Cyclic voltammetry half-wave potential

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

Useful experimental parameters in cyclic voltammetry are (i) the value of the separation of the potentials at which the anodic and cathodic peak currents occur, A = Pia — PiC, and (ii) the half wave potential, 1/2, the potential mid-way between the peak potentials. A value of AE of c. 0.057 V at 25°C is diagnostic of a Nernstian response, such as that shown in Figure 2.87. More generally, if n electrons are transferred from R, then the separation will be 0.057/n V. It should be noted that the expected value for AE of 0.57/nV has no relationship to the usual Nernstian slope of RT/nF = 0.059/n V at 25UC. 

Using, for example, cyclic voltammetry, the cathodic peak current (normalized to its value in the absence of RX) is a function of the competition parameter, pc = ke2/(ke2 + kin), as detailed in Section 2.2.6 under the heading Deactivation of the Mediator. The competition parameter can be varied using a series of more and more reducing redox catalysts so as eventually to reach the bimolecular diffusion limit. km is about constant in a series of aromatic anion radicals and lower than the bimolecular diffusion limit. Plotting the ratio pc = keij k,n + km) as a function of the standard potential of the catalysts yields a polarogram of the radical whose half-wave potential provides the potential where ke2 = kin, and therefore the value of... 

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 ... 

Reduction of sulphonium salts polarographic half-wave potentials, Ey. ref. [54], in water cyclic voltammetry peak potentials, Ep ref. [55], in acetonitrile at glassy carbon, scan rate 50 mV s. ... 

Cyclic voltammetry of thianthrene in the presence of two molar equivalents of pyridine generates thianthrene 5-oxide and not T . Having shown 2-(pyridinium-l-yl)thianthrene to have half wave potentials, 1.45 and 1.74, different from those of thianthrene, the kinetic scheme summarized by Eqs. (16)-(19) was proposed (77JOC976). 

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.)... 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

Binding is evaluated by cyclic voltammetry monitoring the half-wave potential of the Fc group (Fig 10.7). Cation binding causes a shift to lower potential in the following order Na+ >Li+ >K +, Cs +. 49... 

All C60 adducts have low-lying LUMOs that can easily be populated by electrochemical methods. For C60 itself, six reduction couples have been observed by cyclic voltammetry (CV) or square-wave voltammetry (SWV), and as many as four reduction couples have been found for many organometallics (9,84). Most of the studies have been performed in thf or acetonitrile at lower temperatures, which increases the size of the potential window. Table VII lists the half-wave potentials for some metal complexes, and Fig. 7 shows the cyclic voltammogram for [Co(NO)(PPh3)2(i72-C60)]. 

In fact, the potentiometric or voltammetric measurement is carried out using a conventional reference electrode (e.g. Ag+/Ag electrode).3 After measurement in the test solution, Fc or BCr+ (BPhJ salt) is added to the solution and the half-wave potential of the reference system is measured by polarography or voltammetry. Here, the half-wave potential for the reference system is almost equal to its formal potential. Thus, the potential for the test system is converted to the value versus the formal potential of the reference system. The example in Fig. 6.2 is for a situation where both the test and the reference systems are measured by cyclic voltammetry, where E1/2=(Epc+Epi)/2. Curve 1 was obtained before the addition of Fc and curve 2 was obtained after the addition of Fc. It is essential that the half-wave potential of the test system is not affected by the addition of the reference system. 

It is certain that, in the first reduction step in aprotic solvents, an electron is accepted by the LUMO of the organic compound. However, it was fortunate that this conclusion was deduced from studies that either ignored the influence of solvation energies or used the results in different solvents. Recently, Shalev and Evans [55] estimated the values of AG V(Q/Q ) for 22 substituted nitrobenzenes and nine quinones from the half-wave potentials measured by cyclic voltammetry. For quinones and some substituted nitrobenzenes, the values of AG V(Q/Q ) in a given solvent were almost independent of the EA values. Similar results had been observed for other aromatic hydrocarbons in AN (Section 8.3.2) [56]. If AG V(Q/ Q ) does not vary with EA, there should be a linear relation of unit slope between El/2 and EA. Shalev and Evans [55], moreover, obtained a near-linear relation between AG V(Q/Q ) and EA for some other substituted nitrobenzenes. Here again, the Ey2-EA relation should be linear, although the slope deviates from unity.8)... 

Table 8 Half-wave Potentials (E)) (in V vs. see) of Octaethylporphyrins, using Cyclic Voltammetry with a Platinum Button Electrode3...

It will be clear that cyclic voltammetry is a powerful tool for a first analysis of an electrochemical reaction occurring at the surface of an electrode because it will reveal reversibility. Depending on whether the system is reversible, information will be obtained about half wave potential, number of electrons exchanged in the reaction, the concentration and diffusion coefficient of the electroactive species. However, these data can also be obtained for an irreversible system1113 but, in this case, the equations describing the current-potential curves differ somewhat from Equations 2.21 to 2.27. 

Three other methods were used to obtain a value for the charge-transfer coefficient. The coefficient can be obtained from the difference between peak (Ep) and half-wave potential (Ep/2) in cyclic voltammetry at a stationary-disc electrode40,41 ... 

A complete comprehension of Single Pulse electrochemical techniques is fundamental for the study of more complex techniques that will be analyzed in the following chapters. Hence, the concept of half-wave potential, for example, will be defined here and then characterized in all electrochemical techniques [1, 3, 8]. Moreover, when very small electrodes are used, a stationary current-potential response is reached. This is independent of the conditions of the system prior to each potential step and even of the way the current-potential was obtained (i.e., by means of a controlled potential technique or a controlled current one) [9, 10]. So, the stationary solutions deduced in this chapter for the current-potential curves for single potential step techniques are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. Moreover, many of the functional dependences shown in this chapter for different diffusion fields are maintained in the following chapters when multipulse techniques are described if the superposition principle can be applied. 

All general typical variables considered in this chapter for a particular reaction scheme, for example the half-wave potential, are of fundamental interest for its characterization in any electrochemical technique. Moreover, as indicated in the previous chapter, all the current-potential expressions deduced here under stationary conditions (when microelectrodes are used) are applicable to any multipotential step or sweep electrochemical techniques like Staircase Voltammetry or Cyclic Voltammetry. 

Reduction of substituted triarylcarboniumions to the corresponding radicals was studied by polarography 598 and cyclic voltammetry S99 The half wave potential is shifted to more negative values with increasing stability of the carbo-niumion by p-substituents R in the order R ... 

The electron affinity of 3-(iV-methylpiperazino)-5-nitroindazole, 3,5-dinitroindazole, and molecular complex of the last with water is discussed on the basis of their half-wave potentials and in connection with their eventual radiosensitizing properties [667], The mechanism of EC behavior of 2-substituted 5(6)-nitrobenzimidazoles in acetonitrile has been investigated by classical polarography, cyclic voltammetry, and platinum rotating disk electrode with a ring (RDER) [888,991], It is shown that... 

Cyclic voltammetry revealed that all ferrocene derivatives can be reversibly oxidized. The number of reversible waves in the cyclic voltammograms corresponds to the number of the redox sensitive moieties in the molecule. With respect to ferrocene the half-wave potentials of the compounds are shifted anodically. Furoxanes were isolated in minor amounts as the expected byproducts resulting from dimerization of the nitrile oxides. 

The effect of temperature on ion transfer across the water-nitrobenzene interface was studied for a series of six quaternary ammonium and phosphonium cations and two anions using cyclic voltammetry and equilibrium impedance measurements [115]. Standard entropies (A S ) and enthalpies (A iT ) of ion transfer have been evaluated from the experimentally accessible reversible half-wave potential ( "572 and standard Gibbs energy of transfer (A G ),... 

Voltammetric data for ester reductions are available for several aromatic esters [51-54], and in particular cyclic voltammetry shows clearly that in the absence of proton donors reversible formation of anion radical occurs [51]. In dimethylfonnamide (DMF) solution the peak potential for reduction of methyl benzoate is —2.29 V (versus SCE) for comparison dimethyl terephthalate reduces at —1.68 V and phthalic anhydride at —1.25 V [4]. Half-wave potentials for reduction of aromatic carboxylate esters in an unbuffered solution of pH 7.2 are linearly correlated with cr values [51] electron-withdrawing substituents in the ring or alkoxy group shift Ei/o toward less negative potentials. Generally, esters seem to be more easily reducible than the parent carboxylic acids. Anion radicals of methyl, ethyl, and isopropyl benzoate have been detected by electron paramagnetic resonance (epr) spectroscopy upon cathodic reduction of these esters in acetonitrile-tetrapro-pylammonium perchlorate [52]. The anion radicals of several anhydrides, including phthalic anhydride, have similarly been studied [55]. 

Credi, Raymo and coworkers reported in 2002, the preparation of a new type of carbohydrate-coated, dendronized ferrocenes [46], The structures of these den-drimers are illustrated by compounds 14 and 15. Binding interactions with p-CD were investigated by liquid secondary ion mass spectrometry, 11 NMR spectroscopy, voltammetry and circular dichroism. Upon addition of P-CD, cyclic voltammetric experiments showed data similar to those obtained with our dendronized ferrocenes, that is, a CD-induced current level decrease and anodic half-wave potential shift. At 22°C, these authors obtained K values of 2,000 200 and 1,300 200 M-1 for the complexation of 14 and 15, respectively, by p-CD in 0.1MNaClO4. Similar dendrimers in which carbohydrate branches are covalently attached to both cyclopentadienyl rings of the ferrocene residue were not bound at all by P-CD, reflecting the shielding of the core ferrocene center in these more symmetric compounds. 

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