Selective semipermeable

For example, the measurements of solution osmotic pressure made with membranes by Traube and Pfeffer were used by van t Hoff in 1887 to develop his limit law, which explains the behavior of ideal dilute solutions. This work led direcdy to the van t Hoff equation. At about the same time, the concept of a perfectly selective semipermeable membrane was used by MaxweU and others in developing the kinetic theory of gases. 

This is a process mainly used in power plants for removal of dissolved polar compounds. Solute is exchanged between two liquids through a selective semipermeable membrane in response to differences in chemical potential between the two liquids. The process removal efficiency for TDS is about 62-96%. 

Reverse osmosis (RO) is a separation technology that uses selective semipermeable membranes to remove dissolved solids, such as metal salts, from water. In the RO process, the solution containing the contaminant(s) is applied under pressure to one side of a membrane. The water passes through the membrane, leaving behind a solution with a smaller volume and a higher concentration of solutes. 

The PbS04 Layer as a Selective Semipermeable Membrane Electrode Potential... 

It has been established that after a series of galvanostatic polarization runs with low currents, the Pb/PbS04 electrode is passivated permanently and its open-circuit potential remains within the range from —0.40 to —0.60 V [24]. It has been suggested that this behaviour is due to the transformation of the PbS04 layer into a selective semipermeable membrane by the process presented in Fig. 2.12 [24]. 

The relationship between electrical potential and ionic concentrations or, more precisely, ionic activities is frequently considered in electrochemistry. Most commonly, two ionic solutions of different activity are separated by an ion-selective semipermeable membrane that allows one type of ion to pass freely through the membrane. It can be shown that an electric potential E exist between the solutions on either... 

In addition to metal-electrolyte interfaces, electrode potentials can be produced by ion transport through an ion-selective semipermeable membrane. In this case, the membrane is interposed between two liquid phases. Reversible transfer of a selected ion occurs through the membrane. For an ideal membrane, the developed electrode potential E is given by the Nernst equation... 

Membrane separator. A separator that passes gas or vapor to the mass spectrometer through a semipermeable (e.g., silicon) membrane that selectively transmits organic compounds in preference to carrier gas. Membrane separator, membrane enricher, semipermeable membrane separator, and semipermeable membrane enricher are synonymous terms. 

First, we consider the experimental aspects of osmometry. The semiperme-able membrane is the basis for an osmotic pressure experiment and is probably its most troublesome feature in practice. The membrane material must display the required selectivity in permeability-passing solvent and retaining solute-but a membrane that works for one system may not work for another. A wide variety of materials have been used as membranes, with cellophane, poly (vinyl alcohol), polyurethanes, and various animal membranes as typical examples. The membrane must be thin enough for the solvent to pass at a reasonable rate, yet sturdy enough to withstand the pressure difference which can be... 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

A reverse osmosis membrane acts as the semipermeable barrier to flow ia the RO process, aHowiag selective passage of a particular species, usually water, while partially or completely retaining other species, ie, solutes such as salts. Chemical potential gradients across the membrane provide the driving forces for solute and solvent transport across the membrane. The solute chemical potential gradient, —is usually expressed ia terms of concentration the water (solvent) chemical potential gradient, —Afi, is usually expressed ia terms of pressure difference across the membrane. 

An ion-selective electrode contains a semipermeable membrane in contact with a reference solution on one side and a sample solution on the other side. The membrane will be permeable to either cations or anions and the transport of counter ions will be restricted by the membrane, and thus a separation of charge occurs at the interface. This is the Donnan potential (Fig. 5 a) and contains the analytically useful information. A concentration gradient will promote diffusion of ions within the membrane. If the ionic mobilities vary greatly, a charge separation occurs (Fig. 5 b) giving rise to what is called a diffusion potential. 

In the selection of an appropriate cell culture system, a number of criteria must be considered (Table 3). These include not only the characteristics of the cell type but also the controllable parameters of the complete transport system such as the permeants, the filter properties, and the assay conditions. In general, most transport experiments employ the experimental design shown schematically in Figure 4 with modifications as discussed below. Typically, the desired cell is seeded onto some sort of semipermeable filter support and allowed to reach confluence. The filter containing the cell monolayer separates the donor and receiver... 

In the following sections we highlight only selected works that have contributed toward the further development of passive samplers for SVOCs and/or HOCs. The literature related to the development and use of passive samplers for monitoring gases or VOCs in occupational environments is large. However, these publications are discussed only briefly, because lipid-containing semipermeable membrane devices (SPMDs) are primarily designed for SVOCs. 

Smer-Lauridsen, F. and Kjlholt, J. 2000, Identification of selected hydrophobic organic contaminants in wastewater with semipermeable membrane devices (SPMDs). Water Res. 34 3478—3482. 

An overview of the decomposition and dehydrogenation reactions that have been investigated using semipermeable membranes for selective permeation of one of the reaction products is given in Tables 7.1 and 7.3, respectively. An overview of the most interesting studies is given in Tables 7.2 and 7.4. 

Membrane electrochemistry proper began in 1890 with the work of W. Ostwald [79], who formulated the concept of a semipermeable membrane that selectively affects the membrane transport of ions. According to Ostwald, not only electric currents in muscles and nerves, but also the mysterious phenomenon of electric fishes will finally find an explanation in the properties of semipermeable membranes . 

Many simple ions such as K+.Na+.Cl, and Ca are normally kept within a narrow range of concentrations in the body, and they must be monitored during critical care. Potentiometric sensors for ion, also called ion-selective electrodes or ISEs, utilize a membrane that is primarily semipermeable to one ionic species. The ionic species is used to generate a voltage that generally obeys the Nemst equation [cf. Section 3.1.3.2 and Eq. (3.24)]... 

Membranes are semipermeable barriers that permit the separation of two compartments of different composition or even condition, with the transport of components from one compartment to another being controlled by the membrane barrier. Ideally, this barrier is designed to let pass selectively only certain target compounds, while retaining all others—hence the denotation semipermeable . Membrane separations are particularly suitable for food applications because (1) they do not require any extraction aids such as solvents, which avoids secondary contamination and, hence, the necessity for subsequent purification (2) transfer of components from one matrix to another is possible without direct contact and the risk of cross-contamination (3) membrane processes can, in general, be operated under smooth conditions and therefore maintaining in principle the properties and quality of delicate foodstuff. 

A membrane is a semipermeable barrier whose function is to compartmentalize metabolic processes, maintain pH differences on either side, control osmotic pressure and ionic gradients, provide a surface or environment for the stabilization of active biomolecules, provide tissue discrimination, and allow selective access as well as egress to specific metabolites. 

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