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Element Mercury (Hydrargyrum), Hg, Transition Metal

About Mercury (Hydrargyrum)

In its chemical relations mercury, or Hydrargyrum, is most nearly allied to copper, since it forms, like it, two elementary ions, a monovalent and a divalent, which in many respects also are similar to those of copper. With cadmium it shares the tendency to form slightly dissociated halogen compounds of the divalent series.

Metallic mercury occurs free in nature, and by reason of its being liquid at medium temperatures it has attracted attention from remote times. In the older history of chemistry, while the experimental conception of a chemical element was not yet developed, mercury was regarded as the type of the metallic character; this found expression in the fact that mercury was regarded as a constituent of all metals. The endeavours to prepare gold and silver from base metals, which are connected with this view, had generally, for the first purpose, the "fixing" of the mercury, i.e. making it non-volatile. For this reason, and through the discovery made about the fifteenth century of the powerful medicinal actions of the mercury preparations, the chemistry of mercury became known at an earlier period than that of most of the other metals.

During the development of the newer period of chemistry at the end of the eighteenth century mercury again played a considerable role. This is due, in the first place, to the chemical properties of mercury oxide. The possibility of converting the metal into its oxide by heating in the air, and of effecting the separation of this into metal and oxygen by more strongly heating, was of the greatest importance for the correct interpretation of the phenomena of oxidation. On the other hand, the introduction of the mercury pneumatic trough for the investigation of gases at once led to the discovery of a series of hitherto unknown substances.

Up to the present day mercury has not lost its importance for scientific investigation. Its liquid nature, fairly great chemical resistibility, considerable density, etc., assures its unceasing use for physico-chemical apparatus, of which the thermometer and barometer need only be mentioned as the most important. Since, being a liquid metal, it is not subject to the variations which are exhibited by the solid metals in consequence of being wrought, it is employed as a standard metal for electro-chemical apparatus, and many other scientific applications could also be mentioned.

Metallic mercury has the density 13.595 at 0°. Its expansion by heat is, up to the boiling point of water, so nearly proportional to that of the gases that the mercury thermometer agrees well with the gas thermometer over this range. At -39.4°, mercury solidifies to a silver-like solid metal; it thereby readily exhibits in a considerable degree the phenomena of supercooling. At 358° mercury boils under the pressure of the atmosphere. Since, in many measurements, the vapour pressure of mercury at comparatively low temperatures also comes into account, we give the following table of vapour pressures:

0° - 0.00002 cm.
20° - 0.00013 cm.
40° - 0.0007 cm.
60° - 0.0028 cm.
80° - 0.0093 cm.
100° - 0.0280 cm.
158° - 0.29 cm.
200° - 1.82 cm.
250° - 7.58 cm.
300° - 24.2 cm.
350° - 66.3 cm.

From this it is seen that up to 100° the vapour pressure is small, viz. less than 1 mm.

In the air mercury behaves in general as a " noble " metal, i.e. it does not oxidise spontaneously. This is not, however, in all strictness the case, for if it is maintained for a lengthened period at about 300°, it slowly becomes covered with red crystals of mercury oxide. Water, standing in contact with mercury, assumes poisonous properties. Whether this is due to the solution of a trace of oxide formed, or to the solution of metal in water, has not yet been determined (That metallic mercury can dissolve in water must be regarded as indubitable. For, all gases dissolve in water; since mercury has an appreciable, although small, vapour pressure even at room temperature, its vapour must also be soluble in water. There is, however, no difference between a solution of liquid and one of vaporous mercury, since in the case of a solution only the existing state is of importance and not the former states of its components.).

The combining weight of mercury has been found by analysis of the oxide and sulphide to be Hg = 200.0. The vapour density shows the molar weight to be 200; the two are therefore equal. On account of its low boiling point mercury was the first metal in the case of which this remarkable relation was established.

Pure mercury does not wet glass; if, however, it contains foreign metals dissolved in it, it becomes covered with a film of oxide, the effect of which is that the metal no longer flows over glass and other surfaces in round drops, but "leaves a tail." This is a very sensitive test of the purity of the metal.

In order to purify mercury, a task which is constantly occurring in the laboratory, it is shaken with dilute sulphuric acid, to which from time to time some drops of potassium dichromate are added; it is then washed with a large quantity of water and dried by gently heating it. In this way considerable impurities can be quickly removed. The fairly pure metal is allowed to flow in small drops through the apparatus, which is filled with a dilute acid solution of mercurous nitrate (vide infra). These methods depend on the fact that the oxidising agents employed oxidise the contaminating metals rather than the mercury; in order that the object may be attained, fine division is necessary.

Mercury History

Mercury, or Hydrargyrum was known by Chinese and in Hindus even 2000 years BC. Cinnabar (HgS), the mercury ore, was easily isolated and widely used in these countries as well as by Greeks and Romans as a pigment (vermilion), remedy and in cosmetics. Dioscorides, the Greek physician of 1st century AD, extracted it from cinnabar distilling it on the iron lid of the vessel. It was called hydrargyros from hydor meaning water, and argyros that is silver, which was borrowed in Latin as hydrargyrum. The original Latin name was argentum vivum - living silver (quicksilver in English).

Mercury Occurrence

Mercury is a trace element with abundance 7.0x10-6 mass % in Earth's crust, 1.03 mg/m3 in sea water and 2x10-3mg/m3 in atmosphere. It occurs as a native metal and forms more than 30 minerals with cinnabar (HgS) as the most common ore. Mercury minerals as isomorphous additions may be found in quartz, chalcedony, carbonates, micas and lead-zinc ores. Bulks of mercury participate in exchange processes which take place in atmosphere, hydrosphere and lithosphere. It is dispersed in biosphere being accumulated in insignificant quantities in clay and silt, reaching 4x10-5% in clays and micas. Sea water contains 3x10-9% of mercury. Mercury deposits are evaluated as 500 thousand tons as a whole, including 250 in Spain, 100 in Italy, 50 in USA, 15 in Canada, 15 in Mexico, 9 in Turkey, and 8 in Algeria. Significant deposits are located in Japan, Bolivia, Peru, China, and Slovakia. Ores contain from 0.05 to 6-7% of mercury.

The principal ore of mercury is the red sulphide, cinnabar, HgS. It is far more abundant than any other, and is found in rocks of very different ages, generally occurring in a massive or granular condition. Several varieties are known, such as steel ore (which is the richest), hepatic cinnabar, coralline ore, and brick ore. The hardness is 2-2.5 on Mohs' scale, and the density is 8-8.2. The chief European cinnabar mines are at Almaden in Spain, but the ore also occurs in Idria, where it was mined as early as 1497, and, to a less extent, in the Balkan Peninsula, Russia, Siberia, Caucasia, Australia, New Zealand, China, Japan, and certain parts of Central and South America. Important mines are to be found in California and Peru. Metacinnabarite is a black sulphide abundant in some of the Californian mines.

Mercury occurs in the metallic state to a certain extent, generally as a decomposition product of cinnabar. Mercury minerals of less importance are the chloride calomel, Hg2Cl2, the oxide montroydite, HgO, and two oxychlorides, one a mercurous-mercuric compound, terlinguaite, 2HgO.Hg2Cl2, and the other eglestonite, Hg2O.Hg2Cl2, to which the empirical formula Hg6Cl3O2 has also been ascribed. An interesting mineral is kleinite, which was regarded as an oxychloride by Moses, but which is apparently a mercury-ammonium derivative, NHg2Cl, mixed with a small quantity of a mercury oxychloride and sulphate, or oxysulphate.

Amalgams of silver of different concentrations have been found. For example, a crystalline amalgam of the approximate composition Ag5Hg3 occurs in Chili. A crystallised gold amalgam from California, AuHg3, has also been described. There are rare compounds of mercury with antimony sulphide, titanium, and iodine, and the selenide, tiemannite, was formerly commercially worked in Utah.

Mercury and mercury sulphide have been found in hot-spring deposits in New Zealand and Nevada, and it is, in fact, generally considered that cinnabar and other ores of mercury have been deposited originally from aqueous solution by cooling and reduction of pressure. Mercury ores are much more frequently found as narrow veins, or disseminated through siliceous or limestone rocks, than in the massive state.


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