Potential scanning methods

In voltammetry the potential is scanned whilst the current is monitored. The current has two components, one due to changes in the capacitance and the second due to changes in redox states (Faradaic currents). Usually the Faradaic currents are to be detected as they are specific to the element which is being determined, whereas the much less specific capacitance currents are eliminated as an interference (this is not always the case as changes in capacitance currents are used for instance to determine organic surfactants in natural waters Gasparovic and Cosovic, 1994)). 

Element Reagent pH range Buffer (pH) LD(60s) nmol/L Scan type Reference 

Co nioxime/ nitrite 9.1 ammonia 0.003 DP Ostapczuk et al., 1986 Donat and Bndand, 1988 Vega and vim den Berg, 1997 

APDC = ammonium pyrrolidine dithiocarbamate, BBS = lV,V-bis(2-hydroxyethyl)-2-aminoethanesul-phonic acid, DTPA= diethylenetriaminepentaacetic acid, DMG = dimethylglyoxime, HEPES = N-2-hydroxyethyl piperazine-N -ethanesulphonic acid, MES = 2-(7V-morpholino)ethanesulphonic acid, NN = l-nitroso-2-naphtol, oxin = 8-hydroxyquinoline, PIPES = piperazine-lV,iV -bis-2-ethanesulpho-nic acid, TBP = tri-n-butyl phosphate, TTA= 2-thenoyltrifluoroacetone. 

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Other phenothiazine dyes, such as methylene green (MG), brilliant cresyl blue (BCB), janus green (JG), toluidine blue (TB), and azure A (AA), can also be immobilized on Pt electrode surfaces by adsorption and the cyclic potential scan method used to prepare MB CMEs." The MG, BCB, JG, TB, and AA CMEs were all found to facilitate effectively the electrochemical reactions of redox proteins and the modified... 

This chapter on the fundamentals of corrosion is a short introduction in those parts of thermodynamics and electrochemistry, which are required for an understanding of corrosion phenomena and the related mechanisms. To keep the chapter small enough, only a condensed overview could be given and it should be seen as a recapitulation of the basics. Other important topics for corrosion, especially methods for corrosion research, are not mentioned here. Modern corrosion research applies various in situ and ex situ methods, spectroscopic and surface analytical tools like XPS, AES, Raman and IR-spectroscopy scanning techniques like STM, AFM, SEM, and electron microprobe analysis impedance spectroscopy and potential scanning methods like SRET and SVET and theoretical calculations. The application of these methods will be mentioned in the different chapters. Literature describes these methods in detail, which is recommended to the interested reader [5,. ... 

The purity of a sample of K3Fe(CN)6 was determined using linear-potential scan hydrodynamic voltammetry at a glassy carbon electrode using the method of external standards. The following data were obtained for a set of calibration standards. 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

The transient method characterized by linearly changing potential with time is called potential-sweep (potential-scan) voltammetry (cf. Section 5.5.2). In this case the transport process is described by equations of linear diffusion with the potential function... 

Experimental results obtained at a rotating-disk electrode by Selman and Tobias (S10) indicate that this order-of-magnitude difference in the time of approach to the limiting current, between linear current increases, on the one hand, and the concentration-step method, on the other, is a general feature of forced-convection mass transfer. In these experiments the limiting current of ferricyanide reduction was generated by current ramps, as well as by potential scans. The apparent limiting current was taken to be the current value at the inflection point in the current-potential curve. 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

As discussed earlier, the potential is scanned between selected potentials with the measurement of the corresponding electrical current. Another type of electrochemical methods is potential step methods, such as chronoamperometry. In this case, the potential of the working electrode is changed drastically and immediately from a potential Ex to a potential E2, at the... 

The first voltammetric methods met are stationary voltammetries performed on a dropping mercury electrode (polarography) or on a solid rotating disk electrode. The limiting current measured is directly proportional to the concentration of the electroactive species in the solution. Experimental potential scan rate is lower than lOrnVs-1. 

A variety of outer-sphere reactions were studied on diamond electrodes by Swain, Miller, Ramesham, and others, using potentiodynamic curves taken under the linear potential scan. This method is appropriate for both qualitative and quantitative characterization of the electrode kinetics (for details, see monographs [90, 91]). 

In a first part of this section, the synchrotron methods are described as they might still not be so common to many scientists in the field of corrosion research. The scanning methods are discussed only briefly, as they have been introduced by numerous papers on in situ studies of the structure of electrode surfaces. Several good reviews are found in literature, and are recommended to the interested reader they describe the application of STM to adsorption and Under Potential Deposition (UPD) metal dissolution and deposition and nano-structuring by deposition of small metal clusters [103-105]. In a following part, results are presented for a number of systems that have been studied in detail with special attention to Cu. 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

Simultaneous application of spectroscopy in voltammetry — Numerous spectroelectrochemical methods employ spectrometers fast enough to acquire complete spectra within seconds or fractions of a second. Acquisition of spectra during a slow electrode potential scan as employed in -> cyclic voltammetry or during single scan voltammetry can thus be related to a single electrode potential (within a range of a few millivolts or even more narrow). Thus interfacial phenomena as detected with the applied spectroelectrochemical technique can be related to the electrode potential directly even in case of unstable intermediates. 

The scan rate is an important parameter for potential sweep methods such as CV or LSV The current is proportional to the square root of the scan rate in all electrochemical systems—irreversible, reversible, and quasi-reversible systems. Figure 4.4 shows the LSV for EMI—TFSl using a glassy carbon (GC) [49]. Note in this figure... 

Linear sweep voltammetry (LSV), also known as linear sweep chronoamperometry, is a potential sweep method where the applied potential (E) is ramped in a linear fashion while measming cnrrent (i). LSV is the simplest technique that uses this waveform. The potential range that is scanned begins at an initial or start potential and ends at a final potential. It is best to start the scan at rest potential, the potential of zero current. For a reversible couple, the peak potential can be calcnlated nsing equation (6). ... 

Meirovitch developed the scanning method to study a system of many chains with excluded volume contained in a box on a square lattice.With this method, an initially empty box is filled with the chain monomers step by step, with help of transition probabilities. The probability of construction of the whole system is the product of the transition probabilities selected, and therefore, the entropy of the system is known. Consequently standard thermodynamic relations can be used to make highly accurate calculations of pressure and chemical potential, directly from the entropy. In principle, all these quantities can be obtained from a single sample without the need to carry out any thermodynamic integration. 

H. Meirovitch, ]. Chem. Phys., 97,5803 (1992). Entropy, Pressure, and Chemical Potential of Multiple Chain Systems from Computer Simulation. 1. Application of the Scanning Method. 

Yet when applied to current reversal techniques, such as double-step chronampero-metry of cyclic voltammetry, these methods require that an appreciable current be observed during the backward perturbation, that is, for t > 0, in potentiostatic methods or after the potential scan inversion in cyclic voltammetry. This requires that the characteristic time 0 of the method is adjusted to match the half-life ti/2 of the electrogenerated intermediate. Today, owing to the recent development of ultramicroelectrodes, 0 can be routinely varied from a few seconds to a few nanoseconds [102]. Yet with basic standard electrochemical equipment, 0 is usually restricted from the second to the low millisecond range. Thus for experimental situations involving faster chemical reactions, current rever-... 

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