Electrolyte compositions, separation components

Capillary Electrophoresis. Capillary electrophoresis (ce) is an analytical technique that can achieve rapid high resolution separation of water-soluble components present in small sample volumes. The separations are generally based on the principle of electrically driven ions in solution. Selectivity can be varied by the alteration of pH, ionic strength, electrolyte composition, or by incorporation of additives. Typical examples of additives include organic solvents, surfactants (qv), and complexation agents (see Chelating agents). 

The speed of p- and n-type doping and that of p-n junction formation depend on the ionic conductivity of the solid electrolyte. Because of the generally nonpolar characteristics of luminescent polymers like PPV, and the polar characteristics of solid electrolytes, the two components within the electroactive layer will phase separate. Thus, the speed of the electrochemical doping and the local densities of electrochemically generated p- and n-type carriers will depend on the diffusion of the counterions from the electrolyte into the luminescent semiconducting polymer. As a result, the response time and the characteristic performance of the LEC device will highly depend on the ionic conductivity of the solid electrolyte and the morphology and microstructure of the composite. 

Chcrkaoui and Veuthey have used nonaqueous CE for the simultaneous separation of nine nonsteroidal anti-inflammatory drugs. Separation was achieved using methanol/acetonitrile (40/60 v/v) with 20 mM ammonium acetate. Perhaps more important, the elution order of the individual components was shown to change with both solvent and electrolyte composition, indicating that selectivity can be manipulated. 

Process or device development is intimately linked to the availability of materials suitable as active or passive cell components. Design, even in its conceptual stage, is inseparable from what materials are available for electrodes or for containment, what electrolyte compositions may come into consideration, and what separators (if any) are needed. Electrochemical engineering involves not only the cell or cell process but also the often considerable chemical and physical operations (separations, chemical reactors, heat exchangers, control, etc.) that precede and follow the electrochemical step. 

Following this reasoning, by variation of the electrolyte compositions on the two aqueous phases independently, the B2 and the hypothetical B2 components may be separated. An experimental test of this idea is shown in Figure 11 (41). Bacteriorhodopsin was first reconstituted into a lipid bilayer with both aqueous phases maintained initially at pH 7. Acidification of the cytoplasmic aqueous phase caused reversible inhibition of the negative phase as expected for the known behavior of the B2 component. Subsequent acidification of the extracellular aqueous phase leads to the appearance of a new negative phase that has a different time course. Furthermore, this negative component has a pH dependence that is opposite to the pH dependence of the known B2 component and is enhanced by acidification. This new component is thus identified with the hypothetical B2 component. 

The membrane has two purposes. Firstly it separates the internal components of the sensor from the external working environment. This is useful in that the electrolyte composition may be maintained and that fouling of the electrode by components of the analyte mixture may be prevented. Secondly, the membrane forms a well defined diffusion barrier for the analyte to pass through. The steady state current observed under potentiostatic control is a function of the kinetics of electron transfer at the electrode and of mass transport to the electrode surface. At high potentials when electron transfer is fast the current is solely a function of mass transfer. This may be controlled by changing the thickness of the membrane and changing the membrane material. The sensitivity and selectivity of the sensor may therefore be controlled to some extent by judicious choice of the membrane material. 

Fig. 39.1a shows a schematic representation of the HUP/C supercapacitor a pure solid electrolyte membrane separates two composite electrodes composed of a mixture of highly polarizable electrode particles and electrolyte crystallites. Current collectors and plastic encapsulation complete the component. The manufacturing process involves (i) synthesis of electrolyte powder for the pure electrolyte membrane (ii) synthesis of the intimate mixture of composite electrode parts (iii) pressure performing of the membrane (iv) assembly of the two composite electrodes and the... 

The foremnner of the modern methods of asphalt fractionation was first described in 1916 (50) and the procedure was later modified by use of fuller s earth (attapulgite [1337-76-4]) to remove the resinous components (51). Further modifications and preferences led to the development of a variety of fractionation methods (52—58). Thus, because of the nature and varieties of fractions possible and the large number of precipitants or adsorbents, a great number of methods can be devised to determine the composition of asphalts (5,6,44,45). Fractions have also been separated by thermal diffusion (59), by dialysis (60), by electrolytic methods (61), and by repeated solvent fractionations (62,63). 

In addition, EC-ALE offers a way of better understanding compound electrodeposition, a way of breaking it down into its component pieces. It allows compound electrodeposition to be deconvolved into a series of individually controllable steps, resulting in an opportunity to learn more about the mechanisms, and gain a series of new control points for electrodeposition. The main problem with codeposition is that the only control points are the solution composition and the deposition potential, or current density, in most cases. In an EC-ALE process, each reactant has its own solution and deposition potential, and there are generally rinse solutions as well. Each solution can be separately optimized, so that the pH, electrolyte, and additives or complexing agents are tailored to fit the precursor. On the other hand, the solution used in codeposition is a compromise, required to be compatible with all reactants. 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

A number of composition analyzers used for process monitoring and control require chemical conversion of one or more sample components preceding quantitative measurement. These reactions include formation of suspended solids for turbidimetric measurement, formation of colored materials for colorimetric detection, selective oxidation or reduction for electrochemical measurement, and formation of electrolytes for measurement by electrical conductance. Some nonvolatile materials may be separated and measured by gas chromatography after conversion to volatile derivatives. 

Divided cells — Electrochemical cells divided by sintered glass, ceramics, or ion-exchange membrane (e.g., - Nafion) into two or three compartments. The semipermeable separators should avoid mixing of anolyte and - catholyte and/or to isolate the reference electrode from the studied solution, but simultaneously maintain the cell resistance as low as possible. The two- or three-compartment cells are typically used a) for preparative electrolytic experiments to prevent mixing of products and intermediates of anodic and cathodic reactions, respectively b) for experiments where different composition of the solution should be used for anodic and cathodic compartment c) when a component of the reference electrode (e.g., water, halide ions etc.) may interfere with the studied compounds or with the electrode. For very sensitive systems additional bridge compartments can be added. 

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