Electrodes controlled-potential coulometry

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. 

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. 

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. 

Electrochemistry of [VO(acac)2] in DMSO has been studied by cyclic voltammetry and controlled-potential coulometry at a platinum electrode.516 [VO(acac)2] is irreversibly reduced by one electron at —1.9 V vs. SCE (saturated calomel electrode) to a stable Vm product. In the presence of an excess of ligand, [VO(acac)2] is reduced by two electrons to [V(acac)3] with the V111 species mentioned above and [V(acac)3] as intermediates. The one-electron oxidation of [VO(acac)2] at +0.81 V vs. SCE gives a product. 

Controlled-potential coulometry in a three-electrode cell is more selective than constant-current coulometry. Because the working electrode potential is constant, current decreases exponentially as analyte concentration decreases. Charge is measured by integrating current over the time of the reaction ... 

Controlled-potential coulometry has been applied extensively to precision determinations [69] and to the establishment of n values for electrode reactions. The technique has also been used for the elucidation of electrode reactions [70,71]. It is a very useful technique when used in conjunction with a thin-layer cell where complete electrolysis is rapid (see Sec. II.E). 

Figure 9.5 Controlled-potential coulometry cell with a mercury pool working electrode. a, Platinum wire contact to mercury pool working electrode b, mercury pool working electrode c, reference electrode d, auxiliary electrode e, porous Vycor f, sample solution g, inert gas inlet h, stirrer i, reference electrode salt bridge j, clean mercury k, waste. [From Ref. 11, adapted with permission.]...

Controlled-potential coulometry (CPC) provides a direct measure of the number of electrons transferred in an electrode reaction. It is widely employed in molten salts for this purpose. Background information about CPC is given in Chapter 3 of this... 

Controlled-potential coulometry (Chap. 3, Sec. IV.F) has been used in pharmaceutical analysis for determination of pure compounds and compounds in their dosage forms, and in studies to determine the nature of electrode reactions. Controlled-potential coulometry as an analysis tool has exhibited its greatest utility in the measurement of inorganic ions and nitro and halogenated com-... 

Recent studies describe the use of cyclic voltammetry in conjunction with controlled-potential coulometry to study the oxidative reaction mechanisms of benzofuran derivatives [115] and bamipine hydrochloride [116]. The use of fast-scan cyclic voltammetry and linear sweep voltammetry to study the reduction kinetic and thermodynamic parameters of cefazolin and cefmetazole has also been described [117]. Determinations of vitamins have been studied with voltammetric techniques, such as differential pulse voltammetry for vitamin D3 with a rotating glassy carbon electrode [118,119], and cyclic voltammetry and square-wave adsorptive stripping voltammetry for vitamin K3 (menadione) [120]. 

In controlled-potential coulometry, the potential of the working electrode is maintained at a constant level such that only the analyte is responsible for conducting charge across the electrode/solution interface. The charge required to convert the analyte to its reaction product is then determined by recording and integrating the current-versus-time curve during the electrolysis. 

In controlled-potential coulometry, the working electrode potential is maintained at a constant value with respect to a reference electrode. In constant-current coulometry, the cell is operated so that the current is maintained at a constant value. 

In controlled-potential coulometry the total number of coulombs consumed in an electrolysis is used to determine the amount of substance electrolyzed. To enable a coulometric method, the electrode reaction must satisfy the following requirements (a) it must be of known stoichiometry (b) it must be a single reaction or at least have no side reactions of different stoichiometry (c) it must occur with close to 100% current efficiency. 

There have been numerous applications of controlled-potential coulometry to analysis. Many electrodeposition reactions that are the basis of electrogravimetric determinations can be employed in coulometry as well. However, some electrogravimetric determinations can be used when the electrode reactions occur with less than 100% current efficiency, for example, the plating of tin on a solid electrode. Coulometric determinations can, of course, also be based on electrode reactions in which soluble products or gases are formed (e.g., reduction of Fe(III) to Fe(II), oxidation of 1 to I2, oxidation of N2H4 to N2, reduction of aromatic nitro compounds). Many reviews concerned with controlled-potential coulometric analysis have appeared (1, 20-22) some typical applications are given in Table 11.3.2. 

Controlled-potential coulometry is also a useful method for studying the mechanisms of electrode reactions and for determining the n-value for an electrode reaction... 

Controlled-potential coulometry may be applied to the analysis of a wide variety of substances. Clearly, metals like copper could be deposited and determined without the necessity of weighing the electrode. More importantly, mercury electrodes can be used and thus most of the metals more difficult to reduce can be determined. Also, it is possible to apply coulometry to systems in which both oxidized and reduced forms are soluble, such as determining iron by reducing iron(III) to iron(II). Anions such as chloride or bromide may be converted to AgCl or AgBr by deposition on a silver anode. 

Controlled-potential coulometry has also found some use in the study of basic electrochemistry. It is not always obvious how many electrons are involved in a newly studied electrochemical reaction, e.g., in polarography. Thus, coulometry at controlled potential, in which a known quantity of the substance is electrolyzed and Q is measured, is often used to determine values for n and thereby help elucidate electrode mechanisms for a wide variety of compounds, both organic and inorganic. Very slow chemical reactions coupled with the electrochemical reaction may also be studied by controlled-potential coulometry [4] other electrochemical techniques usually are suitable only for much faster chemical reactions, with time scales of jusec to sec. 

Standard Test Method for Plutonium by Controlled-Potential Coulometry Standard Test Method for Determining Plutonium by Controlled-Potential Coulometry in H2SO4 at a Platinium Working Electrode... 

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