Sensing electrode reversibility

Most suitable would be the use of a perfectly NH4+ ion-selective glass electrode however, a disadvantage of this type of enzyme electrode is the time required for the establishment of equilibrium (several minutes) moreover, the normal Nernst response of 59 mV per decade (at 25° C) is practically never reached. Nevertheless, in biochemical investigations these electrodes offer special possibilities, especially because they can also be used in the reverse way as an enzyme-sensing electrode, i.e., by testing an enzyme with a substrate layer around the bulb of the glass electrode. 

It has also been demonstrated that the sensing electrode can be maintained at a predetermined voltage of 1.0 V by electrically biasing (constant voltage source) the sensing electrode vs. a stable counter-reference, such as Pt/Hp, H+ or PbOp/PbSO, H+ (1, 2,3). With the former, the sensing electrode is electrically biased 1.0 V above the Pt/Hp,H+ potential (0.0 V) and with the latter 0.7 V below the PbOp/PbSOi, H+ potential (1.7 V). Both of these counter-reference electrodes exhibit good reversibility, but reliability and life are not adequate. 

The most convenient and reliable electrical biasing method for use with a hydrated SPE cell has been shown to be a three electrode potentiostatic circuit which maintains the sensing electrode at a predetermined potential vs. a stable reference (1 >3.>j0e The most reversible reference is a Pt/Hp, H+, static or dynamic. In practical instruments, however, good accuracy and convenience are achieved using a large surface area platinoid metal black/air (Op). All work reported in this study utilized the air reference which has a potential of approximately +1.05 V vs. a standard hydrogen electrode (SHE). For convenience, all potentials reported are vs. the SHE ... 

Microsensor array arrangements (a) sensing electrode heater (front), (b) etching figure on reverse (back heater), (c) device for sensing thick-film formation, (d) mounted on PCB. 

On the other hand, the reverse half reaction occurs at the sensing electrode in the exhaust compartment. 

In contrast the SHE is an ideal device and cannot be rigorously realized experimentally. For the practical realization the standard potential of the hydrogen electrode is not measured at standard conditions but recalculated to at unit activity and 10 Pa. These electrodes are often called reversible hydrogen electrodes (RHE). The acronym RHE is also used for relative hydrogen electrode. In this special case both electrodes, the hydrogen reference electrode and the electrode to be studied, are immersed in the same electrolyte. In this way the junction potential between the reference and the sensing electrode is reduced [6]. 

Equation (6) therefore gives us a means of calculating chemical equilibria from measurements of electromotive force, and vice versa. It must be remembered that E has a sense only when it refers to a reversible cell if the cell is not reversible this simply means that no equilibrium can be set up at its electrodes between the reacting materials. 

The silver-zinc cell is a storage battery After discharge, it can be recharged by forcing through it an electric cnrrent in the reverse direction. In this process the two electrode reactions (19.3) and (19.4) as well as the overall reaction (19.2) go from right to left electrons flowing in the sense of arrow r in Fig. 19.1. 

It should be noted that the reversibility of the galvanic cell has so far been considered from a purely thermodynamic point of view. Reversible electrode processes are sometimes considered in electrochemistry in a rather different sense, as will be described in Chapter 5. 

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

In polarography, we obtained the half-wave potential E// by analysing the shapes of the polarographic wave. E1/2 is a useful characteristic of the analyte in the same way as E . In cyclic voltammetry, the position o/both peaks (both forward and back in Figure 6.13 cathodic and anodic, respectively, in this example) gives us thermodynamic information. Provided that the couple is fully reversible, in the thermodynamic sense defined in Table 6.3, the two peaks are positioned on either side of the formal electrode potential E of the analyte redox couple, as follows ... 

Reversibility in the context of chemical sensing means that the response follows concentration changes, both up and down. It does not have the usual thermodynamic meaning, despite the fact that a decrease of free energy is always the driving force in all interactions. Thus, sensors can be either thermodynamically reversible, as with ion-selective electrodes, or thermodynamically irreversible, as with enzyme electrodes if, however, they respond to a step up or a step down in the concentration... 

The cyclic voltammetric experiment can give a great deal of information about the redox activity of a compound and the stability and accessibility of its reduced or oxidised forms. For a fully chemically reversible process, ipa must equal rpc, i.e. all of the material oxidised at the electrode surface on the forward scan must be re-reduced on the reverse scan (or vice versa). If this condition does not hold true, then the process may be partially reversible (rpc < ipa) or irreversible (rpc = 0). Observation of processes that are not fully reversible implies decomposition or chemical reaction of the reduced or oxidised species and the ratio of ipa to /p(. will show a strong dependence on scan rate since the reverse current is related to the lifetime of the redox-generated material. Note that processes that are chemically reversible (in the sense that the reduced and oxidised species are both stable) may not be electrochemically reversible (a term that relates to the relative rates of forward and back electron transfer). Electrochemically reversible processes are characterised by a separation between the forward and reverse potential peaks of exactly 59 mV. 

The foregoing has been concerned with the application of SERS to gain information on surface electronic coupling effects for simple adsorbed redox couples that are reversible in the electrochemical as well as chemical sense, that is, exhibit Nernstian potential-dependent responses on the electrochemical time scale. As noted in the Introduction, a major hoped-for application of SERS to electrochemical processes is to gain surface molecular information regarding the kinetics and mechanisms of multiple-step electrode reactions, including the identification of reactive surface intermediates. 

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