Ion-selective properties

Thus in this system, in addition to the usual requirements, the separator has the task of delaying penetration for as long as possible. A membrane would be regarded as perfect which lets hydroxyl ions pass, but not the larger zincate ions. This requirements is best met by regenerated cellulose ( cellophane ) [10,11], which in swollen condition shows such ion-selective properties but at the same time is also chemically very sensitive and allows only a limited number of cycles the protective effects of additional fleeces of polyamide or polypropylene have already been taken into account. 

The name "ion-exchange membrane mainly refers to the fact that this ion-selective property can best be realized by using materials closely resembling the ion-exchange resins. 

Fig. 2. Diagram of the properties of an excitable cell membrane. Voltages across each of the three segments of cell membrane are indicated by -I- and — signs. The ion selectivity properties of the Na+ and K+ channels are represented by negatively charged groups (C and P ) guarding the entrances. See text for description of channel kinetics.

The ion-selective properties of glass/electrolyte interfaces were recognized early in the 20th century, and glass electrodes have been used since then for measurements of pH and the activities of alkali ions (24, 37,46-55). Figure 2.4.1 depicts the construction of a typi-... 

Poly(vinyl chloride) (PVC) is an excellent selective membrane barrier widely used in ion-selective electrodes. " The ion-selective properties of the membrane depend on incorporating a platicizer, ion exchanger, or neutral carrier molecule. Christie et applied PVC membranes (with no incorporated ion... 

The properties of several representative liquid-based ion-selective electrodes are presented in Table 11.3. An electrode using a liquid reservoir can be stored in a dilute solution of analyte and needs no additional conditioning before use. The lifetime of an electrode with a PVC membrane, however, is proportional to its exposure to aqueous solutions. For this reason these electrodes are best stored by covering the membrane with a cap containing a small amount of wetted gauze to... 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

Electroanalytical chemistry is one of the areas where advantage of the unique properties of SAMs is clear, and where excellent advanced analytical strategies can be utilized, especially when coupled with more complex SAM architectures. There are a number of examples where redox reactions are used to detect biomaterials (357,358), and where guest—host chemistry has been used to exploit specific interactions (356,359). Ion-selective electrodes are an apphcation where SAMs may provide new technologies. Selectivity to divalent cations such as Cu " but not to trivalent ions such as Fe " has been demonstrated (360). 

The newest molecular addition to Cl- channels are bestrophins [5]. There are four different bestrophin isoforms in humans. Mutations in Bestl cause Best macular dystrophy, hence their name. All four isoforms induce chloride currents when expressed heterologously. Although they show a dependence on intracellular calcium, their biophysical properties differ from Ca-activated chloride currents typically observed in native cells. Mutagenesis experiments changed the activation of currents and induced slight changes in ion selectivity, lending support to the hypothesis that bestrophins themselves are Cl- channels. 

The measured potential is thus a linear function of pH an extremely wide (10-14 decades) linear range is obtained, with calibration plots yielding a slope of 59 mV per pH unit. The overall mechanism of the response is complex. The selective response is attributed to the ion-exchange properties of the glass surface, and in particular the replacement of sodium ions associated with the silicate groups in the glass by protons ... 

Ion-selective membranes derive their permselective properties from either ion exchange, solubility or complexation phenomena. Current ion-selective electrodes contain membranes which consist of glass, solid or liquid phases. 

Principles and Characteristics A substantial percentage of chemical analyses are based on electrochemistry, although this is less evident for polymer/additive analysis. In its application to analytical chemistry, electrochemistry involves the measurement of some electrical property in relation to the concentration of a particular chemical species. The electrical properties that are most commonly measured are potential or voltage, current, resistance or conductance charge or capacity, or combinations of these. Often, a material conversion is involved and therefore so are separation processes, which take place when electrons participate on the surface of electrodes, such as in polarography. Electrochemical analysis also comprises currentless methods, such as potentiometry, including the use of ion-selective electrodes. 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

Molecular mechanics calculations have become a well established tool in the area of coordination chemistry, including the coordination chemistry of nickel375-379 where they are often applied for the analysis or the prediction of structures,380 the computation of isomer or conformer ratios and metal ion selectivities,381,382 and for simulating spectroscopic properties in combination with AOM calculations or by hybrid quantum mechanics/molecular mechanics (QMMM) methods.383,384 Details of the various approaches, e.g., the incorporation of d-electron stabilization energy... 

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