Voltammetry, stationary conditions

The simplest chronoamperometric technique is that defined as single potential step chronoamperometry. It consists of applying an appropriate potential to an electrode (under stationary conditions similar to those of cyclic voltammetry), which allows the electron transfer process under study (for instance Ox + ne — Red) to run instantaneously to completion (i.e. COx(0,0 —1 0). At the same time the decay of the generated current is monitored.20... 

All general typical variables considered in this chapter for a particular reaction scheme, for example the half-wave potential, are of fundamental interest for its characterization in any electrochemical technique. Moreover, as indicated in the previous chapter, all the current-potential expressions deduced here under stationary conditions (when microelectrodes are used) are applicable to any multipotential step or sweep electrochemical techniques like Staircase Voltammetry or Cyclic Voltammetry. 

If (95) is used to estimate values for the diffusion layer thickness obtained for sonovoltammetry in acetonitrile, values of the order of a few micrometres are obtained - much smaller than encountered in conventional voltammetry under silent (stationary) conditions unless either potential scan rates of hundreds of mVs, or more, are employed or alternatively steady-state measurements are made with microelectrodes with one or more dimensions of the micrometre scale (Compton et al., 1996b). 

The application of this technique (even in its various modes such as cyclic voltammetry) to other electrodes has already been mentioned in the description of LSV at the dme [Section 3.3.1.2.1(5)]. Especially with stationary electrodes LSV becomes fairly simple, under the conditions of sufficient solubility of ox and red, because of the constant and undisturbed electrode surface at an inert electrode the residual faraday current can be adequately eliminated by means of "J compensation (cf., Fig. 3.23) or by subtractive [cf., Section 3.3.1.2.1(3)] and derivative59 [cf., Section 3.3.1.2.1(4)] voltammetry at a stationary mercury electrode (e.g., HMDE), in addition to the residual faradaic current,... 

For other techniques other kinds of data are given Instead. For chronopotentiometry these Include the area of the indicator electrode and the current or current density for stationary-electrode voltammetry they include the area of the indicator electrode and the scan rate for cyclic voltammetry they include the starting and reversal potentials and the area of the indicator electrode, and so on. The list of abbreviations must always be consulted regarding the units of the quantities given In this column. In the space that was available for these purposes it is quite Impossible to give a full description of the experimental conditions, but art attempt has been made to give an accurate Idea of their nature. 

This term denotes a potential whose nature depends on the technique used. Typical characteristic potentials are the half-wave potential in polarography, the quarter-transition-time potential in chronopotentiometry, and the peak or half-peak potential in stationary-electrode voltammetry. Regardless of its nature, the characteristic potential always depends on the identity of the electroactive substance, on the kinetics or thermodynamics of the electron-transfer process, and of course on the experimental conditions for any particular technique and under any completely defined set of experimental conditions the value of any characteristic potential is a reproducible property of the electroactive substance. 

A complete comprehension of Single Pulse electrochemical techniques is fundamental for the study of more complex techniques that will be analyzed in the following chapters. Hence, the concept of half-wave potential, for example, will be defined here and then characterized in all electrochemical techniques [1, 3, 8]. Moreover, when very small electrodes are used, a stationary current-potential response is reached. This is independent of the conditions of the system prior to each potential step and even of the way the current-potential was obtained (i.e., by means of a controlled potential technique or a controlled current one) [9, 10]. So, the stationary solutions deduced in this chapter for the current-potential curves for single potential step techniques are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. Moreover, many of the functional dependences shown in this chapter for different diffusion fields are maintained in the following chapters when multipulse techniques are described if the superposition principle can be applied. 

In this section, we will show that the stationary responses obtained at microelectrodes are independent of whether the electrochemical technique employed was under controlled potential conditions or under controlled current conditions, and therefore, they show a universal behavior. In other words, the time independence of the I/E curves yields unique responses independently of whether they were obtained from a voltammetric experiment (by applying any variable on time potential), or from chronopotentiometry (by applying any variable on time current). Hence, the equations presented in this section are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. 

Stationary electrode voltammetry was used in nitrate melts as early as 1948 by Lyalikov and Karmazin (50), using a platinum microelectrode in the form of a "dipping" electrode with bubbles of an inert gas to produce a periodically fluctuating current. A similar electrode was used by Flengas (51) and by Bockris, et al. (52) in 1956. In 1960, Hills, Inman and Oxley (53) described an improved version of the dipping electrode, which of course is limited by relatively ill-defined mass transport conditions. Later work has involved linear sweep voltammetry as described by Hills and Johnson (54) in 1961 or steady state voltammetry with stationary electrodes by Swofford and Laitinen (55) in 1963. 

The voltammetric method, developed at ORNL for determination the [U(1V)]/[U(111)] ratio in the MSBR fuel salt [47], is based on measuring of the difference between the redox potential of the melt Feq and 1/2, the voltammetric equivalent of the standard redox potential of the U(1V)AJ(111) couple, at [U(IV)] S> [U(Ill)]. In conditions of linear voltammetry, at a stationary electrode and a reversible charge transfer of the melt-soluble oxidized and reduced forms of uranium, is approximately equal to the polarographic half-wave potential 1/2 and corresponds to the potential in the voltammogram, at which the current accounts for 85.2% of the peak current. The relationship between eq, 1/2 and [U(lV)]/[U(in)] ratio is given by the Nernst equation ... 

Both linear sweep voltammetry (oscillographic polarography, stationary electrode polarography, chronoamperometry with linear sweep) and chrono-potentiometry have been extensively applied for studies in molten salts. The advantages of linear sweep voltammetry include (1) extensively developed theory enabling the experimentalist to interpret the mechanisms of relatively complex electrode reactions (2) well-defined mass transfer conditions, particularly when faster scan rates ( 1 V/sec) are employed (3) the decrease of the faradaic charge with the square root of the scan rate and the resulting decrease of any modifications of the solid electrode caused by the faradaic process. Chronopotentiometry, 29) related electroanalytical... 

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