Voltammetric electrodes, commercial

FIGURE 25-3 Some common types of commercial voltammetric electrodes (a) a disk electrode (b) a hanging mercury drop electrode (HMDE) (c) a microelectrode (d) a sandwich-type flow electrode. (Electrodes [a], [c]. and [d) courtesy of Bioanalyticai Systems. Inc.. West Lafayette. IN. with permission.)... 

In Section 23F-2, we described how the specificity of potentiomciric sensors could be enhanced by a(>plying molecular recognition layers lo the electrode surfaces. There has been much research in recent years in ap plying the same concepts to voltammetric electrodes. A number of voltammetric systems are available commercially for the determination of. specific species in industrial, biomedical, environmemal. and research applications. 1 hesc devices are sometimes called electrodes or detectors but arc. in fact, complete voltam metric cells and arc better referred to as sensors. In the sections that follow, we describe two commercially available sen.sors and one that is under development in this rapidly expanding field. 

Electrochemical synthesis and subsequent isolation of gram or greater amounts of a pure sample of product B, via the electrode process A B + e , requires exhaustive or bulk electrolysis of electroactive material A at a large-size working electrode, although, of course, exhaustive electrolysis of A frequently is achieved on the microscale (mg to pg) level in mechanistic studies at smaller-sized electrodes. Thus, bulk electrolysis can be a useful large-scale synthetic tool and, indeed, is widely used for commercial production of metals such as Cu, Zn, and Al. Bulk electrolysis experiments, when the data are analyzed in a coulometric form, also enable the n value in a voltammetric electrode process to be determined, provided no additional reactions occur on the longer time scale (typically minutes to tens of minutes) associated with such experiments. 

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. 

From the foregoing treatment of voltammetry it is clear that for its application sophisticated apparatus is required, especially the electronics moreover, the construction of the electrodes and their mutual positions within the measuring vessel, of adapted size and without or with stirring, requires great care and experience. For this reason it is often advisable to purchase commercial apparatus, which has reached a high level of sophistication and reliability. Here the most desirable property is smooth recording of the voltammetric curve, which necessitates the kind of automation inherently required in the voltammetric method this is different from advanced automation, which is treated in Part C. 

The most popular voltammetric technique is probably cyclic voltammetry (CV), partly because of its early development in theory and the availability of the corresponding commercial equipment. In this technique, the electrode potential is first scanned linearly with time from a starting potential, where no reaction occurs, passing E°, towards another potential, and then reversed back to the starting potential. In this case, the time variable can be conveniently represented by the scan rate, v. 

UMEs of 10 pm in diameter and voltammetric instruments for use with such UMEs are commercially available. Electrodes of smaller dimensions can be prepared in the laboratory, although this requires considerable skill [74], In order to use UMEs successfully for high-speed voltammetry in highly resistive solutions, care must be taken concerning the effects of the ohmic drop and the capacitance of the cell system [65 b, 74, 75]. Moreover, two types of voltammograms, i.e. curves (a) and (b) in Fig. 5.23, should be used appropriately, according to the ob-... 

Boron-doped chemical-vapor-deposited diamond is an exceptionally inert carbon electrode with a very wide potential window and very low voltammetric background current. [A. E. Fischer, Y. Show, and G. M. Swain, Electrochemical Performance of Diamond Thin-Film Electrodes from Different Commercial Sources, Anal. Chem. 2004, 76, 2553.]... 

Through the combination of SPR with a - poten-tiostat, SPR can be measured in-situ during an electrochemical experiment (electrochemical surface plasmon resonace, ESPR). Respective setups are nowadays commercially available. Voltammetric methods, coupled to SPR, are advantageously utilized for investigations of - conducting polymers, thin film formation under influence of electric fields or potential variation, as well as - electropolymerization, or for development of -> biosensors and - modified electrodes. Further in-situ techniques, successfully used with SPR, include electrochemical - impedance measurements and -+ electrochemical quartz crystal microbalance. 

Many designs of calomel electrodes exist, and several types are commercially available. Some of these are constructed for voltammetric work, whereas others are used in connection with glass electrodes. The latter type generally has a high resistance, which must be considered when the choice of voltmeter or potentiostat is made. 

Amperometric Sensors A number of voltammetric systems are produced commercially for the determination of specific species of interest in industry and research. These are usually based on measuring the limiting current at a constant applied potential and relating the measured current to concentration. This technique is often called amperometry. Amperometric devices are sometimes called electrodes but are, in fact, complete voltammetric cells and are better referred to as sensors. Two of these devices are described here. 

O.OIM HCIO4 containing 0.005 M Pb " or Tr. Commercial Ag(lll) electrodes were prepared by mechanical polishing (diamond polish of decreasing grain size), followed by chemical chromate polishing. The electrode was transferred luider electrolyte cover first into a conventional electrochemical cell for test voltammetric measurements, then transferred into the electrolytic STM cell. The STM measurements were performed in a commercial Nanoscope II instrument equipped with a homebuilt electrolytic cell [3]. Electrochemically etched Pt/Ir tunneling tips insulated laterally with Apiezon wax were used for the STM experiments. 

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