Cell, amalgam with transference

Gill and Fitzgerald [481] determined picomolar quantities of mercury in seawater using stannous chloride reduction and two-stage amalgamation with gas-phase detection. The gas flow system used two gold-coated bead columns (the collection and the analytical columns) to transfer mercury into the gas cell of an atomic absorption spectrometer. By careful control and estimation of the blank, a detection limit of 0.21 pM was achieved using 21 of seawater. The accuracy and precision of this method were checked by comparison with aqueous laboratory and National Bureau of Standards (NBS) reference materials spiked into acidified natural water samples at picomolar levels. Further studies showed that at least 88% of mercury in open ocean and coastal seawater consisted of labile species which could be reduced by stannous chloride under acidic conditions. 

Figure 5.25 Hanging-mercury-drop electrode (a) electrode-cell assembly with scoop to transfer mercury drops from DME to the electrode (b, c) details of construction for amalgamated platinum electrode to which mercury drops are attached.

This chapter is concerned with the determination of activity coefficients with the aid of various types of concentration cells, and with the comparison of such activity coefficients with the predictions of the Debye-Hiickel theory, developed in the previous chapter. The types of cells discussed are (a) cells without transference, including those containing amalgam electrodes, (b) cells with transference, and (c) cells without transference containing mixtures of electrolytes. 

Hg CV-AFS state with stannous chloride or sodium tetrahydroborate the vapor generated is collected on an amalgamation surface/ Au or Pt. The concentrated mercury is revolatilized by rapid heating of the amalgamation surface and transferred to the absorption cell for measurement at 253.7 nm Hg is chemically reduced to the elemental and waste waters Applicable to 0.001-10 pg L-1 in tap, rain, 111... 

Amalgam Cells.—If the electrolyte in the concentration cell without transference is a salt of an alkali metal, e.g., potassium chloride, it is necessary to set up some form of reversible alkali metal electrode. This is achieved by dissolving the metal in mercury, thus forming a dilute alkali metal amalgam which is attacked much less vigorously by water than is the metal in the pure state." The amalgam nevertheless reactia with water to some extent, and also with traces of oxygen that may be... 

The use of a sodium amalgam electrode [Na(Hg)/NaC104(s)] in DMF has been reported [205] and a similar lithium amalgam electrode has been employed in DMSO [207]. The potential of the cell, Li(Hg)/Li Cr (DMSO), has been measured for LiCl concentrations from 0.01 to 1.0 M and the Nernst relation was verified within 1 mV the Li(Hg) electrode obeys the Tafel equation with the transfer coefficient a = 0.5 over... 

As with hydride generation, the evolved mercury vapor can either be transferred directly to the optical cell for measurement or trapped for later release. As the mercury is normally liberated slowly from solution over a period of 1-2 min, low intensity signals are generated consequently, vapor trapping is frequently used to improve concentration detection limits for solution analysis. Enhanced separation and concentration of mercury is usually achieved by amalgamation on a noble metal trap, from which it is subsequently thermally desorbed (at 500-700°C). 

As the cold-vapor mercury sample is already in the atomic state, there is no need of an atomizer, per se. The vapor, transferred directly from the cell or desorbed as a plug from a heated amalgamation trap, is commonly swept into a moderately heated (resistance wound heating to 200°C) 10 cm quartz T-tube located within the optical beam of a conventional AA spectrometer. Attenuation of an intense electrodeless discharge lamp line source at 253.7nm is used as a measure of the absorption. Alternatively, dedicated continuum source AA-based spectrometers fitted with long path absorption cells (30 cm) are frequently used to increase sensitivity and detection limit. 

An example of the simplest (in the sense of the number of kinetic parameters) electrochemical reaction is reduction of silver ions (Ag+) from a dilute aqueous solution of a well soluble silver salt (e.g., nitrate) in the presence of excess of an indifferent salt (e.g., potassium nitrate) on a liquid silver-mercury alloy (also called amalgam) electrode. Besides the transfer of a single electron, only diffusion steps are involved in this process. The entire reaction can be very well modeled and the kinetic parameters are determined experimentally with high level of accuracy. The information gleaned while analyzing the mechanism of silver ion reduction can be used in elucidating more complex, multi-step, multiphase processes, such as the electrochemical reaction in a lithium-ion cell. 

To 5 ml of solution in a test-tube containing from 0 02 to 0 10 mg of strychnine and 10 per cent w/w of hydrochloric acid, add 0-2 g of zinc amalgam (in 20 mesh and containing 40 per cent of mercury, recently treated by momentary immersion in a 5 per cent aqueous solution of mercuric chloride followed by a brief wash with water). Immerse the tube in a water-bath for seven minutes, cool under the tap and add 0 05 ml of a freshly prepared, approximately 0 1 per cent, aqueous solution of sodium nitrite. Transfer a portion of the red liquid to a 0 5-cm cell and determine the maximum extinction at about 525 rcifji. Prepare a calibration graph under identical conditions. 

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