Element Iodine, I, Halogene

Iodine is allied to chlorine and bromine, and forms a third similar element. Of the three, it has the highest combining weight, amounting to 126.86, and its properties show deviations from those of bromine chiefly in the same direction as those of bromine deviate from chlorine.

At ordinary temperatures, iodine is a solid, crystalline substance of a purple-black colour with an indication of metallic lustre. Its density is 5. At 114° it melts to a deep brown liquid. Even at ordinary temperatures it emits some vapour, which can easily be recognised by its reddish-purple colour when a little iodine is contained in a fairly large vessel. Iodine, however, does not boil till 184°.

Iodine vapour is of a fine violet colour. For the purpose of observing this colour and at the same time also the great density of iodine vapour, a large, round-bottomed flask is strongly heated in a large flame, being kept diligently turned the while, and a few crystals of iodine are then thrown into the hot flask. The iodine is at once converted into a vapour of a dark violet colour, which remains at the bottom, and which, when the vessel is moved, shows itself in a high degree subject to the force of gravity.

The density of iodine vapour is very considerable, being about nine times as great as that of air. The molar weight is 254 and the vapour has, therefore, the formula I₂. An account of its behaviour at high temperatures will be given presently.

In water, iodine is only sparingly soluble, but still sufficiently for the brown colour with which it passes into solution to be detected in fairly thick layers. If a salt-like iodide is present in the water, much larger quantities are dissolved with a brown colour. This is due to the formation of an ion I₃', as will be immediately discussed.

In other liquids, iodine is generally more soluble. In spirit of wine it dissolves with a brown colour similar to that of the aqueous iodide solutions. This solution is used in medicine, and is called tincture of iodine. Other solvents, such as carbon disulphide and chloroform, dissolve it with a fine violet colour similar to that of the vapour. On what these differences of colour depend is as yet unknown, but it appears that in the brown solutions easily decomposable compounds are formed between iodine and the solvent.

If an aqueous solution of iodine is shaken with carbon disulphide (Carbon disulphide is a compound of sulphur and carbon, and forms a heavy, colourless liquid which does not mix with water), the colour of the iodine disappears from the aqueous solution, and the carbon disulphide is coloured purple. The iodine, therefore, leaves the water in order to dissolve in the carbon disulphide. This is an example of a general phenomenon which is subject to definite laws.

Iodine was discovered in 1811 by Bernard Courtois who was the manufacturer of saltpeter (potassium carbonate) and soap. Since old ages alkaline metals carbonates (soude de varech or salin de varech in French), crucially necessary for gunpowder manufacture were extracted in France and other countries from seaweed (varech in French) gathered at the coasts of Normandy and Brittany. For this purpose seaweed was burned and the ash then washed with water. After the process, the remaining wastes were destroyed by adding sulphuric acid. In 1811 Courtois accidentally added excess sulphuric acid resulting in a violet vapor cloud that condensed on colder objects forming dark, lustrous crystals, which as Courtois suspected, belonged to a new element.

Louis-Joseph Gay-Lussac and Andre M. Ampere also investigated the new element, after which Gay-Lussac offered to name it iode. Ampere had given his sample to Sir Humpry Davy, who visited Paris travelling to Italy. He sent off a paper to the Royal Society of London in 1813 describing his experiments and recognizing the similarities between the new substance and chlorine. He named it iodine, after the Greek ιοειδης [ioeides] = violet coloured (from ιον [ion] = violin), which was analogous to chlorine and fluorine, the name, which is now fully accepted in English-speaking environment.

Iodine is the least abundant of the halogen elements, approximately 4x10^-5% on average of the Earth's crust. It is rarer than thulium the most inaccessible among all lanthanides.

A feature which resembles rare earth elements are it's a literally trace element; being a rare element, it is almost omnipresent. Even in superfine rock quartz crystals iodine micro impurities may be found. In calcite crystals Iodine concentration may reach 5x10^-6%. Iodine occurs in soils, in fresh (rivers) and seawaters, in plant cells and in animal tissues. However minerals rich of iodine, are very rare. Lautarite Ca(IO₃)₂ is the most well-known among them, however it does not form commercially significant deposits.

Iodine may be recovered only from such concentrated natural sources as slat lakes water, associated oil waters, or seaweed. One ton of laminaria contains up to 5 kg of iodine; meanwhile its concentration in seawater is 20 - 30g in 1 ton.

As most vitally important elements iodine is in process cycling turnover in nature. Many of iodine compound a soluble in water; that brings to leaching iodine from magmatic rocks into sea and oceans. Seawater, vaporizing, passes elemental iodine into air. "Elemental" is a key word; compounds of the iodine oxidize until I₂ in the presence of carbon dioxide.

Winds transfer iodine with mass of air from oceans to continents where iodine precipitates getting into soil, ground waters and into living organisms, which accumulate iodine and return it into soil after dying.

This is just the general description of iodine turnover, which is researched very well because of iodine's importance for life.