClO2(-)][IO4(-)][H(+)], where the apparent rate coefficient (kobs) was found to be 70 ± 13 M(-2) s(-1). On the basis of the experiments, a simple four-step kinetic model with three fitted kinetic parameters is proposed by nonlinear parameter estimation. The reaction was found to proceed via a parallel oxygen transfer reaction leading to the exclusive formation of chlorate and iodate as well as via the formation of a short-lived key intermediate OClOIO3 followed by its further transformations by a sequence of branching pathways.

A Simple Kinetic Model for Description of the Iodate-Arsenous Acid Reaction: Experimental Evidence of the Direct Reaction.

The autocatalytic iodate-arsenous acid reaction was investigated by a stopped-flow instrument under strongly acidic medium (pH ≤ 1) by monitoring the absorbance-time profiles at 468 nm. The kinetic traces were found to exhibit a perfect sigmoidal shape in stoichiometric excess of iodate with a well-defined and reproducible induction period that depends on the initial concentration of the reactants as well as on the pH. All the experimental curves can be globally fitted by a simple kinetic model involving the direct reaction between the reactants to produce iodide ion, the Dushman and the Roebuck reactions, and two rapid equilibria. Our measurements along with simultaneous evaluation of the kinetic traces clearly support that indeed the initiation reaction exists at strongly acidic conditions and contributes to the overall kinetics. The measured traces cannot be described adequately by the iodide ion impurity-driven Dushman and Roebuck reactions with assuming no direct reaction at all.

Initial inhomogeneity-induced crazy-clock behavior in the iodate-arsenous acid reaction in a buffered medium under stirred batch conditions.

It is unambiguously demonstrated that in the case of an autocatalytic reaction, initial inhomogeneities induced by the imperfectly mixed part of the overall volume may result in a serious irreproducibility of the individual kinetic runs. A statistically meaningful number of repetitions, however, gives rise to a reproducible cumulative probability distribution curve often referred to as a support of the stochastic feature. The iodate-arsenous acid reaction being autocatalytic with respect to both iodide and hydrogen ions displays clock behavior. However, the time lag necessary for the appearance of iodine, even in buffered solution, varies in an apparently random manner. Careful analysis of the variation of the different parameters like stirring rate, overall volume, geometry of the reactor and the way of mixing the reactants led us to conclude that the fate of the individual samples is determined at the initial stage when the reacting system is per se inhomogeneous. The place, the size of the so-called ignition volume, where the reacting system is imperfectly stirred, as well as the residence time spent there by the imperfectly mixed reactants all seem to depend on external factors.

Kinetics and mechanism of the oxidation of bromide by periodate in aqueous acidic solution.

The periodate­bromide reaction has been studied spectrophotometrically mainly in excess of bromide ion, monitoring the formation of the total amount of bromine at 450 nm at acidic buffered conditions and at a constant ionic strength in the presence of a phosphoric acid/dihydrogen phosphate buffer. The stoichiometry of the reaction was established to be strictly IO4(­) + 2Br(­) + 2H(+) → Br2 + IO3(­) + H2O. The formal kinetic order of the reactants was found to be perfectly one and two in the cases of periodate and bromide, respectively, but that of the hydrogen ion lies between one and two. We have also provided experimental evidence that dihydrogen phosphate accelerates the formation of bromine, suggesting the appearance of strong buffer assistance. On the basis of the experiments, a simple two-step kinetic model is proposed involving BrIO3 as a key intermediate that perfectly explains all of the experimental findings. Furthermore, we have also shown that in huge excess of bromide, the apparent rate coefficient obtained from the individual curve fitting method of the absorbance­time series is necessarily independent of the initial periodate concentration that may falsely be interpreted as the rate of bromine formation is also independent of the concentration of periodate.

Kinetics and mechanism of the oxidation of pentathionate ion by chlorine dioxide in a slightly acidic medium.

The chlorine dioxide-pentathionate reaction has been studied at a slightly acidic medium by conventional UV-vis spectroscopy monitoring the absorbance at 430 nm. We have shown that pentathionate was oxidized to sulfate, but chlorate is also a marginal product of the reaction besides the chloride ion. The stoichiometry of the reaction can be established as a linear combination of two limiting stoichiometries under our experimental conditions. Kinetics of the reaction was found to be also complex because initial rate studies revealed that formal kinetic orders of both the hydrogen ion and chlorine dioxide is far from unity. Moreover, log-log plot of the initial rate against pentathionate concentration indicated a nonconstant formal kinetic order. We also observed a significant catalytic effect of chloride ion. Based on our observations and simultaneous evaluation of the kinetic curves, an 11-step kinetic model is obtained with 6 fitted rate coefficients. A relatively simple rate equation has also been derived and discussed.

A new system for studying spatial front instabilities: the supercatalytic chlorite-trithionate reaction.

We show that the chlorite-trithionate reaction is "supercatalytic"; i.e., the formal kinetic order of the autocatalyst H(+) is 2. A simple model is proposed and discussed to describe the unusual concentration dependencies of one-dimensional front propagation found experimentally. It is also demonstrated that at relatively wide concentration ranges the linear two-dimensional front initiated loses its stability, giving rise to an appearance of cellular structures in a convection-free system. Compared to the chlorite-tetrathionate reaction, however, this system is much more stable at alkaline conditions, even for hours; therefore, no side reactions can be taken into consideration to check whether they may have any side effects on the evolution of spatiotemporal structures.

Kinetics and mechanism of the hypochlorous acid-trithionate reaction.

The trithionate-hypochlorous acid reaction has been studied by the stopped-flow technique and conventional spectrophotometry between pH = 6.59-12.2 monitoring absorbance-time profiles at 285 and 225 nm. We showed that the formal kinetic order of Cl(I) is nearly 2; however, those of hydrogen ion and trithionate are significantly lower than unity, suggesting complex kinetics. It was also demonstrated that both forms of Cl(I) are kinetically active within the concentration range studied. Simultaneous evaluation of the kinetic curves revealed that the reaction was initiated by a formal Cl(+) transfer to the partially negatively charged ß-sulfur of trithionate. S3O6Cl(-) formed in the first step was also found to be equilibrating with S3O6OH(-) via a simple chlorine-OH exchange reaction followed by their subsequent oxidation of hypochlorite and hypochlorous acid, respectively. A six-step kinetic model is proposed and discussed with having four fitted and four fixed parameters.

General pathway of sulfur-chain breakage of polythionates by iodine confirmed by the kinetics and mechanism of the pentathionate-iodine reaction.

The pentathionate-iodine reaction has been studied spectrophotometrically at T = 25.0 ± 0.1 °C and at an ionic strength of 0.5 M in both the absence and presence of an initially added iodide ion at the pH range of 3.95-5.15. It was found that the pH does not affect the rate of the reaction; however, the iodide ion produced by the reaction strongly inhibits the oxidation. Therefore, it acts as an autoinhibitor. The kinetic curves also support the fact that iodide inhibition cannot be explained by the formation of the unreactive triiodide ion, and S(5)O(6)I(-) along with the iodide ion has to be involved in the initiating rapid equilibrium being shifted far to the left. Further reactions of S(5)O(6)I(-), including its hydrolysis and reaction with the iodide ion, lead to the overall stoichiometry represented by the following equation: S(5)O(6)(2-) + 10I(2) + 14H(2)O → 5SO(4)(2-) + 20I(-) + 28H(+). A nine-step kinetic model with two fitted parameters is proposed and discussed, from which a rate equation has also been derived. A brief discussion about the general pathway of sulfur-chain breakage of polythionates supported by theoretical calculations has also been included.

Kinetics and mechanism of the chlorine dioxide-trithionate reaction.

The trithionate-chlorine dioxide reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in acetate/acetic acid buffer monitoring the decay of chlorine dioxide at constant ionic strength (I = 0.5 M) adjusted by sodium perchlorate. We found that under our experimental conditions two limiting stoichiometries exist and the pH, the concentration of the reactants, and even the concentration of chloride ion affects the actual stoichiometry of the reaction that can be augmented by an appropriate linear combination of these limiting processes. It is also shown that although the formal kinetic order of trithionate is strictly one that of chlorine dioxide varies between 1 and 2, depending on the actual chlorine dioxide excess and the pH. Moreover, the otherwise sluggish chloride ion, which is also a product of the reaction, slightly accelerates the initial rate of chlorine dioxide consumption and may therefore act as an autocatalyst. In addition to that, overshoot-undershoot behavior is also observed in the [(·)ClO(2)]-time curves in the presence of chloride ion at chlorine dioxide excess. On the basis of the experiments, a 13-step kinetic model with 6 fitted kinetic parameter is proposed by nonlinear parameter estimation.

On the complexity of kinetics and the mechanism of the thiosulfate-periodate reaction.

The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.

An improved chemical model for the quantitative description of the front propagation in the tetrathionate-chlorite reaction.

It is experimentally proven that the stoichiometry of the tetrathionate-chlorite reaction is 2S4O(2-)6 + 8(1/2)ClO-(2) + 6H2O = 8SO(2-)4 + ClO-(3) + 7(1/2)Cl- + 12H+ near 1:4 molar ratio of the reactants. Re-evaluation of the previously measured front velocity--concentration curves also shows that this stoichiometry along with both the rate equation r = (1.6 x 10(5) M(-3) s(-1) [H+]2 + 3.6 x 10(7) M(-4) s(-1) [H+]3)[S4O(2-)6][ClO-(2)] and the protonation processes existing in the present system allow us to describe the front velocity as a function of the initial concentration of the reactants quantitatively. Some consequences detailed in the conclusions may concern not only uniquely the tetrathionate-chlorite reaction but any front propagation study including H+ as an autocatalyst.