Polarography peaks

In AC polarography, peaks instead of steps are recorded, facilitating the evaluation of the polarogram. The strength of the diffusion current i j is given by the Ilkovic equation ... 

The segments of dh DNA with distorted conformation desorb from the mercury electrode at the potentials by c. 0.2-0.3 V more negative than are the potentials of the peak 1 [102]. This desorption results in an appearance of the peak 2 on a.c. polarograms, which can be detected only for carefully isolated high-molecular mass nucleic acids and with the aid of in-phase component of phase-sensitive a.c. polarography. Peak 2 occurs at still more positive potentials than the peak 3 yielded by the corresponding ss nucleic acids [102]. 

The concentration of As(III) in water can be determined by differential pulse polarography in 1 M HCl. The initial potential is set to -0.1 V versus the SCE, and is scanned toward more negative potentials at a rate of 5 mV/s. Reduction of As(III) to As(0) occurs at a potential of approximately —0.44 V versus the SCE. The peak currents, corrected for the residual current, for a set of standard solutions are shown in the following table. 

Peak currents in differential pulse polarography are a linear function of the concentration of analyte thus... 

The amount of sulfur in aromatic monomers can be determined by differential pulse polarography. Standard solutions are prepared for analysis by dissolving 1.000 mb of the purified monomer in 25.00 mb of an electrolytic solvent, adding a known amount of S, deaerating, and measuring the peak current. The following results were obtained for a set of calibration standards... 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

In a method described by Yoshimura and Uzawa [ 144], cadmium in seawater is coprecipitated with zirconium hydroxide (Zr(OH)4) prior to determination by square-wave polarography. The precipitate is dissolved in hydrochloric acid, and cadmium concentration is determined from the peak height of the... 

Fleet and Fouzder have also observed these distortions when polarography was performed over DME. In fact, CV on GC of Ph3PbAc, 1.084 x 10-4 M in acetate buffer pH 7.0 containing 50% v/v ethanol shows two well-defined peaks, at —1.7 V (a cathodic current) and —0.7 V (an oxidation current), and these are attributed to secondary reactions of the unstable triphenyl radical with imminent stripping of elemental lead ... 

Reduction of triphenyltin piperidyldithiocarbamate in acetone was shown by polarography and voltammetry to consist of two diffusion-controlled peaks and two peaks which seem to reflect adsorption142. Apparently, a dithiocarbamate group dissociates and triphenyltin radical forms by reduction. The latter partly dimerizes and partly reduces to triphenyltin anion. 

Binding of metal ions by amorphous silica. 300 mg Si02 dm-3, [Mej = 10 5 MJ. The fraction of metal ion bound was calculated from peak currents measured with differential pulse polarography. 

E = Faraday constant). The equilibrium potential E is dependent on the temperature and on the concentrations (activities) of the oxidized and reduced species of the reactants according to the Nemst equation (see Chapter 1). In practice, electroorganic conversions mostly are not simple reversible reactions. Often, they will include, for example, energy-rich intermediates, complicated reaction mechanisms, and irreversible steps. In this case, it is difficult to define E and it has only poor practical relevance. Then, a suitable value of the redox potential is used as a base for the design of an electroorganic synthesis. It can be estimated from measurements of the peak potential in cyclovoltammetry or of the half-wave potential in polarography (see Chapter 1). Usually, a common RE such as the calomel electrode is applied (see Sect. 2.5.1.6.1). Numerous literature data are available, for example, in [5b, 8, 9]. 

In polarography, we obtained the half-wave potential E// by analysing the shapes of the polarographic wave. E1/2 is a useful characteristic of the analyte in the same way as E . In cyclic voltammetry, the position o/both peaks (both forward and back in Figure 6.13 cathodic and anodic, respectively, in this example) gives us thermodynamic information. Provided that the couple is fully reversible, in the thermodynamic sense defined in Table 6.3, the two peaks are positioned on either side of the formal electrode potential E of the analyte redox couple, as follows ... 

Current maximum suppresser A chemical, usually a surfactant (detergent), added to polarography solutions in order to decrease the incidence of polaro-graphic peaks (see Section 6.8.1). 

The polymers can be oxidized by differential pulse polarography. Their oxidation is metal-centered and leads to Ru(III) compounds. The potential is located aroimd -1-1.26 V (SCE). It can be stated that the polymers which contain the triphenylamine structure imits in the main chain show, as expected, an additional peak caused by the amine nitrogen. Substitution at the triphenylamine by electron-donating substituents lowers these potentials to 1.05 V (25), whereas acceptor substituents cause an increase of the oxidation potential (23). 

Nevertheless, the mid-peak potentials determined by cyclic voltammetry and other characteristic potentials obtained by different electroanalytical techniques (such as pulse, alternating current, or square wave voltammetries) supply valuable information on the behavior of the redox systems. In fact, for the majority of redox reactions, especially for the novel systems, we have only these values. (The cyclic voltammetry almost entirely replaced the polarography which has been used for six decades from 1920. However, the abundant data, especially the half-wave potentials, 1/2, are still very useful sources for providing information on the redox properties of different systems.)... 

Fig. 4.6. Method for determining the concentration of residual impurities, [Impjj jjj., in a reaction mixture if the impurity is a catalyst or co-catalyst. The observed variable x can be peak-height h for a GLC method, absorbance A for spectroscopy, conductivity k for conductimetry, current i for polarography, or rate constant k for kinetics, etc.

Sybrandt LB, Perone SP (1972) Computerized pattern classification of strongly overlapped peaks in stationary electrode polarography. Anal Chem 44 2331-2339. 

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