Chemical potentiometric type

There are two basic types of solid-state chemical sensor (i) potentiometric devices, and (ii) field effect devices, e.g. ion-selective field effect transistors (ISFETs or CHEMFETs). Electrodes of the potentiometric type usually have a metal as the back contact and they also have a high output impedance. Field-effect devices are a variant of the metal oxide field-effect transistor (MOSFET) familiar in electronics, and they have a low output impedance. Hybrid devices attempt to combine the advantages of both. 

In contrast to the well developed calixarene sensors of the potentiometric type, chemical sensors based on membrane permeability changes are still in its initial stage. In the present study the permeability of LB calix[4]resorcinarene was found to be controlled by the presence... 

Chemical sensing is not a well investigated conventional research field. Research started just a few decades ago. However, some chemical sensors are already in widespread commercial use and contribute greatly to making our lives comfortable. Commercial sensors are roughly divided into three types according to the detection principle (I) conductimetric type, (II) potentiometric type, (III) amperometric type. 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

The majority of potentiometric titrations involve chemical reactions which can be classified as (a) neutralisation reactions, (b) oxidation-reduction reactions, (c) precipitation reactions or (d) complexation reactions, and for each of these different types of reaction, certain general principles can be enunciated. 

In the production of anionic surfactants, the analytical procedures to be adopted for quality control and/or assessment are of particular importance. Their reliability as well as their time and chemical demand is a fundamental topic for the economy and success of the surfactant production cycle. To this end the most important analyses to be done on the various types of anionic surfactants are outlined in Tables 15-19. Mention must be made of potentiometric titration of the sulfonic acid (whatever the processed feedstock), which allows one to obtain reliable results over a very short time. 

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

Agents for chemical bleaching rely on different types of peroxides. Potentiometric or amperometric biosensors that detect the highly specific and sensitive reaction of enzymes like katalases with their corresponding substrates can be used for on-line measurement [84]. The sensors can be manufactured with simple technologies at moderate cost, but their stability is not sufficient for integration in household appliances. 

In the early part of this century, many types of solid electrolyte had already been reported. High conductivity was found in a number of metal halides. One of the first applications of solid electrolytes was to measure the thermodynamic properties of solid compounds at high temperatures. Katayama (1908) and Kiukkola and Wagner (1957) made extensive measurements of free enthalpy changes of chemical reactions at higher temperatures. Similar potentiometric measurements of solid electrolyte cells are still made in the context of electrochemical sensors which are one of the most important technical applications for solid electrolytes. 

Different types of sensor based on solid electrolytes have been developed following a report by Kiukkola and Wagner (1957). These sensors are based on one of two principles (a) the chemical potential difference across the solid electrolyte (potentiometric sensor), or (b) the charge passed through the electrolyte (amperometric sensor). In the following galvanic cell,... 

Ever since an ISFET that was chemically modified by a valinomycin-containing PVC membrane was reported [141], there has been general consensus on the advantages of this type of microsensor over conventional ISEs. Some serious problems have also been acknowledged, though e.g. the low mechanical stability of the membranes, the interference of COj in the potentiometric response, the lack of a stable micro-reference electrode and the relatively high drift rate of ISFETs). Attachment of the membrane can... 

The pH window is very wide in solvents that are weak both in acidity and basicity. The widths of the pH window are well over 30 in such solvents, compared to about 14 in water (Table 6.6). The usefulness of these expanded pH regions is discussed in Section 3.2.2. In particular, potentiometric acid-base titrations in such solvents are highly useful in practical chemical analyses as well as physicochemical studies [22]. Acid-base titrations in lion-aqueous solvents were popular until the 1980s, but now most have been replaced by chromatographic methods. However, the pH-ISFETs are promising to realize simple, rapid and miniature-scale acid-base titrations in lion-aqueous solvents. For example, by use of an Si3N4-type pH-ISFET, we can get an almost complete titration curve in less than 20 s in a solution containing several different acids [17d]. 

Potentiometric measurements are done under the condition of zero current. Therefore, the domain of this group of sensors lies at the zero-current axis (see Fig. 5.1). From the viewpoint of charge transfer, there are two types of electrochemical interfaces ideally polarized (purely capacitive) and nonpolarized. As the name implies, the ideally polarized interface is only hypothetical. Although possible in principle, there are no chemical sensors based on a polarized interface at present and we consider only the nonpolarized interface at which at least one charged species partitions between the two phases. The Thought Experiments constructed in Chapter 5, around Fig. 5.1, involved a redox couple, for the sake of simplicity. Thus, an electron was the charged species that communicated between the two phases. In this section and in the area of potentiometric sensors, we consider any charged species electrons, ions, or both. 

There are three types of reference electrodes discussed reference electrodes of the first kind, reference electrodes of the second kind, and redox reference electrodes. The first two are used with potentiometric chemical sensors, whereas the last one helps us to get around the difficult problem of comparing potentials in different solvents. There is also a pseudo-reference electrode that does not have a stable, defined, reproducible potential. It serves only as the signal return to satisfy the condition of closing the electrical circuit (see Section 5.2). Because the liquid junction always causes some leakage of the internal solution, electrodes of the first kind are particularly affected. 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

With respect to the type of sensors that can be used in an electronic tongue, practically all the main families of chemical sensors have been used to form the sensor array, viz. potentiometric, voltammetric, resistive, gravimetric and optical, if main sensor families have to be quoted [11], Table 30.1 sketches a survey of different approaches that can be recorded when the specialized literature is inspected. Even hybrid systems have been proposed, mainly those combining potentiometric and voltammetric sensors [3,12], The combination of electronic noses and electronic tongues to improve detection or identification capabilities, in a sensor fusion approach, has also been proposed [13,14],... 

Products obtained by propane-selective oxidation have been analyzed by gas sensor systems [19, 26]. Usually, several or multiple kinds of compounds are produced during the selective oxidation of propane. The formation of CO, C02, aldehydes such as acrolein, and ketone were observed over iron-silica catalysts [28, 29]. During the initial stage of catalyst investigation, the conversion of propane and the selectivity toward useful oxygenate products as chemical resources are of interest. Semiconductor-type gas sensors selective toward the oxygenate were employed to estimate the yield of oxygenate products, with a combination of the potentiometric CO sensor and the ND-IR C02 sensor [30]. 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

The titrimetric modifications described above exemplify either an instrumental adaptation (as in the case of the potentiometric and conductometric titrations) or a chemical manipulation (olfactory indicators). The first uses the sense of hearing, while the latter type appeals to the sense of smell. Unfortunately, instrumental adaptations that utilize one-of-a-kind homemade instruments are not readily available to the typical educator or may even be out of circulation. Also considered is the fact that at institutions where the enrollment of handicapped people is low, the justification of specialized equipment can be very difficult to obtain. On the other hand, the chemical manipulations discussed rely on readily available chemicals (onions and cloves) and represent comparable costs and laboratory preparation times as traditional titrimetric experiments. Since all students may perform these... 

This anodic reaction provides sodium ions and electrons to the solid electrolyte and the inert Pt counter electrode, respectively, at the source side. Both the sodium ions and electrons will then travel through the solid electrochemical cell along previously-mentioned ionic and electronic paths to sustain the PEVD cathodic reaction for Na COj product formation at the sink side. Eurthermore, based on anodic reaction 60, the chemical potential of sodium is fixed by the vapor phase at the source side. Under open circuit conditions, this type of source can also serve as the reference electrode for a CO potentiometric sensor. 

Considerable attention has also been paid to the potentiometric response of powdered active carbon electrodes, which in considerable part depends on the type and concentration of functional groups on the surface [7,70,160,161]. The response of a carbon electrode to ionic species in aqueous solution arises from the adsorption behavior of surface functional groups. In addition, physically and/ or chemically adsorbed gases (mainly CO and oxygen) affect this process significantly. 

Abstract Brief historic introduction precedes presentation of main types of transducers used in sensors including electrochemical, optical, mass sensitive, and thermal devices. Review of chemical sensors includes various types of gas sensitive devices, potentiometric and amperometric sensors, and quartz microbalance applications. Mechanisms of biorecognition employed in biosensors are reviewed with the method of immobilization used. Some examples of biomimetic sensors are also presented. 

Three types of end points are encountered in titrations with silver nitrate (1) chemical, (2) potentiometric, and (3) amperometric. Three chemical indicators are described in the sections that follow. Potentiometric end points are obtained by measuring the potential between a silver electrode and a reference electrode whose potential is constant and independent of the added reagent. Titration curves similar to those shown in Figures 13-3, 13-4, and 13-5 are obtained. Potentiometric end points are discussed in Section 21C. To obtain an amperometric end point, the current generated between a pair of silver microelectrodes in the solution of the analyte is measured and plotted as a function of reagent volume. Amperometric methods are considered in Section 23B-4. 

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