Coulometry at constant potential

Coulometry. Coulometry at Constant Potential. Coulometric Titrations. Applications of Coulometric Titrations. 

Coulometry at constant current is often considered as being less attractive than coulometry at constant potential. However, when the current density is low, the potential of the working electrode stays almost constant until approximately 90% of the substrate is consumed. Control of the current rather than the potential has, however, a number of advantages. First, the charge consumed during the reaction is directly proportional to the electrolysis time,... 

During coulometry at constant potential, the amount of charge is obtained by integration of the current-time curve (Fig. 41). 

Coulometry. Coulometry at constant potential. Coulonietric titrations. Applications of coulometric titrations. 

During coulometry at constant potential, the total amount of charge (g) is obtained by integration of the current (7) - time (0 curve or g can be determined directly by using a coulometer (electronic integrator). In principle, the end point 1 = 0, i.e., when the concentration of the species under study becomes zero, can be reached only at infinite time, however, in practice the electrolysis is stopped when the current has decayed to a few percent of the initial values. The change of I and g as a function of time at a constant potential >> e. for a stirred solutions and for an uncomplicated electrolysis, is as follows ... 

Quiescent Solutions. Coulometry at constant current provides a simple method for measuring the quantity of electrogenerated species as long as the reaction proceeds with 100% current efficiency. However, this condition breaks down with depletion of the electroactive material in the diffusion layer (cf. chronopotentiometric transitions see Fig. 4.3). For low values of the applied current, the thermal and density gradients supplement diffusion sufficiently to sustain electrolysis without the potential shifting to a different reaction. This mode of radical generation has been employed successfully in the study of stable species. 

A different way of determining n values is based on the measurement of the amount of charge necessary for the exhaustive electrolysis of a known amount of substrate. This type of experiment, traditionally called coulometry, may be carried out either at constant potential or at constant current. 

Coulometry. Two methods of coulometry are used coulometry at controlled potential and coulometric titrations. The main advantage of the coulometric method is the elimination of the necessity of standardization as the Faraday constant is a standard. In analysis of complicated samples encountered in environmental analysis the coulometric titrations are more advantageous where 100% current efficiency can be more readily attained by suitable choice of the reagent-solvent system. Coulometric titrations are suitable for determining the amount of substance in the range 0.01 to 100 mg (and sometimes below 1 iJg). Under optimum conditions these titrations can be carried out with a precision and accuracy of 0.01%. Automatic coulometric analyzers for the determination of gaseous pollutants (SO2, O3, NO, etc.) have proven to be useful in environmental chemistry. 

Coulometric Detectors. Coulometric detectors are based on potentiostatic coulometry [30]. The signal of the constant-potential measurements, as in amperometric detection, is the current resulting from an electron-transfer process (reduction or oxidation of the analyte arriving with the eluent) while the working electrode is held at constant potential. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

Controlled-potential coulometry involves nearly complete reduction or oxidation of an analyte ion at a working electrode maintained at a constant potential and integration of the current during the elapsed time of the electrolysis. The integrated current in coulombs is related to the quantity of analyte ion by Faraday s law, where the amps per unit time (coulomb) is directly related to the number of electrons transferred, and thus to the amount of analyte electrolyzed. 

Constant potential techniques at steady state Pulse voltammetry Stripping voltammetry Coulometry... 

The number of electrons exchanged on a time scale similar to that of a preparative electrolysis is determined by coulometry. A coulometry experiment involves the complete conversion of the substrate to product(s) and, accordingly, C 0 decreases with time, in principle to zero. This is in contrast to the electro analytical methods where C 0 stays essentially constant during the experiments. Coulometry is carried out at either constant potential or constant current and, usually, the solution is stirred magnetically. 

Reactions of hydrazine (N2Hs ), phenylhydrazine (N2H4Ph ) at pH 2.8 and azide (Nj-) at pH 5.3 with [Ru (HL)(H20)] and [Ru "L(H20)] (where L = ethylenediamminetetraacetate) were studied by voltammetry and spectrophotometry at 25 °C. The resultant complexes were electrolyzed in the presence of nitrogenous ligands in excess by constant potential coulometry at Hg-pool electrode. The turnover rates for the formation of ammonia and/or amine and the catalytic efficiency of these nitrogenous compounds were reported. An appropriate mechanism for the catalytic reduction of hydrazines and azide were proposed. 

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