Electronic potentiostats

Potentiostatic conditions are realized with electronic potentiostats. The potential of the working electrode is monitored continuously with the aid of a reference electrode. When the potential departs from a set value, the potentiostat will adjust the current flow in the cell automatically so as to restore the original value of potential. Important characteristics of potentiostats are their rise time and the maximum currents which they can deliver to the cell. Modem high-quality potentiostats have rise times of 10 to 10 s. 

A. Hickling, Trans. Faraday Soc. 33 1540 (1937). The first paper on electronic potentiostats. 

It is reasonable to ask at this point Are there other approaches to reach stability with grace in true potentiostatic circuits The answer is indeed affirmative, but unfortunately with qualifications. One technique is discontinuous control of cell potential. It is not a new approach it was, in fact, the method used in the very first electronic potentiostat by Hickling in 1942. The principle is quite simple Current pulses are applied to the counterelectrode so that the desired potential is maintained between reference and working electrode. Since the potential can be measured between pulses, there is no iR drop. Cell potential is not steady it depends on the sensitivity of the comparator circuit and the rate at which current demand can be met. 

Since the surface must simultaneously corrode, reduce, and catalyze, it is clear that very specific substrates will normally be necessary for ordinary chemical catalytic processes. They must possess just the right combination of catalytic and reductive properties so that, when exposed to their reactive environment, they produce an electrochemical potential that gives the correct rate (bearing in mind their own electrocatalytic character) for the preparative process desired. The correct products will thereby result. Clearly, it would be much easier if the electrocatalytic and potential characteristics could be separated by the use of specialized conductive electrode surfaces, combined with a suitable electronic potentiostat, respectively. In this way, electrocatalysis could become of great future importance in certain types of preparative organic chemistry. 

Figure 6 shows the different measuring circuits for amperometric two- and three-electrode cells. In the circuit for the three-electrode cell an electronic potentiostat applies exactly the preselected polarization voltage between the cathode and the reference electrode and compensates nearly completely the voltage drop //> / in the cell. 

M. Prazak constructed an original electronic potentiostat which became the basic instrument for study of corrosion. It was used, for example, for following corrosion of corrosion-proof steels and alloys [122-124]. The effects of temperature on potentials of metals, on kinetics of their dissolution, and on activation energy of iron passivation in sulfuric acid were tested in the institute [125-130]. 

Rapid-Scan Corrosion Behavior Diagram (CBD) Basically, all the same equipment used in the conductance of an ASTM G5 slow-scan polarization study is used for rapid-scan CBDs (that is, a standard test cell, potentiostat, voltmeters, log converters, X-Y recorders, and electronic potential scanning devices). The differences... 

The potentiostatic technique discussed here involves the polarisation of a metal electrode at a series of predetermined constant potentials. Potentio-stats have been used in analytical chemistry for some time Hickling was the first to describe a mechanically controlled instrument and Roberts was the first to describe an electronically controlled instrument. Greene has discussed manual instruments and basic instrument requirements. 

Cahan, Nagy and Genshaw examine design criteria for an electrochemical measuring system to be used for potentiostatic transient investigation of fast electrode reactions. They emphasise the importance of co-design of the experimental cell and electronics. 

At all stages of the electrolysis the electronic voltmeter reads the potential difference between the cathode and the reference electrode, and the required limiting value of this potential difference is entered into the potentiostat. If the measured potential at any instant differs from the pre-set value, the potentiostat will adjust the current flowing to restore the required potential difference. 

If a controlled-potential determination is to be carried out, additional equipment will be required, namely an electronic voltmeter, a potentiostat and a reference electrode. The latter is most commonly a saturated calomel electrode, the construction of which is described in Chapter 14. 

The electrode will of course be incorporated in a circuit similar to that previously described in which a potentiostat controls the potential of the working electrode and may also provide a counter-current facility to nullify the background current an electronic integrator will also be included. 

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

Accordingly, the resulting current reflects the rate at which electrons move across the electrode-solution interface. Potentiostatic techniques can thus measure any chemical species that is electroactive, in other words, that can be made to reduce or oxidize. Knowledge of the reactivity of functional group in a given compound can be used to predict its electroactivity. Nonelectroactive compounds may also be detected in connection with indirect or derivatization procedures. 

This is called electrochemical shift and simply stems from the fact that the Fermi level of the reference electrode is not equal to that of the working electrode and thus to the Fermi level of the detector. Furthermore if one changes UWr via a potentiostat the core level electron binding energies of species associated with the reference electrode will shift according to Eq. (5.66), i.e. the XPS analyzer acts also as a (very expensive) voltmeter. 

The development and the very widespread use of the polarographic technique to record i-E curves and the more recent designing of electronic devices known as potentiostats which automatically control the potential of the working electrode at a pre-set value has led to many examples in the literature of organic electrode reactions whose products depend on the potential. Some examples are cited below ... 

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