Alternating-Current Electrode Polarization

Conductivity is a measure of the ability of an aqueous solution to carry an electric current between two chemically inert electrodes. To avoid polarization of the electrodes, an alternating current signal is used. Because the current is carried by ions, conductivity is related to the amount of dissolved electrolytes so that the salinity of the solution can be estimated from the measured conductivity (see Greenberg et al. (1992)). 

FIGURE 7.2 Schematic of a DNA sensor based on a capacitive EIS structure. For operation, a DC (direct current) polarization voltage (VG) is applied via the reference electrode (RE) to set the working point of the EIS sensor, and a small AC (alternating current) voltage (E ) is applied to the system in order to measure the capacitance of the sensor. ssDNA - single-stranded DNA, cDNA - complementary DNA, dsDNA - double-stranded DNA. 

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

Alternating-current and frequency effects. With an AC rather than a DC voltage applied to the electrodes, the processes above reverse themselves with the period of the alternating voltage. But each process proceeds at a different rate (with a characteristic relaxation time) so that their relative contributions to energy dissipation vary with frequency. As the frequency is increased concentration-polarization can be reduced or eliminated, particularly if the electrode reaction is reversible (fast electron transfer in both directions). 

If the polarity of the current is not allowed to change too rapidly, it is possible, since oxidation and reduction occur successively at each pole, to accomplish electrolyses with alternating currents. Experiments with this end in view have been made b Drechsel.1 Dehydration is a case of simultaneous reduction and oxidation. The supposition that in living organisms carbamide is produced from ammonium carbamate by the splitting off of water prompted Drechsel to make experiments in this direction. When an aqueous solution of ammonium carbamate is electrolyzed with a current from a battery of 4-6 Grove cells, and platinum electrodes used, carbamide is obtained independently of the electrode material when alternating currents are employed. The reactions are supposed to be either... 

Both alternating and direct current techniques can be used (see also impedance spectroscopy), but the electrode polarization effects should be minimized or taken into account in all cases. For this goal, a four-electrode method where the potential probes are placed between current probes, is often used. 

If net cathodic current flows then this potential is shifted negatively. Concentration polarization (alternatively called -> mass-transport polarization or - concentration overpotential) is encountered if the rate of transport of the redox reactant to the electrode surface is lower than that of the -> charge-transfer reaction. Together with the charge-transfer or -> activation polarization (overpotential), q3, and the polarization (overpotential) due to a preceding chemical reaction, qrxn> (see... 

Disinfection of municipal water contaminated with coliforms and fecal streptococci was the subject of a study by Patermarkis and Fountoukidis [31]. Disinfection was achieved using titanium electrodes and direct current. The polarity was alternated every minute to eliminate titanium oxide buildup. No additives or supporting electrolytes were used in this room-temperatures process. At a current density of 2.5 mA/cm and an applied voltage of 45 V, no microbial activity was detected after 30 min of operation. Noncontaminated, electrochemically treated water possessed a residual disinfection capacitiy addition of treated water to a contaminated sample destroyed the microbial life in the sample. 

Conductometry is an electrochemical technique used to determine the quantity of an analyte present in a mixture by measurement of its effect on the electrical conductivity of the mixture. It is the measure of the ability of ions in solution to carry current under the influence of a potential difference. In a conductometric cell, potential is applied between two inert metal electrodes. An alternating potential with a frequency between 100 and 3000 Hz is used to prevent polarization of the electrodes. A decrease in solution resistance results in an increase in conductance and more current is passed between the electrodes. The resulting current flow is also alternating. The current is directly proportional to solution conductance. Conductance is considered the inverse of resistance and may be expressed in units of ohm (siemens). In clinical analysis, conductometry is frequently used for the measurement of the volume fraction of erythrocytes in whole blood (hematocrit) and as the transduction mechanism for some biosensors. 

The measured resistance of most cells varies with the applied alternating current, which is due to the polarization of the cell electrodes. Consequently, the use of large electrodes in cells with a high cell constant (capillary cells) will minimize the relative variation. In highly corrosive fluorides, however, most investigators use all-metal cells with low cell constants and thus considerable polarizations, so far. 

Because atmospheric humidity must be avoided, the reaction cell is isolated from the atmosphere by drying tubes. Moreover, since the diluting solvent is rarely anhydrous, owing to its hygroscopic nature, its water content must be determined prior to the measurement. The end point of the reaction is detected by the variation of an alternate current intensity that passes between two small polarized platinum electrodes inserted in the reaction medium (Figure 20.13). 

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