Voltammetric Equipment

The instruments below have all been used for in vivo experiments other designs with equivalent specifications should be usable. 

EG G Princeton Applied Research Models 174A and 374 pulse P.O. Box 2565 polarographs 

In vivo electrochemical recording—a new neurophysiological approach, 

Electrochemistry at Solid Electrodes, Marcel Dekker, New York. 

1969 ), Applications of modern electroanalytical techniques to pharmaceutical chemistry,/. Pharm. Sci. 58 1171-1178. 

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Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

Instruments in Other Countries. A similar pattern of development is displayed in the pulse voltammetric equipment manufactured in other countries. 

Voltammetric equipment is available from several sources BAS (USA), Eco Chemie (Netherlands), EG G (USA), Metrohm (Switzerland), Radiometer (Denmark/France), Sycopel Scientific (UK) and other companies such as in the Czech Republic and China. The equipment consists of a hanging mercury drop electrode (HMDE) and a potentiostat. Nowadays the electrodes have a valve to dispense a new mercury drop. These electrodes and potentiostats are computer controlled which is convenient as it automates the timing. 

Time, Cost, and Equipment Commercial instrumentation for voltammetry ranges from less than 1000 for simple instruments to as much as 20,000 for more sophisticated instruments. In general, less expensive instrumentation is limited to linear potential scans, and the more expensive instruments allow for more complex potential-excitation signals using potential pulses. Except for stripping voltammetry, which uses long deposition times, voltammetric analyses are relatively rapid. 

The electrochemical cell for the polarographic measurements had a four-electrode configuration equipped with a microsyringe, and was connected to a computer-assisted data-acquisition system [7]. On the other hand, the cyclic voltammetric measurements that are also assisted by a computer data-acquisition system were carried out using a gel electrode that contains the aqueous phase [8]. The cell structure was as follows ... 

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. 

Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of normal voltammetric experiments until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method calledjhst scan voltammetry [27]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MV s are possible, and species with lifetimes in the nanosecond scale can be observed. Using this technique, P. Hapiot et al. [28] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation and substituted derivatives. The resulting rate constants for the dimerization of such monomers lie in the order of 10 s . The same... 

Lu et al. have described a single sweep voltammetric method for the determination of chlorpromazine at carbon paste electrodes [176]. Powdered tablet was dissolved in and diluted to 100 mL with water, whereupon a 10 mL portion of the solution was mixed with 10 mL of acetate buffer solution (pH 5.3). This solution was then diluted to 100 mL. The solution was analyzed using an electrolytic cell equipped with a carbon paste working electrode, a reference saturated calomel electrode, and a platimun counter-electrode. The voltammogram was recorded by single sweep scanning from 0 to 1 V, with a scanning rate of 10 mV/sec. 

The electrical resistance of non-aqueous electrolytic solutions is often much higher than that of aqueous ones, and so polarographic and voltammetric measurements in non-aqueous solutions should be made with a three-electrode device. A computer-aided three-electrode instrument, equipped with a circuit for iR-drop compensa-... 

Differential pulse (DP) voltammetry, a voltammetric technique with high sensitivity, is normally performed and the equipment as well as the electrochemical procedures used for the voltammetric studies of DNA-drug interaction are described (see Procedure 29 in CD accompanying this book). 

Electropolymerization and cyclic voltammetric experiments are performed with an EG G PARC, Model 173 potentiostat equipped with a Model 175 universal programmer and a Model 179 digital coulometer in conjunction with a Kipp and Zonen BD 91 XY/t recorder. All experiments are carried out using a conventional three-electrode cell. Instrumental setup for amperometric measurements ... 

The equipment necessary for the determination of n by coulometry as described earlier does not in principle differ from that used for a microscale preparative electrolysis. Therefore, in addition to the determination of n, it is possible also to investigate the voltammetric properties of the product and, for example, whether the starting material may be regenerated by electrolysis after the direction of the current has been changed. 

The requirements for accurate recording of these two types of signals are quite different and lead to different equipment requirements. A sensitive potentiostat is required for a voltammetric probe and an electrometer is needed for a potentiometric probe. 

Voltammetric measurements are not simply restricted to analytical laboratories. The applications of these methods are more numerous than is at first obvious. A large number of analytical instruments, whether portable or not, intended to make precise measurements of substrates present in gas mixtures, vapours or solutions are equipped with electrochemical sensors. These devices operate on the principle of the 2- or 3- electrode cell enclosed in the sensor housing. 

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. 

Stripping voltammetry is an important, but limited technique, which uses a pre-concentration step to enhance the sensitivity of the voltammetric analysis step. This pre-concentration step is electrochemical using the same instrumentation as the subsequent voltammetric analysis. The simplest polarographic equipment can be used to obtain the highest sensitivity which depends on the length of the pre-concentration step as well as on the sensitivity of the voltammetric stripping step. Since the pre-concentration step requires the production of an insoluble product which can be reproducibly stripped from the electrode surface in the determination step, the use of stripping voltammetry is limited to a few analytes, eg transition metal ions, halides and pseudo-halides. 

Carbon, hydrogen, and nitrogen elemental analyses were performed in a Perkin-Ehner microanalyzer. XRD powder diffraction analysis was performed with a Shimadzu XD-3A instrument (Cu Kot, 35 kV, 25 mA). The voltammetric measurements were performed on a BAS equipment, model CV-50W, in a 20 cm Pyrex glass cell with a modified electrode (work electrode), a platinum wire (auxihary electrode), and a Ag/AgCl electrode (reference electrode). As electrolyte, 0.1 mol dm KCl solution was used. 

In addition to on-line and in-line analysis, very often off-line systems are used in industry. Intermediates and products are analysed with laboratory analyzers. An example of this is the equipment constructed for the content uniformity analysis and dissolution rate determination of pharmaceutical tablets.The tablet is dissolved under well-defined conditions and the solution analysed using a flow technique with voltammetric detector (Fig. 11). 

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