Electrodes improving selectivity

The advantages of controlled-potential techniques include high sensitivity, selectivity towards electroactive species, a wide linear range, portable and low-cost instrumentation, speciation capability, and a wide range of electrodes that allow assays of unusual environments. Several properties of these techniques are summarized in Table 1-1. Extremely low (nanomolar) detection limits can be achieved with very small sample volumes (5-20 pi), thus allowing the determination of analyte amounts of 10 13 to 10 15 mol on a routine basis. Improved selectivity may be achieved via the coupling of controlled-potential schemes with chromatographic or optical procedures. 

Another approach to improve selectivity is to use an enzyme electrode. The enzyme ascorbate oxidase has been used successfully to remove ascorbate as an interference of in vivo voltammetric electrodes 219,320) Ascorbate oxidase converts the ascorbic acid to dehydroascorbate which is not electroactive in the potential region used for in vivo analysis. 

Ion-selective electrode research for biomedical analysis is no longer the relatively narrow, focused field of identifying and synthesizing ionophores for improved selectivity and the integration of ion-selective electrodes into clinical analyzers and portable instruments. These efforts have matured now to such an extent that they can teach valuable lessons to other chemical sensing fields that are just emerging technologies. 

E. Bakker, Determination of improved selectivity coefficients of polymer membrane ion-selective electrodes by conditioning with a discriminated ion. J. Electrochem. Soc. 43, L83—L85 (1996). 

Selectivity is one of the most important characteristics of an electrode, as it often determines whether a reliable measurement in the sample is possible or not. In practice, most pH sensitive membranes will also respond slightly to some interfering ions. As can be seen in Eq. (5), the potential of such a membrane is governed mainly by the activity of the hydrogen ion and also by the concentration of other interfering ions. To improve selectivity, advanced membrane compositions or protection membranes with size-exclusion or ion-exchange properties are often utilized. 

In 1985 Jakobs et al. studied polypyrrole (PPy) covered platinum and gold electrodes for the ORR,167,168 One interesting result of the work was that, compared to a bare gold electrode, the PPy covered gold reduced oxygen at a lower overpotential.168 Further, the PPy covered electrodes, when in the oxidized state, catalyzed peroxide decomposition and thus improved selectivity to water.168... 

The first combined HPLC-electrochemical measurements of vitamin K used the reductive mode, but this technique suffered from interference from the reduction of oxygen. A redox method was later developed that eliminated this interference, and provided a 10-fold increase in sensitivity over photometric detection and an improved selectivity. The coulometric detector employed in the redox mode is equipped with a dual-electrode cell in which phylloquinone is first reduced upstream at the generator electrode and the hydroquinone is reoxidized downstream at the detector electrode. 

Abstract Single-chamber solid oxide fuel cells (SC-SOFCs) immerse the entire cell in a mixture of fuel and oxidizer gases within a single chamber, which eliminates the need for high temperature sealant, simplifies construction, and increases reliability over traditional double-chamber cells. However, there are challenges, such as low fuel utilization and electrode catalytic selectivity, that need to be overcome. This brief review paper looks at recent improvements in materials, processing, and operation of SC-SOFCs, which are rapidly approaching the performances of the double-chamber fuel cells and may become attractive for specific fuel cell applications. 

M thiocyanate solution. The lifetime of these electropolymerized electrodes (at least 2 months) is a significant improvement over that of PVC-based electrodes that use cobalt porphyrins (28, 29). The latter electrodes were selective toward thiocyanate, but the slopes of the calibration curves deteriorated substantially in less than one month. The improved lifetimes of the poly[Co(o-NH2)TPP] electrodes may be attributed to the covalent fixation of the ionophore to the polymeric matrix ... 

A crystalline electrode for fluoride ion is available from commercial sources. The membrane consists of a slice of a single crystal of lanthanum fluoride that has been doped with europium(II) fluoride to improve its conductivity. The membrane, supported between a reference solution and the solution to be measured, shows a theoretical response to changes in fluoride ion activity from 10 to 10 M. The electrode is selective for fluoride ion over other common anions by several orders of magnitude only hydroxide ion appears to offer serious interference. 

Complexes of porphyrins with metals as perspective ionophores for anion-selective electrodes with improved selectivity 02MI63. 

One approach to improving selectivity is to use a porous zeolite on the electrode [168, 281, 341, 342]. In addition to using the controlled porosity to preferentially control the relative diffusion rates of different species, the zeolite can be loaded with catalysts to promote the desired chemical reaction. 

E. Bakker, Determination of Improved Selectivity Coefficients of Polymer Membrane Ion-Electrodes by Conditioning with a Discriminating Ion, J. Elec-trochem. Soc., 143(4) (1996) L83. 

By incorporating two or more dc potential electrodes into the transducer, several options for differential mode detection exist. In all cases the goal is to (1) improve selectivity and SNR for a given range of operating potentials (to... 

CA films by using the phase inversion process. These CA films were cast from solvent/nonsolvent solutions to yield size exclusion membranes consisting of a thin permselective outer layer and a more porous sublayer. These membranes permitted the rapid permeation of a 1500-dalton poly (ethylene glycol) ester of ferrocene however the reproducibility of results presents a problem with these CA mem-branes. Christie et demonstrated that thin films of plasticized polyvinylchloride (PVC), normally used for potentiometric ion-selective electrode applications, applied to electrodes over a polycarbonate dialysis membrane offered improved selectivity ratios for the amperometric detection of phenolic compounds and H2O2 in the presence of the common biological interferents, ascorbic acid and uric acid, over those observed at the dialysis membrane alone or at a composite dialysis/membrane. 

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