Construction of Gel Type Reference and Combination Ion-Selective Electrodes

Abstract

A simple and an inexpensive procedure for the construction of the reference electrodes and of the conversion of ion-selective electrodes to the “combination” types is described. It allows the manipulation of the reference electrode filling solution according to the needs of a particular measurement. The electrodes have good reproducibility and stable inter-comparison potentials of ± 0.2 mV. The performance characteristics of these electrodes are satisfactory for potentiometric measurements.     

Sodium Ion-Selective Chalcogenide Glass Electrodes

Abstract

Analytical characteristics and sensing mechanism of sodium ion-selective electrodes based on NaCl-Ga₂S₃-GeS₂ glasses have been investigated. Chalcogenide glass electrodes containing 10 mol.% NaCl in the membrane showed near-Nernstian response in the concentration range from 10-³ to 1 M sodium nitrate solution. These sensors were superior to the conventional pNa oxide glass electrodes in selectivity in the presence of hydrogen ions and in Na⁺ ion sensitivity in fluoride media. Prolonged solution treatment for several days reduces, however, the detection limit of the sensors and the slope of the electrode response. Ionic processes at the membrane surface have been investigated using XPS technique and ²²Na tracer measurements. It was shown that sodium ion-exchange governed Na⁺ ion response of chalcogenide     

Carbon-Filled Polymer Paste Reference Electrodes

Abstract

Inexpensive, disposable, carbon-filled polymer paste reference electrodes are reported. These electrodes are designed for use in conjuction with carbon-filled polymer paste ion-selective probes reported earlier. Attachment of integrated electronics is easily achieved.     

Ion-Selective Electrodes Based on Bilayer Film Coating

The electrode characteristics of ion-selective electrodes (ISEs) for K⁺, Na⁺, NH₄ ⁺, and Ca²⁺ based on bilayer film coatings, where the inner layer films are electroactive electropolymerized ones and the outer layer films are composed of conventional ion-sensitive materials, have been examined. These ISEs of the coated-wire electrode type have no conventional internal reference solution and reference electrode, but the inner films may be considered to function as the “internal standard solution.” The ion selectivity coefficients and the activity range showing Nernstian response were almost comparable to those of conventional liquid-membrane electrodes. The bilayer-coated ISEs showed insensitivity to O₂ and CO₂, long-term stability, and little drift. It was also found that the electrode performance is practically unchanged after sterilization in an autoclave. The results demonstrate that the bilayer-coated ISEs examined are promising for the determination of K⁺, Na⁺, NH₄ ⁺, or Ca²⁺ activity in biological and environmental systems.     

Ion-Selective Electrodes

Summary Current investigations and hypotheses by the authors on the theory and mechanism of operation of ion-selective electrodes are reviewed. Some trends in the applications of ion-selective electrodes are discussed, with particular reference to the interests of the authors.

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Zusammenfassung Die laufenden Untersuchungen der Autoren und daraus abgeleitete Hypothesen zur Theorie und Wirkungsweise ionen-selektiver Elektroden wurden dargelegt. Einige Entwicklungen der Anwendung solcher Elektroden wurden im Zusammenhang mit den Arbeiten der Verfasser diskutiert.

     

Carbon Film Electrodes from the Pyrolysis of Two Aromatic Anhydrides

Abstract

Two new carbon film electrodes have been prepared from the low pressure pyrolysis of aromatic anhydrides onto quartz substrates. The anhydrides employed are safer and more volatile than those previously used for preparing carbon film electrodes. The voltammetric response of the film electrodes both in millimolar dopamine and in background electrolyte was studied. The differential pulse voltammetry response with a mixture of dopamine, homovanillic acid and ascorbic acid was studied. Also, the application of the electrode as an optically transparent carbon electrode was examined. The film electrodes had lower background current than glassy carbon, but the reversibility with dopamine was not ideal. These two new films appear to have utility similar to previously studied films, with the advantage of rapid and safe production.     

Application of amalgam electrodes in studies of heavy metals under natural water conditions

The application of amalgam electrodes for measuring the degree of complexation of metal ions is described with respect to natural water conditions. The amalgam electrodes are compared with the corresponding capabilities of ion-selective electrodes. A special cell is described for preparing the amalgam and for filling a hanging amalgam drop electrode. Factors affecting the reproducibility of the standard potentials and slopes, the response time and detection limits are discussed. Complexation measurements are described with lead and zinc amalgam electrodes. Triethylenetetramine, carbonate and nitrolotriacetic acid are used as ligands, to test the ability of these electrodes to measure correctly8 the degree of complexation even at low total-metal. concentrations (down to ca. 10^?7 M) and at very low concentrations of free metal ion (10^?15 M). Results obtained with well-characterized fulvinc compound and an algal culture medium (AAP) are also reported. The observed results are in compl;ete accordance with theoretical predictions (based on Nernstian behaviour), evven at the lowest concentrations of tltal and free metal ion used. An important limitation is that any oxidant in the solution can interfere by oxidizing the amalgma. Solutions must be carefully degassed to eliminate oxygen. It is shown that the interfering actin of oxidants can be corrected for by means of equations which are theoretically sound, even when the nature of the oxidant is unknown, provided that its content is not too high. Compared to ion-selective electrodes, amalgam electrodes are more reproducible, inexpensive and readily prepared for various metal ions which cannot be measured with ion-selective electrodes.     

Sulfonamides as Ionophores for Ise. III. Guanidinium Ion-Selective Electrodes Based on Lipophilic Bis-Sulfonamide Podands

Abstract

PVC membrane ion-selective electrodes based on bis-sulfonamide podands and DOS (Bis(2-ethylhexyl)-sebacate) as plasticizer are described. They were found to behave as guanidinium ion sensors, exhibiting high selectivity for guanidinium ions over potassium ions.     

Ion-Selective Electrodes With Sensing Membranes Covalently Attached to Both the Inert Polymer Substrate and Conductive Carbon Contact

The use of solid-contact ion-selective electrodes (ISEs) is of interest to many clinical, environmental, and industrial applications. However, upon extended exposure to samples and under thermal and mechanical stress, adhesion between these membranes and underlying substrates often weakens gradually. Eventually, this results in the formation of a water layer at the interface to the underlying electron conductor and in delamination of the membrane from the electrode body, both major limitations to long-term monitoring. To prevent these problems without increasing the complexity of design with a mechanical attachment, we use photo-induced graft polymerization to simultaneously attach ionophore-doped crosslinked poly(decyl methacrylate) sensing membranes covalently both to a high surface area carbon as ion-to-electron transducer and to inert polymeric electrode body materials (i.e., polypropylene and poly(ethylene-co-tetrafluoroethylene)). The sensors provide high reproducibility (standard deviation of E⁰ of 0.2 mV), long-term stability (potential drift 7 μV h⁻¹ over 260 h), and resistance to sterilization in an autoclave (121 °C, 2.0 atm for 30 min). For this work, a covalently attached H⁺ selective ionophore was used to prepare pH sensors with advantages over conventional pH glass electrodes, but similar use of other ionophores makes this approach suitable to the fabrication of ISEs for a variety of analytes.     

Solid-Contact Ion-Selective Electrodes with Copper Hexacyanoferrate in the Transducer Layer

The possibility of using mixed Fe²⁺/Fe³⁺ copper hexacyanoferrate (CuHCF) as the material for the transducer layer of solid-contact ion-selective electrodes (SC-ISEs) with plasticized polyvinylchloride membranes is studied. The study is performed for K⁺-SC-ISEs and water-hardness SC-ISEs. It is shown that CuHCF combines the ion-exchange and redox properties and, hence, in principle, should be suitable for SC-ISEs. However, the reproducibility of SC-ISE potentials from one electrode to another and their stability in time are far below those of conventional ISEs with internal aqueous solution. The potentials of individual SC-ISEs can be brought closer to one another by their polarization using a potentiostat or by their short-circuiting to a saturated silver-chloride reference electrode.     

PVC膜离子选择电极检测下限的优化

价格低廉、携带方便、适用浓度宽、操作简单快捷、能耗低的离子选择电极在医院、分析实验室、野外等领域得到了越来越广泛的应用。尽管如此,由于PVC膜中存在的离子流严重破坏了更低检测下限的获取,限制了离子选择电极的进一步发展。因此,本文从减小甚至消除PVC膜中存在的离子流角度出发,系统论述了优化PVC膜离子选择电极检测下限的原理和优良策略,根据收集归纳的大量数据定量阐述传感膜组成的优化、电极组装和调制、应用旋转电极以及电流极化处理等对检测下限的优化提升作用,进一步总结出各种方法的改善规律,分析它们的优势和面临的问题。提出在PVC铸膜液中要突破传统配方,减小增塑剂和离子交换剂用量,以抑制传感膜两侧的离子流,同时外加电流补偿处理等也是降低电极检测下限的有效方法,对检测下限的改善最好的可降低5个数量级。这一总结为PVC膜离子选择电极的高性能化明确了研究方向。     

Selectivity coefficient changes for liquid-membrane electrodes

The effects of the activity levels of the measured ion and interfering ions, and of the detection limit of the electrode on the values of selectivity coefficients for liquid-membrane ion-selective electrodes are discussed. The coefficients were determined by the mixed-ion solution method. Depending on the activity of the interfering ion, the activity of the main ion for which the selectivity coefficient is determined may differ. The best conditions of measurement are those which involve the largest contribution from the term containing the selectivity coefficient in the Nikolsky equation; the measurements are then most precise, and the values of the selectivity coefficients describe the electrode behaviour most consistently. When the limit of detection of the electrode is comparable with the other terms, it must be taken into account in calculations. Under the optimal conditions, selectivity coefficients were calculated for the Orion calcium and divalent cation electrodes, with calcium as the main ion and alkali metal ions as interfering ions.     

Highly Reproducible Flexible Ion-selective Electrodes for the Detection of Sodium and Potassium in Artificial Sweat

It is well known that potentiometric sensors provide a versatile, cost-effective, and efficient platform for wearable applications. Unfortunately, mass production and commercialization of such devices is often constrained by the requirement of a calibration step, which is due to the poor sensor-to-sensor reproducibility and the need of conditioning the electrodes in the analyte before use. Herein, we fabricated calibration-free flexible sensors including ion-selective electrode and reference electrode by integrating single-walled carbon nanotubes (SWCNTs) with poly(3-octylthiophene) (POT) and applying on polyethylene terephthalate (PET) substrate. The developed sodium and potassium ion-selective electrodes (ISEs) display excellent repeatability, selectivity, stability as well as high sensor-to-sensor reproducibility, with a standard deviation of as low as 1.0 mV in artificial sweat microliter samples volume.     

Determination of Inorganic and Organic Thiophosphates with Ion-Selective Membrane Electrodes

Abstract

The conditions for the determination of sodium thiophosphate and sodium S-(2-amino-ethyl)-thiophos-phate in the presence of phosphate ion using ion-selective membrane electrodes are described. Both thiophosphates (inorganic and organic) are hydrolyzed in acidic medium. The phosphate ion is determined with Pb(II) using a Pb²⁺ – selective membrane electrode.

Cysteamine is determined by potentiometric titration with Hg(I1) using a Ag⁺/S^2? – crystal membrane electrode or by direct potentiometry with a Cu²⁺ selective membrane electrode. The results were verified by the iodination method.     

Determination of Nitrate and Nitrite by Ion-Selective Electrodes in Relation to Gypsum Formation on Some Travertine Buildings in Polluted Atmosphere of Ankara

Abstract

Gypsum formation on calcareous building stones in the polluted atmosphere of cities is thought to be accelerated by the presence of nitrogen-containing compounds. A reliable method for nitrate and nitrite determinations is proposed and the presence of nitrogen compounds is discussed in relation to their possible influence on gypsum formation. Investigations on nitrate and nitrite determinations with ionselective electrodes have been carried out along with gypsum determinations on the samples taken from altered surfaces of some travertine buildings in Ankara. Interfering effects of the ions which may be present in the matrix studied have been examined and the proper masking solutions for their removal have been introduced. Investigations show that nitrate and nitrite determinations with ion-selective electrodes are reliable, rapid and inexpensive in stone extracts. The sulphate to nitrate ratios in this study vary between 32 and 393. These data are compared with earlier results and the possible effects of nitrogen-containing compounds in gypsum formation are discussed.     

Miniature Ion-selective Electrodes with Mesoporous Carbon Black as Solid Contact

Herein, we demonstrated miniature solid-contact ion-selective electrodes (ISEs) using a commercial mesoporous carbon black (MCB) as ion-to-electron transducer. MCB is attractive in its high surface area, good conductivity, relative low cost and availability. ISEs for potassium (K⁺) and nitrate (NO₃⁻) ions were prepared by subsequently coating the sealed glass capillaries (1.5 mm) with MCB and ion-selective membranes. Addition of MCB substantially stabilized electrode response by providing adequate double-layer capacitance and lowering resistance by more than 100× compared to the coated-wire electrodes. The electrodes exhibited near-Nernstian slopes of 59.6 (K⁺ ionophore), 57.8 (K⁺ ion-exchanger) and −54.8 (NO₃⁻ ion-exchanger) with standard solutions in the range of 10⁻⁵ to 10⁻¹ M. Fast response (∼10 s) and reproducible sensitivities were also obtained in a mixed electrolyte containing interfering ions, although with a baseline drift of 2–10 mV/day in the long term. Importantly, the electrodes can be simply stored in air between measurements and used directly without conditioning in solutions. With simple fabrication and free maintenance, these sensors offer a low cost and convenient alternative to bulk ISEs, especially when sample volumes or space are limited.     

Water Hardness Electrodes with Ionophores Containing Oxy- and Ester-Groups

Lipophilic compounds combining oxy- and ester-groups are synthesized and studied as neutral ionophores in plasticized PVC membranes for the development of novel water hardness ion-selective electrodes. Electrodes based on the ionophores studied showed a higher selectivity to calcium over magnesium ions. However, for electrodes based on hexadecyl-4-hydroxybutanoate or decyloxybutanol this preference turned to be rather low: logK_CaMg=–(0.5–0.7). Electrodes with membranes containing hexadecyl-4- hydroxybutanoate, 0.3 M as a neutral ionophore and bis[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate, 0.01 M as a charged ionophore, in combination with Ca²⁺-selective electrode based on ETH 1001 as ionophore, proved to be suitable for measurements of water hardness, also for the measurement of Mg²⁺ ion content in artificial aquarium fish-breeding water samples and in samples modeling electrolyte composition of blood serum.     

Response Amplification of an Ion-Selective Electrode

Abstract

The response of an ion-selective electrode can be amplified by connecting the cell, which is composed of an ion-selective electrode and a reference electrode, in series. For a cell using 1, 2 or 3 valinomycin electrodes connected in series, the response slopes to 1 × 10^?5 ?1 × 10^?1 M K⁺ were 58, 116 and 174 mV/activity decade (a.d.) at 25°C, respectively. This amplification method would especially be useful for accurate determinations with electrodes in the range of low concentrations outside the Nernstian response or for determinations of polyvalent ions, in which both cases exhibit small emf response changes in changing the ion concentration.     

An evaluation of some sodium ion-selective glass electrodes in aqueous solution: Part I. Electrode calibration characteristics and selectivity with respect to hydrogen ions

Some properties of a series of commercial sodium ion-selective electrodes have been investigated and the results compared. In general the potential response of the electrodes was found to approach Nernstian with aging.An improved method for investigating the selectivity of ion-selective electrodes with respect to hydrogen ions is based on the mixed solution method utilizing tris buffers. The selectivity of the sodium ion electrodes with respect to hydrogen ions was also found to depend on the ratio of the primary to interfering ion activity. Some other improvements in technique are also reported.