A. 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). 

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...

A modification of faradaic impedance measurement is a.c. polarography, where a small a.c. voltage is superimposed on the voltage polarizing the dropping mercury electrode (Fig. 5.181). 

M. Saxena, Studies on electrode processes using a.c. polarography, faradaic rectification and transitional potential decay techniques, Ph.D. thesis, Bhopal University, 1978. 

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. 

Accurate speciation of tributylstannyl ions in the presence of other degradation products was carried out by a.c. polarography, directly on the organic extract, without derivatization101. The degradation of tributyltins in aqueous solution was studied by differential pulse polarography102. 

Boberski and Allred reported that oxidation potentials of permethylpoly-silanes determined by a.c. polarography decrease with increasing chain length, and that the oxidation potentials are correlated almost linearly with the energies of the HOMO as determined by MO calculations (Table 7) [63]. 

W.E. Geiger and M.D. Hawley, Cyclic voltammetry, A.C. Polarography and Related Techniques. Physical Methods of Chemistry. Electrochemical Methods. A. Weissberger and B.W. Rossiter eds, Vol. 2., Chapter 1. Wiley Interscience, New York, 1986. 

A.c. polarography Second harmonic a.c. polarography Cyclic voltammetry... 

Thus, the contributions of several orders can be accessed experimentally by means of a detection device tuned to the proper frequency. Applications of this principle have thus far been limited to the so-called second-order techniques second harmonic a.c. polarography [25], faradaic rectification [26], and the recently developed demodulation technique [27],... 

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... 

Generally, sensitivity in the analytical sense is greater if the technique employed is faster, i.e. the electrolysis time is shorter, or the frequency of a periodic electrolysis is higher. Resolution of half-wave potentials, and thus accuracy of standard potentials and stability constants, is better if a derivative technique such as differential pulse polarography, a.c. polar-ography, and, preferably, the second derivative technique second-harmonic a.c. polarography, is employed. 

Theories and discussions on this case have been published with consideration of large-amplitude potential control [127—129], a.c. impedance at equilibrium potential [130], and a.c. polarography [131]. The problem is formulated as follows. 

Reversible, quasi-reversible and irreversible electrode processes have been studied at the RDE [266] as have coupled homogeneous reactions without [267] and with the effect of electrode kinetics [268], The theoretical results are very similar to those of a.c. polarography, being very phase-angle sensitive to coupled chemical reactions in the rotation speed range where convection can be neglected, the polarographic results may be directly applied [269]. 

A.C. Polarography 1 Coulometry - (Constant I) Voltammetry - Solid Electrode... 

Table 12 shows oxidation potentials ( 1/2) of a series of permethylpolysilanes determined by a.c. polarography or cyclic voltammetry50,51. The 1/2 values decrease as the chain length increases. This reflects the HOMO energy levels of these permethylpolysilanes52. 

The foregoing proposed mechanism of reduction of cytosine, cytidine and their derivatives in acid and moderately alkaline media are further supported by the results of a recent study, by means of a.c. polarography, of the orientation of cytosine and cytidine at the mercury electrode. It was shown87) that, whereas at acid pH (about 5) both of these are adsorbed via the positive charge on the ring N3, so that the N3 = C4 bond is adjacent to the electrode surface, the orientation in alkaline medium (pH 9) is modified so that the C5 = C6 bond is located at the surface. This is consistent with the proposed mechanisms of reduction. 

Yasumori, Y. Studies on Polarography with Mercury Drop Control (Part2). Application to A.C. Polarography. J. Electrochem. Soc. Japan 24, 309 (1956). 

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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