Potentiometric Measuring Principle

The sensor body consists of a coil with two connections and an electrode with a tapping point. An aluminium measuring ring is attached to the object to be measured. The measurement is based on the eddy - current principle. The non-con-ductive layer between the core and the coil produces a capacitive circuit. The output signal is related to the position of the target. 

This patented sensor principle allows the integration of the sensor inside a compact friction damper. Compared with the limited space inside the damper, a maximum usable measuring range could be achieved. 

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Where R is the gas constant, T is the temperature, and F is the Faraday constant. Caused by the logarithmic correlation between the gas concentration and the voltage signal, the potentiometric measurement is best suited for measurements of small amounts of oxygen. A well-known application of this principle has been realized in the so called lambda-probe for automotive applications where they are used to control the lambda value within a small interval around 1 = 1. The lambda-value is defined by the relation between the existing air/fuel ratio and the theoretical air/fuel ratio for a stoichiometric mixture composition ... 

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. 

The subject of potentiometric titrations has been exhaustively treated by the classic monograph by Kolthoff and Furman.1 Although the second edition was published in 1931, it is still the definitive work. Unfortunately, it is no longer in print, but it is available in many chemical libraries. A complete and thorough discussion of the principles and theory of potentiometric titrations is provided, together with an extremely extensive summary of the many applications of potentiometric measurements. 

Application and Principle This procedure is used to determine the lipase activity in preparations derived from microbial sources and animal pancreatic tissues. The assay is based on the potentiometric measurement of the rate at which the preparations will catalyze the hydrolysis of tributyrin. 

The gas-sensing configuration described above forms a very useful basic unit for potentiometric measurements of biologically important species. In principle, the immobilized or insolubilized biocatalyst is placed on a conventional ion-selective electrode used to measure the decrease in the reactants or the increase in products of the biochemical reaction. The biocatalyst include... 

The equipment for potentiometric methods is simple and inexpensive and includes a reference electrode, an indicator electrode, and a potential-measuring device. The principles of operation and design of each of these components are described in the initial. sections of this chapter. Following these discussions, we investigate analytical applications of potentiometric measurements. 

The basic principle behind potentiometric measurements is the development of charge related to the analyte activity in the sample through the Nernst relation ... 

Numerous applications have been developed in the field of chemical analysis using potentiometric measurements as indicators, including the production of potentiometric sensors and titration devices. In this chapter, we will focus on the defining principles of these potentiometric methods at zero current when these systems are in thermodynamic equilibrium, which is not necessarily true for all potentiometric measurements. In particular, the following description is confined to electrochemical cells with no ionic junction. In practice, these results will also be applied to many experimental cases in which ionic junction voltages can be neglected . 

Flame atomic emission spectrometry Basic information on FAES is presented elsewhere in this encyclopedia. Sodium measurements are performed at 590 nm with the use of a propane flame (1925°C). Physiological samples for sodium determination are highly diluted before measurement. The diluent and the calibrator solution contain the same concentration of lithium ions so as to balance flame instability by a concomitant measurement of lithium in the reference beam (the so-called lithium guideHne). At the same time, lithium ions inhibit the ionization of sodium atoms. This procedure cannot be used in the case of therapy with lithium salts. That is why some authors prefer the concomitant measurement of caesium to that of lithium. Dilution adjusts the viscosity of the sample to that of the calibrator solution to produce identical aspiration rate and drop size on nebulization. As other electrolytes interfere with sodium measurement, their concentration in the caH-brator solution must be similar to their concentration in the sample. For the measurement of sodium in urine, calibrator solutions different from those for serum measurement are needed as the electrolyte concentrations in urine samples are quite different from those in serum and their relations are very variable. As the concentration of the electrolytes in serum is rather constant, calibrator solutions for serum measurements can fulfill their function better than those for urine in other words, urine determinations are usually less accurate. FAES proved to be sufficiently reliable to be used as the basic principle of the sodium reference measurement procedure. In routine use, however, FAES is less accurate. Its application is given up by most clinical laboratories in favor of potentiometric measurements... 

A General Principles 659 23B Reference Electrodes 660 23C Metallic Indicator Electrodes 662 23D Membrane Indicator Electrodes 664 23E Ion-Selective Field-Effect Transistors 675 23F Molecular-Selective Electrode Systems 677 23G Instruments for Measuring Cell Potentials 684 23H Direct Potentiometric Measurements 686 231 Potentiometric Titrations 691 Questions and Problems 692... 

In the last decade increasing attention has been paid to the electeochemical studies on the kinetics and equilibrium of ion transfer at the oil/water interfaces that are polarized in the sense that the interface is of the ideal-polarized nature for all ions (here Bi, Ai, B2, and A2) except transferring ion or ions (here B3) (for reviews see [31-38].) In such cases the nontransferable ions serve as the supporting electrolytes for the transport process of the transferable ions across the interface. It has been shown that the oil/water interface works as an ion-selective electrode surface for both vol-tammetric (and amperometric) and potentiometric measurements of ions, both based on the same electrochemical principle of ion transfer across the interface [43,44]. There it is essential that the oil/water interface is of the ideal-polarized nature for all ions (such as counter ion and supporting electrolyte ion so-called potential window) except the monitored ion or ions (voltammetry). 

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