Kinetic studies, experimental methods flow systems

There have been remarkably few reviews of the chemistry of decompositions and interactions of solids. The present account is specifically concerned with the kinetic characteristics described in the literature for the reactions of many and diverse compounds. Coverage necessarily includes references to a variety of relevant and closely related topics, such as the background theory of the subject, proposed mechanistic interpretations of observations, experimental methods with their shortcomings and errors, etc. In a survey of acceptable length, however, it is clearly impossible to explore in depth all features of all reports concerned with the reactivity and reactions of all solids. We believe that there is a need for separate and more detailed reviews of topics referred to here briefly. The value of individual publications in the field, which continue to appear in a not inconsiderable flow, would undoubtedly be enhanced by their discussion in the widest context. Systematic presentation and constructive comparisons of observations and reports, which are at present widely dispersed, would be expected to produce significant correlations and conclusions. Useful advances in the subject are just as likely to emerge in the form of generalizations discerned in the wealth of published material as from further individual studies of specific systems. Perhaps potential reviewers have been deterred by the combination of the formidable volume and the extensive dispersal of the information now available. 

The kinetics of formation and disintegration of micelles has been studied for about thirty years [106-130] mainly by means of special experimental methods, which have been proposed for investigation of fast chemical reaction in liquids [131]. Most of the experimental methods for micellar solutions study the relaxation of small perturbations of the aggregation equilibrium in the system. Small perturbations of the micellar concentration can be generated by either fast mixing of two solutions when one of them does not contain micelles (method of stopped flow [112]), or by a sudden shift of the equilibrium by instantaneous changes of the temperature (temperature jump method [108, 124, 129, 130]) or pressure (pressure jump method [1, 107, 116, 122, 126]). The shift of the equilibrium can be induced also by periodic compressions or expansions of a liquid element caused by ultrasound (methods of ultrasound spectrometry [109-111, 121, 125, 127]). All experimental techniques can be described by the term relaxation spectrometry [132] and are characterised by small deviations from equilibrium. Therefore, linearised equations can be used to describe various processes in the system. 

In another study [35], the electrochemical emission spectroscopy (electrochemical noise) was implemented at temperatures up to 390 °C. It is well known that the electrochemical systems demonstrate apparently random fluctuations in current and potential around their open-circuit values, and these current and potential noise signals contain valuable electrochemical kinetics information. The value of this technique lies in its simplicity and, therefore, it can be considered for high-temperature implementation. The approach requires no reference electrode but instead employs two identical electrodes of the metal or alloy under study. Also, in the same study electrochemical noise sensors have been shown in Ref. 35 to measure electrochemical kinetics and corrosion rates in subcritical and supercritical hydrothermal systems. Moreover, the instrument shown in Fig. 5 has been tested in flowing aqueous solutions at temperatures ranging from 150 to 390 °C and pressure of 25 M Pa. It turns out that the rate of the electrochemical reaction, in principle, can be estimated in hydrothermal systems by simultaneously measuring the coupled electrochemical noise potential and current. Although the electrochemical noise analysis has yet to be rendered quantitative, in the sense that a determination relationship between the experimentally measured noise and the rate of the electrochemical reaction has not been finally established, the results obtained thus far [35] demonstrate that this method is an effective tool for... 

The experimental techniques used to study the kinetics of OH radical reactions can be separated Into two distinct methods, namely, absolute and relative rate techniques. The absolute methods have employed discharge flow, flash photolysis, modulation-phase shift, and pulsed radlolysls systems, while a variety of differing chemical systems have been used to determine relative rate data. These techniques are briefly discussed below. 

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