The potentiostatic experiment

In a potentiostatic arrangement, a three-electrode cell is always used connected to a potentiostat, as shown schematically in Fig. 9. The idea is that the potential difference between the indicator electrode and the reference electrode is controlled to be a chosen function of time, for example the step function to be treated in this section. The first potentiostat was devised as early as 1942 by Hickling [40]. In modern designs, use is made of solid-state operational amplifiers for both the part that controls the potential and the part that measures the resulting current. Technical details and further references can be found in recent handbooks, e.g. refs. 21 and 22. 

Ideally, the potentiostat should control the interfacial potential only, without interference of ohmic losses. However, it can be seen in Fig. 9 that the voltage difference between I.E. and R.E. necessarily comprises a contribution / x Ru, where Ru is the ohmic resistance residing between the connection to I.E. and the point where the tip of the R.E. contacts the electrolyte solution. Even if this contact is brought very close to the interface by means of a so-called Luggin capillary [41], this contribution may be not negligible. The most sophisticated way to minimize the 

In the following sections, this condition will be assumed to be fulfilled, in order to discuss the principles of the most important potential step techniques. 

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V vs. Ag/AgCl were collected as functions of decomposition time. The electrode potential was initially held at 0.75 V to produce a clean Pt(lll) surface, and was next switched to monitor the CO uptake. Starting at Os, where CO adsorption (from HCOOH decomposition) had not yet begun, the potentiostatic experiment lasted until about 500 s of the progress of reaction. The spectral position is typical of the... 

On the other hand, during potentiodynamic formaldehyde oxidation (solid line in the upper panel of Fig. 13.3b), there is only a small faradaic current at 0.6 V in the positive-going scan, in contrast to the much higher steady-state value (about 0.55 mA) attained in the potentiostatic experiment. 

A second, easier method to follow is based on the fact that in the potentiostatic experiment on Ox at long times n + n2 electrons have passed so that the current is purely diffusive. Therefore, by the use of such current values, recalling that i is proportional to t l/2 for diffusive processes, one can determine the values of id at the various times of interest. 

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. 

Galvanostatic, potentiostatic as well as potentiodynamic techniques can be used to electropolymerize suitable monomeric species and form the corresponding film on the electrode. Provided that the maximum formation potentials for all three techniques are the same, the resulting porperties of the films will be broadly similar. The potentiodynamic experiment in particular provides useful information on the growth rate of conducting polymers. The increase in current with each cycle of a multisweep CV is a direct measure of the increase in the surface of the redoxactive polymer and, hence, a suitable measure of relative growth rates (Fig. 5). 

The CCD detector and the potentiostat were synchronized in an open-loop configuration by starting the experiment with a common trigger. Careful measurements... 

Electrochemical Equipment. Electrochemical experiments were performed using either a PAR Model 175 universal programmer and a PAR Model 363 potentiostat/galvanostat, or a Pine Instruments RDE-4 bipotentiostat, coupled with a Kipp and Zonen BD 91 X-y-y recorder. The current-time response for the chronoamperometry experiments was recorded with a Nicolet 4094 digital oscilloscope. All potentials were measured vs. a Ag/10"2 M Ag+ reference electrode. 

An EG G PARC 273 Potentiostat/Galvanostat was used in both the electrolysis and the CV experiments, coupled with an HP 7044B X/Y recorder. A Solartron 1255 HF Frequency Response Analyzer and a Solartron 1286 Electrochemical Interface were employed for the a.c. impedance measurements, using frequencies from 0.1 to 65 kHz and a 10 mV a.c. amplitude (effective) at either the open circuit potential (OCP) or at various applied potentials. As the RE can introduce a time delay at high frequencies, observed as a phase shift owing to its resistance and capacitance characteristics, an additional Pt wire electrode was placed in the cell and was connected via a 6.8 pF capacitor to the RE lead [32-34]. 

The central importance of the electrode potential has been discussed in Sect. 2.3.2. The potentiostatic operation mode evidently requires a reliable potential measurement. Only experiments at constant current can operate without knowledge of the potential. But here too the potential of the working electrode imparts interesting information, for example, a change in this potential - probably not detectable in the... 

Galvanostatic, potentiostatic, or poten-tiodynamic techniques can be used to elec-tropolymerize suitable monomeric species and form the corresponding film on the electrode. The potentiodynamic experiment, in particular, provides useful information on the growth rate of conducting polymers. The increase in current with... 

The electronic components for the measurements consisted of EG Q Model 173 Potentiostat equipped with slow sweep option (0.1 mv/sec) and EG G Model 376 Logarithmic Current Converter. An EG G Model 175 Universal Programmer supplied the waveform for running the polarization experiment. The output from the electrometer of the 173 and the log output of the 376 were connected to a Hewlett-Packard Model 7036B X-Y Recorder and the potential plotted versus log current. 

In a potentiostatic experiment, E is held constant so that, according to eqn. (170), the rate of the electrode reaction depends only on the surface concentrations of the species O and R. One of the simplest circumstances that can be imagined would have the reaction occurring bidirectionally with the forward rate proportional to the concentration of O and the backward rate proportional to the concentration of R, so that... 

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). 

A typical example would be a coulometric controlled-potential experiment, where initially a large current exists between the auxiliary and the working electrode (the load of the control amplifier). If a potentiostat is rated to have a maximum output of 20 V at 1 A, it cannot supply more than 20 W of power [power (watts) = current (amperes) x potential (volts)]. If Rt were 100 Q, the potentiostat would not be able to control the potential of the working electrode at -2.0 V (or any other potential for that matter) if 0.5 A were demanded. At least 25 W (I2Rt) of power would be required of the potentiostat for potential control to be maintained. As a result of our 5-W deficiency, the potential of the working electrode would be uncontrolled at a value less than the -2.0 V less than 0.5 A, in fact, would pass through the cell. 

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