Nataša Pejić

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.

Examinations of Cross-Linked Polyvinylpyridine in Open Reactor

Macroporous cross-linked copolymer of 4-vinylpyridine and 25% (4:1) divinylbenzene is analyzed under open conditions, that is in a continuous well-stirred tank reactor (CSTR). With this aim the appropriate bifurcation diagram is found and the behavior of the system with and without polymer in the vicinity of the bifurcation point is used for the polymer examinations. Two different granulations of polymer are considered. Moreover, some physicochemical characteristics of the polymer, such as specific surface area, skeletal and particle density, are determined.

Dynamic behavior of the bray-liebhafsky oscillatory reaction controlled by sulfuric acid and temperature

The non-periodic, periodic and chaotic regimes in the Bray-Liebhafsky (BL) oscillatory reaction observed in a continuously fed well stirred tank reactor (CSTR) under isothermal conditions at various inflow concentrations of the sulfuric acid were experimentally studied. In each series (at any fixed temperature), termination of oscillatory behavior via saddle loop infinite period bifurcation (SNIPER) as well as some kind of the Andronov-Hopf bifurcation is presented. In addition, it was found that an increase of temperature, in different series of experiments resulted in the shift of bifurcation point towards higher values of sulfuric acid concentration.

Kinetic analytical method for determination of uric acid in human urine using analyte pulse perturbation technique

Simple and reliable novel methods for the determination of uric acid (UA) are proposed and validated. For quantitative determination of UA, two matrices were used: the Bray-Liebhafsky (BL) oscillatory reaction in a stable non-equilibrium stationary state close to the bifurcation point (method A) as well as, the BL non-oscillating subsystem (mixture KIO3 and H2SO4), i.e., Dushman reaction (DR) in a steady state (method B). The proposed methods are optimized in a continuously fed well stirred tank reactor (CSTR) and applied with excellent results in the determination of UA in human urine samples. The linear relationship between maximal potential shift ΔEm, and both the logarithm of the UA concentration (procedure A) and UA concentration (procedure B) is obtained in the concentration range 2.98 × 10- 5-2.68 × 10- 4 mol L- 1 and 2.98 × 10- 5-3.58 × 10- 4 mol L- 1, respectively. The methods have an excellent sample throughput of 30 samples h- 1 (method A) and 7 samples h- 1 (method B) with the sensitivity determined to be 1.1 × 10- 5 mol L- 1 (method A) and 8.9 × 10- 6 mol L- 1 (method B) as well as the precision RSD < 3.4% for both methods. Some aspects of the possible mechanism of UA action on the BL oscillating and Duschman non-oscillating reaction systems are discussed in detail.

Quantitative determination of some water-soluble B vitamins by kinetic analytical method based on the perturbation of an oscillatory reaction

A novel procedure for kinetic determination of some water-soluble vitamins of the B-group (thiamine (B1), riboflavin (B2), niacin (B3) and pyridoxine (B6)) by the concentration perturbations of the Bray-Liebhafsky (BL) oscillatory chemical system involving the catalytic decomposition of hydrogen peroxide in the presence of both hydrogen and iodate ions is proposed and validated. The method uses a Pt electrode for potentiometric monitoring of the concentration perturbations of the BL matrix in a stable non-equilibrium stationary state close to the bifurcation point. The proposed method relies on the linear relationship between maximal potential displacements, ΔEm, caused by the additional known quantities of a B species. Under the optimal established analytical conditions, linear calibration curves were obtained over the range of 0.01-1.0, 0.016-0.128, 5.0-50.0 and 0.05-2.5 μmol with the limits of detection of 0.01, 0.018, 2.6 and 0.03 μmol, as well as analytical throughput of 30, 5, 12 and 20 determinations per hour, for B1, B2, B3 and B6, respectively. The used technical approach also provides simple, effective and convenient method to assay the pharmaceutical formulations containing B1 together with other active principles such as nicotinamide and vitamin B12 as well as B3.

Dynamic transitions in the Bray–Liebhafsky oscillating reaction. Effect of hydrogen peroxide and temperature on bifurcation

The temporal dynamics of the Bray–Liebhafsky reaction (iodate-based catalytic decomposition of hydrogen peroxide in an acidic aqueous solution) was experimentally characterized in a continuous stirred tank reactor by independently varying the temperature and the mixed inflow hydrogen peroxide concentration. When the temperature was the bifurcation parameter, the emergence/disappearance of oscillatory behavior via a supercritical Andronov–Hopf bifurcation was observed for different mixed inflow hydrogen peroxide concentrations. An increase in the mixed inflow hydrogen-peroxide concentration resulted in a shift of the bifurcation point towards higher values of temperature, but did not alter the bifurcation type.

Current rates and reaction rates in the Stoichiometric Network Analysis (SNA)

Abstract In stoichiometric network analysis (SNA) the instability condition is calculated by the current rates. Recently, we have shown that in the final result the current rates can be substituted by reaction rates, which is a more appropriate value for the examination of instability from experimental point of view. Here, we elaborate the problem of whether the current rates are necessary parameters in the calculation, with the aim of obtaining the region of instability. All calculations are performed on a model for Belousov-Zhabotinsky (BZ) reaction, which has not been examined by SNA.

Deterministic Chaos in Open Well‐stirred Bray‐Liebhafsky Reaction System

Dynamics of the Bray‐Liebhafsky (BL) oscillatory reaction is analyzed in a Continuously‐fed well‐Stirred Thank Reactor (CSTR). Deterministic chaos is found under different conditions, when temperature and acidity are chosen as control parameters. Dynamic patterns observed in real experiments are also numerically simulated.

Activity of Polymer Supported Cobalt Catalyst in the Bray-Liebhafsky Oscillator*

The infuence of poly-4-vinylpyridine-co-divinylbenzene-Co2+ catalyst on the Bray-Liebhafsky (BL) oscillator used as the matrix for establishing catalyst’s activity was analyzed. The addition of the catalyst do not change the dynamics of the reaction in the BL matrix, but the periods of the oscillatory evolution as well as the preoscillatory period (τ1) and the duration from the beginning of the reaction to the end of the oscillatory state (τend). All experimental results are simulated satisfactory.

Pulse perturbation technique for determination of piroxicam in pharmaceuticals using an oscillatory reaction system

AbstractA simple and reliable novel kinetic method for the determination of piroxicam (PX) was proposed and validated. For quantitative determination of PX, the Bray-Liebhafsky (BL) oscillatory reaction was used in a stable non-equilibrium stationary state close to the bifurcation point. Under the optimized reaction conditions (T = 55.0°C, [H2SO4]0 = 7.60×10−2 mol L−1, [KIO3]0 = 5.90×10−2 mol L−1, [H2O2]0 = 1.50×10−1 mol L−1 and j0 = 2.95×10−2 min−1), the linear relationship between maximal potential shift ΔEm, and PX concentration was obtained in the concentration range 11.2–480.5 µg mL−1 with a detection limit of 9.9 µg mL−1. The method had a rather good sample throughput of 25 samples h−1 with a precision RSD = 4.7% as well as recoveries RCV ≤ 104.4%. Applicability of the proposed method to the direct determination of piroxicam in different pharmaceutical formulations (tablets, ampoules and gel) was demonstrated.