Electrochemical analysis coulometry

Electrochemical analysis is composed of a vast range of techniques such as potentiometry, polarography, amperometry and coulometry. Selection of the technique best suited to the purpose and of the appropriate working conditions ensure a high degree of sensitivity and selectivity. 

Coulometry is a quantitative electrochemical analysis based on counting the total electrical... 

The results of the linearization of the polarization curves using Equation 4.5.2 for current densities corresponding to the range of concentrations in which Henry s law is fulfilled [16] are presented in Table 4.5.1. One can see that the value of n (number of electrons involved in the electrode process) is below two. Based on the conventional electrochemical analysis this indicates that vanadium is partially dissolved following a one-electron reaction. This however contradicts our data obtained previously by coulometry, oxidimetry, and potentiometry [16]. 

The largest division of interfacial electrochemical methods is the group of dynamic methods, in which current flows and concentrations change as the result of a redox reaction. Dynamic methods are further subdivided by whether we choose to control the current or the potential. In controlled-current coulometry, which is covered in Section IIC, we completely oxidize or reduce the analyte by passing a fixed current through the analytical solution. Controlled-potential methods are subdivided further into controlled-potential coulometry and amperometry, in which a constant potential is applied during the analysis, and voltammetry, in which the potential is systematically varied. Controlled-potential coulometry is discussed in Section IIC, and amperometry and voltammetry are discussed in Section IID. 

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

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. 

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. 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

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). 

Now returning to the coulometric analysis proper we can. say that any determination that can be carried out by voltammetry is also possible by coulometry whether it should be done by means of the controlled-potential or the titration (constant-current) method much depends on the electrochemical properties of the analyte itself and on additional circumstances both methods, because they are based on bulk electrolysis, require continuous stirring. 

Instructional Examples of Electrode Mechanisms of Transition Metal Complexes - 683 24. Electrochemical Preconcentration - 719 25. Controlled-Current Coulometry - 739 26. Electrochemistry in Pharmaceutical Analysis - 769... 

Recently a series of dialkylpyrrolidinium (Pyr+) cations have been studied in our laboratory 7-9). These cations are reduced at relatively positive potentials and could be investigated electrochemically as low concentration reactants in the presence of (C4H9)4N+ electrolytes. Using cyclic voltammetry, polarography and coulometry, it was shown that Pyr+ react by a reversible le transfer. The products are insoluble solids which deposit on the cathode and incorporate Pyr+ and mercury from the cathode. Both the cation and the metal can be regenerated by oxidation. Quantitative analysis of current-time transients, from potential step experiments, showed that the kinetics of the process involve nucleation and growth and resemble metal deposition. 

Coulometers, like the balance, are basic instruments for absolute analysis and they are still used as the most reliable and precise instruments for the analysis of absolute standards. Coulometers are frequently used in elucidating electrochemical reactions because they allow determining the number of transferred electrons when the molar amount of electrolyzed compound is known (-> Faraday s law). When the charge is measured as a function of time, the technique is called chrono-coulometry. See also coulometric titration. 

Coupling reactions of the electrogenerated cation radicals can also occur. If the parent molecule is a good nucleophile, this can be considered as a nucleophilic attack by parent on its own cation radical. The nature of the electrochemical responses in CV and coulometry depend upon whether the coupled product can undergo further oxidation. For example, in the oxidation of aromatic aza-hydrocarbons such as acridine (AcH), studied by Marcoux and Adams (1974), the CV was characterized by an irreversible one-electron wave and coulometry showed an napp-value of one. An analysis of rotating disk voltammograms demonstrated that the reaction sequence (79)-(81) was more probable them that involving... 

Neutron activation analysis Mass spectrometry Electrochemical Coulometry... 

An electrochemical analyzer is used to perform polarography, voltammetry, and coulometry experiments using dropping mercury, noble metal, rotating disk, and carbon electrodes. These techniques are amenable to the trace analysis of electroactive anions, cations, metals, and organics. Applications include the determination of individual aldehydes in polymer solutions by voltammetry and the determination of stabilizers in polymer formulations. 

Lewis TD (1961) Columetric methods in analysis a review. Analyst 86 494—506 Bard AJ (2001) Electrochemical methods, fundamentals and applications. Wiley, New York Harrar JE (1987) Analytical controlled-potential coulometry. TrAC Trend Anal (Them 6(6) 152-157... 

In the past, electrochemical detection has found extensive application for the simultaneous determination of a-tocopherol, a-tocopherolquinone, and ubiquinones in plasma, erythrocytes, and platelets (26,27). The use of coulometry with a dual cell configuration to this end has been explained above (see ni.A.3). One recent example of this type of assay is the paper by Finckh et al. (77). Electrochemical detection appears to have been ignored lately as a tool to push the detectability to its ultimately low limit. This approach permits a reduction of the sample size, which is of interest, e.g for monitoring of premature infants. Such miniaturized assays were discussed in a 1988 review paper (26). Finally, evaporative light-scattering detection has been used in connection with the analysis of oils (68,129,155). 

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