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Voltage-step Transient in Redox Systems II. 1D Approximation (revisited)
Wein, Ondřej
Using the well-known Cottrell asymptote, voltage-step transient experiment could provide data on bulk concentration and diffusion coefficient of working depolarizer, essential in the liming-current techniques. However, the Cottrell asymptote is not directly applicable in an early stage of the voltage-step transient process,. There are three additional transport resistances that cannot be neglected at extremely high initial currents: Faradaic resistance at surface of working electrode, Ohmic losses in its intimate neighborhood, and the galvanometric constraint in the outer circuit (galvanometer, current follower, potentiostat). Non-linear effect of these additional transport resistances on the transient current is calculated in a 1D approximation, i.e. assuming uniform accessibility of the working electrode, for finite voltage steps.
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Edge Effects in Voltage-step Transient I. Ohmic Losses in 1D Approximation (revisited)
Wein, Ondřej
Ohmic potential drop between the working and reference electrodes plays an important role in the early stage of the voltage-step transient process. Non-linear effect of the ohmic drop on the transient currents has been calculated for large voltage steps, assuming reversible behavior (negligible surface overpotential) of a redox couple O + ne = R. This report extends an older study by Wein (1991). In particular, it adds a series of analytic approximations and gives more accurate numerical solving.
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Electrodiffusion Diagnostics of Apparent Wall Slip in Polymer Solutions
Wein, Ondřej ; Tovčigrečko, Valentin ; Večeř, M. ; Sobolík, Václav
Two substantially improved experimental methods, the apparent wall slip (AWS) rotational viscometry with a Morse-taper sensor and electrodiffusion (ED) flow diagnostics with autocalibrated friction probes, are used to study near-to-wall velocity profiles in aqueous solutions of high-molecular polysaccharides under viscometric flow conditions. By comparing the velocity data from the both methods, estimates of the depleted layer thickness in dependence on wall shear stress are obtained. The ED data support the two-layer model of AWS effect, with the constant enhanced fluidity across the depleted layer.
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Elektrodifuzní diagnostika proudění. Přechodová konvektivní difuze ke kruhovému noru při nízkých Pekletových číslech
Wein, Ondřej
Autocalibration electrodiffusion (ED) wall friction measurements at low Peclet numbers are improved both theoretically and experimentally. Existing theories are united to a simple semiempirical description of the voltage-step transient at non-ideal circular probes. The final transport model of the voltage-step transient for nearly-circular probes is implemented to the PC software EDSWork PR10 for treating the laboratory transient data. A special ED cell was designed for the voltage-step transient ED experiments under a well-controlled steady viscometric flow. The transient ED measurements with a series of aqueous glycerol solutions cover rather broad interval of the convective-diffusion regime and represent an experimental material suitable for a careful calibration of the probes and for a check of the related unsteady mass-transfer theory.
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