Controlled-Potential Instrumentation

II. CONTROLLED-POTENTIAL INSTRUMENTATION BASED ON OPERATIONAL AMPLIFIERS... 

G. L. Booman and W. B. Holbrook, Electroanalytical Controlled-Potential Instrumentation, Anal. Chem. 35 1793 (1963). (Related publications in same issue.)... 

Amperometric detection Controlled-potential instrumentation Electrochemical detection Oxidation/reduction... 

Time, Cost, and Equipment Controlled-potential coulometry is a relatively time-consuming analysis, with a typical analysis requiring 30-60 min. Coulometric titrations, on the other hand, require only a few minutes and are easily adapted for automated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than 1000. 

The advantages of controlled-potential techniques include high sensitivity, selectivity towards electroactive species, a wide linear range, portable and low-cost instrumentation, speciation capability, and a wide range of electrodes that allow assays of unusual environments. Several properties of these techniques are summarized in Table 1-1. Extremely low (nanomolar) detection limits can be achieved with very small sample volumes (5-20 pi), thus allowing the determination of analyte amounts of 10 13 to 10 15 mol on a routine basis. Improved selectivity may be achieved via the coupling of controlled-potential schemes with chromatographic or optical procedures. 

The basic instrumentation required for controlled-potential experiments is relatively inexpensive and readily available commercially. The basic necessities include a cell (with a three-electrode system), a voltammetric analyzer (consisting of a potentiostatic circuitry and a voltage ramp generator), and an X-Y-t recorder (or plotter). Modem voltammetric analyzers are versatile enough to perform many modes of operation. Depending upon the specific experiment, other components may be required. For example, a faradaic cage is desired for work with ultramicroelectrodes. The system should be located in a room free from major electrical interferences, vibrations, and drastic fluctuations in temperature. 

The way in which these alternatives with their particular measuring characteristics are carried out can be best described by (1) controlled-potential coulometry and (2) coulometric titration (controlled-current coulometry). Both methods require an accurate measurement of the number of coulombs consumed, for which the following instrumental possibilities are available (a) chemical coulometers, (b) electrochemical coulometers and (c) electronic coulometers. 

The simplest of the methods employing controlled current density is electrolysis at constant current density, in which the E-t dependence is measured (the galvanostatic or chronopotentiometric method). The instrumentation for this method is much less involved than for controlled-potential methods. The basic experimental arrangement for galvanostatic measurements is shown in Fig. 5.15, where a recording voltmeter or oscilloscope replaces the potentiometer. The theory of the simplest applications of this method to electrode processes was described in Section 5.4.1 (see Eqs 5.4.16 and 5.4.17). 

The method of complete electrolysis is also important in elucidating the mechanism of an electrode reaction. Usually, the substance under study is completely electrolyzed at a controlled potential and the products are identified and determined by appropriate methods, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis. In the GC method, the products are often identified and determined by the standard addition method. If the standard addition method is not applicable, however, other identification/determination techniques such as GC-MS should be used. The HPLC method is convenient when the product is thermally unstable or difficult to vaporize. HPLC instruments equipped with a high-sensitivity UV detector are the most popular, but a more sophisticated system like LC-MS may also be employed. In some cases, the products are separated from the solvent-supporting electrolyte system by such processes as vaporization, extraction and precipitation. If the products need to be collected separately, a preparative chromatographic method is use-... 

In early 1983, Bioanalytical Systems introduced a new class of integrated processor-driven instrumentation based on a concept first developed by Faulkner and his co-workers [1] at the University of Illinois. This unit (Figs. 6.22 and 6.23) has evolved over the years and now includes a repertoire of some 35 electrochemical techniques, including the most popular large-amplitude (Chap. 3) and small-amplitude (Chap. 5) controlled-potential methods. The unit also is capable of determining electrocapillary curves and can automatically measure and compensate for solution resistance (R in Fig. 6.5). Thus in a single instrument it is possible to utilize virtually all of the diagnostic criteria introduced in Chapters 3 and 5 and also to explore quickly which technique is optimum for... 

The control led-potential three-electrode apparatus can be conveniently used to discuss the matching of the cell, the sample, and the instrument. Controlled-potential experiments are common and the following discussion is relevant to many electroanalytical techniques. The operation of a potentiostat is discussed in Chapters 6 and 7, and the reader should be familiar with the characteristics of potentiostats. In short, feedback of an error signal to the input of the potentiostat maintains control of the potential difference between the working and reference electrodes (Chap. 6). 

Delahay, JACS 75, 555-59 (1953) (Oscillographic polarography with controlled potential) 3)C.A.Streuli W.D.Cooke, AnalChem 26, 963 70 (1953) (Chronoamperometry with linearly varied potential using polarized Hg pool cathode) 4)P.Delahay, "New Instrumental Methods in Electrochemistry ,Inter science,... 

One of the most important, yet latent, applications of controlled-potential electrolysis is electrochemical synthesis. Although electrolysis has been used for more than a century to synthesize various metals from their salts, application to other types of chemical synthesis has been extremely limited. Before the advent of controlled-potential methods, the selectivity possible by classical electrolysis precluded fine control of the products. The only control was provided by appropriate selection of electrode material, solution acidity, and supporting electrolyte. By these means the effective electrode potential could be limited to minimize the electrolysis of the supporting electrolyte or the solvent. Today potentiostats and related controlled-potential-electrolysis instrumentation are commercially available that provide effective control of the potential of the working electrode to 1 mV, and a driving force of up to 100 V for currents of up to several amperes. Through such instrumentation electrochemical syn-... 

This technology has been extended to measurements in electrolyte solutions by independently controlling the potential of the substrate and the tip with respect to a reference electrode located in the solution. This electrochemical STM allows the progress of electrochemical reactions to be monitored in situ under potential control. The instrument uses a four-electrode configuration in which the potentials are controlled so that the current flowing between the substrate and tip is dominated by the tunneling current, while a predominantly Faradaic current flows between the substrate and the counter electrodes. 

A solid or liquid sample can be introduced to the analyzer by thermal desorption. The resultant vapors are swept through the inlet by the carrier gas and ionized by a radioactive 63Ni source. Discreet packets of ions are then pulsed down the flight tube under a controlled potential. The arrival of the ions at the detector is inversely proportional to the mass of the molecule. Thus, the smaller ions arrive at the detector first, and the larger ions arrive later. At that point the signal is amplified and read out via an appropriate computer interface. Both commercially available instruments are capable of generation and detection of both positive and negative ions. 

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