Polarography second harmonic

Fig. 3.42 represents the symmetric bell shape curve of 7, i.e., the genuine fundamental harmonic ac polarogram, which means the curve of only 7F discriminated for 7C, e.g., by means of phase-selective ac polarography. The term "fundamental is related to the character of the polarographic cell as a non-linearized network whose response is not purely sinusoidal but consists of the sum of a series of sinusoidal signals at first harmonic (o>) response, besides that of the second harmonic (2a>), the third harmonic (3a>), etc. 

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],... 

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. 

El 12 for the waves was taken at 85 % of the peak height observed in cyclic voltammetry, at scan rates up to 45 V s " (Refs 73-75). Potentials in Ref. 79 were measured by second harmonic AC polarography. 

By using phase selective measurement of a higher-harmonic alternating current (e.g., 2nd harmonic with a frequency 2j) increased sensitivity (lO mol/L) is obtained because of the marked reduction in the capacitive current component of the higher harmonic (AC2 or second harmonic wave polarography). 

Faradaic rectification — When the electrode potential of the working - electrode is modulated with a sinusoidal -> alternating current the mean potential is shifted from the DC value by a small increment in many cases when the AC modulation is sufficiently large. This effect has been named faradaic rectification, it is caused by the nonlinearity of the electrode response, in particular the variation of current with electrode potential [i]. A theoretical treatment for an electrode in contact with a solution containing a redox system has been provided [ii]. It was extended to reactions where one reactant is present in its element form dissolved in the liquid metallic phase (e.g., Cd2+ + 2e -> Cd(Hg)) [iii]. An improved evaluation technique has been proposed [iv], and some inherent problems have been reviewed [v]. A variant of this method applied to -> polarography has been described [vi]. Second and higher harmonics in - AC voltammetry (polarography) [vii] also arise from this nonlinearity, and hence these techniques also have some characteristics that resemble those found in - faradaic rectification voltammetry. 

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