Electrodes hanging electrolyte drop

A hanging electrolyte drop electrode (fig. 9.4) was constructed for analytical purposes requiring electrolytic preconcentration of the test component [14, 15]. 

Fig. 9.4, The hanging electrolyte drop electrode. Symbols are the same as in fig. 9.2. (After Marebek and Samec [15].)...

Fig. 9.7. Determination of acetylcholine by differential pulse polarography with hanging electrolyte drop electrode. Acetylcholine concentrations 0-0, 1-0.5 ppm, 2-1 ppm, 3-2 ppm, 4-5 ppm.

In this communication our attempts to exploit the electrolysis at ITIES for analytical purposes are described. As examples the determination of acetylcholine, tetraethylammonium, calcium, barium and strontium cations by differential pulse stripping voltammetry (DPSV) at the hanging electrolyte drop electrode (HEDE) will be presented. 

Currently a four-electrode system with automatic ohmic drop compensation[5] has been used for accurate polarization measurements at ITIES. However, this rather complicated experimental set-up and the large area of the water/nitrobenzene interface (about 100 mm )[5] have not permitted the use of the fast pulse technique Therefore, we have developed a simpler three-electrode system with the hanging electrolyte drop electrode.[1]... 

Fig. 1. Assembly for hanging electrolyte drop electrode. CE 1 and CE 2 are the counter electrodes, RE 1 is the reference electrode. Pot. is the conventional three-electrode potentiostat with the IR drop compensation, pC is the microcomputer.

Irreversible potentials obtained with a hanging mercury drop working electrode a Ag/AgI/Me2SO reference electrode, and 0.1 M Et4N BF4 in Me2SO as a supporting electrolyte at a scan rate of 25 V/s. 

All ECi adsorption coupled mechanisms have been verified by experiments with azobenzene/hydrazobenzene redox couple at a hanging mercury drop electrode [86,128,130]. As mentioned in Sect. 2.5.3, azobenzene undergoes a two-electron and two-proton chemically reversible reduction to hydrazobenzene (reaction 2.202). In an acidic medium, hydrazobenzene rearranges to electrochemically inactive benzidine, through a chemically irreversible follow-up chemical reaction (reaction 2.203). The rate of benzidine rearrangement is controlled by the proton concentration in the electrolyte solution. Both azobenzene and hydrazobenzene, and probably benzidine, adsorb strongly on the mercury electrode surface. 

The solution iR drop at the DME will also be time-dependent because rt, the drop radius, is a function of time. For this reason a stationary hanging-mercury-drop electrode (HMDE) is to be preferred or the vertical orifice (Smo-ler) DME can be used (see Figure 5.14). The tip of a platinium-wire quasireference electrode can be placed as close as 0.1 drop diameter (about 0.003 cm) because the drop grows in the downward direction.7 This gives nearly complete compensation in an electrolyte with a specific resistance of 15,000 Q-cm for a cell with total resistance of about 105 12. The effect of the polargrams of placing the quasi-reference electrode at different distances from the electrode surface is shown in Figure 6.3. 

For work with the hanging-mercury-drop electrode (HMDE), a cell has been described63 for use with volumes as small as 0.3 mL. An interesting cell for the microcombustion of small samples has been described.64 After combustion, electrolyte is added and the solution volume is measured by a calibrated sidearm, then displaced by mercury into a compartment that contains a DME. 

Adsorption stripping methods are quite similar to the anodic and cathodic stripping methods we have just considered. Here, a small electrode, most commonly a hanging mercury drop electrode, is immersed in a stirred solution of the analyte for several minutes. Deposition of the analyte then occurs by physical adsorption on the electrode surface rather than by electrolytic deposition. After sufficient analyte has accumulated, the stirring is discontinued, and the deposited material is determined by linear-scan or pulsed voltammetric measurements. Quantitative information is based on calibration with standard solutions that are treated in the same way as samples. 

Figure 1. Potentiostatic (pulse) current-time curves at hanging Hg-drop electrode. Solid line = 10 M Cd in 1 M KNOj dotted line = charging current from 1 M KNOj electrolyte.

Pertcchnetatc and Tc(lV) could be more sensitively analyzed in acidic media in the presence of thiocyanate by adsorption stripping voltammetry at the hanging mercury drop electrode using the differential pulse mode. Determinations down to 5-fO " g Te per ml were feasible. An intense eurrent signal at -1.32 V vs SCE was observed if only technetium and thiocyanate were present in the solution. Larger quantities of salts, e.g. chlorides and sulphates, decreased the sensitivity of the method considerably. This, however, could easily be avoided if, after electrodeposition was completed, the primary electrolyte was replaced by a pure solution of dilute acid for the stripping voltammetric step [100]. 

By electrolytic deposition of Hg at pH <4 on an inert electrode, a layer of metallic mercury is generated, serving as a new electrode surface for subsequent use. Compared with the hanging mercury drop electrode, in the absence of complexants and surfactants n and Pb are resolved, but Cd coincides (Roux et al., 1975). 

Mercury offers an ideal, atomically flat liquid metal surface with no preparation needed and with high affinity for thiol adsorption. SAMs on mercury surfaces are notable for the absence of defects, compact structure, and high reproducibility. One possibility is to use a homemade hanging mercury drop electrode (HMDE), which can be immersed in a thiol solution or passed through a layer of thiols floating on the electrolyte [168-171]. A static mercury... 

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