Electrodes double tube

C. Double tube electrode. Double channel electrode. 

The Double Channel Electrode (DCE) and Double Tube Electrode (DTE)... 

Some electrodes are double-junction electrodes. Such electrodes are encased in another glass tube and therefore have two junctions, or porous plugs. The purpose of such a design is to prevent contamination—the contamination of the electrode solution with the analyte solution, the contamination of the analyte solution with the electrode solution, or both, by the diffusion of either solution through the porous tip or plug. See the next section for tips concerning these problems. 

The half-cell enclosed by the dashed line in Figure 15-1 is called a silver-silver chloride electrode. Figure 15-3 shows how the electrode is reconstructed as a thin tube that can be dipped into an analyte solution. Figure 15-4 shows a double-junction electrode that minimizes contact between analyte solution and KCI from the electrode. The silver-silver chloride and calomel reference electrodes (described soon) are used because they are convenient. A standard hydrogen electrode (S.H.E.) is difficult to use because it requires H2 gas and a freshly prepared catalytic Pt surface that is easily poisoned in many solutions. 

Finally, chloride ions from an ordinary calomel electrode were found to interfere seriously with pH determinations, resulting in non-reproducible readings and end points [9]. Only in this one study was a special doublejunction calomel reference electrode with KN03 in the outer tube employed to prevent interference by chloride ions. Use of this double-junction electrode is labeled as KN03 salt bridge under Conditions in Table 1. 

Figures 8.3, 8.4, 8.5, and 8.6 show typical designs of double hydrodynamic electrodes (two working electrodes)—rotating, wall-jet, tube, and channel. Using only one of the two working electrodes one obtains /L as in Table 8.1. Use of the two electrodes simultaneously is described in Sections 8.5-8.8.

In the case of pipe flow (tubular flow, also tubular or tube electrode), the electrolyte solution is pumped through a circular tube at a rate (flow rate V"f) low enough to secure laminar flow (for the distinction from turbulent flow, see below). The working electrode is embedded as a ring (annulus) in the wall of the pipe a double ring can be mounted also to enable mechanistic studies like with a ring-disc electrode (see above). 

The commercial silver-silver chloride electrode is similar to the SCE in that it is enclosed in glass, has nearly the same size and shape, and has a porous fiber tip for contact with the external solution. Internally, however, it is different. There is only one glass tube (unless it is a double-junction design—see Section 14.5.3) and a solution saturated in silver chloride and potassium chloride is inside. A silver wire coated at the end with a silver chloride paste extends into this solution from the external lead. See Figure 14.5. The half-reaction that occurs is... 

Anomalous effects are often observed due to solution entering the capillary at the instant the mercury drop falls. In ac measurements, this phenomenon leads to anomalous frequency dispersion [12], In addition, the drop time becomes irreproducible. These effects may be diminished to a large extent by coating the internal wall of the capillary with a film of silicone [13]. A tip made of hydro-phobic (solvophobic) material may also be attached to the glass capillary. For example, a polyethylene tip was used [14] to discriminate against the attack of fluorides on glass in the study of double-layer structure in the presence of fluorides. In another study, capillary tips were modified with commercial narrow-bore PTFE tubes to determine arsenic in basic solution [15]. This procedure is also used for the hanging mercury drop electrode discussed in Section III. 

Tube double electrode (TDE) Channel double electrode (CDE) ... 

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