Potential measurement, accuracy

The principle of the measurement is described with the help of Fig. 2-7 [50]. Potential measurement is not appropriate in pipelines due to defective connections or too distant connections and low accuracy. Measurements of potential difference are more effective. Figure 3-24 contains information on the details in the neighborhood of a local anode the positions of the cathodes and reference electrodes (Fig. 3-24a), a schematic representation of the potential variation (Fig. 3-24b), and the derived values (Fig. 3-24c). Figure 2-8 should be referred to in case of possible difficulties in interpreting the potential distribution and sign. The electrical potentials of the pipeline and the reference electrodes are designated by... 

The nature of the reference electrode used depends largely on the accuracy required of the potential measurement. In the case of breakdown of passivity of stainless steels the absolute value of potential is of little interest. The requirement is to detect a change of at least 200 mV as the steel changes from... 

It would normally be necessary to locate more than one monitoring unit on a structure. Thus each unit will only transmit its information on receiving coded instructions. Surface units can commonly accommodate up to 80 different codes. The ability to interrogate and receive over any distance is dependent upon the acoustic operation frequency. Relatively high frequencies are preferable, as they avoid problems of pulse reverberations and echoes from structural members. Typically data can be transferred over a distance of 2 km. This type of acoustic unit gives accuracies in potential measurement of 10mV. 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

For the rapid determination of Tc in a mixture of uranium fission products. Love and Greendale have used the method of amalgam polarography. It consists in a selective reduction of technetium at a dropping mercury electrode at a potential of —1.55 V vs. SCE in a medium of 1 M sodium citrate and 0.1 M NaOH. Under these conditions, technetium is reduced to an oxidation state which is soluble in mercury. The amalgam is removed from the solution of fission fragments and the amount of Tc determined in nitric acid solution of the amalgam by a y count. For Tc the measurement accuracy is within 1 %, and the decontamination factor from other fission products 10 . 

The relative uncertainties of the potential measuring devices are assumed to be correct that is, the sensors considered to measure the measurands included in the mathematical models are assumed to work according to the accuracy specified by the manufacturer of the instrument. Systematic errors associated with the sensors are neglected. 

The logarithmic response of ISEs can cause major accuracy problems. Very small uncertainties in the measured cell potential can thus cause large errors. (Recall that an uncertainty of 1 mV corresponds to a relative error of 4% in the concentration of a monovalent ion.) Since potential measurements are seldom better than 0.1 mV uncertainty, best measurements of monovalent ions are limited to about 0.4% relative concentration error. In many practical situations, the error is significantly larger. The main source of error in potentio-metric measurements is actually not the ISE, but rather changes in the reference electrode junction potential, namely, the potential difference generated between the reference electrolyte and sample solution. The junction potential is caused by an unequal distribution of anions and cations across the boundary between two dissimilar electrolyte solutions (which results in ion movement at different rates). When the two solutions differ only in the electrolyte concentration, such liquid junction potential is proportional to the difference in transference numbers of the positive and negative ions and to the log of the ratio of the ions on both sides of the junction ... 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

Optical radiation is distributed over time, position, direction, wavelength, and polarization, so field measurement accuracies within 10% are normal. There are over 30 potential sources of error in light measurement (D. Ryer, personal communication, 1983-1999), six of these sources involving optical error are considered foremost. 

R. Chirat and G. Pittion-Rossillion employ a simplified Weeks-Chan-dler-Andersen (WCA) perturbation theory while F. Ree uses the Man-soori-Canfield-Rasaiah-Stell (MCRS) hardsphere variational theory. Both methods build on the a-Exp-6 potential and yield the theoretical Chapman-Jouget detonation velocities and pressures, which for a large number of explosives lie within the measurment accuracy of practically obtained values. 

Effect of Meter Resistance on the Accuracy of Potential Measurements ... 

With selenate as ligand no decrease in the limiting current or shift in the halfwave potential, Eyj, was observed. It was concluded that no precipitate was formed and that the complex formation was weak. The review assigns the accuracy of the potential measurement to be + 2 mV, which leads to an upper value of the stability constant of the 1 1 complex of about 20 M" in 0.15 M NaNOs. 

Potentiometric titrations are always more accurate than direct potentiometry because of the uncertainties involved in potential measurements. Whereas accuracies of better than a few percent are rarely possible in direct potentiometry, accuracies of a few tenths of a percent are common by potentiometric titration. We can make some general statements concerning potentiometric titrations ... 

A plot of Et/4 vs. log (kt) is shown in Figure 12.2.1. Note that the limiting diffusion and kinetic zones are described by the solid lines, and the dashed curve represents the exact equation, (12.2.24). Of course, the boundaries of these zones depend on the approximation employed, and the applicability of the limiting equations depends on the accuracy of the electrochemical measurements. For example, if potential measurements are made to the nearest 1 mV, the pure kinetic zone will be reached (for n = I and 25°C) when 25.7 In [erf(A /) ] < 1 mV or when > 1.5. 

It is important to appreciate that an error in measured voltage, such as lhat shown in Example 23-1 ( -0.13.3 V), would have an enormous effect on the accuracy of a concentration measurement based on that polenltal. Thus, as shown in Section 2.3H-2. a 0.001 V uncertainly in poicniial leads to a relative error of about 4% in the dcicrininatum of the hydrogen ion concentration of u solution by potential measurement with a gla.ss electrode. An error of the si/c found in Example 23-1 would result in a concentration uncertainty of two orders of magnitude or more. 

One interesting result of this property is that the relative concentration error for direct potentiometric measurements is theoretically independent of the actual concentration. Unfortunately, the error is rather large—approximately 4n% per mV uncertainty in measurement, perhaps the most serious limitation of ISEs. Since potential measurements are seldom better than 0.1 mV total uncertainty, the best measurements for monovalent ions under near-ideal conditions are limited to about 0.5% relative concentration error. For divalent ions, this error would be doubled and in particularly bad cases where, for example, liquid-junction potentials may vary by 5 to 10 mV (as in high or variable ionic-strength solutions), the relative concentration error may be as high as 507o- This limitation may be overcome, however, by using ISEs as endpoint indicators in potentiometric titrations (Sec. 2.6). At the cost of some extra time, accuracies and precisions on the order of 0.1% or better are possible. 

Commercial solid-state potential measuring devices based on the type of op-amp described are often called pH or plon meters and are designed to work with glass pH electrodes, ion selective electrodes, and other indicator electrodes described earlier. Research quality plon meters have built-in temperature measurement and compensation, autocalibration routines for a three-point (or more) calibration curve, recognition of electrodes (so you do not try measuring fluoride ion with your pH electrode ), and the ability to download data to computer data collection programs. The relative accuracy of pH measurements with such a meter is about +0.005 pH units. Meters are available as handheld... 

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