Cells coulometric efficiency

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... 

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... 

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. 

The OCV depends on the DOD and varies from 2.08 to 1.78 V. The operating temperature of a sodium/sulfur cell must be above 285 °C in order to avoid solidification of the reaction product, sodium polysulfide. This would increase the internal resistance of the cell and could result in cell failures. During charging and discharging the internal resistance is nearly constant, but at the end of the charging process a steep increase of the internal resistance occurs. This resistance increase is used to identify the end of charge. The cell reaction is completely reversible, with no side reactions, which means the coulometric efficiency is 100%. 

Two distinctly different coulometric techniques are available (1) coulometric analysis with controlled potential of the working electrode, and (2) coulometric analysis with constant current. In the former method the substance being determined reacts with 100 per cent current efficiency at a working electrode, the potential of which is controlled. The completion of the reaction is indicated by the current decreasing to practically zero, and the quantity of the substance reacted is obtained from the reading of a coulometer in series with the cell or by means of a current-time integrating device. In method (2) a solution of the substance to be determined is electrolysed with constant current until the reaction is completed (as detected by a visual indicator in the solution or by amperometric, potentiometric, or spectrophotometric methods) and the circuit is then opened. The total quantity of electricity passed is derived from the product current (amperes) x time (seconds) the present practice is to include an electronic integrator in the circuit. 

Voltammetry experiments are occasionally undertaken in the form of a tubular or rectangular channel through which the electrolyte solution is pumped at a more or less constant velocity. The electrode may form the channel itself or be embedded in the wall of an inert material, which defines the flow pattern. Sometimes the channel is packed with small particles of electrode material in contact with each other. The latter situation is designed to improve the conversion efficiency of the cell. When all the electroactive molecules are converted during passage through such a porous bed, the efficiency is 100% and the cell is said to be operating coulometrically (see Sec. IV.F). 

The time required for a simple controlled-potential coulometric determination is determined by the efficiency of mass transport in the cell. The current decays exponentially according to the equation... 

In coulometry the stoichiometry of the electrode process should be known and should proceed with 100% current efficiency, and the product of reaction at any other electrode must not interfere with the reaction at the electrode of interest. If there are intermediate reactions, they too must proceed with the desired accuracy. In practice the electrolytic cell is designed to include isolation chambers. Losses of solute through diffusion, through ionic or electrical migration, and simply through bulk transfer must be minimal. Finally, the end point has to be determined by one of the many techniques used in titrations generally, whether coulometric or not. Both indeterminate and determinate end-point errors limit the overall accuracy achieved. Cooper and Quayle critically examined errors in coulometry, and Lewis reviewed coulometric techniques. 

The coulometric determination of chloride provides another example of an indirect process. Flcrc, a silver electrode is the anode, and silver ions are produced by the current. These cations diffuse into the solution and precipitate the chloride. A current efficiency of 1(X)% with respect lo the chloride ion is achieved even though this ion is neither oxidized nor reduced in the cell. 

In the preceding section, we mostly considered cases wherein only a thin segment of the electroactive region (whether the solution or the film phase) was electro-chemically altered. This situation must be contrasted with those in which exhaustive electrolysis is involved. An example is constant-potential coulometry (Fig. 20.4) wherein the entire solution contained within the cell is electrolyzed. As mentioned earlier, this is ensured by the use of a large A/V ratio and efficient solution agitation. The underlying coulometric equation derives from Faraday s law of electrolysis and can be expressed as... 

When high electrochemical conversion efficiency of dilute solution species is required (e.g., electrochemical detection), a working electrode design common in coulometric flow cells is a porous flow-through electrode. Recently, this type of emitter electrode was implemented into an ES emitter system (Figure 3.9). This electrode design, because of the very small pore size, provided for efficient mass transport to the electrode surface even at flow rates of several hundred microliters per minute. 

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