Double-step chronocoulometry

Fig. 4 Chronocoulometry (a) typical charge response (b) Anson plot for a double-step chronocoulometric experiment.

Double step chronocoulometry also gives information on the kinetics of coupled homogeneous reactions8. For example, any deviation, under diffusion control, from (10.35) and (10.36) implies a chemical complication, which can be compared with the responses for the various possible mechanisms. 

Double-step chronocoulometry is a powerful tool in identifying adsorption phenomena, in obtaining information on the kinetics of coupled homogeneous reactions and for the determination of the capacitive contribution. The double potential step is executed in such a way that after the first step from E to E2, a next step is applied, i.e., the reversal of the potential to its initial value Ei from 2 (see Fig. 3). 

Chronocoulometry — Figure 4. Chronocoulometric plots for double step experiments. Lines 1 and 2 correspond to the case when no adsorption of the reactant or product occurs. Lines 3 and 4 depict the linear responses when the reactant is adsorbed... 

Both the physical stability of the electrodes and the stability of response towards a particular analyte have been considered. The former concern arises out of the mechanism of silver oxidation which includes the formation of a soluble hydroxy species. To measure any loss of electrode upon potential conditioning a coulometric experiment was conducted. A series of double-step chronocoulometry experiments were performed and the limiting charge for the oxidative and reductive steps were compared as a function of potential. A loss of electrode is observed during the initial formation of silver(I) oxide. From the geometrical area of the electrode it is estimated that a loss of about 0.1 pm occurs upon conditioning the surface at 450 mV. The practical consequence is that electrodes need to be repolished daily. 

Double-Step chronocoulometry is also extremely useful for characterizing coupled homogeneous reactions. Any deviation from the coulometric responses described by Eqs. (II.4.3) and (II.4.11) - providing that diffusion control prevails -implies a chemical complication. For example, O rapidly reacts with a component of the solution, and this homogeneous chemical reaction results in the formation of an electrochemically inactive species. Qmit > t) falls less quickly than expected or, at complete conversion within the timescale of the experiment, no backward reaction is seen at all. A quick examination of this effect can be carried out by the evaluation of the ratio of Qm (t = 2x) Q t = x). For stable systems this ratio is between 0.45 and 0.55. 

Double-step chronocoulometry is a powerful tool in identifying adsorption phenomena, in... 

Potential Step Chronocoulometry Double Potential Step Chronocoulometry ... 

The equation of double-step chronocoulometry was given by Anson (1966) and Christie et al. (1967) as follows ... 

Determinations of rate constants for the catalytic process were carried out by double potential step chronocoulometry... 

Reconstruction of Au(lll) is observed in STM images as double rows separated from each other by 6.3 nm [335]. Some model calculations have been performed [362] to show that the energy difference of the reconstructed and unreconstructed Au(lll) is small. The effect of Triton X-100 on the reconstruction process of Au(lll) surface has been studied in chloride media [363] applying CV and double potential-step chronocoulometry. It has been found that adsorption of Triton X-100 stabilizes the reconstructed face of Au(lll). Hobara etal. [364] have used in situ STM to study reconstruction of Au(lll), following reductive desorption of 2-mercaptoethanesulfonic acid SAMs. 

Indicators and Dyes Abdel-Hamid [154] has studied adsorption of phe-nolphthalein at a HMDE in aqueous buffer solutions containing 10% v/v ethanol, applying cychc voltammetry and double potential-step chronocoulometry. At pH =... 

Double potential step Chronocoulometry with current reversal... 

Figure 3.12 Double-potential-step thin-layer electrochemistry. (A) Chronoampero-metry. (B) Chronocoulometry. Dashed line, same experiment for semi-infinite situation.

Potential-step chronocoulometry, see Chronocoulo-metry and Double potentia1-step chronocoulometry... 

To a large extent, the discovery and application of adsorption phenomena for the modification of electrode surfaces has been an empirical process with few highly systematic or fundamental studies being employed until recent years. For example, successful efforts to quantitate the adsorption phenomena at electrodes have recently been published [1-3]. These efforts utilized both double potential step chronocoulometry and thin-layer spectroelectrochemistry to characterize the deposition of the product of an electrochemical reaction. For redox systems in which there is product deposition, the mathematical treatment described permits the calculation of various thermodynamic and transport properties. Of more recent origin is the approach whereby modifiers are selected on the basis of known and desired properties and deliberately immobilized on an electrode surface to convert the properties of the surface from those of the electrode material to those of the immobilized substance. 

The double potential step is very powerful in identifying adsorption phenomena by chronocoulometry. From (10.34),... 

Chronocoulometry — Figure 3. The waveform (a), the chronoamper-ometric (b), and chronocoulometric (c) responses in the case of double potential steps... 

Jan. 20, 1927, Cleveland, Ohio, USA - Aug. 10, 2004, Raleigh, NC, USA) Osteryoung received his bachelor s education at Ohio University and his Ph.D. at the University of Illinois. He was professor and Chairman of the Chemistry Department at Colorado State University, a professor at the State University of New York at Buffalo and research professor and Chair of the Department of Chemistry of North Carolina State University. He published about 225 original scientific papers, and was especially known for his papers on double potential step -> chronocoulometry, -> square-wave voltammetry, and room-temperature molten salt electrochemistry. He also initiated computer-controlled electrochemical measurements, which helped in developing and optimizing - pulse voltammetry. He served as an Associate Editor for the journal Analytical Chemistry. 

Double potential-step chronocoulometry [1,2,221] may be used similarly to DPSCA. The working curves now include the charge ratio —Qb/Qf, which takes the value 0.414 for a simple electron transfer reaction. The reductive cyclization of ethyl cinnamate (see Chapter 21) illustrates the use of the technique [226,227]. 

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