Nicholson Shain

On the other hand, adsorption can have serious negative effects on analytical response. Adsorbed reactants will increase the current over that predicted from theory based on diffusion-controlled mass transport. Thus the usually powerful methods based on Nicholson/Shain voltammetric theory are seriously perturbed by adsorption, particularly at high scan rates. In addition, the adsorption of nonelectroactive impurities or reaction products can eventually deactivate the electrode, thus requiring electrode renewal. 

Nicholson, Shain, Saveant and Vianello Geske and Maki... 

By using cyclic voltammetry, Schiffrin and coworkers [26, 186, 187, 189] studied electron transfer across the water-1,2-dichloroethane interface between the redox couple FefCNls /Fe(CN)6 in water, and lutetium(III) [186] and tin(IV) [26, 187] diphthalocyanines and bis(pyridine)-me50-tetraphenylporphyrinato-iron(II) or ru-thenium(III) [189] in the organic solvent. An essential advantage of these systems is that none of the reactants or products can cross the interface and interfere with the electron transfer reaction, which could be clearly demonstrated. Owing to a much higher concentration of the aqueous redox couple, the pseudo-first order electron transfer reactions could be analyzed with the help of the Nicholson-Shain theory. However, though they have all appeared to be quasireversible, kinetic analysis was restricted to an evaluation of the apparent standard rate constant o. which was found to be of the order of 10 cm s [186, 189]. Marcus [199] has derived a relationship between the pseudo-first-order rate constant for the reaction (8) and the rate... 

Before leaving the ce scheme, we must consider the case where the electron transfer is irreversible. The first thing to be noted is that there is no reverse peak on the voltammogram. The overall behaviour is, however, rather complex since there are several possible limiting cases. Nicholson Shain [3] have also investigated this system and have determined, again empirically, that the ratio, / /z , where /p is the peak current for the simple irreversible electron transfer, should obey the expression... 

For the similar case where the chemical reaction is reversible the reader is referred to the paper by Nicholson Shain [3], whilst for all other possibilities the article by Nadjo 8l Saveant [7] should be consulted. 

In cyclic voltammetry, the potential (E) is initially maintained at a value fy, where no faradaic reaction occurs. Then, the potential is swept linearly at a given scan rate (v) until a certain time L, where the direetion of the potential scan is reversed. Then, the potential is given by (Nicholson Shain, 1964) ... 

The cyclic voltammogram is characterized by several important parameters. Four of these observables, the two peak currents and two peak potentials, provide the basis for the diagnostics developed by Nicholson and Shain (1) for analyzing the cyclic voltammetric response. 

A closer scrutiny of Figure 6 reveals the persistence of small, but consistent curvature in all of the plots. In order to verify the curvature, the transfer coefficient 8 was also determined independently from the width of the CV wave, as described by Nicholson and Shain (10). The potential dependence of 8 obtained in this manner is shown in Figure 7. The slopes 88/3E represent the unmistakable presence of curvature in Figure 6. 

Table 6.5 Simple diagnostic tests for a coupled chemical (EC) reaction, carried out by using cyclic voltammetry (after Nicholson, R. S. and Shain, I. Anal. Chem., 36, 706-723 (1994), and Nadjo, L. and Saveant, J. M., J. Electroanal. Chem., 48, 113-145 (1973))...

The theory for cyclic voltammetry was developed by Nicholson and Shain [80]. The mid-peak potential of the anodic and cathodic peak potentials obtained under our experimental conditions defines an electrolyte-dependent formal electrode potential for the [Fe(CN)g] /[Fe(CN)g]" couple E°, whose meaning is close to the genuine thermodynamic, electrolyte-independent, electrode potential E° [79, 80]. For electrochemically reversible systems, the value of7i° (= ( pc- - pa)/2) remains constant upon varying the potential scan rate, while the peak potential separation provides information on the number of electrons involved in the electrochemical process (Epa - pc) = 59/n mV at 298 K [79, 80]. Another interesting relationship is provided by the variation of peak current on the potential scan rate for diffusion-controlled processes, tp becomes proportional to the square root of the potential scan rate, while in the case of reactants confined to the electrode surface, ip is proportional to V [79]. 

Bard AJ, Faulkner LR (1980) Electrochemical Methods. John Wiley Sons, New York. Nicholson RS, Shain I (1964) Theory of Stationary Electrode Polarography. Anal Chem 36 706-723. 

The first reaction of oxidation of ferrocene occurs at an electrode followed by oxidation of GO(red) in solution with the pesudo-first-order rate constant f=/j[GO(red)]. The values k(la (a = nFv/RT v is a scan rate) estimated from the experimentally measured i ji ratios (Fig. 2) using the Nicholson and Shain approach (73) have been... 

From Ref. 39, reprinted with permission from R. S. Nicholson and I. Shain, Anal. Chem. ... 

Nicholson and Shain [24] have shown that when the same number of electrons is transferred in each step of the ECE mechanism, Equation 23.33 holds, in which Xc and XD are the current functions in the presence and absence, respectively, of the intervening chemical reaction (XD is the hypothetical value of the current function under diffusion-controlled conditions, i.e., if the C step were not present) ... 

AC037-0190 Nicholson, R.S., and Shain, I., AGA9, AH06, AH2X... 

Besides the peak current and the reversibility of the redox system and its current-potential curve, the inclining part of the forward wave has also been a target for extensive studies and characterisation. Numerous mathematical approaches have been presented in the literature however, the approach of Nicholson and Shain has been chosen here11 due to the clarity of the physical-chemical meaning behind the mathematical approach and the ease with which non-electrochemists can understand the shape of the voltammetric curve. In their approach, a current-potential curve of a reversible system is presented as ... 

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