Techniques 3 Amalgamation

A quite different technique for the determination of mercury by ASV was described by Scholz et al. (1987). They liberated mercury from sample solutions by a CV technique, amalgamated it on a gold-plated electrode, and determined the amount of mercury by ASV. A detection limit of 30 ng was obtained. 

Mercury layers plated onto the surface of analytical electrodes serve as Hquid metal coatings. These function as analytical sensors (qv) because sodium and other metals can be electroplated into the amalgam, then deplated and measured (see Electro analytical techniques). This is one of the few ways that sodium, potassium, calcium, and other active metals can be electroplated from aqueous solution. In one modification of this technique, a Hquid sample can be purified of trace metals by extended electrolysis in the presence of a mercury coating (35). 

The phases and their proportions present ia hardened amalgam are controlled by many factors. The composition of the alloy the size, shape, and size distribution of the particles the thermal history of the cast ingot and the comminuted alloy and the surface treatment of the particles are some of the factors for which the manufacturer is responsible. The tooth cavity preparation and the mixing, compacting, and finishing techniques of the dentist can make the difference between satisfactory and unsatisfactory restorations, even with the best of alloys. A minimal amount of residual mercury and porosity are needed to obtain the most serviceable restorations (138). 

The very low Hg concentration levels in ice core of remote glaciers require an ultra-sensitive analytical technique as well as a contamination-free sample preparation methodology. The potential of two analytical techniques for Hg determination - cold vapour inductively coupled plasma mass spectrometry (CV ICP-SFMS) and atomic fluorescence spectrometry (AFS) with gold amalgamation was studied. 

Fig. 3. Evans-diagram for the cementation of Cu2+ and Pb2 with zinc amalgam of different zinc content. If the zinc concentration in the mercury employed for this special extraction technique is low, the anodic zinc-dissolution current density may be diffusion controlled and below the limiting cathodic current density for the copper reduction. The resulting mixed potential will lie near the halfwave potential for the reaction Cu2+ + 2e j Cu°(Hg) and only Cu2 ions are cemented into the mercury.

The dropping electrode, in partieular used with mercury and with amalgams, has been frequently employed in kinetic studies [66But3]. Data obtained with this electrode/method are labelled DME, if no specific technique is applied that is more helpful in qualifying the obtained data. 

The reaction of Na with Hg to form an amalgam that can then be separated from the NajO for oxygen analysis has been compared with the vacuum distillation technique . Ion-exchange techniques in which the sample is dissolved in a suitable solvent and the resulting separation of elements is achieved by an ion-exchange resin is less common. This technique is particularly suited to separating the volatile impurities such as K, Rb and Co. ... 

Radioactive tracer techniques. In electrochemistry, the procedure is essentially the same as in studies of chemical reactions the electroactive substance or medium (solvent, electrolyte) is labelled, the product of the electrode reaction is isolated and its activity is determined, indicating which part of the electroactive substance was incorporated into a given product or which other component of the electrolysed system participated in product formation. Measurement of the exchange current at an amalgam electrode by means of a labelled metal in the amalgam (see page 262) is based on a similar principle. 

In the decomposer, deionized water reacts with the amalgam, which becomes the anode to a short-circuited cathode. The caustic soda produced is stored or evaporated, if higher concentration is required. The hydrogen gas is cooled by refrigeration to remove water vapor and traces of mercury. Some of these techniques are employed in different facilities to maximize the production of chlorine, minimize the consumption of NaCl, and also to prevent the buildup of impurities such as sulfate in the brine.26 The production of pure chlorine gas and pure 50% sodium hydroxide with no need for further concentration of the dilute solution is the advantage that the mercury cell possesses over other cells. However, the cell consumes more energy and requires a very pure brine solution with least metal contaminants and above all requires more concern about mercury releases into the environment.4... 

Interaction to form lithium amalgam is violently exothermic and may be explosive if large pieces of lithium are used [1]. An improved technique, using p-cymene as inerting diluent, is described in the later reference [2],... 

Watling [491] has described an analytical technique for the accurate determination of mercury at picogram per litre levels in fresh and seawater. Mercury, released by tin (II) chloride reduction of water samples is amalgamated onto silver wool contained in quartz amalgamation tubes. The wool is then heated and the mercury thus released is flushed by argon into a plasma where it is excited. The emission signal thus produced results in a detection limit of 3 x 10 17 g and an analytical range 1 x 10 14 g-1 x 10"7 g. 

This particular aspect of analytical chemistry is the outcome of the unique amalgamation of the principles and techniques of analytical chemistry and biochemistry and was initially termed as clinical chemistry> but is more recently and more descriptively known as biomedical analytical chemistry . 

Large scale enrichment of lithium for thermonuclear uses took place at the Oak Ridge National Laboratory in the 1950 s. The enrichment primarily employed ion exchange between aqueous/organic solutions and amalgam, commonly mercury-based (Palko et al. 1976). Electrochemical separation has also been employed for such operations (Umeda et al. 2001). These practices have not been taken up in academic laboratories in the intervening years, partly as they tend to be most effective only with relatively pure starting materials, partly because of the difference in scales involved. Enrichment factors of Li of 1-7% are typical for these techniques (Symons 1985). 

There are many techniques used for the analysis of mercury in natural gas the most commonly used approach is the Jerome Analyzer (Arizona Instrument. Phoenix. Arizona. USA). This collects mercury onto a gold adsorber over a period of time by amalgamation. The 431 -X Mercury Vapour Analyzer is shown in Fig. 3.9. 

To extend the levels of detection for mercury stiU lower, several workers, especially in this area of atomic absorption techniques, have chosen to collect the mercury on gold or other noble metal trapping systems prior to revaporizing the mercury into the measurement technique. Figure 7.14 shows the configuration of a specific system to concentrate mercury onto an amalgam preconcentrator prior to analysis. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

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