Current maxima

The influence of the solvent on the oxidation of film under conformational relaxation control is illustrated in Fig. 47, which shows chronoamperograms obtained by steps from -2000 to 300 mV vs. SCE at room temperature (25°C) over 50 s in 0.1 M LiC104 solutions of different solvents acetonitrile, acetone, propylene carbonate, (PC), dimethyl sulfoxide (DMSO), and sulfolane. Films were reduced over 120 s in the corresponding background solution. Despite the large differences observed in the relative shape of the curves obtained in different solvents, shifts in the times for the current maxima (/max) are not important. This fact points to a low influence of the solvent on the rate at which confor-... 

Figure 65. Correlation between theoretical and experimental values for the current maxima of the voltammetric curves. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, J. Phys. Chem. 101, 8525, 1997, Figs. 3-11, 13. Copyright 1997. Reproduced with permission from the American Chemical Society.)...

Voltammograms with characteristic current maxima are obtained (see Fig. 12.9) when linear potential scans (LPS) which are not particularly slow are apphed to an electrode. The potentials at which a maximum occurs depend on the nature of the reactant, while the associated current depends on its concentration. When several reactants are present in the solution, several maxima will appear in a curve. 

As an example of the form of the information that may be derived from a pyrolysis-MS, Figure 26 [69] shows the structure of the polycarbonate (PC) and the EI-MS spectra of pyrolysis compounds obtained by DPMS of poly(bisphenol-A-carbonate) at three different probe temperatures corresponding to the three TIC (total ion current) maxima shown in Figure 27(b) Figure 27 compares the MS-TIC curve with those obtained from thermogravimetry. (The TIC trace is the sum of the relative abundances of all the ions in each mass spectrum plotted against the time (or number of scans) in a data collection sequence [70].)... 

A passivating oxide is formed under sufficiently anodic potentials in HF, too. However, there are decisive differences to the case of alkaline and fluoride-free acidic electrolytes. For the latter electrolyte the steady-state current density prior to passivation is zero and it is below 1 mA cnT2 for alkaline ones, while it ranges from mA cm-2 to A cm-2 in HF. Furthermore, in HF silicon oxide formation does not lead to passivation, because the anodic oxide is readily etched in HF. This gives rise to an anodic I-V curve specific to HF, it shows two current maxima and two minima and an oscillatory regime, as for example shown in Fig. 4.7. 

Fig. 4.7 The current density-voltage curve of a (100) p-type Si electrode in 0.3 mol kg-1 NH4F (pH = 3.5) recorded with 50mVs 1 showing the characteristic current maxima and minima. At the points indicated by filled...

For galvanostatic anodization a first potential maximum is again observed at about 19 V, and the thickness of the anodic oxide at this maxima has been determined to be about 11 nm, as shown in Fig. 5.4. Note that these values correspond to an electric field strength of about 17 MV cm4. The first maximum may be followed by several more, as shown in Fig. 5.1c and d. Note that these pronounced maxima become smeared out or even disappear for an increase in anodization current density (Fig. 5.Id), a reduction in temperature (Fig. 5.1c), or an increase in electrolyte resistivity. The latter value is usually too large for organic electrolytes to observe any current maxima. A dependence of these maxima on crystal orientation [Le4] or doping kind and density [Pa9] is not observed. The rich structure of the anodization curves is interpreted as transition of the oxide morphology and is discussed in detail in the next section. 

Figure 632 Illustration of polarographic current maxima, where the continuous line is the correct, undistorted polarograph (drawn as the mean current, i.e. without the sawtoothed effect of drop replenishment), and the dashed and dotted lines represent current maxima of the first and second kinds, respectively.

The most serious causes of error are (i) wave and peak distortion caused by excessively fast scan rates, which are themselves caused by diffusion being an inefficient mass transport mechanism, (ii) current maxima caused by convective effects as the mercury drop forms and then grows, and (iii) IR drop, i.e. the resistance of the solution being non-zero. Other causes of error can be minimized by careful experimental design. 

A number of variations of the basic DME have been devised. The vertical-orific capillary, formed by bending a capillary into a right angle and cutting off the capillary near the bend, is claimed to eliminate problems of current maxima and depletion effects (growth of a drop in a solution depleted of electroactive material by the preceding drop), and to provide greater uniformity... 

Current maxima Anomalous peaks in the current of a polaro-graphic cell can often be eliminated by the introduction of surface active agents. 

Fig. 1.3e. Capacitive current as a function of electrode potential ii) Current maxima...

Sketch the appearance of current maxima in dc polarography and explain how these unwanted signals can be avoided. 

The phenomena of current maxima are illustrated the origins explained and a simple method of suppressing the effect given. 

For intermediate ionic strengths, because of higher repulsion, no current maxima occur, i.e., the current density depends mainly on the amount of DNA molecules screened by counterions and their mobility inside the loose adsorption layer. Therefore surface denaturation, ° ° favored by the energy of adsorption, seems unlikely, whereas the high field strength ( 10 V cm ) in 1 M salt solution is effective just in the Helmholtz double layer region (<0.5 nm), i.e., this concerns a small part of the few segments adsorbed and influences merely less than 1 % of the whole DNA molecule, which extends over the diffuse double layer into the bulk of solution. ... 

A detailed study of N2O reduction on a variety of single-crystal Pt-group metal electrodes shows that N2O reduction current maxima occurred exactly at Claviher s Eq=o- Therefore the N2O reduction data have been used for obtaining the Eq values (Table 5). The importance of a local value of Eq=o has been emphasized, especially with respect to reconstructing metal surfaces such as Pt(lOO) and Pt(llO), which can be prepared in a variety of crystallographic states [5, 65-68]. 

The Brdifika basic reaction is realized by simple procedures in which biological material, usually serum, is submitted to polarographic analysis, being added to cobalt solutions without any previous treatment by de-naturation, precipitation, filtration, or by enzymatic or electrophoretic procedures. The polarography of solutions containing Co salts yields polarographic curves with current maxima of cobalt, and after the addition of the serum the typical double wave is obtained. 

Suppose we have a black box with one port = two terminals. Suppose that with sinusoidal excitation, all voltage differences and all currents inside are also sinusoidal. Suppose that inside there are only resistors. Then all current maxima and voltage maxima occur simultaneously, there are no delays, no phase shifts. 

---