Diffusion controlled currents step methods

Mishra and Gode developed a direct current polarographic method for the quantitative determination of niclosamide in tablets using individually three different buffer systems, namely Mcllraine s buffers (pH 2.20 8.00), borate buffers (pH 7.80—10.00), and Britton Robinson s buffers (pH 2.00—12.00) as well as 0.2 M sodium hydroxide. The drug was extracted from the sample with methanol, appropriate buffer was added to an aliquot and the solution then polarographed at the dropping-mercury electrode versus saturated calomel electrode at 25°C [36], The resultant two-step reduction waves observed were irreversible and diffusion-controlled. For the quantitative determination, the method of standard addition was used. Niclosamide can be determined up to a level of 5—10 pg/mL. 

Laplace transformation, 1215 Nemst s equation and. 1217 non-steady, 1254 as rate determining step, 1261 Schlieren method, 1235 semi-infinite linear, 1216, 1234, 1255 in solution and electrodeposition, 1335 spherical. 1216. 1239 time dependence of current under, 1224 Diffusion control, 1248... 

However, when one gets down to detailed quantitative equations to represent real, actual reactions with several steps in consecutive sequence, the mathematics become very complex. Thus, the change in the limiting current with time introduces complications that one tries to avoid in other transient methods by working at low times (constant current or constant potential approaches) or at times sufficiently high that the current becomes entirely diffusion controlled. However, taking into account the... 

The important concept in these dynamic electrochemical methods is diffusion-controlled oxidation or reduction. Consider a planar electrode that is immersed in a quiescent solution containing O as the only electroactive species. This situation is illustrated in Figure 3.1 A, where the vertical axis represents concentration and the horizontal axis represents distance from the electrodesolution interface. This interface or boundary between electrode and solution is indicated by the vertical line. The dashed line is the initial concentration of O, which is homogeneous in the solution the initial concentration of R is zero. The excitation function that is impressed across the electrode-solution interface consists of a potential step from an initial value E , at which there is no current due to a redox process, to a second potential Es, as shown in Figure 3.2. The value of this second potential is such that essentially all of O at the electrode surface is instantly reduced to R as in the generalized system of Reaction 3.1 ... 

A single DBP droplet is positioned in the vicinity of the microelectrode by the laser trapping technique, and the droplet-microelectrode (edge-to-edge) distance (L) is controlled arbitrarily in micrometer dimension. Knowing the oxidation potential of PPD in the water phase to be 30 mV, PPD is oxidized by a potential step method (100 mV) to induce the dye formation reaction. The anodic current relevant to oxidation of PPD reaches a steady-state value within a short electrolytic time (t) because of cylindrical diffusion of PPD to the microelectrode. The dye formation in the droplet can be easily confirmed by the color change from transparent to cyan or yellow. The dye formation reaction in a single microdroplet could be... 

Double-potential step chronoamperometry This method was proposed in 1965 by Schwarz and Shain [18] for the investigation of follow-up reactions especially for the mechanism. During the first potential pulse the product B is produced at a stationary electrode under diffusion-controlled conditions for a timed interval tp. During this interval substance B diffuses into the solution and simultaneously undergoes a chemical reaction. Then, the potential is suddenly switched to a value where B is converted back into A. The backward current indicates the amount of B which has not reacted and can be related to the rate constant kf. The forward current-time dependence is given by the Cottrell equation... 

The Cottrell equation is derived from Pick s second law of diffusion (Section 1.5) and predicts the variation of the current in time, when a potential step is applied under conditions of large overpotential. For this equation to be valid the current must be limited by diffusion of the analyte to the electrode surface, and thus the solution has to be unstirred. The overpotential at which the reaction is driven must be large enough to ensure the rapid depletion of the electroactive species (O) at the electrode surface, such that the process would be controlled by the diffusion to the electrode. This equation is most often applied to potential step methods (e.g., chronoamperometry see Chapter 11) ... 

Applying to the electrolysis cell a constant current step is technically the easiest method of operation. The electrode to be investigated may be inert and at equilibrium with a redox reaction + e Red. An external anodic current will oxidize the Red components immediately after its onset and their concentrations will decrease at the interface, while that of the Ox" components will increase in parallel. For pare diffusion control of the transport. 

---