Voltammetry voltammetric measurements

Scale of Operation Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Most analyses are carried out in conventional electrochemical cells using macro samples however, microcells are available that require as little as 50 pL of sample. Microelectrodes, with diameters as small as 2 pm, allow voltammetric measurements to be made on even smaller samples. For example, the concentration of glucose in 200-pm pond snail neurons has been successfully monitored using a 2-pm amperometric glucose electrode. ... 

In voltammetry we measure the current in an electrochemical cell as a function of the applied potential. Individual voltammetric methods differ in terms of the type of electrode used, how the applied potential is changed, and whether the transport of material to the electrode s surface is enhanced by stirring. 

Wangfuengkanagul and Chailapakul [9] described the electroanalysis of ( -penicillamine at a boron-doped diamond thin film (BDD) electrode using cyclic voltammetry. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, and provided a linear dynamic range from 0.5 to 10 mM with a detection limit of 25 pM in voltammetric measurement. In addition, penicillamine has been studied by hydrodynamic voltammetry and flow injection analysis with amperometric detection using the BDD electrode. 

Without any doubt, cyclic voltammetry is the most popular voltam-metric technique used in the field of inorganic chemistry. Unfortunately, the power of the technique is frequently overestimated in that simple cyclic voltammetric measurements rarely allow one to gain complete electrochemical information. As we will discuss, it must be always coupled with complementary techniques. 

The mechanism of the iodide formation at platinum immersed in aqueous electrode was recently studied by laser-activated voltammetry in a channel flow cell system [161]. In this technique, solid deposits of iodine are removed from the electrode continuously by short nanosecond high-power laser pulses. By removing deposits on electrode surfaces within a channel flow cell, the voltammetric measurements becomes time independent and data can be analyzed and modeled quantitatively. Laser activation using a 10-Hz pulsed Nd YAG 532-nm laser was shown to remove bulk iodine from the electrode surface so that under sustained pulsed... 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

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. 

This is an appropriate point at which to comment on the common practice of evaluating the formal potential from voltammetric measurements. When a reversible response is obtained in voltammetry, what is actually measured is the reversible half-wave potential, E1/2, which (except for hydrodynamic voltammetry) is related to the formal potential by a term involving the diffusion coefficients of the oxidized and reduced forms of the half-reaction, D0 and DR, respectively. 

The potential-time relation for voltammetric measurements is presented in Figure 3.2. With linear-sweep voltammetry, the potential is linearly increased between potentials Ex and E2. Cyclic voltammetry is an extension of linear-sweep voltammetry with the voltage scan reversed after the current maximum (peak) of the reduction process has been passed. The voltage is scanned negatively beyond the peak and then reversed in a linear positive sweep. Such a... 

When applicable, second harmonic a.c. voltammetric measurements give the most reliable reversible potentials. Using the first derivative of the response during cyclic voltammetry allows precision to be attained approaching that for the second harmonic a.c. measurements. 

Unfortunately, it is far from trivial to obtain oxidation potentials for commonly encountered 17-electron metalloradicals M, because many such radicals dimerize at rates approaching diffusion-control, rendering it nearly impossible to observe such species by cyclic voltammetry. The use of ultramicroelectrodes was shown [41] to give a reversible signal for the oxidation of Mn(CO)5 at scan rates of ca 5000 V s , but the fmther oxidation of this radical to the 16-electron cation was not reported. There are, however, certain frequently encountered systems for which such radicals are stable at least on the time-scale of normal voltammetric measurements. Figure 4 shows an example, the oxidation of CpCr(CO)3 in acetonitrile. 

Figure 6.6.5 Application of cyclic voltammetry to in vivo analysis in brain tissue, (a) Carbon paste working electrode, stainless steel auxiliary electrode (18-gauge cannula), Ag/AgCl reference electrode, and other apparatus for voltammetric measurements, (b) Cyclic voltammogram for ascorbic acid oxidation at C-paste electrode positioned in the caudate nucleus of an anesthetized rat. [From P. T. Kissinger, J. B. Hart, and R. N. Adams, Brain Res., 55, 20 (1973), with permission.]...

This experiment, first performed by Barker and Gardner (41), is immediately recognizable as a sampled-current voltammetric measurement exactly on the model described in Sections 5.1, 5.4, and 5.5. Normal pulse voltammetry (NPV) is the more general name for the method, which may also be applied at nonpolarographic electrodes, as discussed in Section 7.3.2(d). 

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