Ion membranes

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

This pardaxin model is not unique. We have developed several similar models that are equally good energetically and equally consistent with present experimental results. It is difficult to select among these models because the helices can be packed a number of ways and the C-terminus appears very flexible. Our energy calculations are far from definitive because they do not include lipid, water, ions, membrane voltage, or entropy and because every conformational possibility has not been explored. The model presented here is intended to illustrate the general folding pattern of a family of pardaxin models in which the monomers are antiparallel and to demonstrate that these models are feasible. 

S.No. Analyte Ion Membrane Composition Cone. Range (M) Recommended pH Range Selectivity Coefficients... 

Test ion Membrane material Major interfering ions... 

The salty taste is primarily due to sodium ions acting directly on ion channels. Amiloride specifically blocks sodium channels however, it does not block all responses to salt, in cating more than one mechanism for salty sensation. A different compound, 4-aminopyridine, blocks potassium channels but not sodium. This suggests that receptor proteins and second messengers are not uired, and that these stimuli act directly on ion membrane channels. The physiology of the response of cells to salt has been reviewed (7). 

To illustrate some of the notions introduced so far, let us consider the Ci and p fields in an electrodialysis cell at equilibrium. For simplicity, let us limit our consideration to a 1,1 valent electrolyte at bulk (feed) concentration Co- Assume a constant fixed charge density N(—N) for the an- (cat-) ion membrane. 

Type of ISE Ions Membrane material Type of ISE Ions Membrane material... 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

In order to obtain potable water from sea water, one must either remove good watex from the solution or remove salt from the solution, leaving the good water behind. Most well known desalinization processes work on the principle of removing good water from solution the ion membrane process is the notable exception. It seems obvious that, since sea water is 96.5% water and only 3.5% salt, it would be preferable to remove the salt. Such processes received special attention in the work reported here. 

Ion Membrane conditioned in 1(T3M NaN03 Membrane conditioned in 10 3M AgNOa ... 

This review summarizes the recent works on syntheses of solid superacids and their catalytic action, including Lewis acids and liquid superacids in the solid state, as discussed in Sections Il-IV. Sections VI and VII describe new types of solid superacids we have studied in this decade sulfate-supported metal oxides and tungsten or molybdenum oxide supported on zirconia. Perfluorinated sulfonic acid, based on the acid form of DuPont s Nafion brand ion membrane resin, is also gaining interest as a solid superacid catalyst Nafion-H-catalyzed reactions are reviewed in Section V. 

A convenient solid of perfluorinated-sulfonic acid can be made readily from DuPont s commercially available Nafion brand ion membrane resins. Powder granules of the 1200-EW polymer, Nafion 501, have been used most frequently in catalytic applications the price in the K+ form of the perfluorosulfonic salt, 501X, was 650/kg in 1981. Because only the potassium salt derivative is commercially available, the salt is converted to the free sulfonic acid by treatment with mineral acid. A standard procedure for the conversion is described below. This procedure also serves to regenerate the resin in various catalytic cycles. 

Organism ATPase Type Ions Membrane helix Motif... 

An intrinsic ionic charge gradient across the membrane exists because of semipermeable nature of membrane, which maintains a difference in the concentration of the ions between the cytosol and the extracellular matrix. This difference results in a definite potential across membrane of the normal cells, which is called the resting potential. Normal plant cells, mammalian muscle cells, and neurons have resting potential values of about —120, —90, and —70 mV, respectively. Along with the resistance to the flow of ions, membrane also exhibits a capacitance. Cm, which is given by... 

Approximately 3000-4000 water molecules per second cross the phospholipid bilayer membrane of a vesicle with a head group area of 70 A, but it takes 70 hours for one sodium ion. Membranes are ion-impermeable and osmotically active. These subjects have been treated in other text books and are of no concern here instead, we concentrate on the organic chemistry of the membrane barrier, and its strengthening, perforation and disruption by synthetic systems. 

Several changes in ion membrane homeostasis also occur from fatty acid and long chain acylcamitines (LCAC) accumulation or from the formation of lysophosphadyl-choline (LPC) and arachidonic acid (AA) due to phospholipid breakdown by lipases. In fact, fatty acids and AA favor activation of K+ outward current while LCAC and LPC favor inward over outward current, reviewed by Carmeliet.55... 

Membrane electrodes are sometimes called p-ion electrodes because the data obtained from them are usually presented as p-functions, such as pH, pCa, or PNO3. In this section, we consider several types of p-ion membranes. 

Another structure/function transport model, often referred to as the capillary or electroki-netic model, predefines the microlevel structure of an ion-exchange membrane as an array of pores of known dimensions with a specified distribution of ion-exchange sites on the pore walls. Equations describing solute and solvent transport and theories for molecularlevel ion/solvent and ion-membrane interactions are then generated, based on this pore structure [151], The fundamental transport equation for the molar flux of ionic species is the Nemst-Planck equation... 

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