Metals, accumulation potentials

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

As mentioned previously, the role of the metal is not specific. Even transition metal free layered double hydroxides are suitable catalysts for the olefin epoxidation with O2 and a sacrificial aldehyde (205). Leaching of the metal from the solid catalyst is a serious problem since organic acids that are potential metal ligands accumulate during the reaction. Leaching occurs, for example, with the polybenzimidazole-supported Ni2+ catalyst (199). 

This books discusses the effects of trace metals on soli quality and potential threats on the transfer of these contaminants to consumers. Topics Include background concentrations In soils and plants metal cycling and transfer to the food chain uptake and accumulation of metals by bacteria, fungi, and Invertebrates mechanistic aspects of metals soli selenium losses and manganese sorption. 

Table II.7.3 Accumulation potentials and stripping peak potentials of some metals at graphite electrodes [2]...

In anodic stripping voltammetry of amalgams and metal deposits, there is no theoretical limit of detection of metal ions. If the accumulation potential is on the plateau of the pseudopolarogram and the solution is stirred, a steady state is established and the concentration of metal ions is linearly proportional to the duration of the accumulation ... 

The separation of the tested adsorptive species from electroactive non-adsorptive compounds in biological samples can be realized by medium exchange (e.g., by the flow injection method [171] or by electrode transfer [157, 158]. The scheme of this procedure is illustrated in Fig. 67. The extent of the interference depends on relative adsorbabil-ities of all species, on their bulk concentration, the choice of the accumulation potential and pH values of the solution. The electrocata-lytic activity of adsorbed substances is favorable for lowering the detection limit. For example, metal complexes of the Pt group extremely enhance the reduction of hydrogen ions in acidic and neutral aqueous media. Thus, the detection limit of these metals is decreased down to 10- " M [172, 173]. 

In the negative potential region (O.OV to -1.8V SCE) passivation is often observed and the electrode surface has to be regenerated periodically or, preferably, continuously. Accumulation of metals (especially Cu, Ni, Fe) on the electrode surface causes a decrease of the overvoltage of the hydrogen evolution and a gradual increase in... 

The function I is called the pseudopolarogram [25,26]. It is constructed by plotting the peak current in anodic stripping voltammetry as a function of the accumulation potential, because the peak current is linearly proportional to the concentration of metal atoms in the mercury electrode [27]. The half-wave potential of a pseudopolarogram depends on the mercury film thickness, the electrode rotation rate and the duration of accumulation. It can be used for the estimation of the optimal accumulation potential [28]. Besides, if metal ions form labile complexes with ligands in the solution, the half-wave potential of a pseudopolarogram depends on the ligand concentration, so that the stability constant of the complexes can be determined from this dependence [29,30]. If... 

Constructed wetlands. Peat bogs and wetlands are known accumulators of metals (Kalin et al. 1991 Kleinmann Pacelli 1991). Constructed wetlands, employ similar techniques to conventional treatment for the removal of metals and buffering of acidic drainage. Treatment of coal and metal mine drainage may involve aerobic or anoxic conditions. An important requirement of a wetland system is its ability to withstand both current and future metal loadings. Of major concern is the gradual infilling of the wetland basin and the potential for remobilization of contaminants. 

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