Mercury drop film electrodes

FIGURE 3-14 Stripping voltammograms for 2 x 10 7 M Cu2+, Pb2+, In3+ and Cd2+ at the mercury film (A) and hanging mercury drop (B) electrodes. (Reproduced with permission from reference 21.)... 

So far, five types of mercury electrode have been used in specular reflection measurements (i) hanging mercury drop electrode (HMDE) [38, 39], (ii) mercury film deposited on a platinum or gold substrate [40-42], (iii) mercury pool electrode with or without an amalgamated platinum ring guide [43, 44], (iv) mercury drop bottom electrode placed on the optical window [45—47], and (v) mercury drop bottom electrode placed on an underlying ion-conductive optically transparent polymer film [48]. To avoid the difficulty due to mechanical vibration and shape change, the mercury drop bottom electrode would be useful. 

The use of a mercury drop bottom electrode placed on an underlying Nation fihn in ER measurement was demonstrated for the reaction of heptyl viologen incorporated in a Nafion film (Fig. 2.13) [48], A mercury drop was placed on a Nation 117 tihn of thickness 0.175 mm. The transparent nature of the Nafion as weU as the high conductivity for cations through its film made it actually possible to measure the ER spectrum of the redox reaction of heptyl viologen with a perpendicular incidence of the hght to the mercury electrode surface through the cell bottom window and the Nafion fihn. 

We have developed nanoparticle label-based electrochemical immunoassay of OP exposure biomarkers, OP-ChE (44). The principle and procedure of this method is shown in Figure 5. Here, zircomia (Zr02) NPs, electrochemically deposited on the electrode surface, which functioned as the capture antibodies in the sandwich immunoassay. The zirconia- NP- modified electrode is then exposed to the sample solution and the ChE-OP in the sample is captured by the Zr02 nanoparticles. The QD-tagged, anti-AChE conjugate is introduced to form the sandwich-like complex on the sensor surface. The captured QDs are then dissolved by a drop of acid to release cadmium ions. This is followed by square wave voltammetric (SWV) [abbreviation already introduced but unexplained] detection of the released cadmium ions at an in situ plated mercury/bismuth film electrode. 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Anodic stripping voltammetry consists of two steps (Figure 11.37). The first is a controlled potential electrolysis in which the working electrode, usually a hanging mercury drop or mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the electrode. For example, with Cu + the deposition reaction is... 

Voltaic cells 64. 504 Voltammetry 7, 591 anodic stripping, 621 concentration step, 621 mercury drop electrode, 623 mercury film electrode, 623 peak breadth, 622 peak current, 622 peak potential, 622 purity of reagents, 624 voltammogram, 622 D. of lead in tap water, 625 Volume distribution coefficient 196 Volume of 1 g of water at various temperatures, (T) 87... 

DME = dropping mercury electrode HMDE = hanging mercury drop electrode MFE = mercury film... 

Anodic shipping voltammetry (ASV) is the most widely used form of stripping analysis, hi this case, the metals are preconcenhated by elechodeposition into a small-volume mercury electrode (a tiiin mercury film or a hanging mercury drop). The preconcenhation is done by catiiodic deposition at a controlled tune and potential. The deposition potential is usually 0.3-0.5 V more negative than E° for the least easily reduced metal ion to be determined. The metal ions reach die mercury electrode by diffusion and convection, where diey are reduced and concentrated as amalgams ... 

The mercury film electrode has a higher surface-to-volume ratio than the hanging mercury drop electrode and consequently offers a more efficient preconcentration and higher sensitivity (equations 3-22 through 3-25). hi addition, the total exhaustion of thin mercury films results in sharper peaks and hence unproved peak resolution in multicomponent analysis (Figure 3-14). 

There are several types of mercury electrodes. Of these, the dropping mercury electrode (DME), the hanging mercury drop electrode (HMDE), and mercury film electrode (MFE) are the most frequently used. 

The above considerations also apply to the ion of an amalgamating metal with the reversible equilibrium M"+ + ne M(Hg) at a stationary mercury electrode such as an HMDE (hanging mercury drop) or an MTFE (mercury thin-film) with the restriction, however, that the solution can contain only ox, so that merely the cathodic wave (cf., eqn. 3.15) represents a direct dependence of the analyte concentration, whilst the reverse anodic wave concerns only the clean-back of amalgam formed by the previous cathodic amplitude. When one or both of the electrodic reactions is or becomes (in the case of a rapid potential sweep) irreversible, the cathodic wave shifts to a more negative potential and the anodic wave to a more positive potential (cf., Fig. 3.10) this may even result in a complete separation of the cathodic and anodic waves (cf., Fig. 3.11). 

The electrodes usually consist of mercury or deposited mercury or occasionally of inert solid material further, they are mainly of a stationary type (in the stripping step as the crucial analytical measurement, but not in the concentration step, where often the solution is stirred or the electrode is rotated). Considering the mercury, only exceptionally has a sessile mercury drop electrode (SMDE)91 or a slowly growing DME(drop time 18 min and phase-selective recording of stripping curve)92 been applied. Most popular are the hanging mercury drop electrode (HMDE) and the mercury film or thin-film electrode (MFE or MTFE). 

Acebal et al. [401] discussed the quantitative behaviour of lead (and copper) when voltammetric determinations are done at mercury film electrodes and hanging mercury drop electrodes. The samples were collected in polyethylene bottles and, generally, were not acidified immediately after collection. This might place some doubt on the results reported. 

Batley [780] examined the techniques available for the in situ electrodeposition of lead and cadmium in seawater. These included anodic scanning voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen, and in situ electrodeposition on mercury-coated graphite tubes. 

At the glassy carbon electrode, using both in situ and preformed mercury films, similar results were obtained, but the sloping baseline interference observed at the hanging mercury drop electrode was less evident because of the higher stripping currents. 

Batley [28] examined the techniques available for the in situ electrodeposition of lead and cadmium in estuary water. These included anodic stripping voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen and in situ electrodeposition on mercury coated graphite tubes. Batley [28] found that in situ deposition of lead and cadmium on a mercury coated tube was the more versatile technique. The mercury film, deposited in the laboratory, is stable on the dried tubes which are used later for field electrodeposition. The deposited metals were then determined by electrothermal atomic absorption spectrometry, Hasle and Abdullah [29] used differential pulse anodic stripping voltammetry in speciation studies on dissolved copper, lead, and cadmium in coastal sea water. 

The conditions for reliable cyclic voltametry determination of trace Sn concentrations in sea water were investigated. All organotin compounds should be converted to Sn(II) by UV-photolysis adsorption on mercury drop in the presence of 40 pM of tropolone (1) cyclic voltametry stripping shows two cathodic peaks, corresponding to the two-step process Sn(IV) — Sn(II) -> Sn(0)29. A complex of Sn ions with catechol can be accumulated in a glassy carbon mercury film electrode, followed by stripping voltametry measurement in the cathodic direction, at pH 4.2-4.7. Interference occurs when Cu, Cd and Cr are present LOD 0.5 pg/L for 300 s accumulation30. 

Derived methods. A mercury-film electrode (MFE) is superior to an HMDE because stirring of the solution can be performed much more vigorously, thereby enhancing the efficiency of the mass transport of analyte to the electrode. Faster stirring is allowed because there is no longer the chance that the mercury drop will be displaced and thus lost by the solution s movement. The usual substrate employed for the mercury film is graphite. ... 

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

The following types of mercury electrodes have been widely used for voltammetry dropping mercury electrode (DME), hanging mercury drop electrode (HMDE), static mercury drop electrode (SMDE), streaming mercury electrode (SME), and mercury film electrode (MFE). We begin our discussion with a description of the construction and properties of the DME because this electrode has a long history and continues to be used for both analytical and fundamental studies. 

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. 

The working electrodes found to be useful at room temperature can also be used at low temperature. There are no special constraints. Platinum is probably the most widely used, simply because it is the most common electrode material for room-temperature work. A mercury electrode can also be employed, either as a hanging mercury drop electrode (HMDE) [23], a thin mercury film on a solid support, or an amalgam [26]. The HMDE was reported to extend the range of usable potentials to somewhat more negative values than found with platinum [23]. Of course, below -39°C, the HMDE is actually a solid electrode however, no detectable change in its voltammetric behavior is noted at the phase transition. 

Various voltammetric waveforms can be employed during the stripping step, including linear scan, differential pulse, square-wave, staircase, or alternating-current operations. The differential pulse and square-wave modes are usually performed at the hanging mercury drop electrode, while linear scan stripping is usually performed in connection with the mercury film electrode. 

Figure 24.8 Stripping voltammograms at the (A) mercury film and (B) hanging mercury drop electrodes for a solution containing 2 x 10 7 M Cd2+, In3+, Pb2+, and Cu2+. Deposition for (A) 5 and (B) 30 min. [From Ref. 47, reproduced with permission.]...

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