Errors electrodes

The relative measurement error in concentration, therefore, is determined by the magnitude of the error in measuring the cell s potential and by the charge of the analyte. Representative values are shown in Table 11.7 for ions with charges of+1 and +2, at a temperature of 25 °C. Accuracies of 1-5% for monovalent ions and 2-10% for divalent ions are typical. Although equation 11.22 was developed for membrane electrodes, it also applies to metallic electrodes of the first and second kind when z is replaced by n. 

Electromagnetic flow meters ate avadable with various liner and electrode materials. Liner and electrode selection is governed by the corrosion characteristics of the Hquid. Eor corrosive chemicals, fluoropolymer or ceramic liners and noble metal electrodes are commonly used polyurethane or mbber and stainless steel electrodes are often used for abrasive slurries. Some fluids tend to form an insulating coating on the electrodes introducing errors or loss of signal. To overcome this problem, specially shaped electrodes are avadable that extend into the flow stream and tend to self-clean. In another approach, the electrodes are periodically vibrated at ultrasonic frequencies. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Departures from the ideal behavior expressed by equation 7 usually are found in alkaline solutions containing alkaH metal ions in appreciable concentration, and often in solutions of strong acids. The supposition that the alkaline error is associated with the development of an imperfect response to alkaH metal ions is substantiated by the successhil design of cation-sensitive electrodes that are used to determine sodium, silver, and other monovalent cations (3). 

Because of the very large resistance of the glass membrane in a conventional pH electrode, an input amplifier of high impedance (usually 10 —10 Q) is required to avoid errors in the pH (or mV) readings. Most pH meters have field-effect transistor amplifiers that typically exhibit bias currents of only a pico-ampere (10 ampere), which, for an electrode resistance of 100 MQ, results in an emf error of only 0.1 mV (0.002 pH unit). 

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. 

In the same way that potential differences can occur due to different mobility, they can also occur due to different adsorption of ions. There are therefore a large number of possibilities for potential errors in the field of reference electrodes [2], which, however, are generally less than 30 mV. Such potential errors can be neglected in the application of protection potential criteria, but they can lead to increased error in the evaluation of voltage cones (see Section 3.3.1). Equation (3-4) can be used for their evaluation in this case. It explains, for example, the increased... 

Point (a) only concerns simple metal electrodes and needs to be tested for each case. Point (b) is important for the measuring instrument being used. In this respect, polarization of the reference electrode leads to less error than an ohmic voltage drop at the diaphragm. Point (c) has to be tested for every system and can result in the exclusion of certain electrode systems for certain media and require special measures to be taken. 

CU-CUSO4 electrodes with saturated CUSO4 solution are recommended for potential measurements in soil. Their potential constancy is about 5 mV. Larger errors can be traced to chemical changes in the CUSO4 solution. These electrodes have been developed for long-life applications in potential-controlled rectifiers and built-... 

The second term of Eq. (3-28) is very prone to errors because the factor AU ff includes electrode errors (see Section 3.2). For small values of AU ff, Eq. (3-28) is not applicable in practice [2]. Applications are described in Section 3.7 along with intensive measurement. 

Since the Wenner formula [Eq. (24-41)] was deduced for hemispherical electrodes, measuring errors appear for spike electrodes. To avoid errors in excess of 5%, the depth of penetration must be less than a 5. Soil resistivity increases greatly under frost conditions. While electrodes can be driven through thin layers of frost, soil resistivity measurements deeper than 20 cm in frozen ground are virtually impossible. 

A further possibility of error is the addition of electrode errors. For this reason examination of the value according to Eq. (3-62) at the next measuring point is necessary. 

The determination of AU values as well as the potential is effected by simple measurements of the difference in potential between two reference electrodes over the pipeline. To reduce possible electrode errors, partial sums can also be measured for a larger span, e.g., (a - O ) = 300 m ... 

To measure the potential, reference electrodes are lowered on unbreakable ropes tensioned with 20 kg of lead as near as possible to the ship s side. IR errors can be neglected because of the good conductivity of seawater [see Eq. (2-34)]. In contrast to fresh water, the switching method in seawater is not necessary (see Section 3.3.1). 

Since usually the reference electrode is not equipped with a capillary probe (see Fig. 2-3), there is an error in the potential measurement given by Eq. (2-34) in this connection see the data in Section 3.3.1 on IR-free potential measurement. The switching method described there can also be applied in a modified form to potential-controlled protection current devices. Interrupter potentiostats are used that periodically switch off the protection current for short intervals [5]. The switch-off phase is for a few tens of microseconds and the switch-on phase lasts several hundred microseconds. 

By using only a single reference electrode in the object to be protected, the potential can be determined only in the vicinity of this electrode and not in more remote areas. Section 3.3.1 together with Eq. (3-27) provides further explanation of this. To improve the current and potential distribution, the number and location of the anodes must suit the geometry of the object to be protected. Occasionally, additional reference electrodes are required for potential control [2]. The optimum nominal potential for potential control can be found by this method by considering remote IR errors. 

The test gas must be of uniform and known composition. This generally requires on-line gas analysis if flammable mixtures are not supplied from a suitable reservoir. If concentration gradients are created in the surrounding air, errors can be introduced by releasing the test gas stream from a perforated probe doubling as an electrode. The maximum effective energy of a... 

If the electrode potential is further reduced to h-350 mV, a hexagonal superstructure with a periodicity of 2.4 0.2 nm is observed. With respect to the interatomic distances in the Au(lll) structure at the surface, this corresponds - within the error limits - to an 8 X 8 superstructure (Figure 6.2-9). 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

It is desirable to measure the potential of a structure without an IR error. It is not always possible to place the reference electrode close to the structure to minimise the IR error but it can be achieved using the so-called... 

Fig. 10.9 Diagram illustrating the source of the IR error in potential measurements on a cathodically protected structure. BA is the absolute electrode potential of the structure CD is the absolute electrode potential of the anode and CB is the field gradient in the environment due to cathodic protection current flux. A reference electrode placed at E will produce an IR error of EFin the potential measurement of the structure potential. If placed at G the error will be reduced to GH. At B there would be no error, but the point is too close to the structure to permit insertion of a reference electrode. If the current is interrupted the field immediately becomes as shown by the dotted line, and no IR is included...

From Fig. 10.40 it will be seen that contact between the electrolyte (soil or water) and the copper-rod electrode is by porous plug. The crystals of CUSO4 maintain the copper ion activity at a constant value should the halfcell become polarised during measurements. The temperature coefficient of such a cell is extremely low, being of the order of 1 x 10" V/°C and can thus be ignored for all practical purposes. To avoid errors due to polarisation effects, it is necessary to restrict the current density on the copper rod to a... 

Voltmeters and potentiometers The instruments described here are generally referred to as corrosion voltmeters. As mentioned previously, the current flowing through any potential-measurement circuit must be small to avoid errors due to polarisation. Moreover, if the current flow is too large, errors will be introduced owing to the voltage drop caused by the contact resistance between the reference electrode and the electrolyte. It is thus clear that the prime requirement of a potential measurement circuit is high resistance. 

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