Coulometry electrode

Brenes, M., Garcia, A., Garcia, R, and Garrido, A. Rapid and complete extraction of phenols liom olive oil and determination by means of a coulometrie electrode array system. Journal of Agriculture and Food Chemistry, 48,5178-5183. 2000. 

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. 

From this equation we see that increasing k leads to a shorter analysis time. For this reason controlled-potential coulometry is carried out in small-volume electrochemical cells, using electrodes with large surface areas and with high stirring rates. A quantitative electrolysis typically requires approximately 30-60 min, although shorter or longer times are possible. 

Instrumentation Controlled-current coulometry normally is carried out using a galvanostat and an electrochemical cell consisting of a working electrode and a counterelectrode. The working electrode, which often is constructed from Pt, is also... 

The ability to control selectivity by carefully selecting the working electrode s potential, makes controlled-potential coulometry particularly useful for the analysis of alloys. For example, the composition of an alloy containing Ag, Bi, Cd, and Sb... 

Another area where controlled-potential coulometry has found application is in nuclear chemistry, in which elements such as uranium and polonium can be determined at trace levels. Eor example, microgram quantities of uranium in a medium of H2SO4 can be determined by reducing U(VI) to U(IV) at a mercury working electrode. 

Controlled-potential coulometry also can be applied to the quantitative analysis of organic compounds, although the number of applications is significantly less than that for inorganic analytes. One example is the six-electron reduction of a nitro group, -NO2, to a primary amine, -NH2, at a mercury electrode. Solutions of picric acid, for instance, can be analyzed by reducing to triaminophenol. 

Controllcd-Currcnt Coulomctry The use of a mediator makes controlled-current coulometry a more versatile analytical method than controlled-potential coulome-try. For example, the direct oxidation or reduction of a protein at the working electrode in controlled-potential coulometry is difficult if the protein s active redox site lies deep within its structure. The controlled-current coulometric analysis of the protein is made possible, however, by coupling its oxidation or reduction to a mediator that is reduced or oxidized at the working electrode. Controlled-current coulometric methods have been developed for many of the same analytes that may be determined by conventional redox titrimetry. These methods, several of which are summarized in Table 11.9, also are called coulometric redox titrations. 

Studies aimed at characterizing the mechanisms of electrode reactions often make use of coulometry for determining the number of electrons involved in the reaction. To make such measurements a known amount of a pure compound is subject to a controlled-potential electrolysis. The coulombs of charge needed to complete the electrolysis are used to determine the value of n using Faraday s law (equation 11.23). 

In voltammetry the working electrode s surface area is significantly smaller than that used in coulometry. Consequently, very little analyte undergoes electrolysis, and the analyte s concentration in bulk solution remains essentially unchanged. 

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. 

Coulometry. If it can be assumed that kinetic nuances in the solution are unimportant and that destmction of the sample is not a problem, then the simplest action may be to apply a potential to a working electrode having a surface area of several cm and wait until the current decays to zero. The potential should be sufficiently removed from the EP of the analyte, ie, about 200 mV, that the electrolysis of an interferent is avoided. The integral under the current vs time curve is a charge equal to nFCl, where n is the number of electrons needed to electrolyze the molecule, C is the concentration of the analyte, 1 is the volume of the solution, and F is the Faraday constant. 

Coulometry measures the amount of cunent flowing dirough a solution in an electrochemical oxidation or reduction reaction and is capable of measuring at ppm or even ppb levels of reactive gases. Thus a sample of ambient air is drawn through an electrolyte in a cell and the required amount of reactant is generated at the electrode. This technique tends to be non-specific, but selectivity can be enhanced by adjustment of pH and electrolyte composition, and by incorporation of filters to remove interfering species. 

Ammonia Coulometry (e.g. Nessler method) Ion selective electrode Oxidation to NO and chemiluminescence... 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

Lingane and Niedrach have claimed that the h-VI states of tellurium (or selenium) are not reduced at the dropping electrode under any of the conditions of then-investigation however, Norton et al. [42] showed that under a variety of conditions, samples of telluric acid prepared by several different procedures do exhibit well-defined (though irreversible) waves, suitable for the analytical determination of the element. The reduction of Te(H-VI) at the dropping electrode was found coulometri-cally to proceed to the -II state (whereas selenate, Se(-i-VI), was not reduced at the dropping electrode in any of the media reported). 

In coulometry, one must define exactly the amount of charge that was consumed at the electrode up to the moment when the endpoint signal appeared. In galvanosta-tic experiments (at constant current), the charge is defined as the product of current and the exactly measured time. However, in experiments with currents changing continuously in time, it is more convenient to use special coulometers, which are counters for the quantity of charge passed. Electrochemical coulometers are based on the laws of Faraday with them the volume of gas or mercury liberated, which is proportional to charge, is measured. Electromechanical coulometers are also available. 

For coulometric analysis, the substance being examined must react in 100% current yields [i.e., other (secondary) reactions must be entirely absent]. In efforts to avoid side reactions, coulometry most often is performed potentiostatically (amperometrically) (i.e., the electrode potential is kept constant during the experiment), and the current consumed at the electrode is measured. The current is highest at the start of the... 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

Coulometry and amperometry can be distinguished by the extent to which the analyte undergoes a Faradaic reaction at the working electrode, namely complete and partial, respectively. Coulometry is essentially high-efficiency amperometry with working electrodes of large surface area. Successful coulometric or amperometric detection can result only if the applied potential is chosen correctly. 

In coulometry these exchange membranes are often used to prevent the electrolyte around the counter electrode from entering the titration compartment (see coulometry, Section 3.5). However, with membrane electrodes the ion-exchange activity is confined to the membrane surfaces in direct contact with the solutions on both sides, whilst the internal region must remain impermeable to the solution and its ions, which excludes a diffusion potential nevertheless, the material must facilitate some ionic charge transport internally in order to permit measurement of the total potential across the membrane. The specific way in which all these requirements are fulfilled in practice depends on the type of membrane electrode under consideration. 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

In electroanalysis, coulometry is an important method in which the analyte is specifically and completely converted via a direct or indirect electrolysis, and the amount of electricity (in coulombs) consumed thereby is measured. According to this definition there are two alternatives (1) the analyte participates in the electrode reaction (primary or direct electrolysis), or (2) the analyte reacts with the reagent, generated (secondary or indirect electrolysis) either internally or externally. 

Coulometry. Even in water, controlled potential or potentiostatic coulometry is a difficult and often time-consuming technique, as the analyte must participate in a direct electrode reaction. Therefore, in non-aqueous media there are only a few examples of its application, e.g., the potentiostatic coulometry of nitro and halogen compounds in methanol (99%) with graphical end-point prediction, as described by Ehlers and Sease153. 

---