György Bazsa

What is and what isn’t a clock reaction?

Clear distinction should be made between the several different causes that may lead to an induction period in a kinetic experiment, and the term ‘clock reaction’ should be used only when an abrupt increase occurs in the concentration of one or more products due to the total consumption of a limiting reagent.

Kinetics and mechanism of autocatalytic nitric acid oxidations

Abstract A wide range of inorganic substrates undergo autocatalytic oxidation by moderately concentrated (1–4 M) nitric acid. In addition to behaving as clock reactions in closed (batch) systems, several of these oxidations exhibit bistability in open continuous-flow stirred tank reactor (CSTR) systems and wave propagation in unstirred configurations. Analysis of experimental studies, combined with detailed numerical simulation, suggests several mechanistic features which are common to nitric acid oxidations. The mechanistic scheme also affords insight into why nitric acid oxidations, in contrast to many other autocatalytic reactions, have defied efforts to convert them into chemical oscillators.

Wave profile in the acidic nitrate–ferroin reaction

The spatial concentration profile of the autocatalytic wave in the acidic nitrate–ferroin reaction has been investigated by a home-built two-dimensional spectrophotometer as a function of the initial reactant concentrations. The formula [graphic omitted] ≈ 0.36 (k1/D)1/2[H+]1/20[Fe(phen)2+3]0, obtained on the basis of a simplified mechanism of the reaction gives a fairly good description of the experimentally found slope of the wave profile at the inflection point.

High temperature copolymerization of styrene and maleic anhydride in propagating polymerization front

The synthesis of poly(styrene-maleic anhydride) copolymers by frontal polymerization is reported. The propagating front can be achieved if the mole fraction of styrene (St) is 0.3 ≤ St ≤ 0.7 in the feed. Depending on the St/MA mole ratio alternating St-MA-St-MA copolymers (St/MA ≤ 1) or (St-MA)n-(St-St-St)m block copolymers (St/MA > 1) are formed. The microstructure of the copolymers obtained was estimated by means of 13C NMR spectroscopy.

Kinetics and mechanism of autocatalytic oxidation of Fe(phen)2+3 and Fe(bpy)2+3 by nitric acid

The kinetics of the title reactions has been studied and the investigation of the oxidation of the iron(II) aqua ion by nitric acid has been extended. All three reactions are autocatalytic in nature. Oxidation of Fe(aq)2+ is practically complete at [HNO3] > 0.5 mol dm–3. In the case of the two other complexes the process is reversible; the conversion in the equilibrium state depends on the initial concentrations of reactants. Values of the equilibrium constants have been determined from experimental data and compared to those calculated from standard electrochemical potentials. The autocatalytic S-shaped kinetic curves have been measured by means of spectrophotometry and potentiometry and are characterised using the time coordinate (ti) and the slope (rmax) of the concentration vs. time curves at the inflection point. The general mechanism of autocatalytic nitric acid oxidations has been applied in order to fit single experimental curves and all concentration dependences. The validity of the proposed mechanism is supported by the agreement between experiments and the results of the numerical integration of differential rate equations derived from the mechanism. An interesting local maximum and minimum are found in the tivs.[Fe2+]0 curves both experimentally and numerically.

Dependence of the inflection time on the initial concentrations in autocatalytic reactions. Maxima, minima and ‘anomalous changes’

The existence and the behaviour of the inflection time (the turning-point in the rate–time dependence) has been investigated as a function of the initial concentrations of the components in autocatalytic reactions. For a group of rate equations it is shown that the inflection time is a strictly decreasing function of the initial concentration of the autocatalyst. When the initial concentration of the autocatalyst is fixed, the inflection time may attain maxima and minima or may be everywhere increasing with increasing initial concentration of the subtrate. The difference in the influence of the two components remains even when the rate equation is symmetric. Examples and some practical consequences are discussed.

Kinetics of the bromate–iodide–ascorbic acid clock reaction: different mechanism of the molybdenum and vanadium catalysis

The kinetics of the molybdenum(VI)- and vanadium(V)-catalysed bromate–iodide–ascorbic acid clock reaction has been studied by means of spectrophotometry and pH-metric measurements. Two different types of catalysis were detected in this clock reaction. It was shown that MoVI catalyses the component process bromate + iodide of the clock system, while VV catalyses the direct reaction between bromate and ascorbic acid. The rate equation r=k2[BrO3–][I–][H+]2+k′2[BrO3–][I–][H+]2[MoVI] describes the kinetics of the Mo-catalysed system, where k2= 44 ± 2 dm9 mol–3 s–1 and k′2=(4.3 ± 1)× 106 dm12 mol–4 s–1. In the vanadium-catalysed clock reaction the equation r=k′1[BrO3–][VV]+k2[BrO3–][I–][H+]2 was found, where k′1= 9.6 ± 1 dm3 mol–1 s–1 and k2= 42.6 ± 2 dm9 mol–3 s–1. At lower acidities (pH > 3) more complex kinetics appeared and further study was abandoned.

Kinetic study of the reaction between ferriin and nitrous acid by a concentration jump–stopped flow technique

The kinetics of the reaction between ferriin and nitrous acid have been studied as an extension of the investigation of the 2Fe(phen)2+3+ NO–3+ 3H+⇌ 2Fe(phen)3+3+ HNO2+ H2O reversible reaction. Owing to the dynamic equilibrium which exists in this system, the equilibrium position can be shifted from the right-hand side by dilution, and this allows an investigation of the kinetics of the backward reaction by means of the concentration jump method. If excess nitrite (nitrous acid) was applied a pseudo-first-order condition was reached for the ferriin–nitrous acid reaction owing to the peculiar behaviour of the mechanism of the overall process. The kinetic results were compared to those obtained previously and confirm the two-step mechanism proposed for the forward reaction.

Ultraviolet radiation and skin disorders in Hungary

The measurements of ozone content, evaluation of UV radiation and their links with skin cancer in Hungary have been studied for the last three decades. The total ozone content of an air column above Hungary has been measured by the Hungarian Meteorological Service since 1969 using Dobson-spectrophotometer. The measurements of UV-B radiation started in 1993 by three Robertson-Berger equipments and LI-1800 spectroradiometer. It was found a decreasing trend of ozone content 1.7% per 10 years. Since 1991/92 winter the ozone conditions of the stratosphere have been perturbed. Such low ozone values that have been observed in two winters successively never occurred. Deficits in monthly averages: 1991 Dec: -6%, 1992 Jan; -17%, Feb; -9%, Dec; - 10%, 1993 Jan; -16%, Feb; -17%. Statistical analysis of the patient material of the Eastern region of Hungary, characterized by a rather high intensity of sunlight and UV rays, has shown a significant increase in light induced skin disorders in recent decades. Ninety-six photoallergic and phototoxic cases followed up in 1966 rose to 336 (1977) and 788 (1993) whereas the numbers of patients with basal cell carcinoma and malignant melanoma increased two- and fivefold, respectively, from 1966 to 1993. A UV personal dosimeter has been developed to measure exposure of the skin to UV-B radiation. These SUNTEST UV-sensitive strips for general public are produced by FORTE Photochemical Company.