Liquid electroactive substances

These include membranes made of liquid electroactive substances or with electroactive substances dissolved in a suitable non-volatile, water-immiscible solvent (mediator). In early designs, the organic phase was placed between two aqueous phases in bulk or with the support of a thin, porous cellulose sheet, sintered glass, or the like. As work with these sensors proceeded, more durable polymer supports were developed, most often poly(vinyl chloride) (PVC). An electroactive... 

Figure 2 Basic types of ISE (A) glass electrode (B) electrode with a solid homogeneous or heterogeneous membrane (C) classical liquid membrane electrode (D) electrode without internal solution (all-solid-state electrode) (E) coated-wire electrode. 1, Internal standard solution 2, internal reference electrode (Ag/AgCI) 3, membrane 4, glass or plastic body of the electrode 5, reservoir of the electroactive substance solution 6, solid-state contact and 7, metal wire.

This section is devoted to the most important theoretical relationships and application of systems with flowing test solutions. Under the defined experimental conditions given below the direct estimation of electroactive substances can be realized. Over the past three decades, at least four reasons for developing electroanalysis in flowing liquids have arisen ... 

The first stage in stripping electroanalytical methods is the accumulation of the electroactive substance either on the surface or in the bulk of a liquid electrode (mercury electrodes). The second stage consists of electrode polarization, obtaining cathodic or anodic voltam-mograms (stripping voltammetry (SV)) that give information on the nature and concentration of electroactive analytes [7]. 

Rotated electrodes are used for studies of the character of electrode reactions and their kinetics and for the estimation of electroanalytical parameters. Convection provides a constant mass transport of the analyte to the electrode surface with changed diffusion characteristics compared to quiescent liquids. The effect is similar to a stirring of the solution, but the rotation of an electrode can be controlled much more precisely than of a solution. Rotating ring-disc electrodes are practically used only for elucidating electrochemical reaction mechanisms where oxidation or reduction on the disc creates (collection experiment) or consumes (shielding experiment) electroactive substances which are monitored with the ring electrode. 

Electroactivity (electroactive or electrochemically active compounds) — The capability of a substance to take part in a faradaic electrode reaction. Electroactive compounds can be in the gaseous, liquid, or solid state or they may be dissolved in (liquid or solid) solutions. Various compounds are also electroactive in the adsorbed state (see -> adsorption, - adsorptive accumulation, -> adsorptive stripping voltammetry) or as polymer films (see -> polymer-modified electrodes) on electrodes. 

Multiphase system — An inhomogeneous system consists of two or more phases of one or more substances. In electrochemistry, where all processes occur at the interface thus all measurement systems must contain at least two - phases. In common understanding so-called multi-phase systems contain more than two phases. Good examples of such systems are -> electrode contacting a solid phase (immobilized at the electrode electroactive material or heterogeneous -> amalgams) and electrolyte solution, and an electrode that remains in contact with two immiscible liquids [i]. All phenomena appearing in such multi-phase systems are usually more complicated and additional effects as - interphase formation and -> mass transport often combined with - ion transfer must be taken into account [ii]. 

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