Measurement potentials

On the other hand, CU-CUSO4 should not be used as built-in electrodes for potential test probes (see Section 3.3.3.2) because there is a danger of copper precipitating on the steel electrode. Calomel electrodes with saturated KCl solution are preferred in this case and present no problems. 

1 Bases of Potential Measurement of Electrodes with Flowing Current 

The possible values for Xp lie between 10 and 10 s according to the data in Section 2.2.3.2, but with a concentration polarization this can be much greater. In Eq. (3-8) 

For a potential measurement immediately after the switch, exp [-t/tp] = 1 and exp = 0 so that from Eq. (3-6) it follows  

This result is immediately confirmed by the results given in Fig. 3-3. A steel electrode was cathodically polarized in a soil sludge. The potential was measured with a capillary probe IR-free as E, and without a probe as E2. The difference directly 

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A circular metal plate of 10-cm area is held parallel to and at a distance d above a solution, as in a surface potential measurement. The temperature is 25°C. 

Streaming potential measurements are to be made using a glass capillary tube and a particular electrolyte solution, for example, O.OIM sodium acetate in water. Discuss whether the streaming potential should or should not vary appreciably with temperature. 

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

Group 12 (IIB) Perchlorates. The zinc perchlorate [13637-61 -17, cadmium perchlorate [13760-37-7] mercury(I) perchlorate [13932-02-0] and mercury(II) perchlorate [7616-83-3] all exist. Cell potential measurements show that zinc and cadmium perchlorates are completely dissociated in concentrations up to 0.1 molar in aqueous solutions (47—49). Mercurous perchlorate forms a tetrahydrate that can be readily converted to the dihydrate on heating to above 36°C (50). 

The activity coefficients of sulfuric acid have been deterrnined independentiy by measuring three types of physical phenomena cell potentials, vapor pressure, and freeting point. A consistent set of activity coefficients has been reported from 0.1 to 8 at 25°C (14), from 0.1 to 4 and 5 to 55°C (18), and from 0.001 to 0.02 m at 25°C (19). These values are all based on cell potential measurements. The activity coefficients based on vapor pressure measurements (20) agree with those from potential measurements when they are corrected to the same reference activity coefficient. 

FIG. 28-8 The potentiostat apparatus and circuitry associated with controlled potential measurements of polarization curves. 

The basic standard for cathodic protection was laid down for the first time in DIN 30676 to which all the application areas of the different branches of protection can be referred. In this the most important point is the technique for accurately measuring the object/soil potential [58]. The usual off-potential measurement method for underground installations has been slowly implemented and enforced in Europe since the 1960s [59]. 

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. 

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-... 

Fig. 3-3 Potentialtime oscillogram for off potential measurement (explanation in the text).

Fig. 3-4 Current-potential measurements on a buried steel storage tank with bitumen coating (surface area 4 m with four defects 5 cm x 20 cm soil resistivity p 30Q. m).

Fig. 3-5 Switching potential measurements for lead (1,2) and steel (3) in soil solution ...

All techniques fail when Eq. (3-8) is not applicable with a small Xy. It has been observed in a few cases with pure sandy soils that electrical polarization of the sand can occur, which falsifies a potential measurement made without probes [3]. The potential is consequently found to be noticeably too negative. Off potentials that are not realistic may be measured they may have ranging... 

Ohmic voltage drops of the protection current were exclusively involved in the processes for 7/ -free potential measurements described above. Besides this, other foreign currents can cause potential drops and falsify the potential measurement (e.g., cell currents, equalizing currents and stray currents). 

Application of this method or Eq. (3-25 ) in the presence of stray currents is conceivable but would be very prone to error. It is particularly valid for good coating. Potential measurement is then only significant if stray currents are absent for a period, e.g., when the source of the stray current is not operating. In other cases only local direct measurements with the help of probes or test measurements at critical points can be considered. The potential test probes described in Section 3.3.3.2 have proved true in this respect. 

For// -free potential measurement with cell or equalizing currents it is assumed, analogous to Eq. (3-26), that current densities and potentials are proportional ... 

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