Alkaline semipermeable membranes

The above approach for measurement of urea using an enzyme-based potentiometric biosensor assumes that the turnover of urea to ammonium at steady state provides a constant ratio of ammonium ions to urea, independent of concentration. This is rarely the case, especially at higher substrate concentrations, resuitmg in a nonlinear sensor response. The hnearity of the sensor is also limited by the fact tiiat hydrolysis of urea produces a local alkaline pH in the vicinity of the ammonium-sensing membrane, partially converting NH to NH3 (pKa = 9.3). Ammonia (NH3) is not sensed by the ISE. The degree of nonlinearity may be reduced by placement of a semipermeable membrane between enzyme and sample to restrict diffusion of urea to the immobilized enzyme layer. 

Trimethylamine determination is shown schematically in Figure 34.4. It differs that system used for TVB-N and ammonia determination in that there is a third channel running through the peristaltic pump that brings FA solution into a mixing coil, where it meets the injected sample (extraction), and from there it sequesters all the non-TMA volatile bases, ammonia, and nontertiary amines in the FA prior to alkalinization. TMA is thus left as the only base diffusing across the semipermeable membrane. A similar system has been used to determine TMAO in fish extracts (Ruiz-Capillas et al., 2001a). Bromocresol purple has been assayed as an indicator for FIA determination of DMA. 

In the product range of alkaline power sources, each manufacturer has developed for each special application an optimum type of separator. Generally, however, these consist of the combination of a relatively small variety of proven materials. They are presented here jointly, even though they can hardly be compared with each other. They may be divided into three groups, depending on their application macroporous wetting fleeces (Table 11.15), microporous separators (Table 11.16), and ion-semipermeable membranes (Table 11.17). 

As ion-semipermeable membranes, which - despite good permeability to hydroxyl ions - hinder the transfer of zincate and silver ions, essentially only regenerated cellulose is being used in alkaline batteries. In a complex conversion process, a pulp of wood cellulose, primarily from eucalyptus trees, is dissolved... 

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