Mercury-film

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... 

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,... 

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. 

Of recent years the use of mercury film electrodes based on substrates other than platinum has been explored, and increased sensitivity is claimed for electrodes based on wax-impregnated graphite, on carbon paste and on vitreous carbon a technique of simultaneous deposition of mercury and of the metals to be determined has also been developed. 

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 ... 

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 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. 

Example 4-2 A rotating mercury film electrode (2 mm diameter) yielded a stripping peak current of 2.2 pA for a 1 x 10 8 M lead(II) solution following a 3 min deposition at —1.0 V with a 1600 rpm rotation. Calculate the peak current for a 2.5 x 10 8M lead(II) solution following a 2 min deposition with a 2500 rpm rotation. 

Membrane, 141, 178 Mercury electrodes, 62, 108 Mercury film electrode, 76, 110 Metals, 75, 81 Metal complexes, 64 Methyl viologen, 43 Michaelis-Menten kinetics, 175 Microbalance, 52, 53 Microcells, 102 Microchip, 194, 195... 

Fig. 6.5 Schematic representation of a bioelectronic protocol for detection of DNA hybridization (A) binding of the target to magnetic beads (B) hybridization with CdS-labeled probe (C) dissolution of CdS tag (D) potentiometric stripping detection at a mercury-film electrode. (Reprinted from [136], Copyright 2009, with permission from Elsevier)...

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). 

Many workers have tried to omit the mercury film by depositing the analyte directly on inert metals such as Pt, Au, Rh, Ir or Ag or on carbon materials such as glassy carbon or wax-impregnated graphite however, in general this was not successful (lack of selectivity for mixtures as a consequence of interdiffusion) and therefore it is rarely applied except for those nobler analyte metals that cannot be measured at mercury, such as Au, Ag and Hg itself. Nevertheless, metals such as Ni and Cr, which do not amalgamate, can be determined on an HMDE100. 

The above conclusions clearly indicate that the use of very thin mercury films gives maximal sensitivity as it enables fast sweep rates to be combined with excellent peak resolution. 

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

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. 

Garcia-Monco Carra et al. [405] have discussed the use of a hybrid mercury film electrode for the voltammetric analysis of lead (and copper) in acidified seawater. 

Hua et al. [595] have described an automated flow system for the constant-current reduction of uranium (VI) onto a mercury film-coated fibre electrode. Interference from iron (III) was eliminated by addition of sulfite. The results obtained for uranium (VI) in two reference seawater samples, NASS-1 and CASS-1, were 2.90 and 2.68 g/1, with standard deviations of 0.57 and 0.75 g/1, respectively. 

It was the development of the rotating glassy carbon electrode with a preplated or co-plated mercury film that gave this technique the sensitivity and resolution required for use in seawater. 

Batley [780] 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 AAS. 

Scarponi et al. [781] studied the influence of an unwashed membrane filter (Millpore type HA, 47 mm diameter) on the cadmium, lead, and copper concentrations of filtered seawater. Direct simultaneous determination of the metals was achieved at natural pH by linear-sweep anodic stripping voltammetry at a mercury film electrode. These workers recommended that at least 1 litre of seawater be passed through uncleaned filters before aliquots for analysis are taken the same filter can be reused several times, and only the first 50-100 ml of filtrate need be discarded. Samples could be stored in polyethylene containers at 4 °C for three months without contamination, but losses of lead and copper occurred after five months of storage. 

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. 

At lower ultratrace levels, the less voluminous mercury film electrode has to be used. It consists of a mercury film of only several hundred nm thickness on a glassy carbon electrode as support. The fabrication of this glassy carbon electrode is critical for obtaining an optimal mercury film electrode suitable to perform determinations down I ng/1 or below. 

Mart et al. [793] and Valenta et al. [794] have described two differential pulse ASV methods for the determination of cadmium, lead, and copper in arctic seawater. After a previous plating of the trace metals into a mercury film on a rotating electrode with a highly polished glassy carbon as substrate, they were stripped in the differential pulse mode. The plating was done in situ. 

Eskilsson et al. [868] have described equipment for automated determination of traces of cobalt and nickel by potentiometric stripping analysis, which used a freshly prepared mercury film on a glassy carbon support as the working... 

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. 

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