Two-electrode technique

An interesting way of illustrating the need to eliminate the contribution of the ohmic drop to electrode potential is offered by the discussion of corrosion rate monitoring by the two-electrode technique introduced by Marsh [22]. 

Bioimmittance is measured in vivo or in vitro. The tissue may be kept alive and perfused under ex vivo conditions. Bioimmittance can be measured with two-, three- or four-electrode systems. With four electrodes, one electrode pair is current carrying and the other pair picks up the corresponding potential difference somewhere else in the tissue. If the measured voltage is divided by the applied current, the transfer impedance is calculated. If no voltage is measured, the transfer impedance is zero. This is equivalent to the bioelectricity case in which a signal from the source, such as the heart, is transferred to the skin surface electrodes. Zero transfer impedance does not mean the tissue conducts well, only that no signal transfer occurs. With the bioimpedance two-electrode technique, the transfer factor is eliminated because current application and signal pickup occur at the same site, which means that measured impedance reflects tissue electrical properties more directly. 

Foster and Schwan (1986), Stuchly and Stuchly (1990), Duck (1990), and Holder (2005). Gabriel et al. (1996a) made a literature survey. Their own measurements (Gabriel et al., 1996b) were made with a two-electrode technique and a coaxial probe in the frequency range of 10 Hz to 20 GHz. In that way, the transfer impedance component was eliminated. 

According to the definition, a passive technique is one for which no appHed signal is required to measure a response that is analytically usehil. Only the potential (the equiHbrium potential) corresponding to zero current is measured. Because no current flows, the auxiHary electrode is no longer needed. The two-electrode system, where the working electrode may or not be an ion-selective electrode, suffices. 

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

Linear polarization instruments provide an instantaneous corrosion-rate data, by utilizing polarization phenomena. These instruments are commercially available as two-electrode Corrater and three electrode Pairmeter (Figure 4-472). The instruments are portable, with probes that can be utilized at several locations in the drilling fluid circulatory systems. In both Corrater and Pairmeter, the technique involves monitoring electrical potential of one of the electrodes with respect to one of the other electrodes as a small electrical current is applied. The amount of applied current necessary to change potential (no more than 10 to 20 mV) is proportional to corrosion intensity. The electronic meter converts the amount of current to read out a number that represents the corrosion rate in mpy. Before recording the data, sufficient time should be allowed for the electrodes to reach equilibrium with the environment. The corrosion-rate reading obtained by these instruments is due to corrosion of the probe element at that instant [184]. 

This technique can be used to measure displacement where, in effect, the two electrodes are connected to the two bodies. It has also other applications (for example, in moisture meters where the presence of water vapor between the electrodes causes the capacitance change). 

From the nature of the process described above it has been referred to as stripping polarography , but the term anodic stripping voltammetry is preferred. It is also possible to reverse the polarity of the two electrodes of the cell, thus leading to the technique of cathodic stripping voltammetry. 

Essentially, stripping analysis is a two-step technique. The first, or deposition, step involves die electrolytic deposition of a small portion of the metal ions hi solution into die mercury electrode to preconcentrate the metals. This is followed by die shipping step (the measurement step), which involves die dissolution (shipping) of die deposit. Different versions of stripping analysis can be employed, depending upon die nature of the deposition and measurement steps. 

In a recent study of the transport of coarse solids in a horizontal pipeline of 38 mrrt diameter, pressure drop, as a function not only of mixture velocity (determined by an electromagnetic flowmeter) but also of in-line concentration of solids and liquid velocity. The solids concentration was determined using a y-ray absorption technique, which depends on the difference in the attenuation of y-rays by solid and liquid. The liquid velocity was determined by a sail injection method,1"1 in which a pulse of salt solution was injected into the flowing mixture, and the time taken for the pulse to travel between two electrode pairs a fixed distance apart was measured, It was then possible, using equation 5.17, to calculate the relative velocity of the liquid to the solids. This relative velocity was found to increase with particle size and to be of the same order as the terminal falling velocity of the particles in the liquid. 

In situ electron transport measurements on conducting polymers are commonly made by using a pair of parallel-band electrodes bridged by the polymer [Fig. 9(A)].141142 Other dual-electrode techniques in which the polymer film is sandwiched between two electrodes [Fig. 9(B)],139,140 rotating-disk voltammetry [Fig. 9(C)],60,143 impedance spectroscopy,144,145 chronoamperometry,146 and chronopotentiometry147 have also been used. 

Our experimental techniques have been described extensively in earlier papers (2, 13). The gamma ray irradiations were carried out in a 50,000-curie source located at the bottom of a pool. The photoionization experiments were carried out by krypton and argon resonance lamps of high purity. The krypton resonance lamp was provided with a CaF2 window which transmits only the 1236 A. (10 e.v.) line while the radiation from the argon resonance lamp passed through a thin ( 0.3 mm.) LiF window. In the latter case, the resonance lines at 1067 and 1048 A. are transmitted. The intensity of 1048-A. line was about 75% of that of the 1067-A. line. The number of ions produced in both the radiolysis and photoionization experiments was determined by measuring the saturation current across two electrodes. In the radiolysis, the outer wall of a cylindrical stainless steel reaction vessel served as a cathode while a centrally located rod was used as anode. The photoionization apparatus was provided with two parallel plate nickel electrodes which were located at equal distances from the window of the resonance lamp. 

These techniques are especially useful for studies of the adsorption of reactants, intermediates and products of electrode reactions. The simplest case corresponds to adsorption that is so strong that the electrode can be removed from the solution, rinsed and its activity measured without interference from desorption. When this procedure is impossible, the activity of the adsorbate can be measured by the electrode lowering method . The radioactive counter is placed under the bottom of the cell, which is made of a plastic foil. The electrode can be located at large distances from the bottom or can be placed so close to the bottom that only a thin layer of solution remains beneath it. The radioactivity values at the two electrode positions permit determination of the adsorbate activity. This procedure can be repeated many times, thus supplying data on the kinetics of the adsorption process. 

Electrodialysis units are constructed using a plate-and-frame technique similar to filter presses. Alternating sheets of anionic and cationic membranes are placed between two electrodes. The plating or rinse solution to be recovered (electrolyte) circulates past the system s electrodes. Hydrogen and oxygen evolve. Positive ions travel to the negative terminal and negative ions travel to the... 

In electrogenerated chemiluminescence, light emission occurs not on the electrode surface but in the solution. Oxygen has to be excluded 5>. In the usual form a one-electrode technique is applied the potential of the electrode is changed periodically. At cathodic potential the radical anion is produced, at anodic potential the radical cation. These two radical ions react in the diffusion layer near the electrode surface the electron transfer from the radical anion to the radical cation causes the light emission 5>2°). 

Among these techniques, the capacitance dilatometer method may be very sensitive. The change in the sample length produces a capacitance change between the two electrodes of a capacitor one plate of the capacitor is kept in a fixed position while the other is fixed onto one end of the sample. At the maximum elongation of the sample, the two capacitor plates are practically in contact. When the sample contracts the capacitance varies as 1 /AL. One of the main difficulties in this measurement is the realization of a dilatation-free support. 

An alternative method to position two electrodes at nanometer distances apart is the mechanically-controlled, break junction (MCBJ) technique. An ultra-thin, notched Au wire on a flexible substrate can be broken reliably by pushing on the Au with a piezoelectric piston, cracking the Au (Fig. 4). This produces a gap between the Au shards whose size can be finely varied to 1 A by a piston or control rod [46, 47]. When UE molecules with thiol groups on both ends are present in a surrounding solution, the gap can be adjusted until the molecules can span it. A dilute solution means the number of spanning molecules will be small, and the least-common-multiple of current flow among many junctions indicates those spanned by a single molecule [47]. 

In virtually all of the simple immersion and two electrode experiments carried out so far, in-diffused H has been detected at the 1016/cm3 level or less. There has been no demonstration that large densities (> 1018/cm3) of defects can be passivated by these methods, and where plasma and electrochemical treatments have been directly compared, the former have been found to be more effective (Tavendale et al., 1986). In contrast to plasma techniques, the electrolyte boiling point limits the temperature range of electrochemical methods, although several hundred degrees Celsius can be utilized for electrolytes like H3P04. 

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