Dual-reference-electrode system

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. 

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...

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.

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. 

FIGURE 8 Electrochemical detector cell (Bioanalytical Systems, Inc.), (a) Diagram of the flow cell. A = auxiliary electrode, W = working electrode. R = reference electrode, (b) Dual thin-layer working electrodes in parallel (1) and series (2) configurations. [From Bratin K., and Kissinger, P. T. (1981). J. Liq. Chromator. 4, 321-57. Reprinted with permission from Marcel Dekker, Inc.]... 

Figure 14.15 shows a photograph of commonly used microelectrodes and miniaturized reference and counter electrode. Note that even a dual microelectrode (double-barrel electrodes68) can be used in the robotic system.69... 

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