Polarographic maximum

Polarographic maxima. Current-voltage curves obtained with the dropping mercury cathode frequently exhibit pronounced maxima, which are reproducible and which can be usually eliminated by the addition of certain appropriate maximum suppressors . These maxima vary in shape from sharp peaks to rounded humps, which gradually decrease to the normal diffusion-current curve as the applied voltage is increased. A typical example is shown in Fig. 16.3. Curve A is that for copper ions in 0.1 M potassium hydrogencitrate solution, and curve B is the same polarogram in the presence of 0.005 per cent acid fuchsine solution. 

Polarograms are sometimes distorted by polarographic maxima, where the current in individual segments of the I vs. E curves is much higher (several times) than the limiting diffusion current. A number of reasons exist for the development of these maxima. 

There is, as yet, no complete theory to explain the phenomenon of polarographic maxima which is manifested by the observed current overshooting the limiting current. The causes are convective mass transport in the solution and adsorption. Three types of maximum have been identified [61] ... 

Studies have also been conducted into the current distribution at tubular electrodes [94]. At the DME, it is non-uniformity of current and potential distributions, during drop growth, which is one of the causes of polarographic maxima [Sect. 2.3.5(d)]. 

Deviation of a capillary from a vertical position changes its drop time considerably. The horizontally positioned DME was originally used by Smoler [9]. Shielding and depletion effects are diminished to some extent, and current oscillations are much smaller. Probably the best performance is observed when the electrode position deviates 45° from the vertical however, opinions about such electrodes vary [10]. In some experiments, electrodes with small mercury flow rates are helpful. For these electrodes, the probability of the appearance of polarographic maxima is diminished. When m is very low, the drop time is usually very long. In such cases, artificial regulation of the drop time should be used (see Sec. II.F). 

Schwabe and Berg (I67) have studied the polaro-graphy of terpene derivatives. The solvents were 80% ethanol, 80% dioxane, 80% acetone or water and the supporting electrolytes were 0.1 M LiCl, LiOH, Et NI, or EthHOH. Camphor was found to depress the polarographic maxima and this could be used for its determination. 

The Frumkin epoch in electrochemistry [i-iii] commemorates the interplay of electrochemical kinetics and equilibrium interfacial phenomena. The most famous findings are the - Frumkin adsorption isotherm (1925) Frumkin s slow discharge theory (1933, see also - Frumkin correction), the rotating ring disk electrode (1959), and various aspects of surface thermodynamics related to the notion of the point of zero charge. His contributions to the theory of polarographic maxima, kinetics of multi-step electrode reactions, and corrosion science are also well-known. An important feature of the Frumkin school was the development of numerous original experimental techniques for certain problems. The Frumkin school also pioneered the experimental style of ultra-pure conditions in electrochemical experiments [i]. A list of publications of Frumkin until 1965 is available in [iv], and later publications are listed in [ii]. 

The half-wave potentials are dependent on substrate concentration as well as on the drop time of the mercury electrode and the height of the mercury column, and frequently the current (i) versus potential ( ) curves exhibit large polarographic maxima due to adsorption processes. These effects can be minimized by addition of maximum suppressors like gelatine or Triton X-100. A consequence of this non-ideal polarographic behaviour is that results obtained under even slightly different conditions only can be compared in a semi-quantitative way. 

Figure 3.5. Various types of observed polarographic maxima, showing actual currents solid lines) and diffusion currents dashed lines). A ... 

Kalvoda has reviewed different electrochemical methods suppression of polarographic maxima, eletrocapillary measurements, Kalousek commutator technique, differential pulse polarography and tensammetric methods for the determination of oily substances in various types of waters. These compounds are neither reducible nor oxidizable but are surface active. The study of oil fractions has shown that the Kalousek technique responds only to higher levels of dissolved petroleum fractions (0.02 to 100 mg/1) found only in pollution sources such as harbors and refinery effluents.Recent results indicate that the water pollution with petroleum can be determined by differential pulse tensammetry. Good calibration graphs were obtained with Diesel oil at a concentration from 30-50 /xg to 0.5 mg/1 and with Saratov petroleum from 0.1-0.2 to 3.3 mg/1. ... 

Polarographic maxima appear as a rule in solutions containing a depolarizer in concentrations higher than If the solution itself con-... 

Other substances found in biological material also suppress polarographic maxima, further diminishing the applicability of this method. Such substances are, for example, adrenaline (epinephrine), thiamine. 

Earlier examinations of cerebrospinal fluid were performed on the basis of suppression of polarographic maxima (214). The estimation of total protein in 0.1 ml of cerebrospinal fluid, calculated from the percentile depression of the maximum, is in fact an estimation of surface-active substances although protein is much active in this respect, the method represents no specific determination of protein. The same author (214) and also further authors (215) also made use of the pre-sodium wave and of the protein wave in buffered solutions of Co salts, but here also no parallelity was found between the wave height and the protein content, since the number of polarographically active groups present was variable. 

Lymph, transudales, exudates, and aqueous humor, with respect to their extremely low protein content, were examined by the method of suppressing polarographic maxima. 

Polarographic maxima may be eliminated by addition to the solution of tensioactive substances. At present, Triton X-100 is the most widely used compound owing to the higher stability of its solutions with time, together with the fact that since it is a nonionic tensioactive agent, it is, thus, absorbed within a wide range of potentials and is unlikely to chemically interact with other compounds of the solution. 

The use of the static mercury electrode enhances the elimination of polarographic maxima, since measurements are carried out with the mercury not flowing. 

Figure 3 Polarographic maxima A, positive first kind maxima B, negative first kind maxima C, second kind maxima D, third kind maxima.

Adsorption Currents. In general, adsorption currents appear in a polarographic current-potential curve in one of the two following ways (i) as one of two kinds of polarographic maxima from which information can be obtained regarding the adsorption of surface active molecules at a dropping mercury electrode, or (ii) due to adsorption of the electroactive species or the product of the electrode process. 

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