Microelectrodes, calibration

By substituting the well-known Blasius relation for the friction factor, Eq. (45) in Table VII results. Van Shaw et al. (V2) tested this relation by limiting-current measurements on short pipe sections, and found that the Re and (L/d) dependences were in accord with theory. The mass-transfer rates obtained averaged 7% lower than predicted, but in a later publication this was traced to incorrect flow rate calibration. Iribame et al. (110) showed that the Leveque relation is also valid for turbulent mass transfer in falling films, as long as the developing mass-transfer condition is fulfilled (generally expressed as L+ < 103) while Re > 103. The fundamental importance of the Leveque equation for the interpretation of microelectrode measurements is discussed at an earlier point. 

The experimental set-up for cellular oxygen measurements (p02) consists of following components p02 measuring micro chamber (volume 0.6 microliter), polarographic microelectrode, water-bath for constant temperature, chemical microsensor connected to a strip-chart recorder and gas calibration unit. 

Calibration curve and linear response slope of pH microelectrodes... 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

Wehrens R, van der Linden WE. Calibration of an array of voltammetric microelectrodes. 

Voltammetry. The voltammetric techniques are based on the current-voltagetime relationship at microelectrodes. To perform voltammetry, the oil/antioxidant sample is dissolved in a solvent containing an electrolyte and a three-electrode system (glassy carbon working electrode, a platinum wire reference electrode, and platinum wire auxiliary electrode) is inserted into an oil/solvent solution. A fresh oil typical of the application (100% standard) and the solvent system (0 % standard) is used to calibrate the voltammetric instrument for % remaining antioxidant determination (Kauffman, 1989 and 1991). 

Acetylcholineesterase and choline oxidase Pt microelectrode, precoated with cellulose acetate, dipped in a buffered solution (pH 6.89) of choline oxidase or AChE containing glu-taldehyde as cross linking agent. The calibration graph was linear from 0.5 pM to 100 pM. Response time was 15-20 s. The optimum pH was 9-10.5. Electrode was stable over 41 days. [96]... 

Blood and urine pH can be measured easily by means of a calibrated glass electrode, whereas pH measurement inside the metabolizing cells is not easily accomplished. Techniques for estimating intracellular pH include glass electrode measurements on homogenates, calorimetric or fluorometric analysis of intracellular distribution of indicator dyes, and microelectrode methods. 

When properly shielded and connected to the appropriate apparatus, ion-selective microelectrodes should show the response and selectivity similar to conventional size electrodes. They should be calibrated in a solution of similar composition to that of the sample solution to be analyzed. The response time of the electrode itself should be comparable to that of conventional electrodes. However, with these electrodes the diffusion of the ions through the stagnant layer near the ion-selective membrane has a smaller effect on the transient response than the time constant of the measuring circuitry. Measuring instruments often contain a variable capacitor to adjust the delay caused by the capacitance of the input stage. 

However, one should not forget that the potentiometric mode has several drawbacks. The fabrication of most potentiometric tips is much more involved than that of the conventional amperometric tips. It is necessary to have the electrode made by a very skilled technician. Despite this, even when following a proven recipe the success rate is relatively low. The response of potentiometric tips is not always Nemstian and a calibration is required before and after performing the experiment. The behavior typically varies from one microelectrode to another. Potentiometric tips cannot rely on positive and negative feedback diffusion, thus it is difficult to assess the tip-substrate distance from the tip response. Several approaches are available, but most are cumbersome. In the potentiometric mode the response... 

The methods described previously correspond to the partial electrolysis of the analytes in contact with the working microelectrode. By contrast coulometric methods are based upon stoichiometrical relationships (quantitative conversion of the analytes). The concentration of an analyte is calculated from the quantity of electricity, using the equation Q = it. Standardization or calibration curves are not required. This is not therefore a comparative method. 

Less frequently, pH-profiles are published in literature when compared to the oxygen profiles. The details as to their shapes and their underlying processes are not yet sufficiently understood. Additionally, a reliable calibration under the given pressure conditions has not been accomplished for in situ pH-measurements. Consequently, only relative values are measured within one profile. Electrodes measuring H S can also be built as microelectrodes. However, they are only suited for in situ measurements when H S reaches close enough to the sediment surface. 

Volumetric Absorption Cells. Often the amount of sample available for trace analysis is rather limited also, it is often desirable to make the final volume as small as possible, so that absorbance is maximized. This may be accomplished by calibrating an absorption cell to a definite volume by putting a mark on the side and using it as a volumetric flask. Color development may be carried out in the cell, in order to keep the final volume at a minimum. If pH adjustment is required, a glass microelectrode may be used. For an ordinary 1-cm cell, volumes of about 3 ml may be used. 

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