Current -potential polarography

The oxidation product in DCE, CQ, has a charge, and hence it may be liable to transfer to W. The transfer of CQ at the W/DCE interface was investigated by current-scan polarography. Here, CQ in DCE had been prepared by reducing CQ in DCE at a column electrode with carbon fiber working electrode [40]. The polarogram (curve 2 in Fig. 8) indicates that CQ transfers from DCE to W in the potential range more positive than -0.7 V. 

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)... 

AgCl electrode) and the cathodic current due to the reduction of hydrogen ion begins to flow at about -1.1 V. Between the two potential limits, only a small current (residual current) flows. In curve 2, there is an S-shaped step due to the reduction of Cd2+, i.e. Cd2++2e +Hg <=t Cd(Hg). In DC polarography, the current-potential curve for the electrode reaction is usually S-shaped and is called a polaro-graphic wave. 

Polarography and Voltammetry Both methods are the same in that current-potential curves are measured. According to the IUPAC recommendation, the tenn polarography is used when the indicator electrode is a liquid electrode whose surface is periodically or continuously renewed, like a dropping or streaming mercury electrode. When the indicator electrode is some other electrode, the term voltammetry is used. However, there is some confusion in the use of these terms. 

Fig. 5.14 The DC and AC polarographic circuits (a) and the current-potential curves for DC and AC polarographies (b).

If the reaction is reversible, the S-shaped current-potential curve in DC polarography is expressed by Eq. (8.1) (Section 5.3) ... 

As described in Chapter 8, current-potential curves in polarography and voltammetry are useful for obtaining mechanistic information on electrode reactions. However, for complicated electrode processes, the information obtained from the current-potential curves is not conclusive enough. In order to get more conclusive information, it is desirable to confirm the reaction products and/or intermediates by some other technique. In this chapter, we focus our discussion on such techniques. We deal with electrolytic and coulometric techniques in Section 9.1 and the combinations of electrochemical and non-electrochemical techniques in Section 9.2. 

If we measure a residual current-potential curve by adding an appropriate supporting electrolyte to the purified solvent, we can detect and determine the electroactive impurities contained in the solution. In Fig. 10.2, the peroxide fonned after the purification of HMPA was detected by polarography. Polarography and voltammetry are also used to determine the applicable potential ranges and how they are influenced by impurities (see Fig. 10.1). These methods are the most straightforward for testing solvents to be used in electrochemical measurements. 

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)... 

Kinetic parameters of dimerization can be determined by polarography, -> chronoamperometry, -> linear potential scan and -> convolution voltammetry, -> rotating disc voltammetry, and alternating current sinusoidal polarography. See also -> association. 

The current-potential relationship of the totally - irreversible electrode reaction Ox + ne - Red in the techniques mentioned above is I = IiKexp(-af)/ (1+ Kexp(-asteady-state voltammetry, a. is a - transfer coefficient, ks is -> standard rate constant, t is a drop life-time, S is a -> diffusion layer thickness, and

logarithmic analysis of this wave is also a straight line E = Eff + 2.303 x (RT/anF) logzc + 2.303 x (RT/anF) log [(fi, - I) /I -The slope of this line is 0.059/a V. It can be used for the determination of transfer coefficients, if the number of electrons is known. The half-wave potential depends on the drop life-time, or the rotation rate, or the microelectrode radius, and this relationship can be used for the determination of the standard rate constant, if the formal potential is known. 

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... 

Current-potential relationships at the dropping Hg electrode (d.c. polarography) constitute the oldest voltammetric measurements. The advantages of the dropping-Hg electrode (dme) include a clean electrode surface undergoing constant and reproducible renewal and highly developed and well-tested polarographic theory. ... 

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. 

That the shape of the curve on the rising portion of the wave remains the same as in polarography can be understood from the factors that govern the current-potential behavior in these voltammetric techniques. Throughout the voltammetric curve the cur-... 

The d.c. polarographic experiment is performed at a slow scan rate, virtually at constant potential. However, charging currents in polarography are appreciable, owing to the time derivative of Eq. (a) at constant applied voltage ... 

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