Slow scan techniques

Rapid-Scan Corrosion Behavior Diagram (CRD) Basically, all the same equipment used in the conductance of an ASTM G5 slow-scan polarization study is used for rapid-scan CBDs (that is, a standard test cell, potentiostat, voltmeters, log converters, X-Y recorders, and electronic potential scanning devices). The differences are in technique the slow scan is run at a potential sweep rate of about 0.6 V/h the rapid-scan CBDs at about 50 V/h. 

Traditionally, the electrochemical analysis of thin layers of electrodeposited nonequilibrium alloys has simply involved either galvanostatic or potentiostatic dissolution of the electrodeposit under conditions where passivation and/or replacement reactions can be avoided [194, 195]. A technique based on ALSY at a RDE has also become popular [196], To apply this technique, a thin layer (a 10 pm) of the alloy of interest is deposited on a suitable electrode in a solution containing the reducible ions of the alloy components. The plated electrode is then removed to a cell containing an electrolyte solution that is devoid of ions that can be reduced at the initial potential of the experiment, and the complete electrodeposit is anodically dissolved from the electrode surface using slow scan ALSV while the electrode is rotated. 

As already stated, other electrochemical techniques have been used to derive thermodynamic data, some of them considered to yield more reliable (reversible) redox potentials than cyclic voltammetry. This is the case, for instance, of second harmonic alternating current voltammetry (SHACV) [219,333], Saveant and co-workers [339], however, concluded that systems that appear irreversible in slow-scan CV are also irreversible in SHACV experiments. We do not dwell on these matters, important as they are. Instead, we concentrate on a different methodology to obtain redox potentials, which was developed by Wayner and colleagues [350-352]. 

The homogeneous catalysis method is suitable to measure rate constants over a very wide range, up to the diffusion limit. The lower limit is determined by interferences, such as convection, which occur at very slow scan rates. It is our experience that, unless special precautions are taken, scan rates below lOOmV/s result in significant deviations from a purely diffusion-controlled voltammetric wave. For small values of rate constants (down to 10 s ), other potentiostatic techniques are best suited, such as chronoamperometry at a rotating disk electrode UV dip probe and stopped-flow UV-vis techniques. ... 

Comparable or larger errors are introduced by unwanted convective mass transport. Convection is caused by physical motion of the solution, sometimes purposefully introduced for techniques such as rotating electrode voltammetry. When a quiet solution is desired, however, convective errors may arise at longer experiment times (slow scan rates) from mechanical vibrations of the solution. Convection is a particular problem for cells inside inert-atmosphere boxes, on which fans and vacuum pumps may be operative. Convection raises the current... 

SECM can also be used as a read-out tool for protein and DNA chips. While any scanning technique is perhaps too slow for a routine application, SECM is ideally suited for testing concepts in prototype applications which are later transferred to microstructured devices without moving parts. 

The EQCM has been most commonly used simultaneously to quasisteady state techniques like slow scan cyclic voltammetry. In this way mass changes during electrolysis can be obtained from A/(Am/.4) vs. potential curves, while A/(AmA4) vs. charge density curves allow evaluation of the number of Faraday exchanged per mole of electro-active species by use of Faraday s law of electrolysis. 

Finally, the basic equivalence of the two measuring techniques should be appreciated. Although there are many ways to approach such a comparison, the following simplified explanation will, we hope, give a more intuitive feeling for the relationship between EIS and PR measurements. As stated above, both techniques rely on the frequency dependence of the impedance of the double-layer capacitance in order to determine the polarization resistance. EIS uses low frequencies to force the capacitor to act like an open circuit. PR measurements use a slow scan rate to do the same thing. To make comparisons, the idea of equivalent scan rate is useful. Suppose that a particular electrochemical system requires EIS measurements to be made down to 1 mHz in order to force 99% of the current through Rp. What would the equivalent scan rate be for PR measurements A frequency of 1 mHz corresponds to a period of 1000 s. If the sine wave is... 

This instrument can be operated as a normal triple quadrupole with all its scan modes or as a trap in various combinations with the use of the other quadrupoles. If a slow scan rate is used to expel the ions a resolution up to 6000 FWHM can be reached by scanning at 5 Th s 1 using q2 and at 100 Th s 1 using Q3, which is at a lower pressure. As fringe field effects are used, only ions close to the exit lens are expelled. In consequence, mass selective ejection in the axial direction based on this technique is characterized by low ejection efficiency. For instance, an ejection efficiency of less than 20 % is achieved at 1 Th ms-1 scan rate. Different techniques have been proposed to improve the axial ejection efficiency [21], but the most promising technique for mass selective axial ejection is the technique named axial resonant excitation (AREX) [22]. Lenses are introduced between each rod of the quadrupole... 

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