D.c. polarography

Clearly, with the apparatus described above we are dealing with direct current, and the technique as thus carried out is termed conventional d.c. polarography to distinguish it from various modifications (see later) and from the use of alternating current (a.c. polarography). 

As already indicated, quantitative conventional d.c. polarography is limited at best to solutions with electrolytes at concentrations greater than 10-5M, and two different ions can only be investigated when their half-wave potentials differ by at least 0.2 V. These limitations are largely due to the condenser current associated with the charging of each mercury drop as it forms, and various procedures have been devised to overcome this problem. These include ... 

The following experiments (Section 16.14-16.16), which can well be performed with a manual polarograph, serve to illustrate the general procedure to be followed in d.c. polarography it is advantageous to use a chart recorder to produce the polarogram. 

Fig. 5.18 Potentiostatic methods (A) single-pulse method, (B), (C) double-pulse methods (B for an electrocrystallization study and C for the study of products of electrolysis during the first pulse), (D) potential-sweep voltammetry, (E) triangular pulse voltammetry, (F) a series of pulses for electrode preparation, (G) cyclic voltammetry (the last pulse is recorded), (H) d.c. polarography (the electrode potential during the drop-time is considered constant this fact is expressed by the step function of time—actually the potential increases continuously), (I) a.c. polarography and (J) pulse polarography...

The electrochemical behavior of niclosamide was described on the basis of d.c. polarography, cyclic voltammetry, a.c. polarography, and differential pulse polar-ography, in the supported electrolytes of pH ranging from 2.0 to 12.0 [32], A tentative mechanism for the reduction of niclosamide is proposed that involves the transfer of 4 e . Parameters such as diffusion coefficients and heterogeneous forward rate constant values were evaluated. 

The addition of hydroxyde ion to nitrosobenzene produces azoxybenzene186. Three techniques (electronic absorption spectroscopy, linear sweep voltammetry and d.c. polarography) have been used to study the equilibrium between nitrosobenzene and hydroxyde ions. The probable reaction pathway to obtain azoxybenzene is indicated by Scheme 4. The importance of the nitroso group in the reduction of nitro derivatives by alkoxide ions, when the electron-transfer mechanism is operating, has been explained187. 

Equation (11) is also applicable as a good, or reasonably good, approximation to a number of techniques classified as d.c. voltammetry , in which the response to a perturbation is measured after a fixed time interval, tm. The diffusion layer thickness, 5/, will be a function of D, and tm and the nature of this function has to be deduced from the rigorous solution of the diffusion problem in combination with the appropriate initial and boundary conditions [21—23]. The best known example is d.c. polarography [11], where the d.c. current is measured at the dropping mercury electrode at a fixed time, tm, after the birth of a new drop as a function of the applied d.c. potential. The expressions for 5 pertaining to this and some other techniques are given in Table 1. 

Although normal pulse polarography was developed mainly for analytical purposes, it is a valuable and simple method to study kinetics of not-too-fast electrode reactions. As the other controlled potential techniques, it has the advantage of being applicable to systems where only one of the redox components is present initially. The technique is closely related to d.c. polarography [11] and the expressions discussed in this section are directly applicable to the case of d.c. polarography performed with the static mercury drop electrode (SMDE) if the correction for the spherical shape of this electrode is negligible [21, 22]. 

Steady-state and pseudo-steady state techniques (d.c. polarography, pulse and differential pulse polarography, a.c. polarography) are especially suitable for analytical purposes, i.e. the determination of the composition of a sample and of concentrations of single species in such a sample. It is less commonly recognized that these techniques are also indispensable for the determination of less interesting properties of the components... 

Since the invention of d.c. polarography [10, 11], numerous inorganic and organic compounds have been studied by means of this method in Heyrovsky s school and extensive knowledge gathered about the electrochemical properties of these compounds. Among them, many cases were discovered where the polarographic wave appeared to be influenced by the existence of chemical equilibria between the electroactive substance and other, in most cases electroinactive, species in the electrolyte solution, more particularly by the finite rate at which these equilibria relax after the electrochemical perturbation. In fact, the chemical reaction serves as either a source or a sink to deliver or to consume the electroactive reactants and products, in addition to diffusion. 

Potential step perturbation chrono-amperometry, pulse polaro-graphy, and d.c. polarography... 

Moreover, it is difficult to find one s way in the overwhelming amount of literature on this subject because the major part of it is focussed on d.c. polarography and thus to the mass transfer problem at the dropping mercury electrode (DME). Neglecting the sphericity, the expansion of the drop has still to be accounted for in the diffusion equation for a species i. Equation (19b), which we have adopted thus far, should therefore be replaced by [11, 147]... 

First, the rate of the establishment of equilibrium (1) must be slow compared with the drop-time. Under the experimental conditions most frequently used in d.c. polarography, where the drop-time is about 3 sec, the establishment of equilibrium (1) can be considered as slow, if the... 

Studies of the electrode reactions of aqueous [Pd(CN)4]2 by d.c. polarography have shown the presence of an irreversible two-electron step, while the osdllopolaro-gram showed three anodic waves.106 It was concluded that cyano-complexes of Pd° and Pd1 were formed, the former of which decomposed rapidly into Pd metal (amalgam) and free cyanide. 

The ability of adenine and cytosine to undergo reduction in aqueous medium is at least partially retained at the level of oligo- and polynucleotides. In both acid and neutral media, such residues in single-stranded RNA, DNA and synthetic polynucleotides, when subjected to d.c. polarography, undergo irreversible reduction in a protonated, adsorbed, state with the transfer of four electrons to adenine... 

Virtually any electrochemical technique may be used for either analytical or mechanistic (our focus) studies. The merits and limitations of each technique and the information that can be gleaned are discussed for direct-current (d.c.) polarography, pulse polarography, alternating-current (a.c.) polarography and cyclic voltammetry. Con-trolled-potential coulometry is technically not a voltammetric technique (there is no variation of potential), and this technique is considered in 12.3.5. 

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