Cesare Lucarini

Oscillatory phenomena: spontaneous alternance of the occurrence of corrosion and passivation states of an iron electrode in bromate/sulphuric acid medi

Abstract Spontaneous oscillations have been found by us in the potential value (vs. a saturated calomel electrode) of an iron electrode in bromate/sulphuric acid media in open electric circuit conditions. We ascribe them to a spontaneous alternance of corrosion and passivation states. Both bromate and H + reductions, in the corrosion state, lead to an increase in the pH value near to the iron surface and to the precipitation of a passivating film of oxide or salt on the electrode surface. The prevalence of H + influx from the bulk of the solution on its disappearance during the passivation state leads to such a low value of pH as to allow the dissolution of the passivating film, and the phenomenon starts again. We have studied the influence of stirring, of the size of the iron electrode surface, and of the bromate and sulphuric acid concentrations on the oscillations.

SPONTANEOUS PERIODIC AND BURSTING CURRENT OSCILLATIONS IN IRON CORROSION BY DICHROMATE: A USEFUL STUDY FOR SIMULATING BIOLOGICAL SYSTEMS

Abstract Studies on chemical and electrochemical oscillating systems are very useful in understanding more complex biological systems. Spontaneous periodic and bursting current oscillations were found in iron/dichromate systems coupled with graphite or zinc electrodes. In this paper, we study some phenomenological features of the two systems: their typical oscillation profiles and the influence of external resistance. The results are explained by the Franck-Fitzhugh model using the mixed potential theory.

397 - On the entropy production in oscillating chemical systems

Abstract Oscillatory biochemical phenomena are of relevant interest in the knowledge of living processes. To approach the study of such phenomena, energy changes in a chemical oscillating model system (the Belousov-Zhabotinskii reaction), under different experimental conditions, have been considered by measuring some electrical entities using a simple potentiometric device. On the basis of the experimental data obtained, it seemed possible to correlate some fundamental features of a chemical oscillator with those of an electrical oscillator, where feedback effects take place involving only electron flows. This is proved by the validity of some general principles of electrotechnics (Thevenins theorem) when applied to galvanic cells. Simple evaluations on the velocity of entropy production in the model system considered, which is an example of a dissipative structure, are in agreement with the decrease of the entropy associated with the steady-state conditions of the oscillating chemical reaction.

Short communication289 - An unusual form of chemical reactions in membranes

Unusual chemical reactions are obtained in membranes, when these are soaked in solution containing only bromate and ferroin. The evolution of a rotating pattern in synthetic membranes seems to be an attractive model for the understanding of the structural organization of chemical reactions in biomembranes.

Nature and concentration of catalysts and electrochemical properties of oscillating chemical reactions

The chemical nature and the concentration of the catalyst affects the behaviour of oscillations in a bromate, malonic acid system. MnSO4, Ce2(SO4)3 and ferroin have been used as catalyst-indicators, and a marked difference between cerium or manganese and ferroin has been pointed out; such a difference can be ascribed to the diversity in the chemical nature of these substances. The study of the system, when considered during its total evolution. gives the possibility of correlating the behaviour of the reaction with the catalyst concentration. Such a correlation can be interpreted according to a scheme which, in agreement with mechanisms previously proposed, indicates the role of the catalyst during the reaction.

Electrochemical generators of pulsating or alternating current

This paper concerns the design of three different electrochemical generators of nonsteady current: two generate a pulsating direct current, the third generates an irregular alternating current. The (Fe|NaBrO3(aq.)+H2SO4(aq.)|Pt) generator induces an alternating current in a secondary circuit connected to a transformer. The influence of electrode size and external resistance on current oscillation parameters and on power is shown. The shape of current oscillation in the generators (Fe|NaBrO3(aq.)+H2SO4(aq.)|Pt), (Zn/ZnSO4(aq.)||NaBrO3(aq.)+H2SO4(aq.)|Fe) and (Fe|NaBrO3(aq.) +H2SO4(aq.)|Fe) are also shown and the working of the three generators is explained.

Spontaneous current oscillations in iron/bromate/platinum system: Some phenomenological features

Abstract If an iron electrode is coupled by an external resistance with a platinum electrode, put in the same solution consisting of bromate and sulphuric acid, a pulsating conventional current goes through the external circuit from the platinum to the iron electrode. In this work we study these spontaneous current oscillations. The influence of the external resistance, the size and the distance between the two electrodes, temperature, stirring and of ultrasound application on oscillatory parameters as period, amplitude and profiles of oscillations have been examined. Results are explained with a Franck and FitzHugh model ascribing oscillations to a spontaneous alternance of corrosion and passivation states, and using the mixed potential theory.

Spontaneous oscillatory phenomena in iron corrosion by bromate

Abstract Spontaneous potential oscillations occur during iron corrosion by bromate in sulphuric acid medium . Bromate reduction leads to the precipitation of a passivating film due to a pH increase, whilst a successive pH decrease du to H + diffusion leads to film dissolution.

A simple device for the evaluation of the UV radiation index

The solar ultraviolet radiation (UV) flux density at the earths surface depends on the incoming solar energy and the transmission properties of the atmosphere. UV radiation is strongly absorbed by ozone in the spectral range 200–310 nm, while the attenuation is increasingly weaker at longer wavelengths. Following the discovery of the Antarctic ozone hole in 1985, the risk of a possible UV increase at ground level, due to the observed stratospheric ozone depletion, has heightened the interest within the scientific community given the potentially harmful effects on terrestrial and aquatic ecosystems. Spectroradiometers, broad-band meters and dosimeters may be used for measurements of solar UV. In addition, radiation transfer models can be used to quantify UV irradiances at various times and locations, provided that the extraterrestrial solar radiation and the state of the atmosphere are known. Information about UV radiation at the earths surface is given by the ultraviolet index ‘UVI’, which is defined as the effective integrated irradiance (280–400 nm) weighted by the erythemal action spectrum. The UV Index is widely used by many international weather services as an indicator of UV levels at the earths surface providing public awareness of the effects of prolonged exposure to the suns rays. The aim of this paper is to present a device capable of estimating the UV Index. This device is a compact disc, used as a sundial, and is based on modelled UV irradiances derived from the STAR radiative transfer model (System for Transfer of Atmospheric Radiation). The device was tested in an urban setting under clear sky conditions. Copyright

Some new aspects of oscillating chemical reactions

Summary Oscillatory reactions have been studied on the basis of a new experimental approach following the evolution of each reaction from beginning to end. Changes in catalyst concentration and in the ratios of the main reagents concentrations ( i.e. bromate-malonic acid) have been considered. Some equations which correlate the oscillating behaviour with thereagent concentration are also proposed and their results have been found in good agreement with the experimental data.