Coulometry bulk electrolysis

The oxidation state of Au in both Au-oxo complexes 3 and 4 was thoroughly investigated by several chemical and physicochemical methods 44). First, bulk electrolysis (coulometry at controlled potential) confirms the Au(III) oxidation state assignment in both 3... 

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. 

Chapter 22 Bulk Electrolysis Electrogravimetry and Coulometry 633 Chapter 23 Voltammetry 665... 

Bulk electrolysis methods are also classified according to purpose. For example, one form of analysis involves determination of the weight of a deposit on the electrode (electrogravimetry). In this case 100% current efficiency is not required, but the substance of interest must be deposited in a pure, known form. In coulometry, the total quantity of electricity required to carry out an exhaustive electrolysis is determined. The quantity of material or number of electrons involved in the electrode reaction can then be determined by Faraday s laws, if the reaction occurred with 100% current efficiency. For electroseparations, electrolysis is used to remove, selectively, constituents from the solution. 

The theory for the different reaction schemes involves ordinary (rather than partial) differential equations, because the electrolyzed solution is assumed to be essentially homogeneous (see Section 11.3.1). The concentrations are functions of t during the bulk electrolysis, but not of x. The measured responses in coulometry are the i-t curves and the apparent number of electrons app consumed per molecule of electroactive compound. From the quantity of electricity passed during the electrolysis Q t), app can be calculated as... 

Another popular mode for transmission experiments involves a thin-layer system (9, 10, 13, 18) like that shown in Figure 17.1.2. The working electrode is sealed into a chamber (e.g., between two microscope slides spaced perhaps 0.05-0.5 mm apart) containing the electroactive species in solution. The chamber is filled by capillarity, and the solution within it contacts additional solution in a larger container, which also holds the reference and counter electrodes. The electrolytic characteristics of the cell are naturally similar to those of the conventional thin-layer systems discussed in Section 11.7. One can do cyclic voltammetry, bulk electrolysis, and coulometry in the ordinary way, but there is also a facility for obtaining absorption spectra of species in the cell. 

Bulk electrolysis for the purpose of electrosynthesis or for a coulometric determination of the number of electrons (w) associated with a half-cell reaction of the kind A B + neT (A is the compound being oxidized and B is product, with charges being omitted for simplicity, and n is the overall number of electrons transferred per molecule of A oxidized as determined by coulometry and application of Faraday s Law of electrolysis). 

Controlled potential bulk electrolysis or coulometry, as it is often called, is widely used to determine the overall number of electrons involved in an electrode process. It is also used to prepare a sufficient quantity of the reaction products to enable them to be identified by the application of conventional analytical techniques. 

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. 

In this present section, we will look at more specific forms of coulometry. Other than electrolysis of a bulk solution, the most common technique is stripping, which is the method of choice when only a tiny amount of analyte is in solution. 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

Kihara et al. employed flow coulometry to study the electrode reactions for Np ions in various acidic media [49]. Flow coulometry has an inherent advantage over the conventional bulk coulometry methods in that the electrolysis can be achieved rapidly to aid in the characterization of unstable electrode products. The resulting coulopo-tentiograms for the NpO2 /NpO2 " and Np +/Np + couples indicate reversible processes in nitric, perchloric, and sulfuric acids. The differences in potentials between the various acids are attributed to the associated stability constants of the electrode products with the anion of the acid in each case. Table 2 contains the half-wave potentials for each couple in the various acids. 

In cases where electrolysis of the solution bulk is not desired (as in voltammetry or chronoamperometry), the working electrode area is kept reasonably small (nominal dimensions of square millimeters). Such an electrode is termed a microelectrode. On the other hand, in electrolysis procedures (as in thin-layer cells or coulometry), the ratio of electrode area to solution volume (A/V) must be maximized. Large-area working electrodes [grids or porous electrodes such as reticulated vitreous carbon (RVC)j are used in such cases. 

A platinum button and a platinum wire served as the working and auxilary electrodes respectively for cyclic and differential pulse voltam-metric measurements made using a conventional three-electrode configuration and an IBM Model EC225 voltammetric analyzer. A saturated calomel electrode (SCE), separated from the bulk of the solution by a fritted glass disk, was used as the reference electrode. A Princeton Applied Research Model 173 potentiostat with Model 179 digital coulometer was used for controlled-potential electrolysis (CPE) and coulometry. All potentials are reported versus aqueous SCE. 

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