Measurement with Potentiometric Sensors

Measurement with Potentiometric Sensors Measuring Instruments 

The lead from the working electrode to the meter must be shielded carefully to avoid distortion by external electric fields of the surroundings. 

The relation between measured value E and the logarithm of concentration c is estabhshed by calibration. This procedure is necessary since every sensor has its own characteristics that must be considered in order to get precise analytical results. Calibration can be done either by plotting a calibration graph or alternatively by the standard addition technique. 

Calibration graphs are produced by performing a second experiment following the literal measurement. In this second experiment, a series of measurements is done with known concentrations of the substance to be analysed. 

All conditions should be as close as possible to the sample analysis. The graph E = /(log c) is utihzed as a reference to determine the concentration Cx which belongs to the measured value m- If potentiometric caUbration graphs are not linear, then a non-Nernstian behaviour is perceptible, which means that the electrode may produce inaccurate results. 

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The force is induced to the sample via clamps and straps. On every side of the square sample, seven straps are fixed in clamps that are attached via force measurement rings to low-friction ball screws which are driven by altogether 28 servo motors. Each servo motor is controlled independently. The servo motors are able to move perpendicularly to the force direction. The force is applied independently and uniaxially into every strap. To control the servo motors it is necessary to provide a control output variable for the force in the spindle. This must then be measured with a resolution better than 1%. Therefore the force measuring rings we use have been developed by ourselves. The biaxial measurement field has dimensions of 0.70 X 0.70 m, and loads of up to 200 kN/m can be applied. Strains are measured with potentiometric sensors. The external control circuit is set up digitally by means of a PC. 

Measurement with Potentiometric Sensors Measuring Instruments... 

Amperometry is a voltammetric method in which a constant potential is applied to the electrode and the resulting current is measured. Amperometry is most often used in the construction of chemical sensors that, as with potentiometric sensors, are used for the quantitative analysis of single analytes. One important example, for instance, is the Clark O2 electrode, which responds to the concentration of dissolved O2 in solutions such as blood and water. 

Besides, potentiometric sensors with ion-selective ionophores in modified poly(vinyl chloride) (PVC) have been used to detect analytes from human serum [128], Cellular respiration and acidification due to the activity of the cells has been measured with CMOS ISFETS [129], Some potentiometric methods employ gas-sensing electrodes for NH3 (for deaminase reactions) and C02 (for decarboxylase reactions). Ion-selective electrodes have also been used to quantitate penicillin, since the penicillinase reaction may be mediated with I or GST. 

Agents for chemical bleaching rely on different types of peroxides. Potentiometric or amperometric biosensors that detect the highly specific and sensitive reaction of enzymes like katalases with their corresponding substrates can be used for on-line measurement [84]. The sensors can be manufactured with simple technologies at moderate cost, but their stability is not sufficient for integration in household appliances. 

Fig. 18a.7. Typical calibration curve of a potentiometric sensor for measuring monovalent cations. From Ref. [70] with permission.

Cytosensor Microphysiometer technology has been used to detect perturbation in mammalian cells (Hafner, 2000). The system measures small changes in extracellular acidification using a light addressable potentiometric sensor. If the metabolism is interfered with, acid excretion will be affected which could be sensitively measured by LAPS. In principle, this system should be suitable for monitoring pathogen interaction with mammalian cells. 

In potentiometric sensors, an electrical potential between the working electrode and a reference electrode is measured at zero current conditions in a solution containing ions that exchange with the surface. The first potentiometric MIP sensor was prepared in 1992 by Vinokurov (1992). The substrate-selective polyaniline electrode was electrosynthesized with polypyrrole, polyaniline, and aniline-p-aminophenol copolymers. The development of an MIP-based potentiometric sensor was reported in 1995 by Hutchins and Bachas (1995). This potentiometric sensor has high selectivity for nitrite with a low detection limit of (2 + l)x 10 M (Fig. 15.10). 

FIGURE 2.11 Example of biplot. The scores (filled symbols) are replicates of analyses of wine samples of three vintages (2004, 2005, and 2006, respectively), while the loadings (stars) represent the potentiometric sensors used for the measurements (A = anion-sensitive sensors, C = cation-sensitive sensors, G = redox-sensitive sensors, pH = pH sensor) (reproduced from Rudnitskaya etal. 2009b, with permission). 

Total oxygen demand (TOD) tells us how much 02 is required for complete combustion of pollutants in a waste stream. A volume of N2 containing a known quantity of 02 is mixed with the sample and complete combustion is carried out. The remaining 02 is measured by a potentiometric sensor (Box 17-1). Different species in the waste stream consume different amounts of O,. For example, urea consumes five times as much 02 as formic acid does. Species such as NH3 and H2S also contribute to TOD. 

Here, the potentiometric selectivity coefficient is given with respect to the hydroxyl ion. Single-crystal lanthanum fluoride is a wide bandgap semiconductor in which the electrical conductivity is due only to the hopping mobility of fluoride ions through the defects in the crystal. It does not respond to the La3+ ion because of the slow ion exchange of that ion. Hydroxyl ion is the only other ion that has appreciable mobility, and is the only known interference. For this reason, the measurements with a fluoride electrode are always done below pH 7, which circumvents this interference. As shown later, the consideration of ionic and/or electronic conductivity of the membrane plays a critical role also in the design of the internal contact in nonsymmetric potentiometric sensors. 

Figure 2 shows the structure of this sensor which is similar to that of the potentiometric sensor reported earlier (10). The only difference is that in this sensor a short circuit current between the sensing electrode and the counter electrode is measured with an ammeter. The proton conductor, antimonic acid (Sb205 2H20), was prepared from antimony trioxide and hydrogen peroxide according to a method described elsewhere (7,14). The sample powder was mixed with... 

G.B. Sigal, D.G. Hafeman, J.W. Parce and H.M. Mcconnell, Electrical-properties of phospholipid-bilayer membranes measured with a light addressable potentiometric sensor, ACS Symp. Ser., 403 (1989) 46-64. J.D. Olson, P.R. Panfili, R. Armenta, M.B. Femmel, H. Merrick, J. Gumperz, M. Goltz and R.F. Zuk, A silicon sensor-based filtration immunoassay using biotin-mediated capture, J. Immunol. Methods, 134(1) (1990) 71-79. 

B. Stein, M. George, H.E. Gaub and W.J. Parak, Extracellular measurements of averaged ionic currents with the light-addressable potentiometric sensor (LAPS), Sens. Actuators B Chem., 98(2-3) (2004) 299-304. 

Immobilization of bioactive material on/in the electrode allows combining bio-reaction selectivity with sensitivity of electrochemical detection. Irrespective of reaction in the biosensor, the electrochemical response is measured, in particular, as current at the given potential (amperometric sensor) or electrode potential (potentiometric sensor). 

Potentiometric redox measurements are often performed in nonaqueous or mixed-solvent media. For such solvents various potentiometric sensors have been developed, which, under rigorously controlled conditions, give a Nemstian response over a wide ranges of activities, particularly in buffered solutions. There are some experimental limitations, such as with solvent purification and handling or use of a reference electrode without salt bridges, but there also ate important advantages. Solutes may be more soluble in such media, and redox... 

It remains an objective to develop potentiometric sensors with longer lifetimes, greater reproducibility and greater stability. The importance of an appropriate statistical treatment of the results in order to determine their precision is stressed. Frequent calibration is necessary, at least at the beginning of each measurement session and in a medium as similar as possible to that where the sensors are to be employed, in order to ensure the accuracy of the analytical determinations. 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

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