BULK ELECTROLYSIS

CHAPTER 11

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., C0(x, t) = CQ as x —> oo) to be maintained over repeated trials. For example, consider a 5 X 10~3 M solution of О with V = 100 cm3 and A = 0.1 cm2. Assume that during 1 hour of experimentation an average current of about 100 fiA flows (i.e., current density, 7, of 1 mA cm"2). During this time period only 0.36 С of electricity will be passed, and the bulk concentration of electroactive species will have decreased by less

than 1%.

There are circumstances, however, where one desires to alter the composition of the bulk solution appreciably by electrolysis; these include analytical measurements (e.g., electrogravimetric or coulometric methods), techniques for removal or separation of solution components, and electrosynthetic methods. These bulk (or exhaustive) electrolysis methods are characterized by large A/V conditions and as effective mass-transfer conditions as possible. Thus, if all of the conditions of the previous example hold, except that the electrode area is 100 cm2 (so that the total current is 0.1 A, assuming the same j = I mA cm~2), the electroactive species can be completely electrolyzed in less than 10 minutes (assuming n = 1 and the occurrence of only a single electrode process, that is, 100% current efficiency; see Section 11.2.2). Although bulk electrolytic methods are generally characterized by large currents and experimental time scales on the order of minutes and hours, the basic principles governing electrode reactions described in the previous chapters still apply.

11.1 CLASSIFICATION OF TECHNIQUES

The methods can be classified by the controlled parameter (E or /) 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 compartment isolated from the working-electrode compartment by a sintered-glass disk, ion-exchange membrane, or other separator.

In controlled-current techniques, the current passing through the cell is held constant (or is sometimes programmed to change with time or in response to some indicator electrode signal). Although these techniques frequently involve simpler instrumentation than controlled-potential methods, they require either a special set of chemical conditions in the cell or specific detection methods to signal completion of the electrolysis and to ensure 100% current efficiency. For preparative electrolysis (or electrosynthesis), constant-current methods can sometimes be used, as long as measures are taken to ensure that the electrode potential does not move into a region where undesirable side reactions occur.

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 (electro gravimetry). 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 A/V ratios are attained by trapping only a very small volume of solution in a thin (20-100 /nm) 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.

General treatments of bulk electrolysis techniques, as well as numerous examples of their application to analysis and separations, are contained in references 1-4.

11.2 GENERAL CONSIDERATIONS IN BULK ELECTROLYSIS

11.2.1 Extent or Completeness of an Electrode Process

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