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A novel and convenient method with good selectivity and high sensitivity for the determination of hydroquinone (HQ) based on the Belousov–Zhabotinskii (B–Z) oscillating chemical reaction has been presented. The optimum condition for the determination has also been investigated. In the B–Z reaction system, when the sample was injected, the change of the amplitude is linearly proportional to the logarithm of the concentration of HQ in the range 1.0×10 −2 –2.0×10 −4 M ( r =0.9965), whereas, when the concentration of HQ is over the range from 2.5×10 −4 to 4.0×10 −7 M, the calibration curve fits a second-order polynomial equation very well ( r =0.9983).

The ring electrode of a Au–Au rotating ring-disk electrode was used to monitor the concentrations of I − and I 2 during oscillations in current that occur during I − oxidation at the disk electrode. With the ring electrode poised at a potential sufficient to oxidize I − , the anodic current at the ring decreased, while the anodic current at the disk increased, as was expected for a typical shielding experiment. However, when the ring was poised at a potential sufficient to reduce I 2 (or I 3 − ) in a collection experiment, the cathodic current also decreased simultaneously with the increase in anodic current at the disk. These results demonstrate convincingly that the oscillating current involves the periodic formation and dissolution of an I 2 film.

Following our previous studies confirming the role of adsorbed electrocatalytic SCN− ions in the formation of negative polarisation resistance in the electroreduction of the Ni(II)SCN− complexes at the mercury electrode, we present further experimental and theoretical investigations of the oscillatory behaviour of this process. Examples of current oscillations collected for various experimental conditions are shown. A numerical model involving partial differential equations of diffusion transport is constructed for simulations of reported oscillations. Comparison of experimental and theoretical oscillations confirms both the electrochemical source of oscillations and the validity of the numerical approach proposed. With this approach it was possible to explain the effect of a side formation of the inhibitory layer of NiS on the shapes of oscillations.

The mechanistic aspects of the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes, responsible for the formation of the negative polarization resistance and oscillatory instabilities, were studied. Of two possible sources of negative resistance–adsorption of an inhibitory layer of NiS and the desorption of catalytic SCN− from mercury surface at negative potentials, the latter was found to predominate under the conditions studied. Nickel(II) sulphide was suggested as one of the causes for the limited lifetime of oscillations only, and not as a source of actually observed instabilities. An inner-sphere bridge mechanism of the electroreduction of thiocyanate complexes of nickel(II) was proposed, as this pathway is considerably faster than the irreversible outer-sphere reduction, occurring at more negative potentials. The proposed mechanism was confirmed quantitatively by its digital simulation and compared with that suggested earlier for In(III)SCN− complexes. An example of current oscillations is presented.

The non-linear responses of the electrochemical oxidations of methanol and ethanol are studied under constant current conditions. The response of each process in acid solution is compared with the response in NaOH solution. In acid solution, the oxidations of methanol and ethanol exhibit a tristable response in which three different stationary potential values exist under the same conditions. For ethanol, a different type of tristable response is also exhibited in a small current range and consists of an oscillatory response that coexists with stationary states. A bistable response is obtained for both oxidation processes in NaOH solution. Two types of bistability occur for ethanol oxidation: the coexistence of stationary states and the coexistence of an oscillatory state with a stationary state. Only stationary states exist in the oxidation of methanol. The results of the current-controlled experiments are explained in terms of known parts of the reaction mechanisms and models describing multistability and anodic oscillations. Simple relations are established between cyclic voltammograms (CVs) and the results of the current-controlled experiments. Although these relations appear obvious, their realization has been elusive in processes involving the oxidation of oxygenated organics. Examples of fundamental oxidation processes are given where these relations do not exist.

Electrochemical oscillations of a new type, named oscillations C and D, appear, in addition to previously reported oscillations, named oscillations A and B, in the H 2 O 2 -reduction reaction on a Pt electrode in acidic solutions when small amounts of halide ions such as Cl − , Br − , and I − are added to the solution. Oscillation A appears in a narrow potential region of −0.25 to −0.32 V versus SCE where adsorbed hydrogen (upd-H) of a nearly full coverage is formed, and oscillation B appears in a region below ca. −0.35 V where hydrogen evolution occurs, whereas oscillations C and D appear in potential regions more positive than −0.30 V where halide ions are adsorbed on Pt. The detailed behavior of oscillations C and D is described and compared with that of oscillations A and B. Mathematical simulation has shown that the essential features of oscillations C (and D) can be understood by a reaction model which includes a kind of autocatalytic mechanism in which adsorbed halogen and OH promote dissociative adsorption of H 2 O 2 .

Possible explanations for the occurrence of instabilities and oscillations in electrocatalytic reactions are studied for three different kinds of external control of the electrochemical cell: potentiostatic control, fixed-potential control in the case where account must be taken of the cell resistance or an external resistor and galvanostatic control. The relation of our models to existing models and experiments is discussed. In this paper we have investigated simple models of electrocatalytic reactions with the aim of determining the possible origins of instabilities and oscillations under three fundamentally different ways of external control of the electrical state of the electrochemical cell. The conclusions can be summarized as follows. Under truly potentiostatic conditions, i.e. when the interfacial potential can be considered constant irrespective of the current flowing through the cell, the origin of instabilities and oscillations should be purely chemical, i.e. autocatalytic surface chemistry, deviations from ideal Langmuir adsorption, (surface) phase transitions etc. Oscillations cannot be explained by simply referring to the negative slope in the current-voltage characteristic of the reaction. Under potentiostatic conditions a simple negative slope in the J-E curve, owing to passivation for example, cannot give rise to oscillations. Only if a suitable ohmic series resistance, be it only the cell's electrolyte resistance or in addition to this an external series resistor, is present can a simple negative slope give rise to spontaneous oscillations. However, not every negative slope will give rise to oscillations in this way. A negative slope in J-E curve due to competitive adsorption between the electroactive species and a poison will not give oscillations, whatever the value of the external resistance. Potential oscillations under galvanostatic conditions cannot be explained solely by a negative polarization resistance. Potential oscillations can arise if some coupled nonelectrochemical process in the system is assumed to be a potential dependent. We have shown this for a simple model in which a potential-dependent adsorption precedes the electron transfer reaction. This potential-dependent adsorption is capable of hiding the negative slope, but the instability is retained and oscillations occur within a potential interval in the J-E curve with positive slope. This property of the model, and its bifurcation structure in general, are in qualitative agreement with several reports of galvanostatic potential oscillations found experimentally. Because of the simplicity of the models presented in this paper, particularly as these are often reduced to two variables, they can only model simple monoperiodic oscillations. Of course, far more complex oscillatory reponses have been observed experimentally. In principle, one can try to model these by introducing a more complex chemistry or by removing some of the steady-state assumptions that we have introduced. However, these extensions will not significantly change our conclusions about the origin of the oscillations. In this respect, our assumption about the uniform current distribution is a much more serious obstacle. It is not intuitively clear how a nonuniform current distribution (whatever its origin) influences the steady-state stability of an electrochemical cell. It seems that this point needs further eleboration in the future. Finally, the reader will have noticed that we have relied heavily on the mathematical conditions for obtaining oscillations. We have deliberately avoided expressing the different states within a single oscillation into chemical or physical terms. Describing the different events taking place sequentially during a spontaneous oscillation is a difficult and rather tricky business, and it is our experience that such a description is either rather untransparent or, when translated into mathematical terms, will not pass the mathematical test for oscillatory behavior.

The electrodissolution of a cobalt rotating disc electrode in sulphuric acid solutions containing iodide ions was studied using both anodic and cathodic polarization measurements. As a result of the presence of iodide ions, significant changes were observed in the characteristic I - E polarization curves of the Co/1.58 M H 2 SO 4 system. Thus, the adsorption of iodide ions results in strong inhibition of cobalt electrodissolution and in the appearance of current oscillations under potentiostatic conditions. The current oscillations are observed within a finite potential regime in the current plateau region. Both the inhibitory effect and the initiation of oscillatory phenomena depend on the concentration of iodide ions in the solution. The rotation rate of the cobalt disc mainly affects the oscillatory response, which was shown to be strongly associated with pitting corrosion phenomena. The effect of iodide ions on the polarization behaviour of cobalt in sulphuric acid solutions was compared with that of other halides (F − , Cl − , Br − ).

Three strategies are described for designing electrochemical oscillators involving electrochemical reactions at inert electrodes. The strategies are based on coupling a negative faradaic impedance with mass transport, and it is argued that the dynamic behaviour of all of these systems is expected to be qualitatively similar. The three “recipes” are illustrated by experiments of simple reduction reactions at the hanging mercury drop electrode.

In this work we report preliminary data for oscillatory behavior found in oxygen and hydrogen peroxide reduction on a rotating gold electrode with (100) orientation in alkaline solutions.

The effects of a periodically perturbed applied potential on the potentiostatic current oscillations observed during the transition of an iron electrode between the active and passive states in 2 M sulphuric acid solutions is studied experimentally in this work. Autonomous current oscillations appear during the electrodissolution of iron in sulphuric acid solutions when the iron electrode turns from the active to the passive state and vice versa. After the application of a potential perturbation the forced oscillations are conducted in a region with such a parameter space that the autonomous limit cycle is observed. The overall study was limited to the hysteresis region. The response of the oscillatory system Fe/2 M H2SO4 to an external perturbation of the potential was studied as a function of two parameters, the frequency, ωp, and the amplitude, Ep, of the forcing potential and it was characterized with the aid of time-delay reconstructions. The various types of response are presented in the phase diagram Ep-ωp/ω0. For low amplitudes of the potential perturbation and for both ωp/ω0 1 the response of the perturbed system exhibits wide zones of quasi-periodicity interrupted by entrainment bands which are harmonic or subharmonic. For higher amplitudes the entrainment bands become broad and as the forcing amplitude increases the system tends to be driven by the forcing frequency. From this experimental study on forced electrochemical oscillations of the system Fe/2 M H2SO4 it is seen that at very positive potentials for sustained periodic current oscillations to occur under stationary conditions, periodic or quasi-periodic current oscillations could be triggered by the local dissolution of the passive film with a potential pulse of a sufficient amplitude in order for the transition to the active state to be feasible. The resulting small uncovered part of the electrode surface allows metal electrodissolution and its propagation along the surface.

The anodic dissolution of silicon in fluoride media has given rise to much excitement in recent years [l-lo], particularly concerning the intriguing current oscillations sometimes observed in the anodic region (2.5 V < Es,, < 7 V). Several authors [1,5,10-121 have found that the current exhibits spontaneous oscillations above a critical potential = 3 Vs,,. On the contrary, other authors [8,9] have found damped oscillations, which appear only upon a perturbation of the electrode potential. This has allowed them to perform a detailed investigation of the electrochemical impedance in that regime, and to propose a model accounting for their experimental results [9,13]. The origin of the discrepancy between the various authors was somewhat of a mystery. However, the compositions of the electrolytes were different in the various groups, and, interestingly, the electrolytes where sustained oscillations had been observed are in a fluoride concentration range higher than those where the damped ones had been reported. We will show here that, irrespective of the electrolyte composition in a wide fluoride concentration and pH range, the oscillations can be made intentionally damped or sustained, depending upon the value of the uncompensated resistance in series with the interface. All our experiments have been performed on low-doped p-Si electrodes (resistivity l-4 R cm). We performed two kinds of experiments. The first experiments demonstrate that, on a given interface, with damped oscillations (“steady” interface), a sustained oscillation can be created by adding an external resistor in series with the working electrode. We have taken the

For the first time, simultaneous photocurrent, Iph, and excess microwave reflectivity, mw*, profiles are recorded during the light-induced oscillatory behaviour of n-Si(111) in aqueous ammonium fluoride solutions. Stationary oscillations in Iph and mw* are complementary to each other with maxima of Iph at minima of mw* and vice versa. At higher positive potentials, irregular oscillatory behaviour is found and the complementarity of the signals is not fully maintained. For the initial time regime a model is suggested tentatively to determine electronic interface parameters from the simultaneous measurement of Iph and mw*. For increased time, the oscillatory behaviour has also been evaluated with respect to surface recombination and charge transfer processes. In this time region, however, the application of the model suggested is problematic, as changes in band bending, trapping of charges and space charge layer recombination are not accounted for. For the observed lowest quantum yields > 0.2, it is assumed that the drift velocity is not the limiting factor.