Trace metals electrodeposition

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

C. Chemical Interactions in Solution. The rather dramatic effect of trace metal ions and additives on electrodeposition was reviewed briefly in the previous sections for copper and zinc. 

Stripping analysis is a two-step technique. The first or deposition step commonly involves the electrolytic deposition of a small portion of the metal ions in solution into the mercury electrode to preconcentrate the metals. Non-electrolytic (adsorptive) accumulation schemes have also been developed for expanding the scope of stripping analysis to trace metals that cannot be electrodeposited. The preconcentration step is followed by the stripping (measurement) step, which involves the dissolution (stripping) of the deposit. Different versions of stripping analysis can be employed, depending upon the nature of the deposition and measurement steps. 

Bismuth-film electrodes (BiFEs), consisting of a thin bismuth-film deposited on a suitable substrate, have been shown to offer comparable performance to MFEs in ASY heavy metals determination [17]. The remarkable stripping performance of BiFE can be due to the binary and multi-component fusing alloys formation of bismuth with metals like lead and cadmium [18]. Besides the attractive characteristics of BiFE, the low toxicity of bismuth makes it an alternative material to mercury in terms of trace-metal determination. Various substrates for bismuth-film formation are reported. Bismuth film was prepared by electrodeposition onto the micro disc by applying an in situ electroplating procedure [19]. Bismuth deposition onto gold [20], carbon paste [21], or glassy carbon [22-24] electrodes have been reported to display an... 

Oxide electrodes have been observed to be almost immune from poisoning effects due to traces of metallic impurities in solution [99]. This is undoubtedly due primarily to the extended surface area. It can be anticipated that the calcination temperature must have a sizable effect. But in addition, a different mechanism of electrodeposition must be operative. Chemisorption on wet oxides is usually weak because metal cations are covered by OH groups [479]. As a consequence, underpotential deposition of metals is not observed on Ru02, although metal electrodeposition does takes place. However, electrodeposited metals give rise to clusters or islands and not to a monomolecular layer like on Pt. Therefore, the oxide active surface remains largely uncovered even if metallic impurities are deposited [168]. Thus, the weak tendency of oxides to adsorb ions, and its dependence on the pH of the solution is linked to their favorable behavior observed as cathodes in the presence of metallic impurities. 

Electrodeposition is a unique concentration technique, because the separation of trace metals from interfering matrix species can be easily carried out at the same time. Thus, the detection limits of AAS can be improved remarkably without further chemical preconcentration steps and spectral and chemical interferences due to major components in seawater can be also eliminated easily. Thus, many applications of the technique to seawater analysis by AAS have been made, especially in flameless AAS [69—78]. The... 

Electrodeposition onto solid electrodes or mercury cathodes is a long established pre-treatment capable of large concentration factors, and provided the cathode potential is carefully controlled it is also of considerable selectivity. When atomic absorption is used as the finish, selective deposition is not usually required. There have been recent reports of electrodeposition of trace metals from water samples directly onto special graphite furnace tubes [8] and this technique should prove to be just as applicable to the analysis of reagents, where the chemical conditions can be more carefully controlled. The utility of electrodeposition for electrothermal atomisation... 

Turner, D.R. and Whitfield, M. The electrodeposition of trace metal ions from multi-ligand systems. I. Theory. 

Turner, D.R. and Whitfield, M., 1979. The electrodeposition of trace metal ions from multi-ligand systems. II. Calculations of the electrochemical availability of lead at trace levels in sea water. J. Electroanal. Chem., 103 61—79. 

Because of its remarkable sensitivity (down to 10-11 M) stripping analysis is the most widely used voltammetric technique for trace metal determination in clinical samples (Wang, 1982A). Stripping analysis can be considered as a two-step technique. In the first step, metals in solution are effectively preconcentrated onto the working electrode by electrodeposition ... 

Electrolysis of solutions can be used for electrodeposition of a trace metal on an electrode. The selectivity and efficiency which would be present for electrolytic deposition of macro amounts of ions at a controlled potratial is not present, however, for trace amounts. The activity of trace amounts of the species is an unknown quantity even if the concentration is known, since the activity coefficient is dependent upon the behavior of the mixed electrolyte system. Moreover, the concentration of the tracer in solution may not be known accurately since there is always the possibility of some loss through adsorption, complex formation with impurities, etc. Nevertheless, despite these uncertainties it has been found that the Nemst equation can be used, with some caution, for calculating the conditions necessary for electrolytic deposition of trace metals. 

The preconcentration of trace metals by electrodeposition is an integral part of anodic-stripping voltammetry. The method consists of the preelectrolysis of the stirred solution with a small mercury drop or solid electrode as the cathode (112-114). The metals, which are deposited and dissolve in the mercury, are then stripped from the amalgam after a suitable rest period by a reversal of the electrode potential. The resulting current-polarization curve is characteristic of the metal and its concentration. Concentrations as low as 10 M of metal ions require a preelectrolysis of about 60 min or longer. Other electrodes such as mercury films, platinum, gold, silver, and various forms of carbon have been used (77 ). 

The analysis for operation of the QCM in a liquid is somewhat more complex because of the viscous coupling. The shear mode vibration of an AT crystal is parallel to the surface and induces the adjacent liquid to move because of the no-slip boundary condition expected at the interface. Applications of the QCM to electrochemical systems have advanced rapidly from the first studies [72-74], which used the technique to investigate the electrodeposition of Au, and of trace metals from solutions. Indeed, the sensitivity of the QCM for liquid investigations has been calibrated by electrochemical methods [75-78] and found to be identical to that for gas phase studies. The general characteristics of the QCM in electrochemical... 

To check the efficacy of grease removal, the alkali solution is rinsed away or neutralised by dipping in dilute acid. If, after removal from the acid, the draining metal surface remains wetted evenly all over for 30-60 s (or until it dries by evaporation), hydrophobic soils have been removed. Traces of grease cause the surface to de-wet, and surface tension draws the water into separate droplets. This is the water-break test. Traces of grease which remain when the work is plated do not prevent electrodeposition, but are detrimental to adhesion and corrosion resistance. 

Chong et al. [742] have described a multielement analysis of multicomponent metallic electrode deposits, based on scanning electron microscopy with energy dispersive X-ray fluorescence detection, followed by dissolution and ICP-MS detection. Application of the method is described for determination of trace elements in seawater, including the above elements. These elements are simultaneously electrodeposited onto a niobium-wire working electrode at -1.40 V relative to an Ag/AgCl reference electrode, and subjected to energy dispersive X-ray fluorescence spectroscopy analysis. Internal standardisation... 

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]. 

Properties Steel-gray, shining, hard, ductile, somewhat malleable metal ferromagnetic, with permeability two-thirds that of iron has exceptional magnetic properties in alloys. D 8.9, mp 1493C, bp 3100C. Attacked by dilute hydrochloric and sulfuric acids, soluble in nitric acid. Corrodes readily in air. Hardness cast 124 Brinell, electrodeposited 300 Brinell. An important trace element in soils and necessary for animal nutrition. Cobalt has unusual coordinating properties, especially the trivalent ion. Noncombustible except as powder. 

Thorough cleaning and the avoidance of any extraneous debris are essential in any membrane fabrication scheme. After EP, a composite membrane can be soaked in hot water or dilute ammonia to help remove any impurities trapped in the porous support that may be detrimental to the palladium film during high temperature operation [72, 73]. However, traces of impurities from the EP bath such as chlorine, sodium, and carbon, inevitably become incorporated into the metal film. Membrane defects can be a consequence of preparation conditions. Fabrication in a dean-room environment has resulted in increased permselectivity [140]. Porous stainless steel (PSS) must be cleaned and pickled before electrodeposition or else activated for electroless plating. 

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