Amalgamated metals

Quick-erz, n. mercury ore. -gold, n. gold amalgam. -metall, n, amalgamated metal. -mUhle, /. amalgamating mill, -wasser, n. Plating) quickening liquid (solution of a mercuric salt). 

Potentiometric stripping analysis (PSA), known also as stripping potenhometry, differs from ASV in the method used for stripping the amalgamated metals (22). hi this case, the potentiostatic control is disconnected following the preconcentration, and the concentrated metals are reoxidized by an oxidizing agent [such as O2 or Hg(II)] that is present in the solution ... 

The red produced by electrolysis diffuses either into the bulk solution (for an ion) or into the mercury (for an amalgamating metal), so that... 

The above considerations also apply to the ion of an amalgamating metal with the reversible equilibrium M"+ + ne M(Hg) at a stationary mercury electrode such as an HMDE (hanging mercury drop) or an MTFE (mercury thin-film) with the restriction, however, that the solution can contain only ox, so that merely the cathodic wave (cf., eqn. 3.15) represents a direct dependence of the analyte concentration, whilst the reverse anodic wave concerns only the clean-back of amalgam formed by the previous cathodic amplitude. When one or both of the electrodic reactions is or becomes (in the case of a rapid potential sweep) irreversible, the cathodic wave shifts to a more negative potential and the anodic wave to a more positive potential (cf., Fig. 3.10) this may even result in a complete separation of the cathodic and anodic waves (cf., Fig. 3.11). 

This equation applies to all reversible electrode reactions with soluble ox and red, so it includes cathodic chronopotentiometry of ions of amalgamating metals such as Cd2+, Cu2+, Pb2+ and Zn2+ at stationary Hg electrodes. For a redox couple such as Fe3+/Fe2+ the diffusion coefficients DTeA and Dox will not differ much, so that Etl4 is approximately equal to E° (770 V). 

A complicated situation arises when ox or red is insoluble, for instance in the reductive chronopotentiometry of an ion of a non-amalgamating metal then the metal precipitates on the stationary Hg electrode surface without further diffusion, so that... 

Considering the second condition, we realize that the deposition of the metal on a solid electrode is in a way comparable to the polarography of ions of non-amalgamating metals (cf., eqn. 3.38 and Fig. 3.16). Hence Ei alters with the analyte bulk concentration, which can be seen from... 

Table 4.6 yield a redox potential range of at least (0.78 4- 0.83) + (1.23 + 0.52) = 3.36 V. Another important effect, already published by Tafel (Ch. 3, ref. 7), is that an increase in the electrolytic current density often results in a further rise of the overpotentials (and therefore of the redox potential range) (cf., eqn. 3.38 and Fig. 3.16 for non-amalgamating metals in polarography). 

The zero-charge potential is determined by a number of methods (see Section 4.4). A general procedure is the determination of the differential capacity minimum which, at low electrolyte concentration, coincides with Epzc (Section 4.3.1). With liquid metals (Hg, Ga, amalgams, metals in melts) Epzc is directly found from the electrocapillary curve. 

The amalgamated metals are thus stripped out of the electrode in an order that is a function of each metal standard potential, and give rise to anodic peak currents that are measured. Figure 24.1 displays the potential-time sequence used in ASV, along with the resulting voltammogram. 

The rate of the reaction is highly dependent on the nature of the alcohol with more sterically demanding (and also less acidic) alcohols such as Bu OH reacting very slowly.1 For sodium and potassium the amalgamated metals can also be used.8-9... 

Following the preselected deposition period, the forced convection is stopped, and an anodic potential scan is initiated, during which the amalgamated metals are reoxidized and stripped away from the surface ... 

The stripping voltammogram, recorded during this measurement step, consists of multiple current peaks, corresponding to the reoxidation of the amalgamated metals, and their stripping out of the electrode. Such output provides the qualitative and quantitative information through measurements of the peak potential and current,... 

The fourth method finds limited application, although amalgamated metallic surfaces are readily prepared by dipping the metal into a solution of a mercuric salt. An illustration of the action of mercury on the solution of a salt of a more noble metal is to be found in the preparation of the silver mercuride, Ag3Hg4.10 The addition of drops of mercury to a solution of silver nitrate yields crystals of this intermetallic compound. 

Under strongly redncing conditions, several transition metals may also be mobilized - from decomposing biomass or beginning with certain enzymes snch as hydrogenases as precnrsors - as homoleptic, volatile carbonyl complexes (Mo, Ni, W, not Fe) this reaction (Feldmann 1999) thus extends beyond the small range of transition metals which directly form carbonyl complexes npon contact of the bnlk or dispersed (dust, amalgam) metal with CO (Elschenbroich and Salzer 1988). 

Most of the experimental techniques for thermal systems apply here. One important exception is that for experiments using Hg 2537A radiation, the vacuum system must be completely free from mercury, i.e. no mercury diffusion pumps or McLeod gauges should be used, unless mercury-sensitised reactions are being studied. Even the use of iodine, gold or similar amalgamating metals does not completely free a particular section of the vacuum system from mercury. There is an excellent treatise on photochemical reactions by Calvert and Pitts. ... 

Preparation. (a) Aluminum turnings (oil-free) are etched with dilute sodium hydroxide to a point of strong hydrogen evolution, the solution is decanted, and the metal is washed once superficially with water so that it retains some alkali. It is then treated with 0.5% mercuric chloride solution for 1-2 min., and the entire process is repeated. The shiny amalgamated metal is washed rapidly In turn with water,... 

Bachmann309 studied closely the reduction of ketones by sodium amalgam. Fluorenone, and benzophenone and its substitution products, were converted into the corresponding alcohols almost quantitatively by 2% sodium amalgam metal ketyl radicals were shown to be intermediates in these reactions. Relevant also are the published detailed descriptions for reduction of xanthone310 and thioxanthone311 by sodium amalgam. 

Anodic stripping voltammetry (ASV) is ideally suited to the determination of metal ions that are reducible to mercury-soluble metals (Heineman et al., 1984 Wang, 1985). The technique consists of two steps. First, a fraction of the sample metal ion is deposited as amalgamated metal by reduction at a mercury electrode such as a hanging mercury drop electrode (HMDE). This is accomplished by applying a potential that is sufficiently negative to reduce the sample metal ion for a carefully measured time. This step, often called the deposition step, serves to preconcentrate sample as metal in the small mercury drop by what is... 

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