Circumventing conductivity problem

In order to evaluate the electrode configuration, similar experiments were performed as described in the previous section, and the results were compared in order to determine whether the electrodes behave identically in the absence and presence of cotton. As expected, similar results were obtained concerning the relationships Equation 10.1 is also valid for electrodes immersed in cotton that act as an immobilising substance for the electrolyte, but the value of k is different. Indeed, all experimentally obtained curves are shifted towards higher resistive behaviour, which can be explained by the fact that the presence of cotton forms a barrier for the conductivity of ions through the electrolyte solution. However, as explained in the previous section, k can be obtained by calibrating the electrode setup, so calibration in the presence of cotton circumvents the problem of different results in the absence and presence of cotton. 

The aqueous biphasic hydroformylation concept is ineffective with higher olefins owing to mass transfer limitations posed by their low solubility in water. Several strategies have been employed to circumvent this problem [22], e.g. by conducting the reaction in a monophasic system using a tetraalkylammonium salt of tppts as the ligand, followed by separation of the catalyst by extraction into water. Alternatively, one can employ a different biphasic system such as a fluorous biphasic system or an ionic liquid/scC02 mixture (see later). 

The choice of isolation method should be based on all available separation methods (see those listed here and in Section V. C). However, it should be pointed out that scaleup to preparative scale might be necessary. Also, special steps may have to be taken in some cases to circumvent particular problems. For example, it is not possible to utilize a destructive detector, such as a flame ionization detector, to isolate a component in GC. It would be necessary to replace it with a nondestructive detector, such as a thermal conductivity detector, or to use a split stream with accurate timing control that would allow collection of the desired separated component. 

Samples containing more than 2% of sulfur did not pick up any iodine even after a 72-hour period. The completely saturated EPDM portions of the blend seem to prevent any iodine molecules from permeating into the polyacetylene moieties. In order to circumvent this problem, we have doped the blend with iodine prior to the crosslinking procedure. Subsequently, the doped material having a conductivity of 60 ft-1 cm-1 was reacted with sulfur monochloride in a toluene solution for 10 minutes. The color of the solution turned from pale yellow to dark red while the polymer film remained insoluble in the toluene solution. 

To circumvent the problem of competitive dehydroboration with ketones, the Alpine-borane reductions can be conducted in neat (excess) reagent [57] or at high pressure (6000 atm, [58]). Experiments done in neat reagent take several days to go to completion, and afford enantioselectivities of 70-98% [57. At pressures of 6000 atmospheres, the reactions are faster and dehydroboration is completely suppressed. Ketones are reduced with slightly higher enantioselectivities (75-100% es) under these conditions [58]. 

Commonly, ac conductivity is measured in the low frequency range where a plateau is expected to appear [20]. However, in our samples, this plateau is not resolved, as a consequence polarization and contact effects cannot be discerned and the correct dc conductivity cannot be calculated by this method (see inset in Fig. 2.10). To circumvent this problem, the alternative procedure described in Section 2.5.5 can be used. 

Cermets for hydrogen separation consist of a ceramic and a metaUic phase contiguous in a dense matrix. In a cermet, one may combine one state-of-the-art pure proton conductor with a highly electron-conducting metaUic phase, and thereby circumvent the problem of having both electronic and ionic conduction in one and the same oxide. 

Photometric detectors are the most popular in CE instruments including diode array detectors. Laser-induced fluorescence (LIE) detection and electric conductivity detectors are also popular. LIE is particularly sensitive and powerful for detecting low concentration analytes. However, most analytes are not natively fluorescent and some derivatizations are necessary. Conductivity detector is useful for the detection of non-ultraviolet (non-UV) absorbing analytes such as inorganic ions or fatty acids. Both LIE detection and conductivity detectors are commercially available and easy to interface with conventional CE instruments. Electrochemical detectors are also useful for selective high-sensitivity detection. Several techniques have been developed to circumvent the problem of strong effects of electrophoretic field on electrochemical detection, but despite this, commercial electrochemical detectors are not used extensively. 

The main drawback of an OTE is instability and poor conductivity of the islandlike metal film. To circumvent this problem, a Pt grid evaporated on the surface of a ZnSe IRE was used in ATR studies of the electropolymerization of... 

The need for high electronic conductivity has meant that work has focused on the use of the so called conducting polymers, exemplified by polypyrrole, polyaniline and polythiophene (Structures 1 to 3). These materials must be in their p-doped (partially oxidized) states (right hand side of eq. 1, for example) to exhibit sufficient electronic conductivity. Unfortunately, the p-doped polymers are cationic and will therefore tend to exclude the protons needed for the fuel cell reactions. To circumvent this problem, composites of conducting polymers with cation exchange polymers have been used. Thus p-doped polypyrrole / poly(styrene-4-sulphonate) (PSS), for example, exhibits both proton and electron conductivity (8). 

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