Perm-selective

In order to explain the working of membrane electrodes, one generally talks of perm-selective membranes, although on this basis not all phenomena can be fully understood. However, some more fundamental remarks on membranes may be useful,... 

Figure 6.30 Schematic representation of a glucose sensor operating by diffusion across a perm-selective membrane (as represented by the vertical arrows) GOD is glucose oxidase.

There are two broad classes of glucose sensor commonly encountered the first employs a mediator (see Chapter 5), while the second employs a perm-selective membrane. 

In comparison with adsorptive/absorptive techniques for aroma recovery from bioconversions, the disadvantage of pervaporation is the fact that both sorption and diffusion determine the overall selectivity. While the sorption selectivity is very high (equal to that of adsorptive/absorption), the diffusion selectivity favours water owing to the simple fact that water is a smaller molecule than aroma compounds and thus sterically less hindered during diffusion (Table 19.1). The overall (perm)selectivity P=SD) is therefore lower than in strictly sorption controlled processes, although it is still favourable compared with that for evaporation. This shortcoming compares, however, with operational advantages of pervaporation as outlined before. 

This interface is also known as the perm-selective interface (Fig. 6.1a). It is found in ion-selective sensors, such as ion-selective electrodes and ion-selective field-effect transistors. It is the site of the Nernst potential, which we now derive from the thermodynamic point of view. Because the zero-current axis in Fig. 5.1 represents the electrochemical cell at equilibrium, the partitioning of charged species between the two phases is described by the Gibbs equation (A.20), from which it follows that the electrochemical potential of the species i in the sample phase (S) and in the electrode phase (m) must be equal. 

Boujtita et al. [11] Beer Alcohol oxidase (AOx) Screen-printed carbon electrode doped with 5% cobalt phthalocyanine (CoPC-SPCE), and coated with AOx a perm-selective membrane on the surface acts as a barrier to interferents/+400mV vs. Ag/AgCl Cobalt phthalocyanine... 

Y. Benmakroha, I. Christie, M. Desai and P. Vadgama, Poly(vinyl chloride), polysulfone and sulfonated polyether-ether sulfone composite membranes for glucose and hydrogen peroxide perm-selectivity in amperometric biosensors, Analyst, 121 (1996) 521-526. 

IEMs can be discriminated according to their mechanical and electrical properties, perm-selectivity, and chemical stability. 

The membrane perm-selectivity (y.m) is defined as the ratio between the actual and theoretical transfer of counterions through any IEM. It can be simply determined as the percentage ratio between the experimental and theoretical Donnan potential differences as measured using a test system consisting of two cells provided with calomel electrodes and filled with well-mixed standardized aqueous solutions of KC1 (at 0.1 and 0.5 kmol/m3), kept at 25 °C, and separated by the IEM sample under testing. 

Type main properties category thickness, burst strength ion-exchange capacity, IEC gel water content area resistance, rm, at 25°C, 0.5 kmol NaCl/m3 membrane perm-selectivity, iv, at 1.0/0.5 kmol/m3 KC1. 

The anionic or cationic membrane perm-selectivity (am) for the corresponding counterions can be expressed as the ratio by the differences between the real and theoretical transport numbers in the membrane and solution phases ... 

When the counterion transport number in the membrane is the same as in the electrolytic solution, the perm-selectivity tends to zero (Strathmann, 1992). 

To design or optimize an ED process several parameters are to be taken into account, namely stack construction and spacer configuration, operation mode, membrane perm-selectivity, feed and product concentration, flow velocities, current density and voltage applied to the electrodes, recovery... 

Collings, C.W., Huff, G.A. and Bartels, J.V. (January 2004) Separation Processes Using Solid Perm-Selective Membranes in Multiple Groups for Simultaneous Recovery of Specified Products from a Fluid Mixture. US Patent Appl. Publ. 20040004040 Al. 

Permeable materials for the production of membranes have been employed for the treatment of a diversity of fluids [16,59-64], A membrane is a perm-selective barrier between two phases capable of being permeated owing to a driving force, such as pressure, concentration, or electric field gradient... 

Regenesys uses DuPont s Nafion (Section 6.1.7) as the perm-selective sodium ion transfer membrane, separating the two half cells. Figure 2.1. Diffusion of sodium ions in the concentration difference across the Nafion membrane is one of the irreversibilities of the system. The low-cost plastic (e.g. polyethylene) tanks and pipework are treated with fluorine to provide bromine resistance, and are able to operate with, and contain, both electrolytes at ambient temperature. 

The ionically bonded sodium bromide receives two electrons, and loses two +ve sodium cations to form sodium tri-bromide (see Porterfield, 1993). An alternative homogeneous reaction route to tri-bromide is to bubble bromine (Br2) through aqueous sodium bromide. Accordingly, the electrochemically formed tri-bromide has an associated atmosphere of bromine from homogeneous dissociation. Such bromine is corrosive, but is contained by the fluorinated plastic tanks and by one side of the (Na ) Nafion perm-selective membrane. Nafion is itself a compound with many fluorine atoms. Bromine is very much lower than fluorine in chemical reactivity, so that the bromine containment strategy is complete, and the selection of the bromide electrolyte seen to be a well-considered choice. 

Imperial College s Centre for Ion Conducting Membranes is working on perm-selective membranes, and perhaps could develop the concentration cells mentioned in Appendix A (Williams, 2002), pp. 878-891. 

Complete fuel cells are engineered for isothermal oxidation by the addition of perm-selective membranes and isothermal concentration cells. They would, if developed, generate much higher Nernst potential difference than an existing incomplete fuel cell. That would give a chance for fuel cells to draw level, a chance the industry does not seem to understand. 

In Figure A.4 water and methane, each proceeding from store, while being driven by circulators via perm-selective membranes, are reacted isothermally in a notional and conceptually quite new isothermal reformer resembling a fuel cell bounded by perm-selective membranes. The circulators are isothermal concentration cells with Nernst potential differences. 

Wire that senses pH changes such as antimony metal can be coated with urease (65) and used to determine urea in pure blood (66). Covering the antimony metal with a gas perm-selective membrane (67) improves the selectivity. The microsensors respond from 0.1 to 10 mM urea in 30-45 s. This ammonia sensor has a faster baseline recovery than commercial gas membrane electrodes. 

Graft copolymers combine the properties of their polymeric constituents and as such are polymer alloys, which open a vast field of new polymeric species. This is why active research along these lines is performed in many academic and industrial research laboratories all over the world. However, only few applications have reached a commercial level today. They involve the production of specific polymeric adhesives, perm-selective membranes, bio-medical devices and the surface modification of certain products. 

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