Polarography electroactive species

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... 

DC Polarography - Methods that Electrolyze Electroactive Species Only Partially (1) I 117... 

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-... 

Residual Current A small current that flows in the solution free of electroactive species (see curve 1 in Fig. 5.10). The residual current in DC polarography is mainly the charging current, which is for charging the double layer on the surface of the DME (Fig. 5.13).6)... 

Tolerable Amount of the Substance Electrolyzed Before the Analyte(s) In DC polarography, several electroactive species can be determined simultaneously, because they give step-wise waves. However, if a substance that is easier to reduce or to oxide than the analyte(s) exists in high concentrations (50 times or more), detennination of the analyte(s) is difficult. This problem is overcome by obtaining the derivative curve of the DC polarographic wave (Section 5.4). 

Here, A is the electrode area, C and D are the concentration and the diffusion coefficient of the electroactive species, AE and co(=2nfj are the amplitude and the angular frequency of the AC applied voltage, t is the time, and j=nF (Edc-Ei/2) / RT. For reversible processes, the AC polarographic wave has a symmetrical bell shape and corresponds to the derivative curve of the DC polarographic wave (Fig. 5.14(b)). The peak current ip, expressed by Eq. (5.24), is proportional to the concentration of electroactive species and the peak potential is almost equal to the half-wave potential in DC polarography ... 

As Equation 3.28 shows, the diffusion current id is directly proportional to the concentration of the electroactive species. Consequently, polarography has been used extensively for the analysis of solutions containing electroactive materials. The optimum concentration range for determinations by conventional polarography as described here is lO -lO-4 M. Reproducibility is generally 3%. 

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. 

Current-Sampled Polarography. As follows from the Ilkovic equation [Eq. (3.10)], the current at the electrode that results from the electrolysis of an electroactive species (Faradaic current) increases proportionally to tU6 during the life of the mercury drop. However, the electrical double layer is a capacitor that must be charged as potential is applied via a charging current (icc)... 

In -> polarography sometimes faradaic currents are observed which cannot be attributed to diffusion-limited reduction of electroactive species under investigation. Sometimes substances (which are not necessarily electroactive themselves) lower the hydrogen overpotential of the mercury electrode in various ways (by adsorption, by acting as a redox mediator), thus a hydrogen evolution current (a catalytic hydrogen wave) is observed [ii—iv]. See also - Mairanovskii. 

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. 

Electrochemical methods are sensitive to the extent that it is possible to detect a trace of electroactive species in electrolyte solutions. Because of this distinctive feature, electrochemical methods have been developed and utilized for analytical purposes. The detection method used is known as polarography. For the electrochemical study purification of the electrolyte solutions is therefore important. As for most aqueous and organic electrolyte solutions, there are various well-established techniques for purifying both solvents and electrolytes. In the case of room-temperature ionic liquids, it is especially important to purify the starting materials used for preparing the ionic liquids. 

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