Mercury stripping

This can be seen in the cyclic curve in Fig. 3. In this voltammogram, relatively large anodic peak of mercury stripped from the paste electrode surface is observed. The presence of relatively stable Flg(I) at mesoporous silica carbon paste electrode may be due to adsorption. 

TIN OF THE PHILOSOPHERS, or WHITE LEAD OF THE PHILOSOPHERS - Their Mercury stripped of its blackness, but before it has arrived at the perfect White. 

The sodium amalgam cathode product of the electrolyzer of the mercury cell is the chief difference between this and the diaphragm cell. The production of sodium hydroxide from this stream uses a separate set of electrochemical reactions conducted in a decomposer or denuder. This unit is usually located below the electrolyzer so as to allow gravity feed of the sodium amalgam from the electrolyzer to the top end of the decomposer (Fig. 8.4). Deionized water is fed into the bottom of the decomposer to provide countercurrent flows of sodium amalgam and water. Mercury, stripped (or denuded) of sodium, is continuously drawn off the bottom of the decomposer, and a hot solution of 50-70% sodium hydroxide in water, plus hydrogen gas, from the top. 

Copper is removed by masking the mercury as strong bromide- or iodide complexes in acid media. Only copper dithizonate is extracted from 1 M HCl containing KI. Alternatively, the mercury and copper can be extracted from acid solution, and the mercury stripped with an acidic solution of KI or KBr. The mercury is re-extracted from the aqueous... 

Table II.7.4 Solubility and diffusion coefficients of some metals in mercury. Stripping peak potentials and standard rate constants of amalgam electrodes in 0.1 M HCl [1]...

Cathodic Stripping Voltammetry (CSV) can be used to detect organic and inorganic compounds. An anodic potential is applied during the pre-concentration step and the compound of interest forms an insoluble salt with mercury. Stripping is carried out with a potential sweep towards negative potentials. 

Even without the addition of gold, however, the analysis of trace mercury on BDD electrodes is feasible, however. For comparison with glassy carbon (GC), we have carried DPASV on both BDD and GC electrodes. Figure 16.10 shows clear evidence for the observation of the mercury stripping peak at the BDD electrode surface, whereas no peak was observed for the GC 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... 

Potential-excitation signal and voltammogram for anodic stripping voltammetry at a hanging mercury drop electrode. 

In adsorptive stripping voltammetry the deposition step occurs without electrolysis. Instead, the analyte adsorbs to the electrode s surface. During deposition the electrode is maintained at a potential that enhances adsorption. For example, adsorption of a neutral molecule on a Hg drop is enhanced if the electrode is held at -0.4 V versus the SCE, a potential at which the surface charge of mercury is approximately zero. When deposition is complete the potential is scanned in an anodic or cathodic direction depending on whether we wish to oxidize or reduce the analyte. Examples of compounds that have been analyzed by absorptive stripping voltammetry also are listed in Table 11.11. 

Anodic stripping voltammetry at a mercury film electrode can be used to determine whether an individual has recently fired a gun by looking for traces of antimony in residue collected from the individual s hands, fn a typical analysis a sample is collected with a cotton-tipped swab that had been wetted with 5% v/v HNO3. When returned to the lab, the swab is placed in a vial containing 5.00 mb of 4 M HCl that is 0.02 M in hydrazine sulfate. After allowing the swab to soak overnight,... 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

Stripping voltammetry procedure has been developed for determination of thallium(I) traces in aqueous medium on a mercury film electrode with application of thallium preconcentration by coprecipitation with manganese (IV) hydroxide. More than 90% of thallium present in water sample is uptaken by a deposit depending on conditions of prepai ation of precipitant. Direct determination of thallium was carried out by stripping voltammetry in AC mode with anodic polarization of potential in 0,06 M ascorbic acid in presence of 5T0 M of mercury(II) on PU-1 polarograph. 

Electrodes. The Hanging Mercury Drop Electrode is traditionally associated with the technique of stripping voltammetry and its capabilities were investigated by Kemula and Kublik.51 In view of the importance of drop size it is essential to be able to set up exactly reproducible drops, and this can be done as explained in Section 16.8 for the S.M.D.E. 

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... 

Essentially, stripping analysis is a two-step technique. The first, or deposition, step involves die electrolytic deposition of a small portion of the metal ions hi solution into die mercury electrode to preconcentrate the metals. This is followed by die shipping step (the measurement step), which involves die dissolution (shipping) of die deposit. Different versions of stripping analysis can be employed, depending upon die nature of the deposition and measurement steps. 

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 ... 

FIGURE 3-13 Concentration gradient of the metal in the mercury film electrode and nearby solution dining the stripping step. 

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.)... 

The major types of interferences in ASV procedures are overlapping stripping peaks caused by a similarity in the oxidation potentials (e.g., of the Pb, Tl, Cd, Sn or Bi, Cu, Sb groups), the presence of surface-active organic compounds that adsorb on tlie mercury electrode and inhibit the metal deposition, and the formation of intermetallic compounds (e.g., Cu-Zn) which affects the peak size and position. Knowledge of these interferences can allow prevention through adequate attention to key operations. 

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