Voltammetric behaviour

Since hydrogen peroxide can decompose faster under the influence of catalysts, this so-called catalyst decomposition was also studied by means of voltammetric methods applied to the oxidation and reduction reactions 

This effect apparently could be ascribed to the occurrence of the so-called ohmic potential drop (IR-drop)71 with concentrations higher than ca. lSxlO molT1. This ohmic potential drop is the product of the electrical resistance of the cell solution between working electrode and reference electrode and the electrical current, resulting in a high value of the last-mentioned parameter in a potential loss that cannot be neglected. Consequently, the effective potential of the working electrode compared 

2 Nature of the relation between current density and hydrogen peroxide concentration observed for different rotating-disc electrode materials with a rotation speed of 16.67 rotations per second. 

The current signal, on the other hand, can be reduced drastically. A first, obvious, method is reducing the working electrode surface. A second possibility involves reducing the amount of reacting electroactive component which is transported to the electrode surface, e.g. by means of a membrane or a porous wall. When the working electrode, the reference electrode and the counter electrode are positioned at the same side of the membrane, the resistance of the solution between the electrodes is not influenced by the membrane, which is not the case when the electrodes are positioned at different sides of the membrane, and the influence on the IR voltage drop can 

On the basis of the results obtained in the preliminary research, three potentially useful electrode configurations were investigated further  

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M.H. Pournaghi-Azar and H. Razmi-Nerbin, Voltammetric behaviour and electrocatalytic activity of the aluminum electrode modified with nickel and nickel hexacyanoferrate films, prepared by electroless deposition. J. Electroanal. Chem. 456, 83-90 (1998). 

In this connection, Figure 8 illustrates the anodic voltammetric behaviour of complex [Fe(CO)2( -C5H5)]2 either in a solution of... 

In this case, it is highly educational to look at the voltammetric behaviour of the three-legged piano stool [(i76-C6H6)Cr(CO)3] in dichloromethane solution,10 see Figure 10. 

Figure 10 X-Ray structure (a) and cyclic voltammetric behaviour (Pt working electrode ...

Figure 21 X-ray structure and voltammetric behaviour (in cyclic (a) and differential pulse (b) voltammetry) of [0 = P 0(CsH4)Fe(C H3) 3J in CH2CI2 solution...

The molecular structure of nickelocene [Ni(f/5-C5H5)2] is illustrated in Figure 49, together with its cyclic voltammetric behaviour.92 93... 

It is the mutual redox ability of the metal and the ligand which hinders a clear definition of the redox character of these complexes and often gives rise to a considerable number of redox processes. For instance, Figure 9 shows the cyclic voltammetric behaviour of the dication [Vn(phen)3]2+ in dichloromethane solution.15... 

As a further confirmation of the extended redox aptitude of polypyridine ligands, Figure 15 shows the cyclic voltammetric behaviour of the heteroleptic chromium(III) complex with 2,2/-6/,2"-6//,2"/-quaterpyr-idine (qpy), together with its molecular structure.28... 

Figure 109 Typical cyclic voltammetric behaviour of a few Ni(II)-tetrazamacrocycles in MeCN solution. Platinum working electrode...

Even more educational is the case of the Cu(II) and Cu(III) complexes of N,N -bis[2-(l -hydroxyimino-2-methyl-1 -phenyl)propyl]dimethylma-londiamide (H4mal). Figure 125 shows the molecular structure and the voltammetric behaviour of the Cu(II) form [Cu(Hmal)]-.182... 

The monofullerene with the highest number of carbon atoms that has been studied electrochemically is C84. Figure 18 shows its cyclic voltammetric behaviour in 1,2-dichlorobenzene.19... 

Figure 55 shows the cyclic voltammetric behaviour of [Nin(OEP)] (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin) in CH2C12 solution,96 together with its solid-state molecular structure (in the planar triclinic... 

Figure 57 (a) Cyclic voltammetric behaviour of [Zn(Pc)] in CH2Cl2 solution. Platinum... 

A Cr-N= 2.06 A. (b) Cyclic voltammetric behaviour exhibited by [Cr2(DPPC)4] in CH2Cl2 solution... 

The important biochemical role played by this type of (non-heme) iron proteins has stimulated efforts to synthesise Fe4S4 complexes bound to thiolate groups. For instance, Figure 10 shows the voltammetric behaviour of two such complexes, [Fe4S4(SPh)4]2 and [Fe4S4(SBut)4]2-.la... 

Figure 14 Cyclic voltammetric behaviour of [Fe4S6(C5H5)4J in CH2CI2 solution...

Its cyclic voltammetric behaviour in dmf solution is illustrated in Figure 48.64... 

Voltammetric features of adsorption coupled EC mechanisms (2.177) [128] and (2.178) [129] are rather unpredictable and deviate strongly from the EQ mechanism of a dissolved redox couple. Their voltammetric behaviour is mainly controlled by the adsorption parameter p, and the dimensionless chemical parameters k"s = j and... 

This could be explained in terms of disproportionation of the radical-anion to dianion with subsequent protonation. However, a much more complete explanation followed the realisation that, in most cases, the radical-anion acts not only as a base but also as a single electron-transfer agent (the so-called DISP mechanisms). In particular a comparison of observed cyclic voltammetric behaviour of substituted azobenzenes in the presence of weak acids with that predicted using digital simulation based on various mechanistic possibilities has established the DISPl route given in Eq. (3) (reactions 1-4). 

Attempts to develop a model for the digital simulation of the cyclic voltammetric behaviour of PVF films on platinum62 electrodes required inclusion of the following features (a) environmentally distinct oxidized and reduced sites within the film (b) interconversion of the above sites and interaction between them (c) rate of electrochemical reactions to depend on the rate of interconversion of redox sites, the rate of heterogeneous electron transfer between film and substrate, intrafilm electron transfer and the rate of diffusion of counter ions and (d) dependence on the nature of the supporting electrolyte and the spacing of electroactive groups within the film. 

In a first set of experiments, the voltammetric behaviour of sodium dithionite was investigated in alkaline solution (pH around 12.5), by variation of the rotation rate of the platinum-disc electrode for different concentrations of sodium dithionite. In Fig. 6.1, current-potential curves are shown, obtained at different rotation rates of the electrode in a solution with constant sodium dithionite concentration. Two anodic waves are observed. In principle, sodium dithionite is the only electroactive species in solution, therefore it is supposed that both well-separated waves can be attributed to the oxidation of sodium dithionite with formation of a relatively stable intermediate product in the first wave. 

R.M. Pemberton, A. Amine and J.P. Hart, Voltammetric behaviour of chlorophyll a at a screen-printed carbon electrode and its potential role as a biomarker for monitoring fecal contamination, Anal. Lett., 37 (2004) 1625-1643. 

M. Diaz-Gonzalez, C. Fernandez-Sanchez and A. Costa-Garcia, Comparative voltammetric behaviour of indigo carmine at screen-printed carbon electrodes, Electroanalysis, 14 (2002) 665-670. 

J. P. Hart, M. D. Norman, and C. J. Lacy, Voltammetric behaviour of vitamins D2 and D3 at a glassy carbon electrode and their determination in pharmaceutical products by using liquid chromatography with amperometric detection, Analyst, 777 1441 (1992). 

On the other hand, the voltammograms observed with the O/W interface showed quite different features. As shown in Figure 8.4B, the positive-current peak was depressed significantly by lowering [Fc]o, suggesting the existence of a kinetically controlled process. Such a difference in the voltammetric behaviour between the ECSOW and O/W systems is due to the fact that no IT can take place in the ECSOW system. The voltammetric behaviour for the O/W interface shown in Figure 8.4B can be elucidated in terms of the IT mechanism described below. 

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