General Cell Designs

The exact design of an electrochemical cell varies with the specific needs of an experiment. On the laboratory scale, typically if the amount of analytes is not a concern, a 25-50 mL cell (or even larger) can be used for the sake of convenience. With limited quantities of samples, a solution volume of a few mL is reasonable. Even smaller volumes of sample solutions (say, oL) are also possible. However, in these cases, electrodes of ultrasmaU dimensions (UMES, see Chapter 6) will have to be used and aligned properly. Complication in the current/potential measurements might occur as a result from solution resistance (see Chapters 1 and 3) and heterogeneity of the electrolyte solutions. These may render data analysis difficult and sometimes ambiguous. 

In this chapter, we will describe several examples of electrochemical cell designs for a variety of systems and applications in a research laboratory. It should be noted that the 

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Fortunately, it turns out that the general cell design that is suitable for room temperature measurements (cf. Chap. 9) will suffice at low temperatures as well. Glass is the preferred material of construction. There are two general cell types dip-type cells that are designed to be immersed in a bath of coolant, and jacketed cells whose contents are cooled by passage of coolant fluid through the jacket. 

Current efficiency depends on operating characteristics, eg, pH, temperature, and cell design, and is generally in the 90—98% range. The cell voltage is a function of electrode characteristics and electrolyte conductivity and can be expressed as... 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

With the multitude of transducer possibilities in terms of electrode material, electrode number, and cell design, it becomes important to be able to evaluate the performance of an LCEC system in some consistent and meaningful maimer. Two frequently confused and misused terms for evaluation of LCEC systems are sensitivity and detection limit . Sensitivity refers to the ratio of output signal to input analyte amount generally expressed for LCEC as peak current per injected equivalents (nA/neq or nA/nmol). It can also be useful to define the sensitivity in terms of peak area per injected equivalents (coulombs/neq) so that the detector conversion efficiency is obvious. Sensitivity thus refers to the slope of the calibration curve. 

The space time yield, therefore, depends on the current density and the specific cathode area. In an electrolytic cell, the specific cathode area is generally very small thus the space time yield is low as compared to most of the pyrometallurgical reactors. A good cell design should aim at obtaining a high value of the space time yield. 

It should be anticipated that there will not be a smooth transition from these idealized, simple systems into the real world. Some precautions and pitfalls have been cited, but usually in a parenthetical manner that lacked proper emphasis. The systems selected to illustrate the general principles of potentiostatic control have shown what can be expected under ideal conditions, but real systems have additional parameters that may tilt the balance from graceful control to chaos. Cell design is of paramount importance, and a guide to transfer characteristics of cells is included in the bibliography. To bring the information in this chapter effectively into use, it is necessary to acknowledge the role that cells play... 

Many variations of the design of the cells shown in Figure 9.3 have been reported. Reasonable cost, simple construction, flexibility, and ease of correct use have led to widespread acceptance of these general designs. These cell designs fit most aqueous and nonaqueous sample requirements where the presence of oxygen or water is not critical. When the removal and exclusion of these contaminants is required, special care must be taken to work in an inert atmosphere. A dry box (Chap. 19) or a cell that can be interfaced to a vacuum line (Chap. 18) may be required. 

In dilute solutions, the avoidance of local or general polarization is the most crucial task of the cell designer. The so-called tortuous path spacer is utilized in Ionics electric membrane systems (8). A tortuous path spacer consists of one or more flow paths—for example, approximately 1 cm. in width—traversing the face of a mem-... 

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