Limiting current, polarography

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

Fig. 3.13. Reductive polarography of Cd2+, (at) undamped waves, (a2) limited current vs. Cd2 + concentration, (b) undamped waves in presence of Zn2+.

Residual current in polarography. In the pragmatic treatment of the theory of electrolysis (Section 3.1) we have explained the occurrence of a residual current on the basis of back-diffusion of the electrolysis product obtained. In conventional polarography the wave shows clearly the phenomenon of a residual current by a slow rise of the curve before the decomposition potential as well as beyond the potential where the limiting current has been reached. In order to establish the value one generally corrects the total current measured for the current of the blank solution in the manner illustrated in Fig. 3.16 (vertical distance between the two parallel lines CD and AB). However, this is an unreliable procedure especially in polarography because, apart from the troublesome saw-tooth character of the i versus E curve, the residual current exists not only with a faradaic part, which is caused by reduction (or oxidation)... 

Holub, K. and van Leeuwen, H. P. (1984). Influence of reactant adsorption on limiting currents in normal pulse polarography. 2. Theory for the stationary, spherical electrode, J. Electroanal. Chem., 162, 55-65. 

A perfect prototype of an ideally cation-permselective interface is a cathode upon which the cations of a dissolved salt are reduced. Experimental polarization curves measured on metal electrodes fit the theory very closely. Since in dimensional units the limiting current is proportional to the bulk concentration, the polarization measurements on electrodes may serve for determining the former. This is the essence of the electrochemical analytical method named polarography. (For the theory of polarographical methods see [28]—[30].)... 

The limiting current is proportional to the concentration of the electroactive species, whereas the half-wave potential is specific to the electroactive species, being close to the standard potential of the electrode reaction. Thus, by measuring polarographic waves, we can run qualitative and quantitative analyses. In DC polarography, many inorganic and organic substances (ions, complexes and mole-... 

Detailed expositions of limiting current deviations may be found in standard polarography texts, e.g. refs. 5 and 6. Here, we try to pick out the more salient points. 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

The first voltammetric methods met are stationary voltammetries performed on a dropping mercury electrode (polarography) or on a solid rotating disk electrode. The limiting current measured is directly proportional to the concentration of the electroactive species in the solution. Experimental potential scan rate is lower than lOrnVs-1. 

The shape of the current-voltage curves recorded by this technique resembles those obtained by DC polarography. The currents reach a limiting value that is a linear function of concentration of the electroactive species The limiting currents are additive and the half-wave potentials characterize the electroactive species qualitatively. The shape of I-E curves is still affected by the charging current and limiting current can be measured for electrochemically reversible systems to somewhat lower concentrations, up to about 1 x 10-7 mol L-1. 

Of the potential sweep methods, polarography at the dropping mercury electrode occupies a special position in that the early work using this technique provided the foundations upon which most of the modern techniques are based. The method involves low potential sweep rates (less than 10 mV s ) and the recording of current-potential curves. The most pertinent data features are the limiting currents (ii) and the half-wave potential (E ) designated in Fig. 1. While the use of polarography has diminished in recent years due to... 

Although measurements of the diffusion-limited current in dc polarography and variations of this technique provide a variety of means to measure the concentration of reducible species in solution, it is also possible to use the dropping mercury electrode to obtain kinetic... 

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