Bulk electrolysis methods

The methods described in Chapters 5 to 10 generally employ conditions featuring a small ratio of electrode area, A, to solution volume, V. These allow the experiments to be carried out over fairly long time periods without appreciable changes of the concentrations of the reactant and the products in the bulk solution, and they allow the semi-infinite boundary condition (e.g., Cq(x, 0 = C q as x oo) to be maintained over repeated trials. For example, consider a 5 X 10 M solution of O with V = 100 cm and A = 0.1 cm. Assume that during 1 hour of experimentation an average current of about 100 fiA flows (i.e., current density, 7, of 1 mA cm ). During this time period only 0.36 C of electricity will be passed, and the bulk concentration of electroactive species will have decreased by less than 1%. 

The methods can be classified by the controlled parameter (E or i) and by the quantities actually measured or the process carried out. Thus in controlled-potential techniques the potential of the working electrode is maintained constant with respect to a reference electrode. Since the potential of the working electrode controls the degree of completion of an electrolytic process in most cases, controlled-potential techniques are usually the most desirable for bulk electrolysis. However, these methods require potentiostats with large output current and voltage capabilities and they need stable reference electrodes, carefully placed to minimize uncompensated resistance effects. Placement of the auxiliary electrode to provide a fairly uniform current distribution across the surface of the working electrode is usually desirable, and the auxiliary electrode is often placed in a separate 

The general considerations and models employed in electroanalytical bulk electrolysis methods are also often applicable to large-scale and flow electrosynthesis, to galvanic cells, batteries, and fuel cells, and to electroplating. 

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. 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

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Bard, A.J., and L.R. Faulkner. 1980. Bulk electrolysis methods, in Electrochemical Methods Fundamentals and Applications, AJ. Bard and L.R. Faulkner (eds.), John Wiley Sons, Inc., New York, pp. 370-428. 

Thin layer — A layer of -+ electrolyte solution (molten salt electrolyte, - ionic liquid) of about 2 to 100 pm thickness is commonly treated as a thin layer because of particular properties and behavior. In bulk - electrolysis methods the amount of convertible species contained in a thin layer is very limited, thus exhaustive electrolysis becomes feasible. In numerous spectroelec-trochemical setups the electrolyte solution confined between the electrode surface under investigation and the... 

Since we have not previously considered, what happens when current is present in an electrochemical cell, we begin with a discussion of this. Then bulk electrolysis methods are discussed in some detail. The voltammetric methods described in Chapter 23 also require a net current in the cell but use such small electrode areas that no appreciable changes in bulk concentrations occur. 

The bulk electrolysis method may be applied in a constant potential (potentiostatic) or constant current (galvanostatic) modes. In a controlled potential electrolysis (CPE) experiment, the working electrode is held at a constant potential until the solution is exhaustively electrolyzed. The... 

Tsujimura S, Kuriyama A, Fujieda N, Kano K, Ikeda T. Mediated spectroelectrochemical titration of proteins for redox potential measurements by a separator-less one-compartment bulk electrolysis method. Anal Biochem 2005 337 325-331. 

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