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19 September 2003

Purifying and Working with Mercury

by John Strong
Excerpted from "Scientific American's The Amateur Scientist", first published in Procedures in Experimental Physics.

Although mercury approaches the noble metals in chemical inertness, it is easily contaminated, especially by other metals. This is because, as a liquid, it is a fairly good solvent. A simple test for the purity of a sample of mercury is to raise a clean glass rod slowly up through the metal surface. If the mercury is clean, the glass will come up without any adhering mercury droplets.

Fig. 9. Click image to enlarge.

The contaminations commonly found in mercury may be classified according to the manner in which they can be easily removed. First come surface contaminations by materials which do not dissolve in the liquid metal and may, accordingly, be removed by filtering the metal through pinholes in filter paper or through a chamois skin. Second, there are the dissolved metals. Those which are oxidizable are first converted to insoluble oxides by the blowing of air through the mercury as shown in Fig. 9(a). The oxides form a scum on the mercury surface and may later be filtered off. Mercury is practically free of impurities of this type if, after air has been blown through the liquid metal for two hours, no scum has formed on the surface. The alkali metals fall into this class of impurities; here also belong zinc, with a high vapor pressure, and copper and lead, with low vapor pressures. These metals, which are more reactive than mercury, can also be removed by exposing the mercury to a solution of 10 per cent HNO₃ or 80 per cent H₂SO₄. This is shown in Fig. 9(b). Thirdly, there are the dissolved metals, such as the noble metals and tin, which cannot be removed by oxidation or acid. Copper and lead may also be considered as belonging to this class of contaminations. These metals are removed by vacuum distillation of the mercury at a temperature of about 180° to 200°C. (at which temperature the mercury distills at the rate of approximately 1/2 g/cm²/sec.) as indicated by Fig. 9(c).

The vapor pressure of mercury is given in Table I. It is to be noted thoughtfully that at room temperature the vapor density of mercury is many times greater than the accepted nonpoisonous concentration limit, which is 1 milligram of mercury per cubic meter. According to Stock, continual breathing of air containing only 15 micrograms per cubic meter of mercury for a few weeks will make most persons ill. The vapor pressure of mercury is hazardously high in many laboratories. In a Berlin physical laboratory the typical concentration of mercury vapor in the air was found to be about 20 to 60 micrograms per cubic meter; in one room it was 500 micrograms per cubic meter. Heat produced by turning on mercury pumps doubled the concentration of mercury vapor in the air.