Diffusion amperometry

SECM-induced transfer [SECMIT Fig. 2(b)] can be used to characterize reversible phase transfer processes at a wide variety of interfaces. The basic idea is to perturb the process, initially at equilibrium, through local amperometry at the UME. Hitherto, diffusion-limited electrolysis has mainly been used in conjunction with metal tips, but ion transfer voltammetric probes (discussed briefly in Section III, and in detail in Chapter 15) can also be used. The application of a potential to the tip, sufficient to deplete the... 

Membranes. Apart from the role of membranes180 in ISEs, there are at least three important applications of membranes as measurement aids in flow analysis. viz., as diffusion membranes in (1) (partial) dialysis and in (2a) membrane amperometry (MEAM) and (2b) membrane voltammetry (MEVA), and as Donnan membranes in (3) differential ionic chromatography. 

The limiting current is called the diffusion current because it is governed by the rate at which analyte can diffuse to the electrode. The proportionality of diffusion current to bulk-solute concentration is the basis for quantitative analysis by amperometry and, in the next section, voltammetry. 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

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. 

Kunnecke and Schmid [40] introduced a gas-diffusion separation system combined with an immobilized alcohol oxidase column used for the determination of ethanol in beverages by amperometry. Ethanol vapour from the samples diffused through a silicone-modified polypropylene membrane and was collected in a potassium phosphate buffer acceptor stream before passing through the immobilized enzyme column where the ethanol was transformed into hydrogen peroxide. The peroxide was determined using an amperometric detector with excellent precision (cf. Sec. 8.4). 

The most important feature in amperometry is the linear relationship between the diffusion current and the concentration of the electroactive species (this also applies to other electroanalytical techniques... 

Cyanide Chloramine-T/pyridine-barbituric acid Spectrophotometry/ Amperometry 0.002-2.0 mg CN Gas-diffusion preconcentration/UV-photolysis for total cyanide determination/ standard method ISO 14403... 

Amperometric sensors are small electrochemical cells consisting of two or three electrodes that are usually combined in a single body. A constant potential is applied, i.e., the sensor operates as a Faradaic cell, and a dependence of the measured current on the analyte concentration in the sample is obtained. As in ordinary amperometry, this requires a diffusion layer on the surface of the working electrode. This diffusion layer, in which the analyte concentration is depleted, arises because the analyte is consumed in the electrode reaction. In order for this depletion to occur, the electrode kinetics has to be faster than the... 

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