Iodine

Iodine: the essentials

 Brief description: iodine is a bluish-black, lustrous solid. It volatilises at ambient temperatures into a pretty blue-violet gas with an irritating odour.

It forms compounds with most elements, but is less reactive than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulphide to form beautiful purple solutions. It is only slightly soluble in water. Iodine compounds are important in organic chemistry and very useful in medicine and photography. Lack of iodine is the cause of goitre (Derbyshire neck). The deep blue colour with starch solution is characteristic of the free element. It is assimilated by seaweeds from which it may be recovered, and is found in Chilean saltpetre, caliche, old salt brines, and salt wells.

The result of touching nitrogen triiodide (NI₃)! Nitrogen triiodide is dangerously percussion sensitive (only to be demonstrated by a professionally qualified chemist following a legally satisfactory hazard asessment). Improperly done, this reaction is dangerous!

Isolation

Isolation: iodine is available commercially so it is not normally necessary to make it in the laboratory. Iodine occurs in seawater but in much smaller quantities than chloride or bromide. As for bromine, with suitable sources of brine, it is recovered commercially through the treatment of brine with chlorine gas and flushing through with air. In this treatment, iodide is oxidized to iodine by the chlorine gas.

2I^- + Cl₂ → 2Cl^- + I₂

Small amounts of iodine can be made through the reaction of solid sodium iodide, NaI, with concentrated sulphuric acid, H₂SO₄. The first stage is formation of HI, which is a gas, but under the reaction conditions some of the HI is oxidized by further H₂SO₄ to form iodine and sulphur dioxide.

NaI (s) + H₂SO₄ (l) → HI (g) + NaHSO₄ (s)

2HI (g) + H₂SO₄ (l) → I₂ (g) + SO₂ (g) + 2H₂O (l)

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