Sensing electrode oxidation/reduction reactions

Other sensing technologies based on oxidation-reduction reactions at electrodes were extensively pursued in the 1940s and 1950s providing analytical... 

There is nothing in the foregoing discussion that restricts it to reactions at the cathode or to ions it holds, in fact, for any electrode process, either anodic, i.e., oxidation, or cathodic, i.e., reduction, using the terms oxidation and reduction in their most general sense, in which the concentration of the reactant is decreased by the electrode process, provided the potential-determining equilibrium is attained rapidly. The fundamental equation (10) is applicable, for example, to cases of reversible oxidation of ions, e.g., ferrous to ferric, ferrocyanide to ferricyanide, iodide to iodine, as well as to their reduction, and also to the oxidation and reduction of non-ionized substances, such as hydroquinone and qui-none, respectively, that give definite oxidation-reduction potentials. 

The reduction-oxidation reactions create a Nernst potential (E) between the sensing electrode and the reference electrode that is related to the CO2 concentration and given by Equation [16.5]. 

Chlorination is usually measured by oxidation-reduction potential (ORP). The sensing electrode directly contacts the solution, therefore it responds much faster than pH electrodes. It is also much more mechanically robust. ORP measures the reduction of hypochlorite by the following reaction ... 

The three types of reversible electrodes described above differ formally as far as their eonstruction is concerned nevertheless, they are all based on the same fundamental principle. A reversible electrode always involves an oxidized and a reduced state, using the terms oxidized and reduced in their broadest sense thus, oxidation refers to the liberation of electrons while reduction implies the taking up of electrons. If the electrode consists of a metal M and its ions M" ". the former is the reduced state and the latter is the oxidized state similarly, for an anion electrode, the A ions are the reduced state while A represents the oxidized state. It can be seen, therefore, that all three types of reversible electrode are made up from the reduced and oxidized states of a given system, and in every case the electrode reaction may be written in the general form... 

The reactant and product species for both the reduction half-reaction and the hydrogen oxidation half-reaction are specified to be in their standard states. Recall that the standard state of a gas is an ideal gas at 1 bar, a liquid is a 1 m ideal solution in the Henry s law sense, and a solid is the pure solid with an activity of 1. In terms of our shorthand notation, we can measure the standard potential of any reduction half-reaction with a standard hydrogen electrode (S.H.E.) ... 

At the sensing electrode, the electrochemical oxidation (Eq. 17.13) and reduction (Eq. 17.12) occur simultaneously and establish a local cell. When the rates of these reactions are equal to one another, the potential of the sensing electrode is a mixed potential. 

There are several species in this reaction that can be used for electrochemical sensing. Detection of proton released from the gluconic acid was used in the poten-tiometric glucose electrode (Section 6.2.1). The amperometric sensor can be based on oxidation of hydrogen peroxide, on reduction of oxygen, or on the oxidation of the reduced form of glucose oxidase itself. 

This complex has been widely used in sensing applications since both radical ions of the complex are relatively stable to decomposition reactions. Many systems using this chromophore exist in which ECL is produced at a single electrode via coreactant oxidation or reduction schemes as discussed in the first segment of this section [Eqs. (5) through (9)]. For example, the reduction product of the peroxydisulfate dianion, S2Og, can function as an oxidant in the ECL reaction by annihilation with the electrochemically generated Ru1+ to yield the MLCT excited state of the Ru(II) complex by the mechanism [24] ... 

Any sign convention must be based on expressing half-cell processes in a single way—that is, either as oxidations or as reductions. According to the lUPAC convention, the term electrode potential (or, more exactly, relative electrode potential ) is reserved exclusively to describe half-reactions written as reductions. There is no objection to the use of the term oxidation potential to indicate a process written in the opposite sense, but it is not proper to refer to such a potential as an electrode potential. 

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