Potentiostat described

The potentiostat described earlier is used to control the potential of the working electrode and to measure the corresponding current for an RDE. A bipotentiostat is... 

Using a three-electrode systan and a potentiostat, described in Chapter 6, a constant potential can be set up and the current density vs. time can be measured. This organization of measuronents is called the potentiostatic regime. 

Protection current devices with potential control are described in Section 8.6 (see Figs. 8.5 and 8.6) information on potentiostatic internal protection is given in Section 21.4.2.1. In these installations the reference electrode is sited in the most unfavorable location in the protected object. If the protection criterion according to Eq. (2-39) is reached there, it can be assumed that the remainder of the surface of the object to be protected is cathodically protected. 

Since usually the reference electrode is not equipped with a capillary probe (see Fig. 2-3), there is an error in the potential measurement given by Eq. (2-34) in this connection see the data in Section 3.3.1 on IR-free potential measurement. The switching method described there can also be applied in a modified form to potential-controlled protection current devices. Interrupter potentiostats are used that periodically switch off the protection current for short intervals [5]. The switch-off phase is for a few tens of microseconds and the switch-on phase lasts several hundred microseconds. 

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. 

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. 

A certain potential is applied to the electrode with the potentiostatic equipment, and the variation of current is recorded as a function of time. At the very beginning a large current flows, which is due largely to charging of the electrode s EDL as required by the potential change. The maximum current and the time of EDL charging depend not only on the electrode system and size but also on the parameters of the potentiostat used. When this process has ended, mainly the faradaic component of current remains, which in particular will cause the changes in surface concentrations described in Section 11.2. 

The DBMS setup and experimental procedures used in this study were the same as described in more detail elsewhere [Jusys et al., 2001]. Briefly, the DBMS setup consisted of two differentially pumped chambers, a Balzers QMS 112 quadrupole mass spectrometer (MS), a Pine Instruments potentiostat, and a computerized data acquisition system. 

Several questions must be addressed with respect to the simple examples just outlined. Despite their formal similarity, it is important to keep in mind that the first instability describes an isolated system, where charge is a controlled variable. In contrast, the EC as introduced becomes unstable only in contact with a potentiostat (battery), when V is fixed and charge can change. Thus, the collapse of an EC leads to infinite growth of /l, accompanied by electric current from a battery to the EC. 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

Coulometry. Even in water, controlled potential or potentiostatic coulometry is a difficult and often time-consuming technique, as the analyte must participate in a direct electrode reaction. Therefore, in non-aqueous media there are only a few examples of its application, e.g., the potentiostatic coulometry of nitro and halogen compounds in methanol (99%) with graphical end-point prediction, as described by Ehlers and Sease153. 

Various types of controlled-potential pulsing are shown in Fig. 5.18. The simplest case is the single-pulse potentiostatic method (Fig. 5.18A). The current-time (I-t) curves obtained by this method have already been described in Section 5.4.1, Eq. (5.4.10) and (5.4.14). 

Srinivasan et al. [17] have described a four-electrode potentiostat system which is suited to maintaining a constant voltage drop across a membrane in a two-chamber diffusion cell. This system was evaluated in connection with trans-dermal iontophoretic drug delivery of polypeptides. 

In a number of works, a potentiostatic regime has been used for the experimental and theoretical study of the anodization of aluminum and other valve metals.80 Upon the application of a constant potential step, Va, barrier-forming electrolytes are characterized by a sharp increase in the anodic current to a certain maximum. Both the slope and the maximum are determined by the impedance of the cell circuit. Subsequently, there is a continuous decrease in the anodic current, which is due to oxide growth. The decay of the anodic current can be described by the expression81... 

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemically controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemically by applying a constant current pulse. 

Polymer films were produced by anodic deposition by potentiostatic deposition onto a platinum electrode. Deposition was done from 1 M solutions of the monomer in 1M LiC104 in acetonitrile. The films were characterized by cyclic voltammetry and reflection infrared spectroscopy in an apparatus described elsewhere [15]. 

Using the unique four-electrode STM described above, Bard and coworkers (Lev, 0. Fan, F-R.F. Bard, A.J. J. Electroanal. Chem.. submitted) have obtained the first images of electrode surfaces under potentiostatic control. The current-bias relationships obtained for reduced and anodically passivated nickel surfaces revealed that the exponential current-distance relationship expected for a tunneling-dominated current was not observed at the oxide-covered surfaces. On this basis, the authors concluded that the nickel oxide layer was electrically insulating, and was greater than ca. 10 A in thickness. Because accurate potential control of the substrate surface is difficult in a conventional, two-electrode STM configuration, the ability to decouple the tip-substrate bias from... 

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