Mercury vapor pressure, high temperature

It is one of four metals — mercury, cesium, and rubidium — which can be liquid near room temperature and, thus, can be used in high-temperature thermometers. It has one of the longest liquid ranges of any metal and has a low vapor pressure even at high temperatures. 

Liquid Metals. If operating temperatures rise above 250—300°C, where many organic fluids decompose and water exerts high vapor pressure, hquid metals have found some use, eg, mercury for limited appHcation in turbines sodium, especially its low melting eutectic with 23 wt % potassium, as a hydrauhc fluid and coolant in nuclear reactors and potassium, mbidium, cesium, and gallium in some special uses. 

Wc have seen that molecular substances tend to have low melting points, while network, ionic, and metallic substances tend to have high melting points. Therefore, with a few exceptions, such as mercury, a substance that is liquid at room temperature is likely to he a molecular substance. Liquid solvents are heavily used in industry to extract substances from natural products and ro promote the synthesis of desired compounds. Because many of these solvents have high vapor pressures and so give off hazardous fumes, luinids that have low vapor pressures hut dissolve... 

Liquids with high vapor pressures at ordinary temperatures are said to be volatile. Methanol (vapor pressure 98 Torr at 20°C) is highly volatile mercury (1.4 mTorr) is not. Solids also exert a vapor pressure, but their vapor pressures are usually much lower than those of liquids because the molecules arc gripped more tightly in a solid than they are in a liquid. Nevertheless, solids vaporize in the process called sublimation (Section 6.11), which we can observe in the presence of some pungent solids—such as menthol and mothballs. 

A mixture of 2-iodotoluene (8.78 g, 0.04 mol) and trimethyl phosphite (24.8 g, 0.20 mol) was placed in a 45-ml, double-jacketed silica reaction vessel. The mixture was degassed by flushing with dry nitrogen for 5 min and irradiated with a 450-watt Hanovia (Model 679A-10) high-pressure quartz mercury vapor lamp fitted with an aluminum reflector head. The lamp was placed 5 cm from the inner portion of the reaction vessel. The reaction temperature was maintained at 0°C by the circulation of coolant from a thermostatically controlled refrigeration unit. Irradiation was continued at this temperature for 24 h. At the end of this time, the volatile materials were removed with a water aspirator, and the residue was vacuum distilled (96 to 97°C/0.25 torr) to give the dimethyl 2-methylphenylphosphonate (7.28 g, 91%). 

Mercury is a very suitabie pump fluid, it is a chemicai eiement that during vaporization neither decomposes nor becomes strongiy oxidized when air is admitted. However, at room temperature it has a comparativeiy high vapor pressure of 10 mbar. if iower uitimate totai pressures are to be reached, coid traps with iiquid nitrogen are needed. With their aid, uitimate totai pressures of 10 ° mbar can be obtained with mercury diffusion pumps. Because mercury is toxic, as aiready mentioned, and because it presents a hazard to the environment, it is nowadays hardiy ever used as a pump fluid. LEYBOLD suppiies pumps with mercury as the pump fluid oniy on request. The vapor pressure curves of pump fluids are given in Fig. 

When gases are collected over mercury, no correction is needed for the vapor pressure of Hg because it is so small (about 2 x io-3torr at room temperature). Collection over Hg has the further advantage that gases are insoluble in it. Disadvantages are its high cost and the toxicity of its vapor. 

The apparatus in Fig. 9.7.b is the simplest to operate because the entire manometer system, the sample, and its vapor are immersed in the constant-temperature bath. The mercury reservoir permits the removal of mercury from the U-manometer portion of the apparatus, and the material to be measured is then condensed into the terminal bulb. While this material is still condensed and there is a high vacuum in the system, the mercury is reintroduced to the U, thus isolating the sample. The apparatus is immersed in a constant-temperature bath to the level of the wavy lines. With a vacuum on the upper portion of the apparatus the vapor pressures can be measured directly. If vapor pressures beyond the range of the immersible manometer must be measured, the mercury in the... 

Apparently monochromatic resonance radiation of mercury which passes through mercury vapor at the saturated pressure at 25 °C is about half absorbed in four millimeters distance. Beer s law is not obeyed at all because the incident radiation cannot be considered to be actually monochromatic, and absorption coefficients of mercury vapor vary many times between zero and very high values in the very short space of one or two hundredths of an Angstrom unit. Moreover, absorption of mercury resonance radiation by mercury vapor is sufficiently great even at room temperature to make radiation imprisonment a very important phenomenon. If the reaction vessel has any dimension greater than a few millimeters the apparent mean life of Hg(63P ) may be several fold the true radiative life of 1.1 x 10"7 sec, reaction (27), because of multiple absorption and re-emission. 

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... 

Mercury is directly below cadmium in the periodic table, but has a considerably more varied and interesting chemistry than cadmium or zinc. Elemental mercury is the only metal that is a liquid at room temperature, and its relatively high vapor pressure contributes to its toxicological hazard. Mercury metal is used in electric discharge tubes (mercury lamps), gauges, pressure-sensing devices, vacuum pumps, valves, and seals. It was formerly widely used as a cathode in the chlor-alkali process for the manufacture of NaOH and Cl2, a process that has been largely discontinued, in part because of the mercury pollution that resulted from it. 

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