Coulometric efficiency

The reciprocal value, / = l/V/Ah of the coulometric efficiency is called the charging factor. The coulometric efficiency for electrochemical energy conversion is about 70-90 percent for nickel/cadmium and nearly 100 percent for lithium-ion accumulators [14]. 

The multichannel coulometric detection system serves as a highly sensitive tool for the characterization of antioxidant phenolic compounds because they are electroactive substances that usually oxidize at low potential. The coulometric efficiency of each element of the array allows a complete voltammetric resolution of analytes as a function of their oxidation potential. Some of the peaks may be resolved by the detector even if they coelute (Floridi and others 2003). 

The existence of a non-faradaic component to the overall current explains why the amount of material formed by electrochemical formation will generally be less than the theoretical amount, since the theoretical amount relates only to ffaradaic- Clearly, the coulometric efficiency should be maximized, i.e. /non-faradaic should be minimized by careful ehoice of experimental design, reagents and apparatus. Note that coulometric efficiency is also called faradaic efficiency. 

Coulombic efficiency - coulometric efficiency Coulombmeter - coulometer... 

In electrogenerated chemiluminescence (see -> electrochemiluminescence) coulometric efficiency is related to the amount of photons generated per electrons passing in the electrochemical process. See also -> faradaic efficiency. 

Faradaic efficiency (or coulometric efficiency) — Relates the moles of product formed in an -> electrode reaction to the consumed -> charge. The faradaic efficiency is 1.00 (or 100%) when the moles of product correspond to the consumed charge as required by -> Faraday s law. See also -> current efficiency, -> coulometry. 

In a separate set of experiments, the artificial cell model was used to develop models of coulometric efficiency as a function of electrode size and to determine the size of the space between the electrode and the membrane (80). In this case, coulometric efficiency is defined as the ratio of the total number of molecules detected from a vesicle to the total... 

To model coulometric efficiency and determine the size of the electrode membrane gap, a simple geometric representation of the system was devised (Figure 17.1.9). The space between the electrode and the membrane was modeled as a cylinder and the ratio of the vesicle volume to cylinder volume determines the coulometric efficiency. If the vesicular volume is smaller than the cylinder volume, 100% oxidation is expected. 

Figure 17.1.9 Simple model of coulometric efficiency for artificial exoc5dosis. This first-stage model assumes that the efficiency of oxidation for material released is simply the ratio of the memhrane-electrode space (calculated as nrj-h) over the volume of the vesicle (4 nr l i). This assumes that aU the catechol that is present in the membrane-electrode space after exocytosis will be oxidized, but also that only catechol in the solution that fits in this volume will be oxidized. Reproduced with permission from reference (80). (for colour version see colour section at the end of the book).

Thus the predicted coulometric efficiency is the ratio of the sum of the charge from the two stages of opening over The resulting equation that predicts percent coulometric efficiency is given as... 

Only 25% of the Cottrell equation is used in the final expression to correct for the diminished diffusion in the narrowing frustum. This model correctly predicts the shape of the percent coulometric efficiency vs. vesicle radius however, the predicted magnitude is 2.6 times larger than that observed. To correct this a factor of 0.38 has been applied to the theoretical prediction. Experimental data, predicted coulometric efficiency, and the best fit of the experimental data is shown in Figure 17.1.11. 

Applying this model to exocytosis in biological systems permits the prediction of coulometric efficiency for any size electrode if the vesicle size is known. Two cell types commonly used in exocytosis experiments, adrenal chromaffin and PC12 cells, have average vesicle radii of 99 and 125 run, respectively (33, 48). The model set forth here predicts... 

Figure 17.1.11 Coulometric efficiencies for a data set obtained with a beveled 33- tm electrode ( ) compared to data obtained with a 5-pm electrode ( ) for release of catechol measured from a range of vesicle sizes. This is compared to the theoretical coulometric efficiencies for these electrode dimensions calculated with the conditions outlined in Figure 17.1.5 and with the model discussed in the text (dashed lines). These are compared to each set of experimental data, which are shown with best-fit equations (thick lines). The equations for all lines, and their correlation coefficients, are given with the symbols for experimental data sets and for the modeled efficiencies with that for the 5- jm electrode to the left and that for the 33-pm electrode to the right. Reproduced with permission from reference (80). (for colour version see colour section at the end of the book).

Commercially available cells with rate constant of 500 s and a cell volume of about 5 pi assure coulometric efficiency for typical HPLC flow rates with minimal extra-column band broadening. Each electrochemical unit has a central porous carbon electrode, on either side of which is situated a reference electrode and an auxiliary electrode. The characteristics of porous graphitic carbon electrode facilitate the construction of electrode arrays, lypical commercial systems include two units placed in series but arrays of up to 16 units are commercially available (Thermo Scientific, formerly ESA/Dionex). These cells have some degree of resistance to flow and with use can develop a significant back pressure. To minimize such back pressure changes, they need to be protected from particulate materials. Their intrinsic back pressure should also be borne in mind when connecting other types of HPLC detector cell in series. 

This mode is characterized by an incomplete electrochemical conversion of the electroactive analytes that reach the working electrode. In fact, a coulometric efficiency of only 5-10% is usually attained. Consequently, during the residence time of an analyte band flowing through the detector, there is always unreacted analyte in the vicinity of the working electrode. Therefore, the amperometric current measured under these conditions is controlled by the analyte diffusion from the bulk toward the electrode surface which is characterized by a limiting current defined as... 

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