A new sulfate-selective electrode and its use in analysis

The improvement of the steric accessibility of exchange centers of higher quaternary ammonium salts (QAS) by replacing several long-chain alkyl substituents at the nitrogen atom with methyl groups enhanced the selectivity of ion-selective electrodes (ISEs) based on the specified ion exchangers for the SO ₄ ^2? ion up to six orders of magnitude in the presence of singly charged anions. This can be qualitatively explained by the specific features of ion pair formation between quaternary ammonium cations and singly and doubly charged anions. The effect of the steric accessibility of the QAS exchange center on the selectivity of ISEs is partially retained in the presence of a neutral anion carrier, hexyl p-trifluoroacetylbenzoate (HTFAB), which is used for enhancing the selectivity for the sulfate ion in the presence of singly charged anions. A sulfate-selective electrode with a reasonable selectivity for practical purposes was proposed. It is based on the HTFAB-higher QAS ion pair bearing three methyl substituents at the nitrogen atom. The ISE was used in the analysis of natural water.     

Design of a new dodecyl sulfate-selective electrode based on conductive polyaniline.

A new, simple, sensitive, low cost and rapid potentiometric method for direct determination of ultra trace amounts of sodium dodecyl sulfate (SDS) with a new DS(-)-selective electrode is reported. The electrode was prepared by electropolymerization of aniline in acidified DS- ion on the surface of a Pt electrode. The cyclic voltammetry (CV) was used for electropolymerization of polyaniline (PA) in the potential range of -200 to +1000 mV vs. Ag/AgCl. This sensor showed a Nernstian behavior (59.0 +/- 2.3 mV/decade) over a very wide linear range (1.0 x 10(-9)-3.0 x 10(-6) M) with a detection limit of 1.0 x 10(-9) M. The response time of the electrode was 15 s for 1.0 x 10(-7) M of analyte; the electrode can be used for 4 weeks without any major deviation. This electrode can be used in the pH range of 3.5-9.8. The selectivity of electrode to DS- over some organic, inorganic and anionic surfactants was investigated with the fixed primary ion method. The results show that the electrode is highly selective to DS- ion over other ions. The proposed electrode was applied to the determination of DS- in real samples.     

A lithium-selective glass electrode

We pioneered the study of the electrode properties of glasses containing 80 and 85 wt % LiF and 20 and 15 wt % A1(PO₃)₃. LiF-80-based glass electrodes functioned as lithium-selective electrodes in 1-0.0001 M LiCl solutions. Contrary to aliminosilicate glasses, these glasses responded to the Li⁺ ion better than to the Na⁺ ion and were nonselective to the Hi⁺ ion. This made them promising for the development of sensors for Li⁺ ions in different solutions. Forward and reverse transitions of these glasses from the hydrogen to the lithium function and from the lithium to the sodium function are described by the equation of the simple Nikol’skii theory. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EMA-99), Moscow, December 6–8, 1999.     

A propranolol-responsive coated-wire electrode

Propranolol-responsive electrodes of both the coated wire and conventional types based on the didodecylnaphthalene sulfonic acid salt were prepared and their operating characteristics were compared. The detection limit of the coated-wire electrodes is about 10^-6 M; interferences from common inorganic ions and from the analagous product isoproterenol are small. The response time varies from 15 s to 30 min depending on concentration; readings are reproducible to within 0.5 mV. Electrodes are still operational after six months of intermittent use.     

A trimethoprim-selective membrane electrode

An electrode selective for trimethoprim is based on the ion-pair complex of trimethoprim with silicotungstate, and gives rapid response to change in trimethoprim concentration over the range 3 × 10⁻⁵−2 × 10⁻² M with a response slope of 51.0 ± 1.5 mV/decade. No significant interferences are caused by sulpha-drugs. The electrode can be used for the potentiometric determination of trimethoprim.     

A porous-jet flow-through electrode

An electrochemical flow detector, based on a jet of solution directed at a thin porous carbon electrode, is described, along with its electrochemical characteristics and analytical applications. The electrode has a volume of 19 mul and surface area of 1.26 cm(2). The detector exhibits better sensitivity and detectability than a wall-jet detector. Dopamine and ferrocyanide were used as test systems to give detection limits at nanomolar concentration levels. Applications indicated include continuous-flow analysis (utilizing stopped-flow voltammetry) and flow-injection analysis.     

A specific enzyme electrode for urea

A truly specific, simple enzyme electrode is described for the assay of urea in blood serum. The sensor used is the newly developed air-gap electrode of R??i?ka and Hansen, and has advantages of speed of response and specificity over earlier enzyme electrodes for urea. Potassium, sodium and ammonium ions and other organic and inorganic species present in blood do not interfere. Linear curves are obtained from 2 · 10^-2M to 1 · 10^-4M urea with slopes close to Nernstian (about 0.90 pH/decade). Urea in blood was assayed with an accuracy of 2.2% and a precision of 2.0% with immobilized urease; only 3–5 min is required per assay. The electrode was used for a month and almost 500 assays with excellent results. Since the sensor never touches the sample solution, problems caused by blood components which block membrane pores are avoided.     

A glucose electrode for fermentation control

An oxygen-stabilized enzyme electrode was applied to monitor the glucose concentration in a fermentor during a batch culture ofCandida utilis. The electrode contains an electrolysis circuit for generation of oxygen within the enzyme layer that keeps the oxygen activity in that layer at the same level as that of the surrounding broth. The electrolysis current is used as a measure of the glucose concentration in the broth. The glucose analysis continued without major disturbances when the dissolved oxygen pressure gradually decreased during the fermentation and also when the broth was subjected to a sudden increase in dissolved oxygen tension. The electrode could also be used in an anaerobic broth. Then the reference electrode was replaced by a constant reference potential that simulated a reference oxygen activity.     

A simple solid-state pH glass electrode

A modernized version of the thompson electrode is described. A copper wire is connected to the inner surface of the membrane of a glass electrode with a conductive adhesive resin.     

A theory of glass electrode response

Summary The response of glass electrodes to rapid changes in ion concentrations is considered from a theoretieal viewpoint. An expression is derived which describes the potential vs. time behavior of glass eleotrodes in terms of a new constant which may be useful in the characterization and selection of glass compositions. Agreement between glass electrode response predicted by the theory and available experimental data is excellent.

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Zusammenfassung Die Ansprechbarkeit von Glaselektroden auf rasche Änderungen der Ionenkonzentration wird vom theoretischen Standpunkt aus erörtert. Eine Gleichung wird abgeleitet, die das Elektrodenpotential als Funktion der Zeit erfaßt und eine neue Konstante enthält, die zur Beurteilung und zur Auswahl der Zusammensetzung von Gläsern dienen kann. Es besteht eine sehr gute Übereinstimmung zwischen theoretisch vorausgesagter und experimentell geprüfter Ansprechbarkeit von Glaselektroden.

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The financial support of NIH Grant GM-10086-03 and NSF Grant GP-1093 is gratefully acknowledged.

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Alfred P. Sloan Research Fellow.     

A specific enzyme electrode for L-phenylalanine

A specific enzyme electrode procedure is described for the rapid assay of L-phenyl- alanine. The enzyme L-phenylalanine ammonia lyase is used, which cleaves L-phenyl-alanine to ammonia. The ammonia liberated is measured with an air-gap electrode. The procedure is specific; L-tyrosine and other amino acids do not interfere, nor do Na⁺ or K⁺ ions. As little as 5 · 10^-5 M L-phenylalanine can be determined.     

A completely spray-coated membrane electrode assembly

We present a proton exchange membrane fuel cell (PEMFC) manufacturing route, in which a thin layer of polymer electrolyte solution is spray-coated on top of gas diffusion electrodes (GDEs) to work as a proton exchange membrane. Without the need for a pre-made membrane foil, this allows inexpensive, fast, large-scale fabrication of membrane-electrode assemblies (MEAs), with a spray-coater comprising the sole manufacturing device. In this work, a catalyst layer and a membrane layer are consecutively sprayed onto a fibrous gas diffusion layer with applied microporous layer as substrate. A fuel cell is then assembled by stacking anode and cathode half-cells with the membrane layers facing each other. The resultant fuel cell with a low catalyst loading of 0.1 mg Pt/cm² on each anode and cathode side is tested with pure H₂ and O₂ supply at 80 °C cell temperature and 92% relative humidity at atmospheric pressure. The obtained peak power density is 1.29 W/cm² at a current density of 3.25 A/cm². By comparison, a lower peak power density of 0.93 W/cm² at 2.2 A/cm² is found for a Nafion NR211 catalyst coated membrane (CCM) reference, although equally thick membrane layers (approx. 25 μm), and identical catalyst layers and gas diffusion media were used. The superior performance of the fuel cell with spray-coated membrane can be explained by a decreased low frequency (mass transport) resistance, especially at high current densities, as determined by electrochemical impedance spectroscopy.