Polarographic current-potential wave

The polarographic current-potential wave illustrated by Figure 3,3 conforms to the Nemst equation for reversible electrochemical processes. However, it is more convenient to express the concentrations at the electrode surface in terms of the current i and the diffusion current jd. Because id is directly proportional to the concentration of the electroactive species in the bulk and i at any point on the curve is proportional to the amount of material produced by the electrolysis reaction, these quantities can be directly related to the concentration of the species at the electrode surface. For a generic reduction process [Eq. (3.1)] the potential of the electrode is given by the Nemst equation ... 

Hence, the equation of -function type (2.10) describes the polarographic current-potential wave of a many-electron electrochemical process with depolarization. For example, it was established [2] that this kind of equation provides good approximation for the electrochemical wave of reduction of Si(IV) on a platinum electrode in KCl-KF-K2SiF6 melt (Fig. 2.6). 

Figure 14.3.15 Polarographic current-potential curves showing (a) prewave and b) postwave. Curves 1, 2 only adsorption wave observed. Curve 3 current attains...

Polarographic reduction studies of aromatic hydrocarbons [Wawzonek ft at, (18-21), Hoijtink et ed. (22-27) and potentiometric titration experi-naents (2S) proved that besides mononegative ions, dinegative ions of aromatic hydrocarbons can also be formed. The polarographic current-potential curves showed in many instances two one-electron waves, indicating that the two following reactions occurred ... 

The polarographic wave of TBA or TPenA is observed at potentials where the polarographic current (curve 1) due to the eleetron transfer appears, whieh suggests that the coupling of the transfer of TBA" " or TPenA with the electron transfer occurs when W containing NADH and TBA or TPenA is shaken with DCE containing CQ [44]. Since... 

Fig.7.99. The polarographic wave is the current-potential dependence in which, at some sufficiently high potential values, the current becomes entirely diffusion controlled.

The Half-Wave Potential.—In the preceding description of the analytical applications of the dropping mercury cathode it has been supposed that the nature of the reducible substance has been determined and that the position of the corresponding wave on the current-potential curv c is known. If the substance has not been previously identified, however, it is possible to do so by means of the polarographic curve. The reducible material is characterized by its half-wave potential this is the potential... 

Direct-current (d.c.) polarography is approximately a constant-potential experiment in which the current passed during the lifetime of a single Hg drop (ca. 1-7 s) is measured at a succession of potentials. The potential applied to the dme is scanned slowly (1-5 mV s ), and the resulting current-potential profile is recorded. Figure 1 shows two polarographic scans, one on the residual electrolyte (0.1 M [n-Bu N][PF ] in dimethoxyethane) and one to which cobaltocinium hexafluorophosphate (5 X 10 M) is added. The half-wave potential, E, and the limiting current, i, are derived from this S-shaped curve. 

Polarographic data (half-wave potentials, diffusion current constants, etc.) on several oxazole derivatives have been obtained by Bezuglyi... 

You have already learnt that the height of a polarographic wave can be limited or controlled by the rate at which the analyte diffuses to the electrode. In this Part you will learn how to identify other types of polarographic waves and understand their role. These new types of polarographic current should generally be seen as potential interferences to be identified and avoided. 

Voltammetric analysis at solid electrodes usually involves slower scan rates, typically 5-10 minutes per volt. In addition the solution is often stirred. Both of these factors lessen the depletion effect and the peak in the.current potential curve is much less pronounced and the signal is more wave like (Fig. 2.7c). The precise shape depends on the scan and stirring rates. With a fast enough stirring rate the conventional dc polarographic wave is attained. 

The resultant current/potential curve is similar in form to the classical dc polarographic curve with equivalent halfwave potential and limiting current (Fig. 3.3b). In practice the curve is made up of very short flat segments and is of a clearer, cleaner , less noisy form. The height of the pulse polarographic wave, the analytical signal, is... 

However, only a few organic compounds behave in a polarographically reversible manner although many may involve a reversible electron transfer step, this is often followed by irreversible chemical reactions. Irreversible processes are those for which the current is limited mainly by the kinetics of the process at the electrode surface and not by diffusion. The nature of such current-potential curves can be described by reference to Figure 6. If electrochemical equilibrium obtains at the electrode surface, then a reversible wave is obtained (curve a). The irreversible wave (curve b) is more drawn out, i.e., for a given current, say, /i or I2, a higher cathodic potential is required. 

Comparison of polarographic curves at the dme with the current-potential curves obtained at a stirred mercury pool is Important with regard to the identification of products of electrolysis. In most cases, the number, the shape, and the half-wave potentials of the waves are Identical, e.g., as in the behavior of p-diacetylbenzene referred to by Zuman. However, cases have been reported in which differences are observed and in some there is no resemblance at all between the results obtained at a dropping mercury electrode and at a stationary mercury pool. This may arise because extents of adsorption of the reactant are different, since studies at the dme are usually conducted with solutions of low concentration, 10 -10 M, while preparative experiments at a pool electrode are conducted at much higher concentrations. 

Fig. 30. Potential-time profiles (A) of the alternating currents, (B) of the square-wave polarographic techniques, both on a negative ramp. Parameters the drop time, A and dEsw amplitudes of the potential waves. The relation z)Esw = dEAc holds.

This situation arises when the rate of removal A by the forward electrode reaction is compensated by the rate of its generation by the chemical step. In the pure kinetic zone (Fig. 15) when A becomes large the LSV response looks similar to a polarographic wave of nernstian shape. The current-potential dependence resembles that of the reversible reduction wave... 

The current-potential relationship for irreversible polarographic waves is more complex than that for the reversible situation. Instead of Eq. (4.20), one can use the approximate expression... 

---