Conventions polarography

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

In just the same way as differential pulse polarography represents a vast improvement over conventional polarography (see Section 16.10), the application of a pulsed procedure leads to the greatly improved technique of differential pulsed anodic (cathodic) stripping volammetry. A particular feature of this... 

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)... 

The difficulties in conventional polarography as mentioned in Section 3.3.1.1, especially the interference due to the charging current, have led to a series of most interesting developments by means of which these problems can be solved in various ways and to different extents. The newer methods concerned can be divided into controlled-potential techniques and controlled-current techniques. A more striking and practical division is the distinction between advanced DC polarography and AC polarography. These divisions and their further classification are illustrated in Table 3.1. In treating the different classes we have not applied a net separation between their principles, theory and practice, because these aspects are far too interrelated within each class. 

Brugmann [784] discussed different approaches to trace metal speciation (bioassays, computer modelling, analytical methods). The electrochemical techniques include conventional polarography, ASV, and potentiometry. ASV diagnosis of seawater was useful for investigating the properties of metal complexes in seawater. Differences in the lead and copper values yielded for Baltic seawater by methods based on differential pulse ASV or AAS are discussed with respect to speciation. 

The electrodes used in conventional polarography and voltammetry are electronic conductors such as metals, carbons or semiconductors. In an electrode reaction, an electron transfer occurs at the electrode/solution interface. Recently, however, it has become possible to measure both ion transfer and electron transfer at the interface between two immiscible electrolyte solutions (ITIES) by means of polarography and voltammetry [16]. Typical examples of the immiscible liquid-liquid interface are water/nitrobenzene (NB) and water/l,2-dichloroethane (DCE). 

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%. 

Conventional polarography with a dropping mercury electrode can conveniently and routinely be carried out in the dry box, although due to height limitations it may be necessary to use a lower mercury head than is possible outside the box. 

The sensitivity of this technique is excellent and its accuracy is greater than that of conventional polarography. This method has found wide application in analytical chemistry. For addnl info on this subject see Refs 6,7,8,10,12,14,16,17,18,19,20,21,22,24,25,26 28,29,30,31 32... 

The oxidation-reduction behavior, ascertained by direct current (conventional) polarography, and its dependence on pH in aqueous solution of the [P2Mo18062] 6, [As2Moi8062]-6, and [P2Wi8062]-6 anions may be found in Refs.1S9 161>. The polarographic behavior of the dimeric 9-molybdophosphate anion of (NH4)6 [P2Moi 8062] was examined by cyclic voltammetry and alternating current... 

In addition to discriminating against i, normal pulse polarography results in larger currents resulting in chemical change than found in conventional polarography. It can be shown that ... 

Because these methods are so deeply rooted in the polarographic tradition and even now are frequently used with polarographic electrodes, we begin with a discussion of phenomena at dropping mercury electrodes and then develop the subject through conventional polarography and into various forms of pulse voltammetry. 

Since the potentiostat is always active and the potential is constant during a drop s lifetime, the actual current flow at the electrode is the same as that observed in conventional polarography with a controlled drop time. The difference is that the recording system is fed only signals proportional to the sampled currents. The faradaic component of the limiting sampled current must be... 

The improvements in this method yield detection limits near 10 M, perhaps slightly lower than those of conventional polarography. Since tast measurements are only sampled-current presentations of conventional polarographic currents, all conclusions about the shapes of waves and all diagnostics developed for conventional measurements of maximum currents apply to the tast technique. 

The range of time scales for the differential pulse experiment is the same as for normal pulse voltammetry, hence a given system ordinarily shows the same degree of reversibility toward either approach. However, the degree of reversibility toward pulse methods may differ from that shown toward conventional polarography for reasons discussed in Section 7.3.2. 

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