Linear potential sweep with hydrodynamic electrodes

9 Linear potential sweep with hydrodynamic electrodes 

Linear potential sweep at a hydrodynamic electrode can lead to two extreme situations  

The forms of the cyclic voltammograms have been deduced theoretically for the rotating disc1516, tubular17, and wall-jet disc18 electrodes, normally by numerical calculation. There is good agreement between theory and experiment. 

Advantages of using hydrodynamic electrodes in linear sweep voltammetry are weak dependence on the physical properties of the electrolyte, suppression of natural convection, and the possibility of obtaining values of /p and /L in only one experiment. 

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Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

Electrochemical systems can be studied with methods based on impedance measurements. These methods involve the application of a small perturbation, whereas in the methods based on linear sweep or potential step the system is perturbed far from equilibrium. This small imposed perturbation can be of applied potential, of applied current or, with hydrodynamic electrodes, of convection rate. The fact that the perturbation is small brings advantages in terms of the solution of the relevant mathematical equations, since it is possible to use limiting forms of these equations, which are normally linear (e.g. the first term in the expansion of exponentials). 

The next problem lies in the determination of interrelations between the concentrations of individual species. With this aim, the results of analysis performed in Section 3.4.2 could be applied to the present problem. They show that the image of ideally labile system seems to be acceptable, when the rate constants for LH2 or LH dissociation are sufficiently high. The lower limit of k could be estimated at 10 s (steady-state conditions) for linear potential sweep conditions, this limit is supposed to be one to two orders higher. According to the data obtained with hydrodynamic modified rotating disc electrode (RDE) [1], the k - values for formic, acetic, and propionic acids are 4.83 X10, 3.46 X10 , and 2.88 X10 s respectively, but polarographic measurements produce one order lower quantities [2]. Anyway, these acids and similar acids could be treated as sufficiently labile. 

The potential profile associated with hydrodynamic techniques usually takes the form of a linear sweep between two potentials in which the oxidation or reduction processes of interest occur. As for cyclic voltammetry, the gradient of the ramp represents the scan rate. However, for steady-state techniques, the scan rate used must be sufficiently slow to ensure that the steady state is attained at every potential during the course of the voltammetric scan. The upper value of the scan rate that may be used under the steady-state regime is therefore restricted by the rate of convective mass transport of material to the electrode surface. The faster the rate of convective mass transport the faster the scan rate that may be used consistent with the existence of steady-state conditions. 

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