Mercury electrode cell

Discussion. The indicator electrode employed is a mercury-mercury(II)-EDTA complex electrode. A mercury electrode in contact with a solution containing metal ions M"+ (to be titrated) and a small added quantity of a mercury(II)-EDTA complex HgY2- (EDTA = Na2H2 Y) exhibits a potential corresponding to the half-cell ... 

In the classical version one uses a two-electrode cell with DME and a mercury AE (the pool) at the bottom of the cell (see Fig. 23.2). The latter, which has a large surface area, is practically not polarized. The current at the DME is low and causes no marked ohmic potential drop in the solution and no marked polarization of the AE. Hence, to change the DME potential, it will suffice to vary the external voltage applied to the cell. During the measurements, 7 vs. % rather than 7 vs. E curves are recorded. 

Udapa et al.16 showed that C02 was reduced to formic acid at a mercury electrode in a 0.05 M phosphate buffer (pH 6.8) solution. A current efficiency of 81.5% was obtained at a current density of 20 mA/cm2 and a cell voltage of 3.5 V. On the other hand, Bewick and Greener17 reported that malate and glycolate were produced at Hg and Pb electrodes, respectively, using aqueous quartenary... 

A hanging mercury drop electrodeposition technique has been used [297] for a carbon filament flameless atomic absorption spectrometric method for the determination of copper in seawater. In this method, copper is transferred to the mercury drop in a simple three-electrode cell (including a counterelectrode) by electrolysis for 30 min at -0.35 V versus the SCE. After electrolysis, the drop is rinsed and transferred directly to a prepositioned water-cooled carbon-filament atomiser, and the mercury is volatilised by heating the filament to 425 °C. Copper is then atomised and determined by atomic absorption. The detection limit is 0.2 pg copper per litre simulated seawater. 

Voltammetry is a part of the repertoire of dynamic electrochemical techniques for the study of redox (reduction-oxidation) reactions through current-voltage relationships. Experimentally, the current response (i, the signal) is obtained by the applied voltage (.E, the excitation) in a suitable electrochemical cell. Polarography is a special form of voltammetry where redox reactions are studied with a dropping mercury electrode (DME). Polarography was the first dynamic electrochemical technique developed by J. Heyrovsky in 1922. He was awarded the Nobel Prize in Chemistry for this discovery. 

A voltaic cell is composed of a copper electrode (Cu) dipping into a solution of copper ions (Cu+2) and a mercury electrode in a solution of Hg2+ ions. The cell reaction is... 

Figure 17.2 (a) and (b) illustrates the schematic diagram of amperometric titrations with the dropping mercury electrode having a titration-cell and an electric circuit respectively. 

The titration-cell Figure 17.2 (a) essentially comprises of apyrex 100-ml, four-necked, flat-bottomed flask. A semimicro burette (B) (graduated in 0.01 ml), a 2-way gas-inlet tube (A) to enable N2 to pass either through the solution or simply over its surface, a dropping mercury electrode (C) and an agar-potassium salt-bridge are duly fitted into the four necks with the help of air-tight rubber stoppers. 

The apparatus is assembled and electrical connections are duly completed with dropping mercury electrode (C) as cathode and saturated calomel half-cell as anode,... 

Figure 6.6 shows a schematic diagram of the apparatus required as a working electrode for polarography. Such a set-up is almost universally called a dropping mercury electrode (DME), with the mercury drop being immersed in a cell that is essentially the same as that shown in Figure 6.1. 

Figure 6.6 Schematic representation of a typical dropping-mercury electrode (DME) for polarography, where the DME acts as a working electrode in a cell such as that shown in Figure 6.1. The platinum electrode at the top right of the diagram is needed to give an electrical connection. The rate of mercury flow is altered by adjusting by changing the height h.

So far we have considered electrodes whose potentials are determined through the cell reaction of the ions with which they are in contact. Such a potential cannot be formed on an ideally polarized electrode, for example a mercury electrode in a KCl solution within a certain potential region. In this case the electrode potential is determined by the electrode charge. 

Ebel et al. have used a microliter vessel in the voltammetry and polarographic determination of small sample volumes of chlorpromazine [166]. The concentration of cells in glass or PTFE was described for use with a dropping-mercury electrode (sample volume 180 pL), a rotating disc electrode (sample volume 1 mL), or a stationary vitreous-carbon electrode (sample volume 80 pL). Chlorpromazine was determined using oxidative voltammetry at a 3 mm vitreous-carbon or a rotating electrode. 

In a polarographic experiment, a potential difference E is applied across the cell consisting of the dropping-mercury electrode and a nonpolarizable interface (e.g., a calomel electrode). In response to this potential difference, a current density i flows across the drop/solution interface. As each drop grows and falls, however, the surface area of the drop also grows, and then becomes effectively zero when the drop falls. Thus, the instantaneous current (current density times surface area) shows fluctuations, but the mean current is a unique function of the potential difference across the drop/solution interface, and therefore of that across the cell. 

---