Ferroin

Abstract: Measurements are reported of velocities, amplitudes, and profiles in space and time of chemical waves in the Belousov–Zhabotinsky reaction at various temperatures, depths of solution, and initial reactant concentrations. The measurements are made in a thin layer of a quiescent, but excitable solution by means of light absorption by ferroin, the tris(1,10‐phenanthroline) ferrous sulfate complex. Propagating wave profiles are recorded on a linear photodiode array (25.6 mm length) with a spatial resolution of 50 μ. The determinations of velocity corroborate previous experimental findings. New results include experimental verification of the constancy of the concentration profile of the wave in space and time; determination of two characteristic time constants in the relaxation of the wave profile; trends in wave amplitude with variation of initial reactant concentrations and age of the reaction mixture; wave velocity as a function of temperature, and solution depth; and measurements of wave annihilation. Observations of additional structure include the onset of mosaic structure, that is the transition from a homogeneous to an inhomogeneous state due to the passage of the wave, and initiation spikes.

Abstract: BELOUSOV1 reported that the ratio [Ce(IV)]/[Ce(III)] undergoes repeated temporal oscillations in a stirred sulphuric acid solution containing bromate ion and malonic acid. Busse2 and Zaikin and Zhabotinsky3 reported that spatial bands of alternating oxidizing and reducing character propagated through an unstirred solution if it were initially subject to a concentration gradient. Existence of these bands was demonstrated by the redox indicator ferroin (the 1,10-phenanthroline complex of ferrous ion), and bands could be generated in the absence of cerium ions if indicator were present, because the ferroin indicator itself serves as an oxidation-reduction couple in the absence of cerium ions.

Abstract: In the last few years many new reaction routes and intermediates have been discovered in the mechanism of the Belousov–Zhabotinsky (BZ) reaction with the aid of high performance liquid chromatography (HPLC). These previous HPLC studies, however, were limited to the Ce4+–organic substrate (malonic or bromomalonic acid) systems only. Very recently some measurements were made on a cerium catalysed full BZ system but only in its induction period. The present work follows the evolution of the main chemical components in a cerium and in a ferroin catalysed full BZ system from the start until the end of the oscillatory regime in a batch reactor. While recording the potential oscillations of a bromide selective electrode we measured from time to time the concentration of the following components: malonic and bromomalonic acids and bromate as main components; malonyl malonate, ethanetetracarboxylic and bromoethenetricarboxylic acids which are recombination products of organic free radicals; oxidized intermediates: tartronic, oxalic (OA) and mesoxalic (MOA) acids, and brominated products: dibromoacetic and tribromoacetic acids. Recombination products are generated in the intervals when the autocatalytic reaction is “switched off ”. In the course of the autocatalytic periods, however, the organic radicals react with the inorganic bromine dioxide radical mainly which leads to the formation of MOA and OA. Due to a very fast Ce4+–MOA reaction, MOA can be detected in the ferroin catalysed BZ system only. Our model calculations deal exclusively with the cerium catalysed system. The suggested new Marburg–Budapest–Missoula (MBM) model includes both negative feedback loops (bromous acid–bromide ion Oregonator type and bromine dioxide–organic free radicals Radicalator type feedback) and the recently discovered radical–radical recombination reactions. Comparison of the experimental data with the model calculations shows a good qualitative agreement but some open problems still remain. To overcome these problems oxygen atom transfer and other redox reactions are proposed.