Homogeneous chronopotentiometry

Such autocatalyzed redox reactions (effective when > E l) can, in the case of a rather fast SET, be followed by means of voltammetric techniques [abnormally large variation of (dp)/(cllogv) where v is the sweep rate] and chronopotentiometry ( unexpected maximum in E—t curves)]. It thus appears that a careful stereochemical study of an elimination process (possibly influenced by the physical and/or chemical nature of the interface) must take into account a possible concomitant occurence of a homogeneous reaction. Knowledge of the redox potential of the produced olefin may provide valuable infomation on its implications in a possible homogeneous process. 

Thus as shown on Fig. 2.21, r measurements are imprecise. In view of this, whilst investigations of both heterogeneous and homogeneous kinetics are theoretically possible using chronopotentiometry, they are not frequently undertaken and are not recommended. Further details can be obtained from the literature [28-31]. 

There are several electrochemical methods, such as cyclic voltammetry, polarography, chronoamperometiy and chronopotentiometry, which can be used to measure homogeneous reaction rates. It is beyond the scope of this text to explore all the variations and intricacies of electrochemical methods, but they are described in several sources. The purpose here is to give some basic background and some examples of the technique. 

Before attempting to measure diffusion coefficients, some basic information regarding the electrochemical behavior of the redox species must be known. This is particularly important for newly prepared compounds. First, one should evaluate the reversibility of the electron transfer reaction. Certain techniques, such as LSV, can only be applied to measure diffusion coefficients for nemstian systems. Second, the presence of any coupled homogeneous reactions should be established. The current for each technique is often dependent on such reactions, thus making measurements of the diffusion coefficient unreliable. Finally, the adsorption of reactants or products can produce faiadaic current that can greatly affect the measurement of the diffusion coefficient. For example, measuronent of the critical time in chronopotentiometry is less reliable when adsorption is present For these reasons, the electrochemical behavior of the compound must be factored into the selection of a technique. 

Polyaniline/Silver Nanocomposites He and coworkers [56] employed chronopotentiometry to prepare polyaniline/silver (PANI/Ag) nanocomposite films in water-in-IL and IL-in-water microemnlsions, by simultaneous oxidative polymerization of aniline to PANI and rednction of silver nitrate to Ag nanoparticles. The PANI/ Ag prepared in water-in-IL microemulsion was nanofibrous, and the Ag nanocrystals with 5 nm diameter were dispersed homogeneously, whereas the PANI/Ag prepared in IL-in-water microemulsion exhibited dendritic structure, and the diameter of Ag nanocrystals was 50-100 nm. The special structures of the PANI/Ag nanocomposite resulted in more excellent electrochemical activity than that of the pure PANI. 

Ruac I, Blitz D (1991) Consistency proof of the sequential algorithm for the digital simulation of systems involving first-order homogeneous kinetics. Acta Chem Scand 45 1087-1089 Feldberg SW, Auerbach C (1964) Model for current reversal chronopotentiometry with second-order kinetic complications. Anal Chem 36 505-509... 

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