Glass response time

With respect to pH sensitivity and an adequate speed of response (time constant r = RC where R is the resistance of the measuring circuit and C the capacitance of the electrode), a certain degree of superficial swelling is needed however, the gel layer thus formed should remain thin in order to minimize the solubility of the glass and to guarantee sufficient durability of the electrode. In this respect lithium barium silicates offer an attractive compromise32. 

Therefore, the ISE potential depends on the CO2 partial pressure with Nernstian slope. Contemporary microporous hydrophobic membranes permitted the construction of a number of gas probes, developed mainly by the Orion Research Company (for a survey see [143]. The most important among these sensors is the ammonia electrode, in which ammonia diffusing through the membrane affects the pH at a glass electrode. Other electrodes based on similar principles respond to SO2, HCN, H2S (with an internal S ISE), etc. The ammonia probe has a better detection limit than the ammonium ion ISE based on the non-actin ionophore. The response time of gas probes depends mostly on the rate of diffusion of the test gas through the microporous medium [77,143]. 

By its chemical nature, the very thin dry layer of glass permits ion exchange without any possibility of electrochemical redox reactions. If the membrane is a sufficiently thin layer, one can achieve millisecond response times to changes in pH, and these properties can be put to full advantage with rapid mixing devices to study chemical reactions with half-lives in the 5-10 msec range. 

Although the glass electrode displays sufficient sensitivity and time response for many chemical reactions, special circumstances may require an ultrathin glass electrode to obtain millisecond response times. Less sensitive types of the pH Stat are also used in fermentation chambers to determine the rate/extent of fermentation or to maintain the viability and efficiency of microbes during fermentation. 

For instance, conductometric transduction has been used for determination of an atrazine herbicide in the concentration range 4.6-231.8 mM [165]. A sintered glass frit was used as the support for the MIP film prepared by thermo-radical polymerization. However, both the response time of 30 min and the chemosensor recovery time of 12 h were long. For better performance, the MIP film was prepared by photo-radical co-polymerization of a chloroform solution, which contained tri (ethylene glycol)dimethacrylate (TEGDMA), MAA, and oligourethane acrylate, sandwiched between two quartz slides (Table 6). 

The promethazine imprinted MIP particles were used as the recognition element to fabricate a relevant potentiometric chemosensor [200], These particles, prepared with the vinylbenzene functional monomer and the divinylbenzene cross-linker, effectively bound promethazine as compared to the MIP particles prepared with the MAA or vinylbenzene monomer and the EGDMA cross-linker. The MIP particles were embedded in a PVC membrane attached to one end of a glass tube, which was filled with 1 mM promethazine. The chemosensor was able to detect promethazine potentiometrically with LOD as low as 1.0 x 10-7 M and the dynamic linear concentration range from 5.0 x 10-7 to 1.0 x 10-1 M. With this chemosensor, promethazine concentration can be determined in syrup samples and biological fluids with a response time of 50 s. 

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