Homogeneous kinetic measurements processes involving

Most SECM measurements involve steady-state current measurements. This can be a significant advantage in the measurement of kinetics, even for rapid processes, because factors like double-layer charging and adsorption do not contribute to the observed currents. However, one can also carry out transient measurements, recording iT as a function of time. This can be of use in measurements of homogeneous kinetics (Chapter 7) and for systems that are changing with time. It can also be used to determine the diffusion coefficient, D, of a species without knowledge of the solution concentration or number of electrons transferred in the electrode reaction (23). 

Since the 1960s, cyclic voltanunetry has been the most widely used technique for studies of electrode processes with coupled chentical reactions. The theory was developed for numerous mechanisms involving different combinations of reversible, quasi-reversible, and irreversible heterogeneous ET and homogeneous steps. Because of space limitations, we will only consider two well-studied examples—(i.e., first-order reversible reaction preceding reversible ET) and E Ci (i.e., reversible ET followed by a first-order irreversible reaction)—to illnstrate general principles of the coupled kinetics measurement. A detailed discussion of other mechanisms can be found in Chapter 12 of reference (1) and references cited therein, including a seminal publication by Nicholson and Shain (19). 

It is clear that the experimental curves, measured for solid-state reactions under thermoanalytical study, cannot be perfectly tied with the conventionally derived kinetic model functions (cf. previous table lO.I.), thus making impossible the full specification of any real process due to the complexity involved. The resultant description based on the so-called apparent kinetic parameters, deviates from the true portrayal and the associated true kinetic values, which is also a trivial mathematical consequence of the straight application of basic kinetic equation. Therefore, it was found useful to introduce a kind of pervasive des-cription by means of a simple empirical function, h(a), containing the smallest possible number of constant. It provides some flexibility, sufficient to match mathematically the real course of a process as closely as possible. In such case, the kinetic model of a heterogeneous reaction is assumed as a distorted case of a simpler (ideal) instance of homogeneous kinetic prototype f(a) (1-a)" [3,523,524]. It is mathematically treated by the introduction of a multiplying function a(a), i.e., h(a) =f(a) a(a), for which we coined the term [523] accommodation function and which is accountable for a certain defect state (imperfection, nonideality, error in the same sense as was treated the role of interface, e.g., during the new phase formation). 

The first kinetic investigation [1], see Fluor Erg.-Bd. 1, 1959, p. 229, showed that OFg decomposes in a homogeneous monomolecular reaction between 523 and 543 K at some hundred Torr. The measured rate constant k was assumed to correspond to the activation process in reaction (1), see below (formation of OF2, followed by decomposition). The constant A=10 L mor s" found, is regarded to be a factor of six too high for a monomolecular reaction. On this basis it was supposed that the activation energy Ea = 39 was too high (this was not confirmed later, see below) or that chain reactions were involved [2]. The reaction is discussed in light of the theory of monomolecular reactions (see, e.g. [3, 4]). 

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