Response time, glass electrode

A glass electrode response test can use the same buffer solutions as employed for the span test. After standardization of the pH meter in pH 9.18 buffer, the reference electrode tip is immersed in pH 4.01 buffer to preequilibrate for a period of 5 minutes. This eliminates any response time due to the reference electrode. After this time period, rinse the glass electrode with pH 4.01 buffer and immerse the bulb in the same buffer solution with the reference electrode. Record the pH value versus time or observe the reading after 10 seconds. The reading after the 10-second period should be 98% of the final reading that is, the meter should read 4.11 or less within the 10 seconds. If the electrode fails this test, rejuvenation may help to increase its response. Response time for electrodes is discussed in detail in Section 5.3. 

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. 

Most suitable would be the use of a perfectly NH4+ ion-selective glass electrode however, a disadvantage of this type of enzyme electrode is the time required for the establishment of equilibrium (several minutes) moreover, the normal Nernst response of 59 mV per decade (at 25° C) is practically never reached. Nevertheless, in biochemical investigations these electrodes offer special possibilities, especially because they can also be used in the reverse way as an enzyme-sensing electrode, i.e., by testing an enzyme with a substrate layer around the bulb of the glass electrode. 

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]. 

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. 

It is well known that solid-state LECs exhibit a significant response time since electroluminescence can only occur after the ionic double-layers have been built up at the electrode interfaces [79,82]. Since in this case only the PFg anion is mobile, the double-layers are formed by accumulation and depletion of PFg at the anode and cathode, respectively. The LEC device with 45 started to emit blue-green light at a bias of 5 V after several minutes. The electroluminescence spectrum, as shown in Fig. 36 (trace a), is very similar to the photoluminescence spectrum recorded for a spin-coated film on glass and of a solution of the complex. For comparison, the electroluminescence... 

Before using the pH electrode, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modern pH meters, such as the one shown in Figure 5.8, are microcomputer-controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to +0.001 pH unit, all with few basic steps. The electrode must be stored in an aqueous solution when not in use, so that the hydrated gel layer of the glass does not dry out. A highly stable response can thus be obtained over long time periods. As with other ion-selective electrodes, the operator should consult the manufacturer s instructions for proper use. Commercial glass electrodes are remarkably... 

Acetylcholineesterase and choline oxidase Prepared by mounting a carbon fiber (200 pm diam) in a glass capillary with silver paste and epoxy resin, electrochemically pretreating the electrode from 0 to 1.2 V for 15 min, and dipping the electrode in 11% PVA-Styryl pyridinium solution containing AChE and ChO. The calibration graph was rectilinear from 0.2 to 1 mM of ACh. The response time was 0.8 min. [76]... 

M, the pH glass electrode being an exception with a very broad linear range (pH 0-14). However, recently the detection limit of several ISEs has been dramatically improved towards picomolar (10-12 M) concentrations, which is promising concerning potentiometric trace-level analysis. For characteristics of ion-selective electrodes see also response time, - response volume, and - selectivity coefficient. 

Highly specific sodium electrodes have been developed in which the selectivity for sodium may be 10 times greater than that for potassium (C3, M19). With urine, the pH and potassium concentration should preferably be controlled, but this is unnecessary for blood. The potassium glass electrode is less selective and responds to NH/ and Na. Its selectivity may vary with age (M19). It can be used with blood only if corrections are made for sodium concentration according to Eq. (2) (M19, N2, N3), but when this is done, the electrode shows a linear response to potassium concentration. The precision of serum sodium and potassium measurements with electrodes was found to be better than those obtained by flame photometry (M19, N3). To compare the accuracy of the two methods, the results by flame photometry must be converted to concentrations in serum water. For most specimens, it was found that concentrations could be calculated satisfactorily from activity measurements and results by the two methods agreed (N3), but differences were noted with some samples. So far the cause of this has not been resolved, but it is possible that in future ionic activity will be recognized as a better diagnostic feature than ionic concentration (N3). 

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