Sweep diffusion process

A typical cyclic voltammogram is shown in Fig. 2 for a polypyrrole film. The polypyrrole film was electrochemically grown on a 0.5-cm platinum electrode in a solution of 0.1 M tetraethylammonium tetrafluoroborate in acetonitrile [46]. The oxidation wave in the anodic sweep produced a reduction wave on the reverse cathodic sweep. Different diffusion processes involved most likely account for the different shapes of the oxidation and reduction waves. For example, when lithium perchlorate is the electrolytic salt, perchlorate anions diffuse into the polymer upon oxidation. However, upon reduction the more mobile lithium cations diffuse in... 

In this section we use the well established theory of simple electron processes to re-emphasise the advantages obtained by implementing convolution techniques as opposed to the more conventional linear sweep diffusion controlled chronoamperometric criteria based directly on the current. The brief review also serves to stress the feature that such expressions obtained need not depend on any particular experimental voltammetric technique indeed one can combine data from say cyclic chronoamperometric experiments as a composite test of adherence to the proposed mechanistic scheme. 

Recall from Chapter 3 that the open-circuit potential is the corrosion potential Ectyrr) in the absence of an externally applied potential and the corrosion current density icorr) cannot be measured or determined in this case. However, electrochemical techniques can be used for determining icorr from a polarization curve generated by sweeping the WE surface with an applied potential and for measuring the response current or current density. This means that icorr cannot be measured but graphically estimated. Also, polarization is a measure of the overpotential and the electrochemical process on the WE is basically a combination of kinetics and diffusion processes. 

Mass difihsion in a ternary s em In the mass difflision or sweep diffusion separation process for isotopes 1 and 2 (Benedict et al., 1981, chap. 14), an inert vapor, e.g. steam, difflises radially outward while the mixture of two isotopes difflises against the vapor. The two isotopes should have different rates of diffusion due to different diffusion coefficients (as well as different concentration differences). We use Maxwell-Stefan formalism to obtain the governing equations (subscript g not used here). 

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. 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

The transient method characterized by linearly changing potential with time is called potential-sweep (potential-scan) voltammetry (cf. Section 5.5.2). In this case the transport process is described by equations of linear diffusion with the potential function... 

Thus, cyclic or linear sweep voltammetry can be used to indicate whether a reaction occurs, at what potential and may indicate, for reversible processes, the number of electrons taking part overall. In addition, for an irreversible reaction, the kinetic parameters na and (i can be obtained. However, LSV and CV are dynamic techniques and cannot give any information about the kinetics of a typical static electrochemical reaction at a given potential. This is possible in chronoamperometry and chronocoulometry over short periods by applying the Butler Volmer equations, i.e. while the reaction is still under diffusion control. However, after a very short time such factors as thermal... 

The first cathodic wave was studied by cycling the potential across it at various scan rates and the peak potentials were found to increase as indicative of a reversible, diffusion-controlled system, with ° = — 1.43 V vs. SCE. However, at sweep rates 20mV/s the peak anodic current is much smaller than expected which was interpreted by the authors as indicating that the reduced species undergoes a subsequent chemical reaction, i.e. an EC process. 

For the investigation of charge tranfer processes, one has the whole arsenal of techniques commonly used at one s disposal. As long as transport limitations do not play a role, cyclic voltammetry or potentiodynamic sweeps can be used. Otherwise, impedance techniques or pulse measurements can be employed. For a mass transport limitation of the reacting species from the electrolyte, the diffusion is usually not uniform and does not follow the common assumptions made in the analysis of current or potential transients. Experimental results referring to charge distribution and charge transfer reactions at the electrode-electrolyte interface will be discussed later. 

Baranski and Lu [209] have carried out, applying microelectrodes, voltammetric studies on ammonium amalgam in propylene carbonate solutions at room temperatures. The sweep rates up to 80 V s were appropriate for the analysis of the formation kinetics of this compound. Experimental and numerical simulation results have shown that ammonium amalgam was formed via fast charge-transfer process and its first-order decomposition was characterized by the rate constant of about 0.6 s . Diffusion coefficient of NH4 radical in mercury was estimated to be about 1.8 X 10 cm s k The formal potential of NH4+ (aq)/NH4(Hg) couple was determined as—1.723 V (SHE). 

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