Plateau current

With a well-defined polarographic wave where the limiting current plateau is parallel to the residual current curve, the measurement of the diffusion current is relatively simple. In the exact procedure, illustrated in Fig. 16.6(a), the actual... 

It is simpler, though less exact, to apply the extrapolation method. The part of the residual current curve preceding the initial rise of the wave is extrapolated a line parallel to it is drawn through the diffusion current plateau as shown in Fig. 16.6(h). For succeeding waves, the diffusion current plateau of the preceding wave is used as a pseudo-residual current curve. 

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

The method permits the simultaneous determination of reaction order, m, and reaction rate constant, k, from the slope and the intercept of the straight line. The procedure can be repeated for various potential values below the limiting current plateau to yield k as a function of electrode potential. The exchange current density and the Tafel slope of the electrode reaction can be then evaluated from the k vs. potential curves. 

This simple experiment illustrates the basic features of the limiting-current method. A particular electrode reaction proceeds at the highest possible rate, indicated by a current plateau. From the limiting current thus recorded the mass-transfer rate and the mass-transfer coefficient at the electrode in question may be determined. 

The ohmic contribution to the overpotential can be minimized by suitable placement of the reference electrode, but the surface overpotential cannot be reduced similarly. In making limiting-current measurements, the surface overpotential, or rather its rate of increase with current density, should be low enough to permit observation of a long, clearly defined limiting-current plateau. 

Therefore, criteria in the selection of an electrode reaction for mass-transfer studies are (1) sufficient difference between the standard electrode potential of the reaction that serves as a source or sink for mass transport and that of the succeeding reaction (e.g., hydrogen evolution following copper deposition in acidified solution), and (2) a sufficiently low surface overpotential and rate of increase of surface overpotential with current density, so that, as the current is increased, the potential will not reach the level required by the succeeding electrode process (e.g., H2 evolution) before the development of the limiting-current plateau is complete. 

Figure 3a is an illustration of the effect of surface overpotential on the limiting-current plateau, in the case of copper deposition from an acidified solution at a rotating-disk electrode. The solid curves are calculated limiting currents for various values of the exchange current density, expressed as ratios to the limiting-current density. Here the surface overpotential is related to the current density by the Erdey Gruz-Volmer-Butler equation (V4) ... 

In limiting-current measurements, the counterelectrode is sometimes used as a reference electrode. In that case, the surface overpotential of the counterelectrode contributes to the recorded overpotential that is, the potential of the reference electrode is now current dependent. Unless precautions are taken (e.g., the area of the counterelectrode is much larger than that of the working electrode), a properly defined limiting-current plateau may not be obtained. 

In principle, the accuracy with which mass-transfer rates may be measured is limited by the precision with which the limiting-current plateau or inflection point can be read. Furthermore, the electrode area, the current... 

Operation at 2 would detect X, operation at 3 would detect both X and Y. Operation at 4 would detect both X and Y, but at this potential the solvent or background electrolyte is oxidised as well. At best, there would be a large background current it might be impossible to get the recorder on scale. At j, X would be detected, but the sensitivity would be low. It would be much better to work at a potential on the limiting current plateau, such as . 

Polarographic waves often show a peak followed by a sharp fall to the limiting current plateau, the cause of which is related to streaming of the solution past the mercury drop. Known as a current maximum, it can be eliminated by adding a surfactant such as gelatin or methyl-red to the sample solution. 

A dead-stop titration curve is produced if Ag+ is titrated with a halide using a pair of identical silver electrodes. Only whilst both Ag+ and Ag are present will a current flow in the cell, and this is linearly related to the Ag+ concentration. Bi-amperometric titrations require only simple equipment but generally give poorer precision because the currents measured are not necessarily on the limiting current plateau. 

We saw above that the polarographic current rises from zero to a current plateau. The plateau may be horizontal, or it might be gently sloping upwards we called this rise a residual current. Occasionally, there is also a current peak superimposed on the wave (see Figure 6.32). Such peaks are of two types, i.e. maxima of the first kind and maxima of the second kind. Both are caused by enhanced rates of mass transport at the Hg solution interface, as described in the following. 

In voltammetry, the electrode is a solid conductor. The surface of the electrode is not refreshed constantly as it is for a DME, so voltammograms do not have a sawtoothed shape, but are smooth. Rather than a current plateau. Id, voltammograms contain a peak current. Ip, with the magnitude of the peak being directly proportional to the bulk concentration of analyte, according to the Randles-Sev5ik equation (equation (6.13)). 

Two cathodic processes can also be observed in the voltammogram. One is characterized by a current plateau between -600 and -750mV (SCE) and the other by a sharp increase of the current in the cathodic direction at potentials more negative than -850mV (SCE). The former process is expected to be oxygen reduction ... 

A major fallacy is made when observations obeying a known physical law are subjected to trend-oriented tests, but without allowing for a specific behaviour predicted by the law in certain sub-domains of the observation set. This can be seen in Table 11 where a partial set of classical cathode polarization data has been reconstructed from a current versus total polarization graph [28], If all data pairs were equally treated, rank distribution analysis would lead to an erroneous conclusion, inasmuch as the (admittedly short) limiting-current plateau for cupric ion discharge, albeit included in the data, would be ignored. Along this plateau, the independence of current from polarization potential follows directly from the theory of natural convection at a flat plate, with ample empirical support from electrochemical mass transport experiments. 

Conclusion drawn from physical considerations The steadily rising current density reaches a limiting current plateau in the 483 - 700 mV polarization range, prior to a further increase at higher polarization causing proton discharge at the cathode. 

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