Two-electrode method

Fedor (1990), and Fedor et al. (1991). The density, surface tension, electrical conductivity, and viscosity have been measured at the temperature of 1573 K and in a relatively wide concentration range. The density and surface tension were measured by means of the maximum bubble pressure method using a device similar to that described in Section 6.2.2. The viscosity was measured using the rotational method, and the electrical conductivity, by means of the two-electrode method. 

This method employs the basic principles previously described for the EIS method but with the use of two identical working electrodes. The method does not use an auxiliary electrode nor a reference electrode. With reference to Fig. 6.21, the two working electrodes (A and B), ideally, are identical in all aspects—geometry, chemical composition, microstructure, surface condition, etc. The method involves application of a low-amplitude (e.g., 20 mV) AC potential across the two electrodes, at a very low frequency (If) and at a very high frequency (hf), and measurement of the impedance of the system at each frequency, Z lf and Z hf. The assumed equivalent electrical circuit for the system also is indicated in Fig. 6.21. This circuit assumes that the simplest equivalent electrical circuit, as shown in Fig. 6.18, is applicable to each of the electrodes in the two-electrode method. In this case, Rs is the solution resistance (normalized with respect to specimen area, for example, ohms-m2) between the two electrodes. With reference to Fig. 6.21 (and also with reference to the previous discussion of the EIS method), it is seen that ... 

Once Rp is determined, icorr is evaluated with the polarization-resistance (or Stem-Geary) equation, Eq 6.28. The two-electrode method is a relatively simple and fast method for evaluating when compared with the standard polarization-resistance and electrochemical-impedance methods. 

With reference to the two-electrode method (Fig. 6.21), why is the low-frequency impedance equal to 2Rp + Rs and the high-frequency impedance equal to Rs ... 

In the two-electrode method, why should the two working electrodes be identical in every way ... 

Figure 2.7 Measurement of electrical resistivity of concrete by two-electrode method...

In the execution stage, quahty control is applied on site using simple methods with known correlation to the RCM test. For quality control with regard to chloride-penetration resistance, DuraCrete has adopted an electrical resistivity test, the two-electrode method (TEM), to be carried out on specimens kept in a fog room by a test described in Chapter 2. The correlation between the RCM and TEM values is based on theoretical and empirical work mentioned in Section 2.6 [26,27] and was documented for a wide range of concretes [28]. 

The resistance of the concrete against chloride penetration was mainly tested by use of the Rapid Chloride Migration (RCM) method [6] and electrical resistivity measurements based on the two-electrode method [7], The testing was carried out on 50 mm cut slices from 0100 x 200 mm concrete cylinders, and the concrete samples were mainly cured in water with a temperature of 20 2 C and tested at regular intervals of up to 90 days. In addition, some samples from two of the concrete mixtures (PO O and Pl. O) were wrapped in plastic sheeting after demoulding and stored in air with a temperature of 20 2°C until time of testing. 

An alternative approach measures the resistivity of the cover concrete by a two electrode method using the reinforcing network as one electrode and a surface probe as the other tubular sensor B in Figure 4.13 (Feliu et ai, 1988 Broomfield et aL, 1993, 1994). 

Figure 4.3 Dielectric dispersion for glycine crystals with water content (0.67% and 0%) as a parameter. Two-electrode method, dashed line without correction for electrode polarization. From Takashima and Schwan (1965), by permission.

To analyze the situation with a tetrapolar electrode system in contact with, for example, a human body, we must leave our simplified models and turn to lead field theory (see Section 6.4). The total measured transfer impedance measured is the ratio of recorded voltage to injected current according to Eq. 6.39. The impedance is the sum of the impedance contributions from each small volume dv in the measured volume. In each small volume, the resistance contribution is the resistivity multiplied by the vector dot product of the space vectors (the local current density from a unit reciprocal current applied to the recording electrodes) and (the local current density from a unit current applied to the true current carrying electrodes). With disk-formed surface electrodes, the constrictional resistance increase from the proximal zone of the electrodes may reduce sensitivity considerably. A prerequisite for two-electrode methods is therefore large band electrodes with minimal current constriction. 

One of the first to introduce the method was Thomasset (1965), using a two-electrode method and 1 kHz signal frequency. With just two electrodes, it is important to use large-area band electrodes to reduce the contribution from the current constrictional zones near the electrodes. With a tetrapolar electrode system, it is easier to select the preferred... 

Figure 5.36 Two electrodes method of measurement of potential. (By kind permission of Cheveron, USA)...

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