Electrode dual-polarized

Fig. 9. Voltammograms demonstrating a potentiometric titration using dual-polarized electrodes, where the dashed lines indicate the anodic and equal-but-opposite cathodic currents that must be carded by the two opposing electrodes during the titration. Terms are defined in text.

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

The use of dual-polarized electrodes was first suggested more than 70 years ago 2 the subject has been reviewed thoroughly by two more recent publications.3,4 Almost all modem commercial pH meters have provision for imposing a polarizing current of either 5 or 10 nA to make possible measurements by dual-polarized electrode potentiometry. Such a provision is included because dual-polarized potentiometry is by far the most popular endpoint detection method for the Karl Fischer determination of water. For this titration a combination of reagents is used, including iodine the response curve is similar to that of Figure 4.3b. In practice the response is many times more sensitive than... 

Figure 4.3 Dual-polarized electrode potentiometric titration curves (a) titration of Fe(II) by Ce(TV) (b) titration of As(III) by I2.

Figure 4.6 Dual-polarized electrode amperometric titration curves. Both curves result from the application of a 0.25-V potential across two identical platinum electrodes that are immersed in the titration solution.

Figure 4.9 Coulometric titration cell with generator [II (generator anode, 0.7 x 0.7 cm)] and isolated auxiliary [I (generator cathode, 0.7 x 0.7 cm)] electrodes on the left side and a pair of identical platinum electrodes [III, IV (1.4 x 1.8 cm and 2.5 X 1.8 cm)] on the right for dual-polarized electrode amperometric endpoint detection.

The most common end-point detection method for the Karl Fischer titration for determining water (see Section 20C-5) is the amperometric method with dual polarized electrodes. Several manufacturers offer fully automated instruments for use in performing these titrations. A closely related end-point detection method for Karl Fischer titrations measures the potential difference between two identical electrodes through which a small constant current is passed. 

The application of dual polarity DC potential, both positive and negative to the high gradient area between the electrodes successfully coalesces the majority of even the one and two micron droplets resulting in a much lower water content in the clean oil. 

Because the generator electrodes must have a significant voltage applied across them to produce a constant current, the placement of the indicator electrodes (especially if a potentiometric detection system is to be used) is critical to avoid induced responses from the generator electrodes. Their placement should be adjusted such that both the indicator electrode and the reference electrode occupy positions on an equal potential contour. When dual-polarized amperometric electrodes are used, similar care is desirable in their placement to avoid interference from the electrolysis electrodes. These two considerations have prompted the use of visual or spectrophotometric endpoint detection in some applications of coulometric titrations. 

The DPET is designed to hold 1720 L (10.8 bbl) and operates safely at pressures up to 517 kPa (75 psi) and temperatures up to 150 C (300 T). The application of a high-voltage, dual-polarity electric potential to electrodes inside the vessel is used to coalesce and remove small droplets of water in the oil emulsion. The oil should be degassed and have a water content less than 15% before entering the vessel however, the treater does have the capability for free-water knockout. Preheated emulsion is pumped into the bottom portion of the vessel, below the electrodes, where free water generated by heating or chemical treatment may drop out. As more emulsion is... 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

Capillary gap cell — The undivided capillary gap (or disc-stack) cell design is frequently used in industrial-scale electroorganic syntheses, but is also applicable for laboratory-scale experiments when a large space-time yield is required. Only the top and bottom electrodes of c.g.c. (see Figure) are electrically connected to - anode and cathode, respectively, whereas the other electrodes are polarized in the electrical field and act as -> bipolar electrodes. This makes c.g.c. s appropriate for dual electrosynthesis, i.e., pro duct-generating on both electrodes. 

Voltammetrlc end-point detection with the Memotltrator calls for the use of a dual platinum pin electrode and a polarized power supply. 

Dual coulometric detection was used with online SPE with LiChrolut EN [63] for the determination of polar priority phenols at ng/L levels. The first electrode was intended for sample cleanup (normally set at a low potential), and the detection of the phenols was made at the second electrode. 

Cardwell and Christophersen reported a dual channel FI system with amperometric detection for the determination of ascorbic acid and sulfur dioxide in wines and fruit juices (Cardwell and Christophersen, 2000). Here, the ascorbic acid was detected at a glassy carbon electrode polarized at 0.42 V (vs. Ag/AgCl), whereas the sulfur dioxide was detected at a Pt electrode polarized at 0.90 V (vs. Ag/AgCl) after separation of the analytes by a gas diffusion unit. The determination of ascorbic acid showed a linear range between 3 and 50 mg L with an FOD of 1.5 mg L for sulfur dioxide the linear range was between 0.25 and 15 mgF i and an FOD of 0.05 mgF" was obtained. The sample frequency achieved with the system was 30 h b The proposed method showed a good agreement with a reference method in the results obtained for white wines and juice samples, while for red wines and sweet wines an extraction procedure of the analytes by solid-phase extraction was required. 

Cheng and Kohl (29) demonstrated that fiducial patterns on one side of an n-lnP wafer, defined in photoresist, could be photoelectrochemically etched through to the other side of a wafer about 90 pm thick. The metallized mask served the dual purpose of the electrical contact, which was at the front surface in this case. The process was carried out by illumination with a collimated beam from a HeNe laser and an electrode polarization of 0.2 - 0.4 volt (SCE) in 2 - 4 N HCI. Best results were obtained at light intensities <260 mW/cm due to reduced effects of diffraction and light scattering within the growing hole. 

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