Ion-selective electrode reproducibility

FIGURE 5-5 Determination of die detection limit of an ion-selective electrode. (Reproduced with permission from reference 12.)... 

Coated-wire ion-selective electrode. (Reproduced with permission from... 

Figure 14. Three-dimensional surface plot of the interfacial capacitance as a function of BSA concentration and applied interfacial potential. Water 0.01 mol/L LiCl nitrobenzene 0.01 mol/L TBATPB t = 25.0 °C. Interfacial potential U given vs. TBA + ion selective electrode. (Reproduced with permission from reference 32. Copyright 1990 Elsevier.)...

The properties of a pH electrode are characterized by parameters like linear response slope, response time, sensitivity, selectivity, reproducibility/accuracy, stability and biocompatibility. Most of these properties are related to each other, and an optimization process of sensor properties often leads to a compromised result. For the development of pH sensors for in-vivo measurements or implantable applications, both reproducibility and biocompatibility are crucial. Recommendations about using ion-selective electrodes for blood electrolyte analysis have been made by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) [37], IUPAC working party on pH has published IUPAC s recommendations on the definition, standards, and procedures... 

Buchberger et al. [104] carried out a selective determination of iodide in brine. The performance of a potentiometric method using an ion-selective electrode and of liquid chromatography coupled with ultraviolet detection at 230 nm were compared as methods for the determination of iodide in the presence of other iodide species. Satisfactory results were obtained from the potentiometric method provided the solution was first diluted tenfold with 5 M sodium nitrate, and external standards were used. Better reproducibility was, however, achieved with HPLC, provided precautions were taken to prevent reduction of iodine to iodide in the mobile phase, for which extraction of iodine with carbon tetrachloride prior to analysis was recommended. This was the pre-... 

Direct determination of surfactants in complex matrices can also be carried out using ion-selective electrodes. Depending on the membranes and additives used, the detergent electrodes are optimized for the detection of anionic surfactants [81], cationic surfactants [82], and even nonionic surfactants [83]. The devices are sensitive to the respective group of surfactants but normally do not exhibit sufficient stability and reproducibility for their use in household appliances. With further optimization of membrane materials, plasticizers and measurement technology, surfactant-selective electrodes offer high potential for future applications. 

Figure 4.12 — (A) Flow-cell for the simultaneous determination of four analytes (1) ion-selective electrodes (2) reference electrode (3) Pt wire for grounding (4) Teflon gasket (5) carrier (6) inlet for reference solution (7) waste (8) screws (9) diecast box (10) rubber sheet for sealing. (B) Seven-electrode holder (the reference electrode is placed from the top into die central bore). ISEs for Na, K, Ca ", NOj", Cr, and HCO, (NH, electrode with internal buffer of 0.1 mmol/L NaHCOj) are placed horizontally around the reference electrode, the metal waste tubes being connected to the waste via filter-paper strips. (Reproduced from [124] and [108] with permission of Elsevier Science Publishers and VCH Publishers, respectively).

Figure 4.14 — (A) Flow injection system for the preconcentration and determination of copper P peristaltic pumps A 0.5 M HNOj B sample q = 2.5 mL/min) C water (jq = 0.5 mL/min) E 1 M NaNOj/O.l M NaAcO, pH 5.4 q = 0.5 mL/min F 1 M NaAcO/2 x 10 M Cu pH 5.0 (9 = 1.0 mL/min) 3-5 valves ISE copper ion-selective electrode W waste I and II 2 and 3 mL of chelating ion exchanger for purification III 100 fil of chelating ion exchanger for metal ion preconcentration. (B) Scheme of the flow system for the determination of halides A 4 M HAcO/1 M NaCl/0.57 ppm F B 1 M NaOH/0.5 M NaCl C, mixing coil (1 m x 0.5 mm ID PTFE tube) Cj stainless-steel tube (5 cm x 0.5 mm ID) ISE ion-selective electrode R recorder. (Reproduced from [128] and [129] with permission of Elsevier Science Publishers and the Royal Society of Chemistry, respectively).

Figure 4.15 — (A) Tubular flow-through electrode 1 Perspex body 2 conducting epoxy cylinder 3 mobile carrier PVC membrane 4 electric cable 5 channel (1.2 mm ID) 6 holders 7 screws 8 0-rings. (B) Schematic diagram of a system for on-line monitoring of ammonia ISE tubular flow-through ammonium ion-selective electrode R reference electrode W waste. (Reproduced from [137] with permission of the Royal Society of Chemistry).

Direct analysis with the fluoride ion-selective electrode requires addition of total ionic strength adjustor buffer solution (TISAB) to the standard and to unknown samples Some advantages of this addition are that it provides a constant background ion strength, ties up interfering cations such as aluminum or iron, which form a complex with fluoride ions, and maintains the pH between 5 0 and 5 5 According to the manufacturer s claim, reproducibility of direct electrode measurement is 2 0%, and the accuracy for fluoride ion measurement is 0 2% [27]... 

The GECE sensors were used for lead determination in real water samples suspected to be contaminated with lead obtained from water suppliers. The same samples were previously measured by three other methods a potentiometric FIA system with a lead ion-selective-electrode as detector (Pb-ISE) graphite furnace atomic absorption spectrophotometry (AAS) inductively coupled plasma spectroscopy (ICP). The results obtained for lead determination are presented in Table 7.1. The accumulation times are given for each measured sample in the case of DPASV. Calibration plots were used to determine the lead concentration. GEC electrode results were compared with each of the above methods by using paired -Test. The results obtained show that the differences between the results of GECE compared to other methods were not significant. The improvement of the reproducibility of the methods is one of the most important issues in the future research of these materials. 

The simplest method of measurement with ion-selective electrodes is direct potentiometry by use of the Nemst equation. However, this makes extreme demands on the reproducibility of the junction potential, and there is the problem of variation of activity with ionic strength. Concentration-cell techniques have proved to be very precise, especially in terms of null-point potentiometi... 

The ion-selective electrode technique has been examined by several workers [ 1, 2,3], but primarily on waters of relatively high purity, and under carefully controlled laboratory conditions. Typical electrode performance in normal laboratory use is shown in Figure 1, and reproducibility of approximately —0.5 mV can be expected over the... 

Other types of electrodes have been designed to measure the concentrations of ions other than H30. The simplest example of such an ion-selective electrode is a metal wire, which can be used to detect the concentration of the corresponding metal ion in solution. Silver and copper wires can be used reproducibly in this way to determine the concentrations of Ag and Cu, respectively. Still other electrodes have been developed to detect specific ions. For example, glasses of chemically modified composition are used to construct membrane electrodes to determine potassium and sodium ions or halogen ions. 

More recently, it was demonstrated that the thermistor approach could be used to monitor specific interactions of fluoride ions with silica-packed columns in the flow injection mode. A thermometric method for detection of fluoride [56] was developed that relies on the specific interaction of fluoride with hydroxyapatite. The detection principle is based on the measurement of the enthalpy change upon adsorption of fluoride onto ceramic hydroxyapatite, by temperature monitoring with a thermistor-based flow injection calorimeter. The detection limit for fluoride was 0.1 ppm, which is in the same range as that of a commercial ion-selective electrode. The method could be applied to fluoride in aqueous solution as well as in cosmetic preparations. The system yielded highly reproducible results over at least 6 months, without the need of replacing or regenerating the ceramic hydroxyapatite column. The ease of operation of thermal sensing and the ability to couple the system to flow injection analysis provided a versatile, low-cost, and rapid detection method for fluoride. 

Potentiometric detection of anions is feasible when an electrode is available that responds quickly, reversibly and reproducibly to the concentration (or more precisely to the activity) of sample ions. It is often possible to detect a given ion or class of ions with excellent selectivity. For example, solid-state or crystalline ion selective electrodes have been used in IC to detect halide anions. The fluoride ion-selcclivc electrode is particularly selective [20,21]. A copper wire electrode has been used to detect anions such as iodate, bromide and oxalate [22]. 

The ion-selective electrode is the basis of instruments that are used for the rapid determination of potassium during surgical procedures or the subsequent recovery. This method is relatively cheap and convenient to use at the patienfs bedside and, in comparison with reference methods, is both accurate and reproducible. The direct potentio-metric determination of potassium (and sodium) in small blood samples is also possible this is a useful development of micro-analytical techniques. The naturally occurring radioactive isotope °K is used to measure total body potassium (Schmidt 1992, Birch and Padgham 1993). 

The selectivity of any chemical assay is its ability to provide a result unbiased by the presence of foreign species, and it is therefore crucial to the assay s practical application. Surprisingly, however, very few attempts have been made to describe selectivity of chemical assays in exact terms—except for ion-selective electrodes. To quantify the extent of interference, it is essential to be able to conduct all measurements at precisely and reproducibly maintained conditions so that the interfering species are treated, physically and chemically, in exactly the same manner as the substance to be determined (analyte). Such identical treatment can be achieved in FI A, and it thus makes sense to express the selectivity of a method for species A toward an interfering species by a numerical value. Since any interfering species will always appear as a pseudoanalyte, it was suggested [378] that the selectivity coefficient for any FI A method be defined by... 

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. 

Finally, electrochemical techniques including voltammetric techniques and ion-selective electrodes (ISE) for Cd have also been reported in the literature for free Cd determinations in waters, allowing the differentiation between labile Cd species from strong Cd complexes (e.g., boimd to humic and fulvic acids and colloids). Mercury is still the electrode material of choice for detection of Cd due to its large hydrogen overvoltage and its remarkable reproducibility. 

Atomic spectrometric techniques such as flame atomic absorption spectrometry (FAAS), electrothermal AAS (ETAAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and ICP-MS are used for the determination of elements, particularly metals. ICP-MS is the most sensitive, typically with microgram per liter detection limits and multielement capability but it has high start-up and operating costs. UV-visible spectrophotometry is also used for the determination of metal ions and anions such as nitrate and phosphate (usually by selective deriva-tization). It is a low cost and straightforward technique, and portable (handheld) instruments are available for field deployment. Flow injection (FI) provides a highly reproducible means of manipulating solution chemistry in a contamination free environment, and is often used for sample manipulation, e.g., derivatization, dilution, preconcentration and matrix removal, in conjunction with spectrometric detection. Electroanalytical techniques, particularly voltammetry and ion-selective electrodes (ISEs), are... 

The ion-selective electrode (ISE) for potassium is now well established [29-31] and is the basis of instruments that are used in intensive care units and operating theaters for rapid determination of potassium during surgical procedures or following recovery. Usually sodium and potassium concentrations are determined simultaneously and the result is displayed within a very few seconds of the sample loading. The ISE is relatively cheap and convenient for use at the patient s bedside or in the operating theater. In comparison with reference methods it is both accurate and reproducible. 

The Royal Society of Chemistry for permission to reproduce the diagram of a surfactant-selective electrode (Figure 3.4) from Development of ion-selective electrodes for use in the titration of ionic surfactants in mixed solvent systems, Dowle, C. J., Cooksey, B. G. and Ottaway, J. M., Analyst, 112 (1987), 1299-1302. 

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