Cold trap, mercury vapor

Table IV gives all of the values obtained for mercury in coal, which range from 0.057 to 0.198 ppm with most values in the range of 0.07 ppm. Our attempt at a mercury balance for runs 5 and 9 is shown in Table V. From these numbers it is clear that very little mercury ( — 12% ) remains with the slag and fly ash particles. The cold trap was not effective in trapping mercury vapor (.—11% ). The results are in qualitative agreement with those of Billings and Matson (5), except that these authors were able to collect the mercury in the gas phase. Their data shows that most of the mercury is in the gas phase. This can also be implied from our results.

For the pumping of large quantities of gas in this pressure region, vapor ejector pumps are by far the most suitable. With mercury vapor ejector pumps, completely oil-free vacua can be produced. As a precaution, the insertion of a cold trap chilled with liquid nitrogen is recommended so that the harmful mercury vapor does not enter the vessel. With the medium vacuum sorption traps described under a), it is possible with two-stage rotary vane pumps to produce almost oil-free vacua down to below 10" mbar. 

Diffusion pumps operate at very low pressures. The ultimate vacuum attainable depends somewhat upon the vapor pressure of the pump liquid at the temperature of the condensing surfaces. By providing a cold trap between the diffusion pump and the region being evacuated, pressures as low as 10" mmHg absolute are achieved in this manner. Liquids used for diffusion pumps are mercury and oils of low vapor pressure. Silicone oils have excellent characteristics for this... 

A vacuum system may consist of a diffusion pump and a backing pump, together with bailies, cold traps and isolation valves. The cold trap is essential if a mercury vapor diffusion pump is used and is best filled with liqnid nitrogen ( — 196°C) otherwise, the system will be exposed to the mercury vapor pressure, which is torr at 18CC. 

Mercury diffusion pumps may be used in the lab to produce a high vacuum. Cold traps are generally placed between the pump and the system to be evacuated. The traps cause the condensation of mercury vapor, which prevents diffusion back into the system. The maximum pressure of mercury that can exist in the system is the vapor pressure of mercury at the temperature of the cold trap. Calculate the number of... 

Cold traps must be used if mercury is used in your system (such as manometers, diffusion pumps, bubblers, or McLeod gauges) and if your mechanical pump has cast aluminum parts. Mercury will amalgamate with aluminum and destroy a pump. Even if your mechanical pump does not have aluminum parts, the mercury may form a reservoir in the bottom of the mechanical pump, which may cause a noticeable decrease in pumping speed and effectiveness. Aside from a cold trap between the McLeod gauge and the system, place a film of low vapor pressure oil in the McLeod gauge storage bulb. This oil will limit the amount of mercury vapor entering the system that makes its way to the mechanical pump. In addition, an oil layer should be placed on the mercury surface in bubblers and other mercury-filled components. 

A liquid trap can be placed between the McLeod gauge and the rest of the system to prevent mercury from accidentally spraying throughout your system. If you do not want condensable vapors affecting the McLeod gauge readings or do not want mercury vapors to enter your system, a cold trap can be placed between the liquid trap (shown in Fig. 7.41) and the main vacuum line. 

Solvent extraction, coprecipitation and ion-exchange techniques are the main concentration methods used for seawater analysis. Other interesting concentration techniques, such as electrodeposition, amalgam trap (for mercury), a cold trap-vaporization system for hydride generation, and recrystallization, are often used by marine and analytical chemists. The first three methods are briefly reviewed here. 

Two very different kinds of pump fluids have been employed in diffusion pumps. For many years, mercuiy diffusion pumps, were used in small laboratory-bench glass vacuum systems. Mercury pumps are now seldom used owing to the health hazards associated with mercury and the high probability of contamination of the vacuum system with mercury unless a cold trap is used (the vapor pressure of mercury at room temperature is —1.5 mTorr). The oil diffusion pump eliminates the safety hazard and can serve for both small glass and larger metal vacuum systems. 

This composite calibration curve for seawater demonstrates the applicability of the cold-trap pre-concentration technique to low concentration ranges of mercury. Approximately 0.2 ng of mercury can be determined with a 25x scale expansion. Since the response depends on the vaporization and elution of trapped mercury from the column, the calibration curves were similar for other aqueous media including acidified (nitric acid) distilled deionized water. Therefore, this cold-trap procedure appears to separate effectively reducible mercury species from interfering substances that might be associated with differing solution matrices. 

Probably the most commonly used instruments for cation impurity analysis of silicates are flame atomic absorption spectrophotometers and ion selective electrodes. In most cases, separation of silica is required to reduce interferences. The sample may also have to be diluted to bring the analyte concentration within the linear operating range. For cations, the atomic absorption spectrophotometer is more versatile than ion specific electrodes. If the analyst is concerned with the presence of heavy metals, then accessories such as a hydride system for the elements that form high vapor pressure compounds, e.g., Sb, and a mercury vapor cold trap are useful. If a large number of elements are to be determined, a substantial investment in hollow cathode and electrode discharge lamps must be made. Several gas mixtures will also be required. 

Figure 4.4.8. Isopiestic vapor-sorption apparatus with built-in manometer using a quartz spring 1 - connection to the vacuum, 2-9 -stop corks, 10, 11, 12 - connections to nitrogen, 13 - degassing flask for the pure solvent, 14, 18 - buffers, 15 - cold trap, 16,19 - Hg-ma-nometers, 17,20 - mercury float valves, 21 -pure solvent reservoir at temperature Ti provided by 22 - thermostat, 23 - temperature controlled air box, 24 - measuring cell, 25 - quartz spring (four quartz springs can be inserted into the equilibrium cell, only one is shown), 26 - pan with the polymer solution, 27 - closing plug sealed with epoxy resin, 28 - heating to avoid solvent condensation.

Figure 4.4.9. Schematic diagram of an isopiestic vapor sorption apparatus using an electronic microbalance PC - personal computer, MB - microbalance, WBl-3 - water bath thermostats with T3>T2>Ti, Vl-3 - valves, WM - W-tube mercury manometer, S - polymer sample/solution, SV - solvent reservoir, MS -magnetic stirrer, CT - cold trap, VP - vacuum pump. [Reprinted with permissitm from Ref. 92, Copyright 1998, American Chemical Society].

The most sensitive technique for solution analysis is cold vapor AAS or AFS using collection of the mercury vapor on a gold trap or directly in the graphite tube, treated with a permanent modifier. 

Note—A cold trap can be inserted before the pressure transducer in Option No. 2, if desired, or if the design of the transducer, such as a mercury McCieod gage, would require vapor protection. 

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