Potentiometric Ion Selective Electrodes ISEs

The theory that governs the operation of ISEs was described early on by the Nernst equation  

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Electrochemical ion-selective sensors (ISSs), including potentiometric ion-selective electrodes (ISEs) and potentiometric or amperometric gas-selective sensors (GSSs), attracted the interest of clinical chemistry because they offer fast, reliable, inexpensive analytical results in service-free automated analyzers. In this way, the electrochemical sensors satisfy the present demands of central hospital laboratories and peripheral point-of-care medical service points, such as bedside, emergency or first-contact healthcare centers. 

Potentiometric ion-selective electrodes (ISEs) are one of the most important gronps of chemical sensors. The application of ISEs has evolved to a well-established rontine analytical technique in many fields, inclnding clinical and environmental analysis, physiology, and process control. The essential part of ISEs is the ion-selective membrane that is commonly placed between two aqueous phases, i.e the sample and inner solutions that contain an analyte ion. The membrane may be a glass, a crystalline solid, or a liquid (1). The potential difference across the membrane is measured with two reference electrodes positioned in the respective aqueous phases... 

The sufficient selectivity to a principal component is the most important condition determining the possibility of ion-selective electrodes (ISEs) practical appliances. In this work, the relationship between the potentiometric selectivity of alkylammonium-selective electrodes and factors such as the nature of plasticizer, ion-exchanger and substitution degree of cationic nitrogen atoms of the principal and foreign ions, is discussed. 

The equipment required for direct potentiometric measurements includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Figure 5-1). Conventional voltmeters cannot be used because only very small currents are allowed to be drawn. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not... 

Electrodes which only respond to certain free (not bound) measured ions are called ion-selective electrodes (ISE). The term is now usually applied to all potentiometric measuring electrodes that are capable of providing data concerning the concentration or activity of... 

Fig. 18a.l. Schematic diagram of a potentiometric cell with an ion-selective electrode (ISE) as the indicator electrode. EM is the electrical potential of the sensing membrane and IFS the internal filling solution. 

These measure the potential difference between the transducing electrode and a reference electrode under conditions of zero current. Three types of potentiometric detectors are commonly employed ion-selective electrodes (ISE), gas-sensing electrodes and field effect transistors (FET). 

Potentiometric measurements are based on the determination of a voltage difference between two electrodes plunged into a sample solution under null current conditions. Each of these electrodes constitutes a half-cell. The external reference electrode (ERE) is the electrochemical reference half-cell, which has a constant potential relative to that of the solution. The other electrode is the ion selective electrode (ISE) which is used for measurement (Fig. 18.1). The ISE is composed of an internal reference electrode (IRE) bathed in a reference solution that is physically separated from the sample by a membrane. The ion selective electrode can be represented in the following way ... 

Potentiometry is one of the versatile electroanalytical techniques widely applied for sensing. A classical example of its application is potentiometric determination of pH, or activity of other inorganic and organic ions by using a pH electrode or ion-selective electrodes (ISEs), respectively. Generally, potential difference across a... 

Chapters 1 to 5 deal with ionophore-based potentiometric sensors or ion-selective electrodes (ISEs). Chapters 6 to 11 cover voltammetric sensors and biosensors and their various applications. The third section (Chapter 12) is dedicated to gas analysis. Chapters 13 to 17 deal with enzyme based sensors. Chapters 18 to 22 are dedicated to immuno-sensors and genosensors. Chapters 23 to 29 cover thick and thin film based sensors and the final section (Chapters 30 to 38) is focused on novel trends in electrochemical sensor technologies based on electronic tongues, micro and nanotechnologies, nanomaterials, etc. 

The potentiometric biosensor is a combination of an ion-selective electrode (ISE) base sensor with a vegetable tissue (the source of enzyme), which provides a highly selective and sensitive method for the determination of a given substrate. Advantages of such potentiometric biosensors are simplicity of instrumentation (only a pH meter is needed),... 

Ion selective electrodes (ISEs) or, in a wider sense, potentiometric sensors have demonstrated its usefulness to yield information of chemical species in automated and autonomous operation. This feature has fostered their use in the monitoring of numerous processes, in the industrial, clinical and environmental fields, among others. Current practice with these devices relies on sensors with high selectivity only in this way, a simple determination of a single ion is possible in presence of its interferents. Some reluctances on the broadening of their use are surely due to the fact that ISEs are not specific but show high selectivity towards a reduced number of ions. 

The electrochemical analyzers are another important family of liquid analyzers. They include potentiometric, wherein an electric potential is measured and the solution remains unchanged conductive, in which a minute current is measured but the system is essentially unchanged and amperometric, in which a chemical reaction occurs during the course of the measurement. Potentiometric analyzers can measure the presence of dissolved ionized solids in a solution. These measurements include pH, oxidation-reduction potential (ORP), and ion-selective electrodes (ISEs) or probes. 

Isopotential point — In potentiometric measurements with the use of a -> ion-selective electrode (ISE) cell, the isopotential point is the potential difference between the internal and external reference electrodes which is independent of temperature. The isopotential point is governed by a particular activity of the ion being determined. Both ISE and the outer reference electrode must be specified. When an isothermal cell is used with identical reference electrodes, the isopotential point is defined by the activity of the sensed ions that gives zero net - membrane potential, e.g., sensed activity is the same in the inner and outer (test) solution. Calibration lines for different cell temperatures have different slopes, but they intersect at a common activity point. Cells with temperature gradients are not recommended. 

Potentiometric transducers measure the potential under conditions of constant current. This device can be used to determine the analytical quantity of interest, generally the concentration of a certain analyte. The potential that develops in the electrochemical cell is the result of the free-energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition is satisfied. For electrochemical cells containing an anode and a cathode, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell. If the reaction is conducted under standard-state conditions, then this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must use the Nernst equation to determine the cell potential. Physical phenomena that do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion-concentration gradients across a semi-permeable membrane this can also be a potentiometric phenomenon and is the basis of measurements that use ion-selective electrodes (ISEs). 

The mechanism of functioning of potentiometric chemical sensors, of which most commonly are developed and used membrane ion-selective electrodes (ISE), is based on change of distribution of ions on the interface membrane-electrolyte. Thus, there are designed solid-state and plasticized membrane of different chemical composition, which can be tailored to choose and transport ions with a selected... 

Pipet-based electrodes have been used for a wide range of applications, e.g., micropipet ion-selective electrodes (ISEs) for potentiometric determination... 

Potentiometric ion-selective electrodes are passive probes, which in contrast to voltammetric sensors do not convert the analyte in the sample. The response of an ISE depends linearly on the logarithm of the activity (concentration) of a potential determining ion (primary ion) in the presence of other ions. The schematic layout of a complete potentiometric cell including an ion-selective electrode is shown in Figure 2. The electrochemical notation of the cell assembly is given as ... 

A gravimetric method was proposed for Ag and other precious metals, based on reduction with dipyrone (39)121. Determination of Ag(I) ions, strongly complexed with ammonia, sulfite, thiocyanate or thiosulphate, was carried out by potentiometric titration with sodium A.A-diethyldithiocarbamate (11). The titration end-point was better defined when using a sulfide ion-selective electrode (ISE)137. Polypyrrole-based ISEs are poisoned by Ag(I) cations leading to modified selectivities. Ag(I) ions can be implanted... 

Ionophore-based solvent polymeric membranes are widely used as sensing membranes in ion-selective electrodes (ISEs) [24,25]. This type of potentiometric sensor has attracted great interest in the last decade due to the extraordinary improvement in the detection limit down to picomolar (10-12 M) levels [26,27], Furthermore, solid-contact ISEs have been developed by using various conducting polymers, including PEDOT, as the ion-to-electron transducer [28-31],... 

The application of potentiometric detection in ion-chromatography is favoured by the progress in the field of membrane ion-selective electrodes (ISE). The electrodes with solid-state membranes were mostly employed for determination of halides, pseudohalides and some other anions binding silver ions. The use of fluoride electrode in multidetector, chromatographic system offers very low detection limit of 1.2 ng fluoride in injected samples. Application of bromide electrode in the same system provides even five-fold better detectability. The same level of detectability was reported by Butler and Gershey for iodide with iodide ISE. In the system with preconcentration step the detectability can be lowered by an order... 

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