Dual-reference electrode

If the three-electrode instrument is equipped with an iR-drop compensator, most of the iT-drop caused by the solution resistance can be eliminated. However, in order to minimize the effect of the iT-drop, a Fuggin capillary can be attached to the reference electrode with its tip placed close to the indicator electrode. Moreover, for a solution of extremely high resistance, it is effective to use a quasi-reference electrode of a platinum wire (Fig. 8.1(a)) or a dual-reference electrode (Fig. 8.1(b)), instead of the conventional reference electrode [12]. 

In the dual-electrode techniques, the potential of each electrode is controlled with a bipotentiostat so that a small constant potential difference is maintained across the polymer film as its potential is slowly scanned, relative to a reference electrode. Figure 10 shows the results of this type of experiment for poly(3-methylthiophene) in SO20).37... 

Rabenstein and Yamashita [52] determined penicillamine and its symmetrical and mixed disulfides by HPLC in biological fluids. Plasma and urine were deproteinized with trichloroacetic acid, and HPLC was performed on a column (25 cm x 4.6 mm) or Biophase ODS (5 pm) with a mobile phase comprising 0.1 M phosphate buffer (pH 3) and 0.34 mM Na octylsulfate at 1 mL/min. Detection was with a dual Hg-Au amalgam electrode versus a Ag-AgCl reference electrode. (z>)-penicillamine and homocysteine were determined at the downstream electrode at +0.15 V, and homocystine, penicillamine-homocysteine, and penicillamine disulfides were first reduced... 

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 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. 

Fig. 14.15 Miniaturized electrodes for usage in the electrochemical robotic system. From left to right 3M Ag/AgCl reference electrode, platinum microelectrode, dual platinum microelectrode, stainless steel cannula that serves as pump outlet and counter electrode 14...

As an alternative to a bipotentiostat, two separate potentiostats can be used to control two working electrodes. For conventional potentiostats, both working electrodes would be held at circuit common, which requires that one of the potentiostats be operated in a floating mode. With potentiostats that allow a floating working electrode, the circuit common can be shared at the reference electrode and no isolation is required. Whether to use a single bipotentiostat or dual potentiostats in a SECM experiment is up to... 

Detector E, Bioanalytical systems Model LC-4B, dual glassy-carbon working electrode used in parallel mode, +0.65 V and +0.80 V (monitored), stainless steel auxiliary electrode, Ag/AgCl reference electrode following post-column reaction. The column effluent flowed at 0-5° through a 2 mL knitted coil of 0.5 mm i.d. PTFE tubing irradiated with a low pressure mercury lamp (Photronix Model 816) to the detector. 

Fig. 20. (A) Thin-layer dectrochemical detector with external rererenceoompattment. (B) Condensed detector cell with internal reference electrode and negligible uncompensated resistance. In both schemes, the working electrode block is interchangeable to provide various dual-and single-electrode configurations. The condensed cell is directly suital Ibr mkrobore LC. Reproduced with permission from Bioanalytkal Systems Inc.. West Lafayette, Indiana.

Hordenine was detected with a dual-electrode coulometric cell. The potential of the first electrode was set to -I-0.50V [vs. solid palladium reference electrode (Pd)], which was at the base of hordenine s hydro-dynamic voltammogram. This electrode cleaned the sample by removing easily oxidizable impurities. Hordenine and its precursors were subsequently detected by a second electrode at a potential of -I-0.75V. This method of detection is called the screen mode . In addition, the mobile phase was purified by connecting a coulometric cell between the pump and injector as a guard cell. This additional cell operated at +0.80 V. The detection limit of hordenine was at 1.1 ng, which was 25 times better than detection by UV at 275 nm. 

A simple two-electrode electrochemical cell consisting of either single or dual polarizable electrode(s) is normally required for amperometric titrations of various organic and inorganic substances. By definition, a polarizable electrode is a suitable electronic conductor whose potential changes even with the passage of relatively small current. In contrast, the potential of a nonpolarizable electrode, such as the saturated calomel and silver - silver chloride electrodes that are commonly employed as reference electrodes, remains reasonably constant even when a large current is passed. 

Fiber optic pH sensors have distinct advantages over pH electrodes. They are small, not interfered by electromagnetic flelds and have remote sensing capability. They can be used in extreme environments, such as deep-water analysis, chemical reactors, or wastewater. Moreover, they can be intrinsically referenced due to the dual wavelength measurement capabiUty and do not require a reference electrode [90]. Optical pH sensors also pave the way for imaging applications, whereas pH electrodes only enable punctiform pH measurements. Sensors for pH determination are also of high significance in environmental and marine research because they provide the basis for CO2 sensors. 

Rg. 3 Cross section through a dual coulometric electrode showing the placement of working, counter and reference electrodes. 

As basic sensor a pH-l FET chip was developed which can be used for different ISM depositions. It contains a dual ISFET structure (n-channel FET, LP-CVD Si3N4 gate membrane, gate size 16 x 400 pm) and a temperature sensitive diode. The chip is fabricated in a standard MOS>production line. For using the chip in a ISFET difference measuring mode without a conventional reference electrode an integrated pseudo reference electrode has been designed too. 

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