Double-step kinetics

Double step chronocoulometry also gives information on the kinetics of coupled homogeneous reactions8. For example, any deviation, under diffusion control, from (10.35) and (10.36) implies a chemical complication, which can be compared with the responses for the various possible mechanisms. 

Double-step chronocoulometry is a powerful tool in identifying adsorption phenomena, in obtaining information on the kinetics of coupled homogeneous reactions and for the determination of the capacitive contribution. The double potential step is executed in such a way that after the first step from E to E2, a next step is applied, i.e., the reversal of the potential to its initial value Ei from 2 (see Fig. 3). 

Since the diffusion layer is not renewed at the beginning of each measurement cycle, it is not possible to treat each cycle in isolation, and theoretical treatments of SWV are intrinsically much more complex than for other forms of pulse voltammetry. The initial condition for each cycle is the complex diffusion layer that has evolved from all prior pulses, and it is a function, not only of the details of the waveform, but also of the kinetics and mechanisms of the chemistry linked to the electrode process. The considerations are similar to those that we encountered in Section 5.7 as we treated double-step responses, and the mathematical device of superposition can be applied to the extended step waveforms of SWV in the simpler cases. 

Marques and co-workers considered the two limiting situations, a single monomer coordination and a double monomer coordination, and their combination. " In the first case a two-step kinetic scheme is reported the coordination of the monomer... 

Kinetics of Double-Step Plasma Pyrolysis of Hydrocarbons... 

Figure 9-2. Kinetics of the double-step pyrolysis of hydrocarbons in plasma-chemical jet reactor. Step 1, methane pyrolysis inplasmajet of hydrogen Step 2, propane pyrolysis, injection of propane is delayed with respect to start of the process (1) 0.17 ms, (2) 0.37 ms, (3) 0.73 ms. Propane injection temperature 293 K propane-to-methane flow rate ratio 1 2.

Lidiiim.— Ligand exchange between Me InL and HL, where L = [CFaCOCHCOR] with R = Me or Bu, obeys kinetics that are first-order in each reactant, and is sug ted to involve a species with two unidentate diketonate ligands, the slow step being rotation of one of these about a partial double bond. Kinetic data on the formation of a complex between indium(m) and murexide lead to a somewhat different stability constant to that obtained by other methods. The authors speculate on the possibility that more than one type of complex may be involved. Several further accounts of the formation and reactions of indium(i) species have appeared. ... 

Dip 5 X 10 cm [16] and the height of a characteristic double-step 10 cm. These result in a required equilibrium rate of < 5 x 10 cm. The actual step growth velocity along dislocation-free 111 silicon planes can be estimated by the relation J st = Pi AT with Pi the kinetic step coefficient (50cm g-i j -i supercooling, which has been determined at growing... 

Gmucova, K Thurzo, L Orhcky, J. Pavlasek, J. (2002). Sensitivity Enhancement in Double-Step Voltcoulometiy as a Consequence of the Changes in Redox Kinetics on the Microelectrode Exposed to Low Frequency Sound. Electroanalysis, Vol.l4, No.l3, Quly 2002), pp. 943-948, ISSN 1521-4109 Gmucova, K. Orhcky, J. Pavlasek, J. (2004). Non-Cottrell Behaviour of the Dopamine Redox Reaction Observed on the Carbon Fibre Microelectrode by the Double-Step Voltcoulometiy. Collect Czech. Chem. Commim., Vol.69, No.2, (February 2004), pp. 419-425, ISSN 0010-0765... 

Thurzo, L Gmucova, K. Orlicky, J. Pavlasek, J. (1999). Introduction to a Kinetics-Sensitive Double-Step Voltcoulometry. Reo. Sci. Instnim, Vol.70, No.9, (September 1999), pp. 

The treatment may be made more detailed by supposing that the rate-determining step is actually from species O in the OHP (at potential relative to the solution) to species R similarly located. The effect is to make fi dependent on the value of 2 and hence on any changes in the electrical double layer. This type of analysis has permitted some detailed interpretations to be made of kinetic schemes for electrode reactions and also connects that subject to the general one of this chapter. 

Double potential steps are usefiil to investigate the kinetics of homogeneous chemical reactions following electron transfer. In this case, after the first step—raising to a potential where the reduction of O to occurs under diffrision control—the potential is stepped back after a period i, to a value where tlie reduction of O is mass-transport controlled. The two transients can then be compared and tlie kinetic infomiation obtained by lookmg at the ratio of... 

For the catalyst system WCU-CsHbAICIs-CzHsOH, Calderon et al. (3, 22, 46) also proposed a kinetic scheme in which one metal atom, as the active center, is involved. According to this scheme, which was applied by Calderon to both acyclic and cyclic alkenes, the product molecules do not leave the complex in pairs. Rather, after each transalkylidenation step an exchange step occurs, in which one coordinated double bond is exchanged for the double bond of an incoming molecule. In this model the decomposition of the complex that is formed in the transalkylidenation step is specified, whereas in the models discussed earlier it is assumed that the decom-plexation steps, or the desorption steps, are kinetically not significant. 

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