Membrane impedance

Anotlier teclmique used for stmctural inference is dielectric dispersion in tlie frequency [25] or time [26] domains. The biopolymer under investigation must have a pennanent dipole moment p. It is first dissolved in a dielectrically inert solvent, e.g. octanol, which may be considered to bear some resemblance to a biological lipid membrane, and tlien tlie complex impedance i +j( is measured over a range of frequencies / typically from a... 

Because of the very large resistance of the glass membrane in a conventional pH electrode, an input amplifier of high impedance (usually 10 —10 Q) is required to avoid errors in the pH (or mV) readings. Most pH meters have field-effect transistor amplifiers that typically exhibit bias currents of only a pico-ampere (10 ampere), which, for an electrode resistance of 100 MQ, results in an emf error of only 0.1 mV (0.002 pH unit). 

The aperture impedance principle of blood cell counting and sizing, also called the Coulter principle (5), exploits the high electrical resistivity of blood cell membranes. Red blood cells, white blood cells, and blood platelets can all be counted. In the aperture impedance method, blood cells are first diluted and suspended ia an electrolytic medium, then drawn through a narrow orifice (aperture) separating two electrodes (Fig. 1). In the simplest form of the method, a d-c current flows between the electrodes, which are held at different electrical potentials. The resistive cells reduce the current as the cells pass through the aperture, and the current drop is sensed as a change in the aperture resistance. 

Figure 4. Impedance versus temperature behavior of Celgard microporous membranes,...

The hot-ER test refers to measuring the impedance of a separator while the temperature is linearly increased this technique was first used by Laman et al. [5]. Figure 4 shows actual measurements for some Celgard membranes [20], The single-layer materials exhibit a sharp rise in impedance near their respective melting points the impedance goes back down after a maximum value has been reached. 

The intrinsic ionic conductivities of hydrated chitosan membranes investigated using impedance spectroscopy were as high as 10 S cm [232]. 

In this chapter, the focus is on how zeohte membranes can be appHed in the field of catalysis and to what extent this is successful. The latter is illustrated by reviewing some commonly studied zeohte membrane applications. Finally, the current hurdles that impede industrial application are discussed and some remarks... 

Water produced by reverse osmosis (RO) is forced by an osmohc pressure through a semi-permeable membrane which acts as a molecular filter. The difiusion of solubles dissolved in the water is impeded, and those with a molecular weight in excess of 250 do not difftise at all. The process, which is the reverse of the natural process of osmosis, thus removes microorganisms and their pyrogens. Post-RO contaminahon m occur if the plant after the membrane, the storage vessel or the distribuhon system is not kept Ifee Ifom microorganisms. 

De Marco R, Pejcic B, Prince K, van Riessen A (2003) A multi-technique surface study of the mercury(ll) chalcogenide ion-selective electrode in saline media. Analyst 128 742-749 Pejcic B, De Marco R (2004) Characterization of an AgBr-Ag2S-As2S3-Hgl2ion-selective electrode membrane a X-ray photoelectron and impedance spectroscopy approach. Appl Surf Sci 228 378-400... 

Diard J-P. 1998. Impedance measurements of polymer electrol)4e membrane fuel cells running on constant load. J Power Sources 74 244-245. 

FIG. 3 Comparison of membrane impedance among silicone-rubber-based ion-sensing membrane containing calixarene neutral carriers (5) (0) (6) ( ) and (2) (A)- (From Ref. 21.)... 

Apart from the necessity of excluding interferences from any diffusion potential, normal potentiometry requires accurate determination of the emf, i.e., without any perceptible drawing off of current from the cell therefore, usually one uses the so-called Poggendorff method for exact compensation measurement the later application of high-resistance glass and other membrane electrodes has led to the modern commercial high-impedance pH and PI meters with high amplification in order to detect the emf null point in the balanced system. 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

Although the absence of paracellular transport across the BBB impedes the entry of small hydrophilic compounds into the brain, low-molecular-weight lipophilic substances may pass through the endothelial cell membranes and cytosol by passive diffusion [7]. While this physical barrier cannot protect the brain against chemicals, the metabolic barrier formed by the enzymes from the endothelial cell cytosol may transform these chemicals. Compounds transported through the BBB by carrier-mediated systems may also be metabolized. Thus, l-DOPA is transported through the BBB and then decarboxylated to dopamine by the aromatic amino acid decarboxylase [7]. 

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