Evaporation of mercury

But does mercury evaporate at room temperature? Because the boiling point is very high – 357 ° C. In fact, at room temperature, the mercury vapor pressure does not exceed 0.001 mm Hg (this is about a million times less than atmospheric pressure). But even such a low pressure means that each cubic centimeter of air contains 30 trillion mercury atoms! And that’s bad: because the forces of attraction between mercury atoms are small (which is why this metal is liquid), mercury evaporates fairly quickly, although at first glance it seems that the spilled drops of mercury for a long time do not diminish in size at all. And the lack of color and smell of mercury vapor leads to the fact that many underestimate the danger. To make this fact obvious in the literal sense of the word, in 1942 the United States conducted such an experience. In a small plastic cup poured a little mercury so that a puddle about 2 cm in diameter was formed. This puddle was sprinkled with a fine fluorescent powder (the word “fluorescent” comes from the Latin root fluor-flow and the suffix escentia, meaning a weak action) – about the same as that covered from the inside TV picture tubes or fluorescent tubes. If such a powder is illuminated with invisible ultraviolet rays, it starts to glow brightly. When such a powder was simply poured into a cup and irradiated with ultraviolet, a uniform glow of the bottom of the cup was seen. But when there was mercury under the powder, dark moving “clouds” were visible on a bright background. This was most clearly seen in the case when there was a slight movement of air in the room.

The explanation is simple: the mercury in the cup is continuously evaporated and its vapors pass freely through a thin layer of fluorescent powder. Mercury vapor has the ability to strongly absorb ultraviolet radiation. Therefore, in those places where invisible “mercury trickles” rose above the cup, the ultraviolet rays stayed in the air and did not reach the powder. In these places, and there were dark spots.

Later this experience was perfected so that many spectators could watch it at once in a large audience. Mercury this time was in an ordinary bottle without a test tube, from where her bunks freely emerged. A screen was placed behind the bottle, covered with fluorescent powder, and before it – an ultraviolet lamp. When the lamp was turned on, the screen began to glow brightly, and moving shadows were clearly visible on a light background. This meant that in these places the ultraviolet rays were delayed by mercury vapor and could not reach the screen.

As shown by special measurements, after equilibrium is established between liquid mercury and its vapors at room temperature, the concentration of mercury vapor in the air is hundreds of times higher than permissible for respiration. But if the open surface of mercury is covered with water, the rate of its evaporation decreases in fitting, in million ral. This is because mercury is very poorly soluble in water: in the absence of air, 0.06 mg of mercury can dissolve in one liter of water. Accordingly, the concentration of mercury vapors in the air should be very much reduced, provided that it is ventilated (in the absence of ventilation, the concentration of mercury vapor in the air will be the same as in the absence of a protective water layer). This was carried out in the company “Bethel Device” in Pennsylvania (USA), in the shops of which over the years of their existence, thousands of tons of liquid mercury were distilled and packed. In one experiment about 100 kg of mercury were poured into two identical trays measuring 78 x 21 x 7 cm, one of which was filled with a layer of water about 2 cm thick and left overnight. In the morning, the concentration of mercury vapor was measured at a height of 10 cm from each tray. Where mercury was flooded with water, it was in the air 0.05 mg / m3 – slightly more than in the room (0.03 mg / m3). And over the free surface of mercury the instrument went off scale.

All this became known relatively recently, and in the past, mercury was treated rather blithely. The ancient Indians, the Chinese, the Egyptians knew about mercury. Greek doctor Dioscorides, who lived in the I century BC, gave her the name hydrargyros, i.e. “Water silver”. The close-by-name name – Quecksilber (ie “moving silver”) was preserved in German (it’s interesting that quecksilberig means “restless” in German). An ancient English name for mercury is quicksilver (“fast silver”).

Mercury and its compounds in antiquity and in the Middle Ages were used in medicine, as well as for the preparation of paints. But there were quite unusual applications. Thus, in the mid-tenth century, the Moorish King Abdarrahman III built a palace near Cordoba in Spain, in the courtyard of which was a fountain with a continuously flowing stream of mercury (recall that the rich mercury deposits in Spain were known in ancient times, and now mercury mining this country Takes a leading place). Even more original was another king, whose name the story did not save: he slept on a mattress that floated in the pool of mercury! Mercury was poisoned not only by kings, but also by many scientists, including Isaac Newton (at one time he was very interested in alchemy). And in our time, careless handling of mercury often leads to sad consequences. From all that has been said, mercury spilled in the room should be collected in the most careful way. Especially a lot of vapor is formed if mercury is scattered into a multitude of small droplets, which are hammered into various cracks, for example between tiles of parquet. Therefore, all these droplets must be collected. It is best to do this with a tin foil, to which mercury easily adheres, or a copper wire washed in nitric acid. And those places where mercury could still be delayed, pour 20% solution of ferric chloride. A good preventive measure against mercury vapor poisoning is to carefully and regularly, for many weeks or even months, ventilate the room where mercury has been spilled.

In the sense of poisoning with mercury bunks, daylight hazards are a big danger. Who did not see the white tubes of burnt-out lamps in the dump? However, each such tube contains up to 0.2 g of liquid mercury, which, if broken, begins to evaporate and pollute the air.