Redox-activated reactions reference electrodes

In MET, a low-molecular-weight, redox-active species, referred to as a mediator, is introduced to shuttle electrons between the enzyme active site and the electrode.In this case, the enzyme catalyzes the oxidation or reduction of the redox mediator. The reverse transformation (regeneration) of the mediator occurs on the electrode surface. The major characteristics of mediator-assisted electron transfer are that (i) the mediator acts as a cosubstrate for the enzymatic reaction and (ii) the electrochemical transformation of the mediator on the electrode has to be reversible. In these systems, the catalytic process involves enzymatic transformations of both the first substrate (fuel or oxidant) and the second substrate (mediator). The mediator is regenerated at the electrode surface, preferably at low overvoltage. The enzymatic reaction and the electrode reaction can be considered as separate yet coupled. 

During a redox reaction, a potentiometric titration can be employed to determine a concentration of analyte rather than an activity, since we are only using the emf as a reaction variable in the accurate determination of an end point volume. For this reason, an absolute value of reference electrode need not be known, as we are only concerned with changes in emf. It is, however, advisable to titrate at high ionic strength levels in order to minimize fluctuations in the mean ionic activity coefficients. 

The value of the constant V, and hence the values of standard potentials, depend on the choice of the reference electrode and on the character of electrode reaction, which takes place on it With the reference electrode potential conventionally taken as zero, we can choose, for example, the normal hydrogen electrode (NHE), i.e., an electrode, for which the equilibrium at the interface is attained due to the reversible redox reaction H+ + e = H2, provided the activity of H+ ions in the solution is 1 mol/liter and the pressure of gaseous hydrogen above the solution is 1 atm. Many of the measured potentials are given below relative to the saturated calomel electrode (SCE) its potential relative to the NHE is 0.242 V. 

Another opportunity to realize constant activity of the potential determining ion at the reference interface appears when one chooses the solid electrolyte in such a way that the ion of the redox couple is the same as one ion of the major component of the electrolyte. In that case, the change of the activity due to the electrode reaction with the gas can be neglected against the overall constant activity of that ion in the salt. This is the solid-state reference arrangement. An example is the chlorine sensor (Fig. 6.40), in which the reference potential is set up by the constant activity of CP in the solid AgCl electrolyte. This arrangement is equivalent to a reference electrode of the second kind, discussed in Section 6.2.2.1. 

A transducer is selected with respect to the features of the biochemical reaction. In amme-tering transducers, constant potential applied to the reference electrode and the current generated in the redox transformation of the electrochemically active compound present on the enzymatic electrode surface is measured. Electron transfer rate is controlled by increasing or reducing the potential drop between electrodes. 

The Marcus Theory can also be applied for heterogeneous electron transfer reaction at electrode surfaces [24 and references therein]. The electronic coupling between the protein and the electrode can be varied using different self-assembled monolayers controlling the orientation of the redox active protein on the surface and the distance between the redox active site of the protein and the electrode. The driving force is related to the appHed potential and the redox potential of the protein. In many cases the rate of electron transfer across the protein-electrode interface is limited by conformational reorganization. This has focussed the efforts of many groups on tailored interaction between proteins and enzymes and electrode surfaces. 

In the sense used in this chapter, generation-collection (GC) mode refers to experiments where the tip is used simply to sense redox active or electroactive species produced by the specimen under study. This usage is essentially the same as elsewhere in this volume except that the generator is a biochemical reaction or organism rather than another electrode. 

Coupling between a biologically catalyzed reaction and an electrochemical reaction, referred to as bioelectrocatalysis, is the constructional principle for enzyme-based electrochemical biosensors. This means that the flow of electrons from a donor through the enzyme to an acceptor must reach the electrode in order for the corresponding current to be detected. In case a direct electron transfer between the active site of an enzjane and an electrode is not possible, a small molecular redox active species, e.g. hydrophobic ferrocene, meldola blue and menadione as well as hydrophilic ferricyanide, can be used as an electron transfer mediator. This means that the electrons from the active site of the enzyme reduce the mediator molecule, which, in turn, can diffuse to the electrode, where it donates the electrons upon oxidation. When these mediator molecules are employed for coupling of an enzymatic redox reaction to an electrode at a constant potential, the resulting application can be referred to as mediated amperometry or mediated bioelectrocatalysis. 

Redox potential (Eh). The potential that is generated between an oxidation or reduction halfreaction and the standard hydrogen electrode (SHE) (0.0 V at pH = 0). In soils, it is the potential created by oxidation-reduction reactions that take place on the surface of a platinum electrode measured against a reference electrode minus the Eh of the reference electrode. This is a measure of oxidation-reduction potential of redox active components in the soil (see Chapter 4). 

Here Vq is the standard electrode potential of the redox system (with respect to the hydrogen reference electrode at 1 mol concentration), n is the number of electrons in the unit reaction, R is not resistance but the universal gas constant, and F is the Faraday constant (see Section 7.8). aox and area are activities, a = yc, where c is the concentration and y is the activity coefficient. Y = 1 for low concentrations (no ion interactions), but <1 at higher concentrations. The halfcell potentials are referred to standardized conditions, meaning that the other electrode is considered to be the standard hydrogen electrode (implying the condition pH = 0, hydrogen ion activity 1 mol/L). The Nernst concept is also used for semipermeable membranes with different concentration on each side of the membrane (see Section 7.6.4). 

The most cited reference electrode is the platinum-hydrogen electrode, and electrode DC potentials are often given relative to such an electrode. It is an important electrode for absolute calibration, even if it is impractical in many applications. The platinum electrode metal is submerged in a protonic electrolyte solution, and the surface is saturated with continuously supplied hydrogen gas. The reaction at the platinum surface is a hydrogen redox reaction H2 2H (aq) + 2e, of course with no direct chemical participation of the noble metal. Remember that the standard electrode potential is under the condition pH = 0 and hydrogen ion activity 1 mol/L at the reference electrode. Thus the values found in tables must be recalculated for other concentrations. Because of the reaction it is a hydrogen electrode, but it is also a platinum electrode because platinum is the electron source or sink, and perhaps a catalyst for the reaction. 

Also known as the standard hydrogen electrode (SHE), it is a redox reference electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. The potential of the NHE is defined as zero and based oti equilibrium of the following redox half-cell reaction, typically on a Pt surface 2H+(aq) + 2e H2(g). The activities of both the reduced form and the oxidized form are maintained at unity. That implies that the pressure of hydrogen gas is 1 atm and the concentration of hydrogen ions in the solution is 1 M. 

For both metal-based or gas-based reference electrodes, these definitions in practical terms mean that the concentration of the dissolved ions within the reference electrode remains essentially constant. The electrochemical changes of the reference system should be part of an equilibrium reaction and overall no net reaction should occur during the measurement. However, if a reaction or alternative equilibrium occurs which can change the activities of the reference species in a detectable manner, then the redox couple chosen can be problematic and stability of the reference electrode can be compromised. Therefore, care must be taken to ensure this type of reaction does not take place with the chosen ions. 

As can be seen from Table 7.5, similar to the case of the AglAg" couple, when the H2lH is referenced to an internal redox couple a shift in the occurs and is dependent on the IL studied. Additionally, Compton et al. have reported that the electrochemical reversibility of the H2lH couple is dependent on the identity of the IL [33]. This is most likely due to the difference in proton activity in the different ILs. As such it is important if the reader wishes to use the H2lH couple in a reference electrode then the reversibility of this reaction is checked prior to use. 

The oscillations in most demonstration experiments produce periodic color changes. However, other properties of the solution, like the electrical potential, oscillate as well. This is due to changes in the concentrations of the redox active species. The electrical potential changes can be observed by measuring the potential of a platinum electrode versus a reference electrode. The voltage oscillates in phase with the color changes. The range of oscillations in the classic Belousov-Zhabotinsky reaction is about 200 mV. If the solution is poured in a petri dish and left unstirred, mosaic patterns appear as spatial oscillations. 

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