Polarography polarographic wave

Taking into account the results obtained by polarography as well as controlled potential electrolysis, the reaction which proceeded in Range A and gave the polarographic wave was estimated to be composed of two-electron oxidation of NADH and one-electron reduction of CQ at the W/DCE interface. The oxidation of NADH is accompanied by the dissociation of one H in W. 

Leucovorin, since it is totally reduced, is polarographically inert in a pH 9 buffered solution.63 After acid treatment, three polarographic waves are generated, corresponding to an anodic oxidation of a tet-rahydro compound and two cathodic reductions of unreduced pteridines presumably at least one of these three is a dihydro species. Polarography is useful as a technique in structural elucidation, but analytical data would be difficult to obtain from an acid-treated solution containing several species, each with its own polarographic behavior. 

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 ... 

This explanation for the two polarographic waves seen in Figure 3.2 suggests that the region of transition between the two waves will be sensitive to buffer concentration and composition. Such effects are seen in the polarography-pH profiles of steroid enones, some of which [88] show behaviour like that of cyclohexenone while others show only a linear variation of half-wave potential over the whole pH range of 2 — 11 [89, 90]. 

The limiting current is proportional to the concentration of the electroactive species, whereas the half-wave potential is specific to the electroactive species, being close to the standard potential of the electrode reaction. Thus, by measuring polarographic waves, we can run qualitative and quantitative analyses. In DC polarography, many inorganic and organic substances (ions, complexes and mole-... 

Tolerable Amount of the Substance Electrolyzed Before the Analyte(s) In DC polarography, several electroactive species can be determined simultaneously, because they give step-wise waves. However, if a substance that is easier to reduce or to oxide than the analyte(s) exists in high concentrations (50 times or more), detennination of the analyte(s) is difficult. This problem is overcome by obtaining the derivative curve of the DC polarographic wave (Section 5.4). 

Here, A is the electrode area, C and D are the concentration and the diffusion coefficient of the electroactive species, AE and co(=2nfj are the amplitude and the angular frequency of the AC applied voltage, t is the time, and j=nF (Edc-Ei/2) / RT. For reversible processes, the AC polarographic wave has a symmetrical bell shape and corresponds to the derivative curve of the DC polarographic wave (Fig. 5.14(b)). The peak current ip, expressed by Eq. (5.24), is proportional to the concentration of electroactive species and the peak potential is almost equal to the half-wave potential in DC polarography ... 

Polarography and cyclic voltammetry26 of [V(bipy)3]z (z =2+, 1+, 0, 1-, 2-, 3-) gave Elt2 values of -0.8, -1.01, -1.11, -1.55 and —2.23 V for these five steps, and smaller polarographic waves at -2.10 and -2.6 V were attributed to the reduction of free bipy. From comparison of the Em values for the bipy complexes with those for 4,4 -dimethyl-2,2 -bipy and 5,5 -dimethyl-2,2 -bipy, the electron added (or removed) in the reactions of [V(L)3]Z (z = 2+, 1+, 0) was at t2g orbitals.26 On the other hand, for V(bipy)3/V(bipy)J and V(bipy)J/V(bipy)2-the added electron would occupy orbitals with a predominant n character. However, [V(bipy)3] is diamagnetic, and an intermediate character has been proposed for it. 

Phosphorescence, 191, 223 Photoacoustic detector, 177 Photoelectric effect, 238 Photoelectron, 238 Photovoltaic, 176 Plasam, 274 Plate, 10 Polarogram, 361 Polarographic wave, 361 Polyethylene glycols, 32 Polysiloxanes, 31 POPOP, 333 PPO, 333 Precession, 129 Precision, 386 Pulse polarography, 364 Pulsed NMR, 155 Pyroelectric, 175... 

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. 

The 1,4-benzodiazepine class of pharmaceuticals (see Fig. 26.6), which are in widespread clinical use today, all possess intrinsic electrochemical activity that renders them ideal for voltammetric studies. Senkowski et al. were the first to perform a comprehensive study of this class of compounds by polarography [77] and demonstrated that the 1,4-benzodiazepines show a wave for the two-electron reduction of the 4,5-azomethine electrophore in a supporting electrolyte of 0.1 M HCI containing 20% methanol. Secondary polarographic waves... 

Substituted purines, such as 6-alkylaminopurines and 6-methylpurine, were studied using dc and ac polarography to evaluate the effects of substitution on the electrochemical behaviour of these compounds 155). When the 1,6 and 2,3 double bonds are present, a 4e polarographic wave, due to reduction of these bonds, is generally observed 1S5). 

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