Measurement of Potential

There are two commonly used instruments for making potential measurements. One is the potentiometer and the other is the pH meter (a voltmeter), with the latter almost always used today. pH measurements with a glass (or other) electrode involve the measurement of potentials (see below). 

The potentiometer can be used for measurements of low-resistance circuits. The pH meter is a voltage measuring device designed for use with high-resistance glass electrodes and can be used with both low- and high-resistance circuits. Electrometers can also be used with high-resistance circuits. 

A pH meter is a voltmeter that converts the unknown voltage to a current that is amplified and read out. These are high-input impedance devices. (Impedance in an ac circuit is comparable to resistance in a dc circuit. These devices convert the signal to an ac signal for amplification.) Because of their high-input resistance, very little current is drawn, typically 10 to 10 A, and so chemical equilibrium is not greatly disturbed. A voltmeter must be used for irreversible reactions that do not return to the prior state when disturbed by appreciable drawing of current. 

High-input impedance circuits must be used with high-resistance electrodes (e.g., several megohms—10 O). Also, the current drawn must be very small in order for the voltage drop across the cell =iR or current X cell resistance) to be low enough not to cause error in the measurement the cell resistance is high since it includes the glass electrode. 

Expanded-scale pH meters are available that will measure the potential to a few tenths of a millivolt, about 10 times more closely than conventional pH meters. They are well suited for direct potentiometric measurements with ion-selective electrodes. 

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Any measurement of potential must describe a reference point, and we will take as this point the potential of an electron well separated from the metal and at rest in vacuo. By reference to figure A2.4.8 [16], we can define the following quantities. 

Reference electrodes are used in the measurement of potential [see the explanation to Eq. (2-1)]. A reference electrode is usually a metal/metal ion electrode. The electrolyte surrounding it is in electrolytically conducting contact via a diaphragm with the medium in which the object to be measured is situated. In most cases concentrated or saturated salt solutions are present in reference electrodes so that ions diffuse through the diaphragm into the medium. As a consequence, a diffusion potential arises at the diaphragm that is not taken into account in Eq. (2-1) and represents an error in the potential measurement. It is important that diffusion potentials be as small as possible or the same in the comparison of potential values. Table 3-1 provides information on reference electrodes. 

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 switching-off method for 7/ -free potential measurement is, according to the data in Fig. 3-5, subject to error with lead-sheathed cables. For a rough survey, measurements of potential can be used to set up and control the cathodic protection. This means that no information can be gathered on the complete corrosion protection, but only on the protection current entry and the elimination of cell activity from contacts with foreign cathodic structures. The reverse switching method in Section 3.3.1 can be used to obtain an accurate potential measurement. Rest and protection potentials for buried cables are listed in Table 13-1 as an appendix to Section 2.4. The protection potential region lies within U[[Pg.326]

However, total quantities of hazardous materials do not, on their own, provide an entirely reliable measure of potential hazard. It is more useful to consider quantities of material within sections of the plant that can be isolated. The amount of material within these individual plant sections usually represents the largest credible release that could occur. Some examples of plant sections that may be isolated include tank farms, unloading racks, and separate process buildings. 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

Another technique for flatband determination is based on the measurement of potential-modulated microwave conductivity signals and is described further in the next section. 

The measurement of potential Hg methylation and MeHg demethylation is significantly more complex than the measurement of HgT and MeHg concentrations in ambient samples. Methodological considerations include the maintenance of redox and temperature condition of samples during measurement, an understanding of the time course of both processes, and an understanding of the impact of spike level on the methylation and demethylation rate constants. Measurement of methylation and demethylation also requires the use of a tracer, and the abiUty to measure that trace analytically. 

Assays of soil enzyme activities are usually carried out in soil slurries, since efficiencies of enzyme extraction from soil and purification are still low (49). Such assays, under these conditions, will only give a measure of potential rather than actual activities moreover, they constitute integrated measures of activity as enzymes come from a variety of sources and are in several states in the soil (50). Enzyme activities may vary substantially with the season according to the synthesis, release into soil, and persistence of plant, animal, and microbial enzymes (57). 

FIG. 13 Determination of the speed of action potentials in soybean by measuring of potential differences between two Ag/AgCl electrodes in the stem of a soybean plant 24 h after adding 50 mL of 5 X 10 " M pentachlorophenol to soil. Distance between electrodes was 8 cm. The plants were given water every other day and kept at 24°C. Volume of soil was 0.5 L. (a) and (b) were taken from Fig. 11(a). (From Ref. 19.)... 

The development of surface-sensitive techniques. The classical electrochemical methods involve the measurement of potential and current. While these are extremely useful in the study of reaction rates and mechanisms, they give no information on the structure of the interface. A variety of surface-sensitive techniques has now been adapted to the electrochemical situation and applied to the investigation of electrode surface structure. 

Measurement of Potential Effects on Cell-Mediated Immunity.72... 

Risk a measure of potential economic loss, environmental damage, or human injury in terms of both the probability of the loss, damage, or injury occurring and the magnitude of the loss, damage, or injury if it does occur... 

Choice of Potential Bioavailability Criterion. It is usually assumed that calcium must be soluble and probably ionized in order to be available for absorption ( ). For the in vitro procedure, as a first approximation we chose calcium solubility after centrifugation at 18,000 x g as the measure of potential bioavailability (Figure 1). We assumed that this would probably overestimate the available calcium and later work based on fractionation might define the bioavailable calcium more precisely. The data in Table IV illustrate how the choice of criterion for "solubility" could affect the in vitro estimate of potential availability, even if in vitro conditions closely resembled in vivo conditions. Since our in vitro criterion unexpectedly underestimated calcium bioavailability for two of the three foods in the direct in vivo - in vitro comparison (8), it was necessary to determine the in vitro digestion conditions which might be limiting solubility before addressing the choice of appropriate criterion. 

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