Second-harmonic AC voltammetry

Recently, Lenhard (43) has used phase-selective second-harmonic ac voltammetry to obtain more nearly reversible redox potentials for a group of cyanine dyes. The main advantage of this approach is the short time scale of the ac measurements. 

A third electrochemical technique, phase selective second harmonic AC voltammetry has recently been successfully used for determining reversible redox potentials for systems where species formed undergo fast follow-up reactions (Ahlberg et al., 1978 Ahlberg and Parker, 1980 Jaun et al., 1980). 

As indicated above, phase-selective second-harmonic AC voltammetry has provided accurate and thermodynamically meaningful redox data for a number of dyes. One interesting consequence of obtaining such accurate electrochemical data... 

Shono and coworkers studied the electrochemical formation of triphenylcyclopropenyl radical and its chemical behavior in the presence of hydrogen donors and olefins (Table 10). Wasielevsky and Breslow studied the reduction of various cyclopropenyl cations by second harmonic AC voltammetry (Table 11). They detected adsorption effects for some of the cations at the Pt but not at the Au electrode. The electrochemical data were used in thermodynamic sequences to evaluate the basicity of various cycloproenyl anions as well as the C-O bond dissociation energy of cyclopropenols. 

An exact treatment of higher harmonic response is straightforward, but it is rather lengthy, so we will leave it to the specialized literature. Instead we will follow an intuitive approach that will reveal most of the distinctive features of second-harmonic ac voltammetry. For simplicity, we consider only a reversible system in which R is initially absent. 

Both fundamental and second-harmonic ac voltammetry are attractive as analytical techniques because they offer good sensitivities. Detection limits for the polarographic variants can reach the order of 10 M. Such performance is possible because both methods have ready means for discrimination against capacitive currents (5, 7). 

Second-harmonic ac voltammetry gains its freedom from nonfaradaic interference from the relative linearity of the double-layer capacitance as a circuit element. There is consequently only a very small second-harmonic capacitive current, although it too can become important at low analyte concentrations. 

Lenhard J (1986) Measurement of reversible electrode potentials for cyanine dyes by use of phase-selective second-harmonic AC voltammetry. J Imag Sci 30 27-35... 

Subsequently, the electrochemical reduction of a cyclopropenium ion using second harmonic AC voltammetry was studied. This led to an estimated p/C of 61.3 0.5 for cyclopropene as compared to 38.8 0.4 for cycloheptatriene, which corresponds to a difference in acidity of 31 kcal/mol. The calculated gas-phase value (Table 3) is somewhat higher (43 kcal/mol), but the difference will probably be smaller in solution because the small cyclopropenyl anion should be better stabilized by a solvent than is the larger cycloheptatrienyl anion. 

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. 

The major voltammetric methods employed for studies of the anodic oxidation of aromatic hydrocarbons are linear sweep voltammetry (LSV), cyclic voltammetry (CV), and derivative cyclic voltammetry (DCV). Second harmonic ac (SHAC) voltammetry and... 

The methods of ac voltammetry are widely used for kinetic studies of different electrochemical reactions. The sensitivity for analytical purposes is about 10 M. It can be raised by about an order of magnitude when versions are used in which the ac signal is recorded not at the fundamental frequency of the ac voltage, but at its second harmonic, or when still more complicated effects are used. 

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. 

Second harmonic — Any nonlinear oscillating system produces higher harmonic oscillations. The second harmonic is the response having twice the frequency of the basic oscillation. The - current response of a faradaic electrode reaction (- faradaic reaction) to perturbations of the - electrode - potential is generally nonlinear, and thus higher harmonic oscillations of the - alternating current (AC) are produced in - AC voltammetry. Since the -> capacitive current is a much more linear function of the electrode potential, the capacitive contribution to higher harmonic currents are rather small which allows a desirable discrimination of theses currents in electro-analytical applications. 

AC voltammetry using the fundamental and second harmonic wave of Ti at a semistationary mercury drop electrode has been used for the direct determination of Ti [29]. This method has the distinct advantage of being able to tolerate large quantities of metal ion impurities. In another method, Ti salts were chelated with dihydroxyazo dyes, adsorbed onto a hanging mercury drop electrode, and then determined... 

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