Mercury vapour apparatus

Figure 7.2. Schematic diagram of the static mercury vapour apparatus. A Absorption cell B metal support C PTFE tubing D reduction vessel E silicone rubber F magnetic bar G magnetic stirrer H PTFE tubing Iq incident beam intensity I transmitted beam intensity and J exhaust. From [34]...

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

A diagram of a suitable apparatus is shown in Fig. 21.7. The mercury vapour is flushed out of the reaction vessel by bubbling argon through the solution, into the absorption tube. 

The Apparatus and General Methods section has been increased by the addition of an outline of the mercury vapour lamp and its uses in polari-metry and in catalytic reactions. A laboratory-scale apparatus, devised by us, for the distillation of solids is included. The section on the use of the library has been extended, and attention has been drawn to the changes which have taken place in the German reference literature. 

If mercury ions in solution are reduced to the free element, and a current of air or inert gas is passed through the solution, mercury vapour, which is monatomic, will be swept out of the solution into the gas phase. This provides a very sensitive basis for the determination of this toxic element.1 The apparatus required is illustrated in Figure 1. The flame is replaced by a glass tube atom cell with silica end windows in atomic absorption. Usually, for convenience, the atom cell is clamped to the top of a conventional AAS burner head. If atomic fluorescence is... 

In the same manner, temperatures exceeding the ignition temperatures (see Table 1.3 in Section 1.2.2) or the maximum surface temperatures according to the temperature class of the apparatus (see Table 4.1 in Chapter 4) are permissible inside of d. This covers windings and rotors in motors, especially for low temperature classes T4, T5 and T6 as well as the discharge tubes of high pressure sodium or mercury vapour lamps in case of a broken lamp bulb, or the ovens of gas chromatographs. 

The most common technique for the determination of mercury in environmental samples is cold vapour atomic absorption spectrometry (CV-AAS) due to its simplicity and sensitivity. The flameless procedure was investigated by Hatch and Ott (1968) with a view to simplifying the apparatus required and improving the sensitivity of the method. The method is based on the unique properties of mercury. Elemental mercury has an appreciable vapour pressure at ambient temperature and the vapour is stable and monatomic. Mercury can easily be reduced to metal from its compounds. The mercury vapour may be introduced into a stream of an inert gas and measured by atomic absorption or atomic fluorescence of the cold vapour without the need of atomiser devices. 

Clinton, O.E. (1974) Static vapour apparatus for the determination of mercury by flameless atomic absorption. Lab. Pract., 1974, 705-706. 

Factors Affecting the Accuracy of the Measurement.—Mercury Vapour. Often mercury is used as a pressure transferring medium in apparatus used for determining critical properties. If a sample is confined over mercury at temperatures for which the vapour pressure is appreciable the measured pressure may be considerably different from the pressure which would be measured in the absence of mercury. Relatively little work has been undertaken on the effect of mercury having a catalytic effect on slow reactions which take place in the sample. Secondly, the partial pressure of mercury may differ considerably from the vapour pressure of mercury at the same temperature. The usual procedure for correcting for the mercury-vapour effect is to subtract the vapour pressure of mercury from the experimental measured pressure. Jepson and Rowlinson have questioned this procedure and Kay and co-workers have made measure-... 

The principles of atomic absorption have been known since the early nineteenth century and the technique has been used by astronomers for the approximately quantitative analysis of stellar atmospheres for many years. However, apart from the determination of mercury vapour, it was not used by analytical chemists until 1955 when Walsh realised its potentialities and devised a simple apparatus which could be used for routine analysis. Since that time other equipment has been built and a commercial attachment is available for use with a spectrophotometer. 

In a similar procedure [32] the sediment is wet oxidised with dilute sulphuric acid and nitric acids in an apparatus in which the vapour from the digestion is condensed into a reservoir from which it can be collected or returned to the digestion flask as required. The combined oxidised residue and condensate are diluted until the acid concentration is IN and nitrate is removed by addition of hydroxylammonium chloride with boiling. Fat is removed from the cooled solution with carbon tetrachlodithizone in carbon tetrachloride. The extract is shaken with 0.1M hydrochloric acid and sodium nitrite solution and, after treatment of the separated aqueous layer with hydroxylammonium chloride a solution of urea and then EDTA solution are added to prevent subsequent extraction of copper. The liquid is then extracted with a 0.01% solution of dithizone in carbon tetrachloride and mercury estimated in the extract spectrophotometrically at 485nm. 

Figure 1 Apparatus for cold vapour mercury determination by AAS...

---