A microcomputer-based system for potentiometric measurements with ion-selective electrodes

A comparison, on completely equal terms, of a microcomputer-based system with manual measurements is described in detail. The main features compared are the precision and accuracy of the results and the number of determinations per time unit. The example used is a standard addition measurement of fluoride in toothpaste. The microcomputer system and its interfacing to the peripheral units are also described. The microcomputer system for control, supervision and evaluation of the measurements results in a significant gain in precision and speed. Accuracy requires the use of a non-theoretical value of the Nernstian slope. The memory effect of the ion-selective electrode is large when the type of sample is changed.     

Simple antilog converter for conventional and flow-injection measurements with ion-selective electrodes

The antilog converter for potentiometric measurements produces an output voltage which is linearly dependent on the concentration of the sensed species over two orders of magnitude. Signals obtained in measurements with chloride, fluoride, potassium and copper ion-selective electrodes range from 1.0 to 10.0 V. The converter is successfully applied for flow-injection potentiometry within one concentration decade but its sensitivity depends on the dispersion in the flow system.     

Bipolar pulse conductivity measurements applied to ion-selective electrodes

Bipolar pulse conductivity (BICON) measurements were evaluated as a means of using calciumion-selective electrodes with an upoised reference electrode. The study shows a change in total cell conductivity with Ca²⁺ concentration at low concentrations in the absence of other electrolytes but no change in conductivity with Ca²⁺ concentration in the presence of 0.1 M KCl. The computed voltage at zero current varied with Ca²⁺ concentration but electroactive species interfered with the measurements. For the conditions used, it is concluded that there is no change in conductivity of the ion-selective membrane with Ca²⁺ concentration and that reliable quantitation of Ca²⁺ is not feasible.     

Coated-wire ion-selective electrodes

Coated-wire ion-selective electrodes were first developed in 1971, and comprise a film of polyvinyl chloride or other suitable polymeric matrix substrate containing a dissolved electroactive species, coated on a conducting substrate (generally a metal, although any material with conductivity substantially higher than that of the film can be used). Electrodes of this sort are simple, inexpensive, durable and capable of reliable response in the concentration range of 10^?1 to 10^?6 M for a wide variety of both organic and inorganic cations and anions. The principles on which these electrodes are based, as well as their application to a variety of analytical problems, will be discussed.     

Neutral-carrier-based ion-selective electrodes

Neutral-carrier-based ion-selective electrodes for the assay of alkali and alkaline earth metal cations have advanced to become the most frequently used potentiometric sensors in clinical chemistry. The major developments since the realization of the first potentiometric cell assemblies utilizing electrically neutral complexing agents (valinomycin and the macrotetrolides) in 1966 are presented.     

Comparison of copper(II) ion-selective electrodes for measurements at micromolar concentrations

Four types of copper ion-selective electrodes have been tested for determining copper at concentrations below lO^-6 mol 1^l-1. None of the electrodes has a Nemstian response in dilute copper solutions in this concentration range, though their responses are linear in pCu buffer solutions. The causes of the deviations are a direct redox effect in the case of an electrode with a Cu_1.8,Se single crystal membrane, production of copper ions by oxidation of the membrane itself in Ag₂S—CuS membrane electrodes, and a combination of the two in the case of the R??i?ka Selectrode. The electrode potentials are affected by the oxygen content and pH of the sample solution and the condition of the membrane surface. Precision tests on two types of electrode are described.     

Linear titration plots with ion-selective electrodes

A computational method for simultaneous determination of the equivalence volume and the Nernstian slope of an ion-selective electrode is described. Treatment of data from linear titration and standard addition procedures is simplified by the use of equal additions of standard solution. Equivalence volumes and concentrations were determined with errors of 0.5-2% by the proposed method.     

Experience with mixed-salt ion-selective sulphate electrodes

Sulphate ion-selective electrodes with mixed-salt, pressed-pellet membranes have been manufactured and tested for response, stability and selectivity. Acceptable performance was obtained only with selected specimens. The addition of small amounts of cuprous sulphide to the membrane had no significant effect.     

Single-point potentiometric titrations with ion-selective electrodes

A single-point potentiometric titration method is described for the determination of silver(I), chloride and sulphate with ion-selective electrodes. Relative standard deviations in the range 0.1–1.25% can be obtained, depending on the quality of the electrodes used.     

Calixarene-based potentiometric ion-selective electrodes for silver

Four lipophilic sulphur and/or nitrogen containing calixarene derivatives have been tested as ionophores in Ag(I)-selective poly (vinyl chloride) membrane electrodes. All gave acceptable linear responses with one giving a response of 50 mV/dec in the Ag(I) ion activity range 10(-4)-10(-1)M and high selectivity towards other transition metals and sodium and potassium ions. This ionophore was also tested as a membrane coated glassy-carbon electrode where the sensitivity and selectivity of the conventional membrane electrode was found to be repeated. The latter electrode was then used in potentiometric titrations of halide ions with silver nitrate.     

Application of azoles as neutral carriers in liquid membrane ion-selective pH electrodes

New synthetic H⁺-selective carriers, derivatives of pyrazole, have been used as ionophores in liquid membrane electrodes for pH measurements in the acidic range. Selectivity coefficients toward sodium, potassium and calcium have been determined in this range and they are comparable to or better than those for previously described carriers.     

Screen-printed copper ion-selective electrodes

A simple, low-cost and reproducible method for the mass production of potentiometric ion-selective electrodes for copper ions is presented. These planar, strip sensors were obtained by screen-printing. The application of pastes cured at low temperature allows printing of the sensors on low-cost, plastic substrates. The pastes for printing of ion-sensitive thick-film membranes were based on copper (1) and copper (II) sulfides. The analytical characteristics of the thick-film electrodes were compared. The analytical properties (range of determination, sensitivity, selectivity, response time) of the copper (I) sulfide-based sensors were comparable with those for conventional ion-selective electrodes.     

Screen-printed copper ion-selective electrodes

A simple, low-cost and reproducible method for the mass production of potentiometric ion-selective electrodes for copper ions is presented. These planar, strip sensors were obtained by screen-printing. The application of pastes cured at low temperature allows printing of the sensors on low-cost, plastic substrates. The pastes for printing of ion-sensitive thick-film membranes were based on copper (I) and copper (II) sulfides. The analytical characteristics of the thick-film electrodes were compared. The analytical properties (range of determination, sensitivity, selectivity, response time) of the copper (I) sulfide-based sensors were comparable with those for conventional ion-selective electrodes. Received: 12 January 2000 / Revised: 13 March 2000 / Accepted: 16 March 2000     

Potentiometric determination of penicillins with ion-selective electrodes

Quaternary ammonium, phosphonium and arsonium membrane electrodes sensitive to benzylpenicillin, ampicillin and oxacillin have been investigated. The order of merit of electrode performance is cetyltrioctylammonium> cetyltrioctylphosphonium> cetyltrioctylarsonium. The electrodes are suggested for use in rapid determination of penicillin drugs by direct potentiometry.     

Determination of sulpha-drugs with ion-selective membrane electrodes

A liquid-membrane mercury(II)-sensitive electrode is used for determination of various sulpha drugs by addition of excess of mercuric nitrate and potentiometric back-titration of the surplus mercury(II) with EDTA. The performance of the electrode is compared with that of an Ag(2)S crystal-membrane electrode. Attempts to prepare a sulphamethoxazole-sensitive electrode failed.     

Fullerene-based electrochemical buffer layer for ion-selective electrodes

In this work, C(60) fullerene is used as an electrochemical mediator for the development of an all-solid-state ISE. The unique electrochemical characteristics of the fullerenes allow for the facile ion-to-electron transduction across the ionically active polymeric ion-selective membrane and the electrochemically active glassy carbon transducer. The interfacial ion-to-electron charge transfer was investigated by Electrochemical Impedance Spectroscopy. The study of the analytical characteristics of a model potassium-selective electrode, together with the EIS studies, reveals that, indeed, the interfacial C(60) electrochemically active layer facilitates the ion-to-electron transduction, providing a stable and reversible solid-state ISE system. This finding is a significant contribution to the efforts aiming at overcoming one of the most significant drawbacks of the solid-state ISEs, that is the potential drift observed during continuous measurements, and could lead to the development of both cation- and anion-sensitive systems.