Mercury film glassy carbon electrodes

The electrochemical behaviour of trace concentrations of triphenylstannyl acetate, using a mercury film-glassy carbon electrode, was investigated by various measuring methods cyclic voltametry, differential pulse voltametry and controlled-potential electrolysis. Determination by DPASV (differential pulse ASV) of water and fish samples has LOD 2.5 nM98. 

Many applications of modified electrodes with [Ni (cyclam)] + or derivatives have been reported in the past years either on mercury or glassy carbon electrodes Modifications include cubic phases, Langmuir-blodget films, ... 

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. 

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. 

Using a glassy carbon electrode modified with a mercury film, Weber et al. [66] measured the association and dissociation rate constants for the complex formed between Pb + and the 18-crown-6 ether. It was found that Pb + forms a complex with 18-crown-6 with a stoichiometiy of 1 1 in both nitrate and perchlorate media. The formation constant, for the nitrate and perchlorate systems are (3.82 0.89) X 10 and (5.92 1.97) x lO mol Ls , respectively. The dissociation rate constants, are (2.83 0.66) x 10 with nitrate and (2.64 0.88) x 10 s with perchlorate as counter ion. In addition, the binding of Pb + with benzo-18-crown-6 embedded in a polymerized ciystalline colloidal array hydrogel has been also analyzed [67]. 

Film Electrodes and Related Hg Electrodes Mercury films were prepared on reticulated vitreous carbon flowthrough electrodes by Hg deposition from Hg + solutions in acetic buffer [31]. Such an electrode was designed for the purposes of trace metal analysis. Mercury film de-position/oxidation on reticulated vitreous carbon and glassy carbon electrodes were compared. 

Fig. 6.3. Stripping voltammograms for 50ppb Zn, Cd and Pb at glassy-carbon electrodes coated with bismuth (a) and mercury (b) films. Two-min deposition at -1.4 V. (Based on Ref. [12], with permission obtained from The American Chemical Society).

The potential danger associated with mercury has led to the development of other strategies that avoid the use of a mercury solution. These strategies use glassy carbon electrodes (GCE) coated with a mercury film modified with Nafion [3,4], cellulose acetate [5], naphthol derivative [6], etc., where mercury is generated in situ and this way avoiding the manipulation of mercury solutions as done previously. Composite electrode containing HgO as a built-in mercury precursor, which supply mercury-film formation, has even been reported to avoid the use of mercury solution [7]. 

Braun and Metzger [52] showed that trace amounts of nickel obtained from natural environmental samples could be determined voltammetrically as nickel dimethyl glyoximate following adsorptive enrichment onto a rotating glassy carbon electrode, on which a thin mercury film has been deposited electrolytically. See Sect. 7.34.1. 

Lead and mercury are deposited as micron-sized clusters, predominantly at intercrystallite boundaries [105] so does lithium from the polyethylene oxide solid electrolyte. What is more, Li intercalates into the sp2-carbon [22, 138], Thus, observations on the Li intercalation and deintercalation enable one to detect non-diamond carbon on the diamond film surface. Copper is difficult to plate on diamond [139], There is indirect evidence that Cu electrodeposition, whose early stages proceed as underpotential deposition, also involves the intercrystallite boundaries [140], We note that diamond electrodes seem to be an appropriate tool for use in the well-known electroanalytical method of detection of traces of metal ions in solutions by their cathodic accumulation followed by anodic stripping. The same holds for anodic deposition, e.g. of, Pb as PbCh with subsequent cathodic reduction [141, 142], Figure 30 shows the voltammograms of anodic dissolution of Cd and Pb cathodically predeposited from their salt mixtures on diamond and glassy carbon electrodes. We see that the dissolution peaks are clearly resolved. The detection limit for Zn, Cd, and Pb is as low as a few ppb [143]. 

Bismuth is one of the elements most easily determined with anodic stripping voltammetry (Florence 1972) as it can be deposited onto electrodes at potentials at which most other elements are in solution. Glassy carbon electrodes coated with films of mercury, but also of gold, have been used (Florence 1974). In biological materials with low concentrations of Bi, interference from other metals is not to be expected because of the low levels of most metals in these materials. Best results are reached with solutions containing 0.1 M hydrochloric acid, with detection limits in the range of 1 (IfQand 1993). 

Another novel system is presented in Figure 5.9 . This is a crystalline cubic phase formed with monoolein and Myverol . This structure was used to host [Ni (cyclam)] + or the derivative l-hexadecyl-1,4,8,111 tetraaza-cyclotetradecane. After obtaining a mixture of the lipids and the catalyst, the mixture was spread over a glassy carbon electrode or a thin mercury silver film. The best catalyst for the reaction was the substituted complex because the hydrophilic [Ni(cyclam)] + is easily removed from the cubic phase. The voltam-metric results indicate that the best response is obtained on the thin mercury film with formation of CO and regeneration of the catalyst. A further reaction of [Ni(cyclam)] + in the presence of CO generates [Ni(cyclam)CO]. 

A variety of voltammetric methods can be used for the stripping step. Usually, three of them are applied LSV, SWV and DPV. Responses of these techniques are compared in Fig. 65. Mostly mercury electrodes with stationary surface are used (static MDE). The surface of these electrodes is well reproducible (1-5%) and the background current is low. Mercury film and glassy carbon electrodes usually exhibit high background current (5-15% near the detection limiting concentration). 

Cathodic stripping voltammetry methods have been used in the determination of the OP insecticides Fenitrothion, Malathion " and Methyl Parathion. The influence of selected surfactants on the voltammetric determination of Fenitrothion and Malathion has been studied in detail, " and a new method described for the cathodic electrochemical determination of Methyl Parathion and other OPs using a bismuth film-modified glassy carbon electrode. The electrode behaviour was similar and favourable compared to that of mercury and a bare electrode, and its voltammetric response proportional to the concentration of Methyl Parathion in the 3.0 to lOOng/mL range with a detection limit of 1.2ng/mL. 

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