Dragomir R. Stanisavljev

Determination of Cl–, Br–, I–, Mn2+, malonic acid and quercetin by perturbation of a non-equilibrium stationary state in the Bray–Liebhafsky reaction

A new method applying a non-linear chemical system under conditions far from thermodynamic equilibrium in microvolume/microconcentration quantitative analysis is described. The chemical system used as a matrix is the Bray–Liebhafsky reaction in a non-equilibrium stationary state close to a bifurcation point. The method is based on monitoring the response of this system to perturbations by Cl–, Br–, I–, Mn2+, malonic acid and quercetin analyte solutions, which are followed potentiometrically either by an Ag+/S2– ion-sensitive or by a Pt electrode. A linear response of the potential shift versus the logarithm of the analyte concentrations is found in the following ranges: 1.3 × 10–6 mol dm–3 ≤ [Cl–] ≤ 1.6 × 10–4 mol dm–3, 1.0 × 10–6 mol dm–3 ≤ [Br–] ≤ 8.3 × 10–5 mol dm–3, 2.0 × 10–6 mol dm–3 ≤ [I–] ≤ 1.0 × 10–4 mol dm–3, 8.4 × 10–7 mol dm–3 ≤ [Mn2+] ≤ 8.3 × 10–5 mol dm–3, 3.8 × 10–7 mol dm–3 ≤ [malonic acid] ≤ 2.1 × 10–5 mol dm–3 and 1.5 × 10–8 mol dm–3 ≤ [quercetin] ≤ 3.7 × 10–5 mol dm–3. Under the investigated conditions an improved detection limit for all halides tested is obtained. Unknown concentrations of the analytes can be determined from a standard series of calibration curves to an accuracy within ±5%. In addition, the application of potentiometric measurements in microvolume/microconcentration quantitative analysis is diversified.

Oxygen centered radicals in iodine chemical oscillators.

The existence of free radicals in iodine-based oscillatory systems has been debated for some time. Recently, we have reported the presence of reactive oxygen species (ROS) in the iodide-peroxide system in acidic medium, which is common to all iodine--based oscillatory systems ( J. Phys. Chem. A 2011 , 115 , 2247--2249 ). In this work, the goal was to identify the ROS produced in this system using an EPR spin trap which can distinguish between hydroxyl (HO(•)) and hydroperoxyl (HOO(•)) radicals. The formation of the hydroperoxyl radical was observed and a possible explanation for the low EPR signal of hydroxyl radical was proposed.

Radicals in the Bray–Liebhafsky Oscillatory Reaction

This study investigates the formation of free radicals in the Bray-Liebhafsky (BL) oscillatory reaction. The results indicate that radicals are produced during both monotonous and oscillatory dynamics observed as the change of the electron paramagnetic signal (EPR) of the spin-probe TEMPONE. EPR spin-trapping with DEPMPO suggested that the most abundant radical produced in the BL reaction is an iodine-centered radical. The EPR spectrum of the DEPMPO/iodine-centered radical adducts has not been previously reported. This study may aid in establishing a more realistic reaction mechanism of the BL reaction and related chemical oscillators.

Role of free radicals in modeling the iodide-peroxide reaction mechanism.

The mechanism of monotonous decomposition of hydrogen peroxide by iodide in acidic medium is investigated at room temperature. For this purpose, O(2) pressure, I(-), I(2), and I(3)(-) concentrations are simultaneously monitored, establishing a useful framework for better understanding of the reaction mechanism and testing various models. The possibility of nonradical and radical approaches to describe experimental data is examined. Results suggest that the best description of experimentally recorded components is obtained by introducing free radicals into the model of the reaction.

The kinetics of iodide oxidation by hydrogen peroxide in acid solution

The kinetics of the complex reaction between I− and H2O2 in acid media was investigated. The particular attention was focused on the determination of the rate constant of the reaction between HIO and H2O2 involved in the investigated complex process. The examination of the whole kinetics was performed by simultaneously monitoring the evolution of O2 pressure, I3− and I− concentrations. We modeled the behavior of experimentally followed components based on Liebhafsky’s research. Our preliminary results suggest a significantly higher rate constant (3.5 × 107 M−1 s−1) of the reaction between HIO and H2O2 as those proposed in the literature.

A Potential Source of Free Radicals in Iodine-Based Chemical Oscillators

The iodide-peroxide system in an acidic medium was investigated as a potential source of free radicals in iodine-based chemical oscillators. The radicals were detected by EPR spin-trapping using spin-trap 5-(tert-butoxycarbonyl)-5-methyl-1-pyrroline N-oxide (BMPO), which forms stable spin-adducts with oxygen-centered radicals. The iodide-peroxide system is introduced as an easily available laboratory source of free radicals.

Energy Dynamics in the Bray−Liebhafsky Oscillatory Reaction

Energy dynamics of the well-stirred, isothermally conducted Bray-Liebhafsky reaction is followed by monitoring the population of the first two vibration states of hydrogen peroxide. Excitations are detected by Raman spectroscopy showing periodical changes of the energy flow through the system, matching the periodicity of chemical oscillations. Well before chemical oscillation, rearrangement of energy provokes excessive excitation of the first vibration state of hydrogen peroxide followed by the phase-shifted excitation of the second state. The observed populations of excited states highly exceed equilibrium values, suggesting that the nonequilibrium distribution of energy related to the peculiar hydrogen bond network dynamics may be an important part of the reaction mechanism.

RESPONSE SURFACE OPTIMIZATION FOR ACTIVATION OF BENTONITE USING MICROWAVE IRRADIATION

Microwave irradiation as a means for heating bentonites during acid activation has been investigated in the past but it has never been optimized for industrial applications. The purpose of this study was to apply a factorial 23 experimental design to a Serbian bentonite in order to determine the influence of microwave heating on the acid-activation process. The effect of acid activation under microwave irradiation on the textural and structural properties of bentonite was studied as a model reaction. A mathematical, second-order response surface model (RSM) was developed with a central composite design that incorporated the relationships among various process parameters (time, acid concentration, and microwave heating power) and the selected process response of specific surface area of the bentonite. The ranges of values for the process parameters chosen were: time, 5–21 min; acid concentration, 2–7 M; and microwave heating power, 63–172 W. The effect of individual variables and their interaction effects on the textural and structural properties of the bentonite were determined. Statistical analysis showed that the duration of microwave irradiation was less significant than the other two factors. The model showed that increasing the time and acid concentration improved the textural properties of bentonites, resulting in increased specific surface area. This model is useful for setting an optimum value of the activation parameters for achieving the maximum specific surface area. An optimum specific surface area of 142 m2g−1 was achieved with an acid concentration of 5.2 M, activation time of 7.4 min, and microwave power of 117 W.

Influence of Chemically Inert Cations on the Hydrogen-bond Network in the Bray-Liebhafsky Oscillatory Reaction

The influence of the chemically inert alkali metal cations on the Bray-Liebhafsky oscillatory reaction dynamics was investigated by the addition of the same amount of various sulfates to the reaction mixture. Beside the expected changes related to the altered acidity of the sulfuric acid solution, subtle changes dependent on cation dimensions were noticed. Larger cations have more impact on the Bray-Liebhafsky reaction dynamics. Analysing the mean ionic activity of salts, it is suggested that the effects may be related to the altered extent of water hydrogen bonding and specific role of bulk water in the reaction mechanism.

Different influences of adrenaline on the Bray–Liebhafsky and Briggs–Rauscher iodate based oscillating reactions

The influence of adrenaline on two iodate based oscillating chemical reactions, the Bray–Liebhafsky (BL) and Briggs–Rauscher (BR) reactions was investigated. It was observed that the addition of adrenaline has different effects on the examined systems. Its addition to the BR system significantly changes the dynamics, while in the BL system, the presence of adrenaline does not show any effect. In order to find out the cause of such a different response of two similar oscillators, UV/VIS spectroscopy was used. Results obtained from recorded UV/VIS spectra indicated that, from all investigated stable reaction species, adrenaline interacts only with potassium iodate and iodine, which are present in both oscillating reactions. Because of this, the different response of the oscillators cannot be ascribed to the reactions between adrenaline and common iodine (stable and unstable) components. As a result, the considerable response of the BR system to adrenaline may be related to the reactions between adrenaline (or its oxidation products) and non-common derivatives of malonic acid or manganese intermediate species.