Compatible mechanism to characterize three independent but cross-coupled reactions of chlorite ion.

Individually proposed kinetic models of the key subsystems of the chlorite-thiosulfate reaction, such as the hypochlorous acid-chlorite, tetrathionate-chlorite, and tetrathionate-chlorine dioxide reactions, have been unified to be able to describe all the main characteristics of these systems simultaneously. A complex 38-step kinetic model is composed in which the subsystems are coupled by the necessary short-lived intermediates and such species that is products or reactants in one system but transients in the other. Such a cross-coupling between the individual systems as well as the sound agreement between the measured and calculated absorbance-time profiles in 367 experimental curves strongly validates the proposed kinetic model.

Classification of clock reactions.

Autocatalytic systems are sometimes designated as clock reactions or reactions that exhibit clock behavior. To resolve the recent dispute over the term clock reaction, we describe a new approach to classify systems featuring clock behavior into three distinct groups: substrate-depletive clock reactions, autocatalysis-driven clock reactions, and systems that have pseudo clock behavior. Many of the well-known classical and recently discovered reactions can conveniently be put into these categories. We also provide a convincing argument for classifying some autocatalytic processes as clock reactions, but it does not necessarily mean that all autocatalytic processes should be classified as autocatalysis-driven clock reactions. This classification can be conveniently performed if the kinetic nature of the given system has been completely elucidated and understood.

Complex kinetics of a Landolt-type reaction: the later phase of the thiosulfate-iodate reaction.

The thiosulfate-iodate reaction has been studied spectrophotometrically in slightly acidic medium at 25.0 +/- 0.1 degrees C in acetate/acetic acid buffer by monitoring the absorbance at 468 nm at the isosbestic point of iodine-triiodide ion system. The formation of iodine after the Landolt time follows a rather complex kinetic behavior depending on the pH and on the concentration of the reactants as well. It is shown that the key intermediate of the reaction is I(2)O(2), its equilibrium formation from the well-known Dushman reaction along with their further reactions followed by subsequent reactions of HOI, HIO(2), S(2)O(3)OH(-), and S(2)O(3)I(-) adequately accounts for all the experimentally measured characteristics of the kinetic curves. A 19-step kinetic model is proposed and discussed with 13 fitted and 7 fixed parameters in detail.

Equilibria of 3-pyridylmethanol with copper(II). A comparative electron spin resonance study by the decomposition of spectra in liquid and frozen solutions.

The copper(II)-3-pyridylmethanol (L) system was investigated in aqueous solution by two-dimensional ESR evaluation at 298 K, and computer simulation of the individual anisotropic spectra at 77 K. The data revealed that the paramagnetic copper(II) complexes [CuL] (2+), [CuL 2] (2+), [CuL 3] (2+), and [CuL 4] (2+) are formed up to pH approximately 7 at a moderate or high excess of ligand. As compared with chelating ligands, two differences were observed for the complexation of 3-pyridylmethanol with copper(II): (1) In contrast with the well-resolved spectra in frozen solution, considerable line-broadening and distortion of the spectral shapes were seen at 298 K, which was interpreted in terms of isomeric equilibria and the medium-rate interconversion of various complexes on the ESR time-scale. (2) At low temperature, there were dramatic changes in the concentration distribution, the minor complexes with higher numbers of coordinating ligands ([CuL 3] (2+) and in particular [CuL 4] (2+)) becoming strongly favored. This phenomenon is explained by the significant differences in the formation enthalpy values of various species, shifting the equilibria according to the van't Hoff equation, and a significant undercooling in the course of fast freezing of the solution, which enhances the changes of the concentration distribution.

Effect of chloride ion on the kinetics and mechanism of the reaction between chlorite ion and hypochlorous acid.

The effect of chloride ion on the chlorine dioxide formation in the ClO 2 (-)-HOCl reaction was studied by following .ClO 2 concentration spectrophotometrically at pH 5-6 in 0.5 M sodium acetate. On the basis of the earlier experimental data collected without initially added chloride and on new experiments, the earlier kinetic model was modified and extended to interpret the two series of experiments together. It was found that the chloride ion significantly increases the initial rate of .ClO 2 formation. At the same time, the .ClO 2 yield is increased in HOCl but decreased in ClO 2 (-) excess by the increase of the chloride ion concentration. The two-step hydrolysis of dissolved chlorine through Cl 2 + H 2O left harpoon over right harpoon Cl 2OH (-) + H (+) and Cl 2OH (-) left harpoon over right harpoon HOCl + Cl (-) and the increased reactivity of Cl 2OH (-) compared to HOCl are proposed to explain these phenomena. It is reinforced that the hydrolysis of the transient Cl 2O 2 takes place through a HOCl-catalyzed step instead of the spontaneous hydrolysis. A seven-step kinetic model with six rate parameters (constants and/or ratio of constants) is proposed on the basis of the rigorous least-squares fitting of the parameters simultaneously to 129 absorbance versus time curves measured up to approximately 90% conversion. The advantage of this method of evaluation is briefly outlined.

Theoretical investigation on the concentration dependence of the Landolt time.

Based on the modified kinetic model of the Landolt and the Dushman reactions published recently, an exact expression has been derived for the concentration dependence of the Landolt time. It is also shown that all the apparently contradictory formulas, regarding the concentration dependence of the Landolt time, reported previously at quite different experimental circumstances can easily be reconciled from the simplification of the newly proposed equation. It also means that the formula derived may satisfactorily be applied to calculate the Landolt time basically with no restrictions for the concentration of the reactants.

Simultaneous investigation of the Landolt and Dushman reactions.

The Dushman reaction taking place after the abrupt change of the absorbance in the Landolt reaction was followed by monitoring the concentration of triiodide spectrophotometrically in buffered solutions. The joint evaluation of the experimental data on the kinetic system (Landolt) and subsystem (Dushman) has revealed that the inverse of the Landolt induction time ( t i ) is proportional to the combination of the concentration and the square of the concentration of both the iodide and hydrogen ions, while the inverse of t i is linearly proportional to the iodate concentration. The hydrogen sulfite dependence, however, is found to be more complex; the Landolt induction time goes through a minimum as a function of hydrogen sulfite concentration; the phenomenon that has not been reported yet. A kinetic model is suggested in which the rate law of both the Landolt and Dushman reactions is supplemented to take all the characteristics of the measured curves into account. It is demonstrated that the supercatalytic term of the Landolt reaction with respect to hydrogen ion discovered recently and the rate term of the Dushman reaction being first order with respect to iodide ion play an important role in explaining the dependencies of the Landolt induction period.

Inherent pitfalls in the simplified evaluation of kinetic curves.

It is shown and explained in detail by four examples generated from known kinetic models that simplified evaluation procedures--initial rate studies, individual exponential curve fitting method--may inherently lead to inappropriate chemical conclusions, even in the case of relatively simple kinetic systems. It is also shown that in the case of all four examples the simultaneous curve fitting immediately reveals the defectiveness of the kinetic model obtained from the simplified evaluation procedures. We therefore propose the extensive usage of the simultaneous curve fitting of all the kinetic traces to avoid these pitfalls and to find the appropriate kinetic models.

Three autocatalysts and self-inhibition in a single reaction: a detailed mechanism of the chlorite-tetrathionate reaction.

The chlorite-tetrathionate reaction has been studied spectrophotometrically in the pH range of 4.65-5.35 at T = 25.0 +/- 0.2 degrees C with an ionic strength of 0.5 M, adjusted with sodium acetate as a buffer component. The reaction is unique in that it demonstrates autocatalysis with respect to the hydrogen and chloride ion products and the key intermediate, HOCl. The thermodynamically most-favorable stoichiometry, 2S(4)O(6)2- + 7ClO2- + 6H2O --> 8SO(4)2- + 7Cl- + 12H+, is not found. Under our experimental conditions, chlorine dioxide, the chlorate ion, or both are detected in appreciable amounts among the products. Initial rate studies reveal that the formation of chlorine dioxide varies in an unusual way, with the chlorite ion acting as a self-inhibitor. The reaction is supercatalytic (i.e., second order with respect to autocatalyst H+). The autocatalytic behavior with respect to Cl- comes from chloride catalysis of the chlorite-hypochlorous acid and hypochlorous acid-tetrathionate subsystems. A detailed kinetic study and a model that explains this unusual kinetic behavior are presented.

Response reactions: equilibrium coupling.

It is pointed out and illustrated in the present paper that if a homogeneous multiple equilibrium system containing k components and q species is composed of the reactants actually taken and their reactions contain only k + 1 species, then we have a unique representation with (q - k) stoichiometrically independent reactions (SIRs). We define these as coupling reactions. All the other possible combinations with k + 1 species are the coupled reactions that are in equilibrium when the (q - k) SIRs are in equilibrium. The response of the equilibrium state for perturbation is determined by the coupling and coupled equilibria. Depending on the circumstances and the actual thermodynamic data, the effect of coupled equilibria may overtake the effect of the coupling ones, leading to phenomena that are in apparent contradiction with Le Chatelier's principle.

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

The sulfite-chlorine dioxide reaction was studied by stopped-flow method at I = 0.5 M and at 25.0 +/- 0.1 degrees C in a slightly acidic medium. The stoichiometry was found to be 2 SO(3)(2-) + 2.ClO(2) + H(2)O --> 2SO(4)(2) (-) + Cl(-) + ClO(3)(-) + 2H(+) in *ClO(2) excess and 6SO(3)(2-) + 2*ClO(2) --> S(2)O(6)(2-) + 4SO(4)(2-) + 2Cl(-) in total sulfite excess ([S(IV)] = [H(2)SO(3)] + [HSO(3)(-)] + [SO(3)(2-)]). A nine-step model with four fitted kinetic parameters is suggested in which the proposed adduct *SO(3)ClO(2)(2-) plays a significant role. The pH-dependence of the kinetic traces indicates that SO(3)(2-) reacts much faster with *ClO(2) than HSO(3)(-) does.

Unexpected formation of higher polythionates in the oxidation of thiosulfate by hypochlorous acid in a slightly acidic medium.

It has been clearly shown that not only tetrathionate but also pentathionate or even higher polythionates is formed in the oxidation of thiosulfate by hypochlorous acid in a slightly acidic medium. In thiosulfate excess, the absorbance-time curves registered at 250 nm may go through a maximum followed by a minimum, suggesting the presence of a short-lived absorbing intermediate proposed to be S(2)O(3)OCl(3-). Matrix rank analysis (MRA) studies have revealed that altogether five independent absorbing species are present in the wavelength range 240-400 nm. A kinetic model is suggested to take all of the experimental observations into account.

Autocatalysis and self-inhibition: coupled kinetic phenomena in the chlorite-tetrathionate reaction.

The initial rate of formation of chlorine dioxide in the chlorite-tetrathionate reaction changes in an unusual fashion. The formal kinetic order of both reactants varies over a very wide range. Moreover, chlorite ion behaves not just as a simple reactant, but also as a self-inhibitor. A five-step scheme, derived from an eight-step mechanism, is proposed in which the autocatalytic formation of HOCl plays a central role in accounting for this kinetic behavior.

Great structural variety of complexes in copper(II)-oligoglycine systems: microspeciation and coordination modes as studied by the two-dimensional simulation of electron paramagnetic resonance spectra.

A series of isotropic EPR spectra recorded at various concentrations and pH (in the range 2-12) on equilibrium systems containing copper(II) and diglycine, triglycine, or tetraglycine were analyzed. A purely mathematical method, matrix rank analysis gave the number of independent EPR-active species. Two-dimensional evaluation then resulted in the formation constants and magnetic parameters of 14 metal complexes (including microspecies) in each system. The independent paramagnetic species formed with each ligand are as follows: Cu(2+) (aqua complex), [CuLH](2+), [CuL](+), [CuLH(-1)], [CuLH(-2)](-), [CuL(2)H(2)](2+), [CuL(2)H](+), [CuL(2)], [CuL(2)H(-1)](-), and [CuL(2)H(-2)](2-). Moreover, for diglycine, the diamagnetic complex [Cu(2)L(2)H(-3)](-), and for triglycine and tetraglycine, the EPR-active species [CuLH(-3)](2-) were identified. Further, equilibria of two microspecies were demonstrated for [CuL(2)], [CuL(2)H(-1)](-), and [CuL(2)H(-2)](2-). The magnetic parameters allowed a detailed description of the coordination modes. The most important findings: (1) For the mono complexes, the in-plane sigma-bonds between copper(II) and the equatorial N donors are particularly strong when the same ligand forms several adjacent chelate rings with the participation of amino N, deprotonated peptide N(s), and the carboxylate group. (2) Structures with coupled chelate rings are likewise favored in the bis complexes. Different protonation states of the two ligands are observed in the major isomer of [CuL(2)] ((LH(-1) + LH) coordination), and in the isomers of [CuL(2)H(-2)](2-) ((LH(-2) + L) coordination) for triglycine and tetraglycine.

Oscillatory photochemical decomposition of tetrathionate ion.

When subject to illumination, tetrathionate ion decomposes in an oscillatory fashion with a period of about 1 h to yield products identified as colloidal sulfur and trithionate ion. This system represents the first experimental example of a stirred, batch photochemical oscillator. A simple model consisting of four reaction steps and four adjustable rate parameters gives good agreement with the observed oscillatory behavior.