In sea-weeds there exist sodium & potassium iodide along with other compounds. From these weeds iodide is extracted & from that iodine is released. The following procedure is used in this regard
Principle : The ash found by burning sea-weeds is called kelp. In kelp there exist soluble chloride, iodide, carbonate & sulphate of potassium. When kelp is heated gradually with water, these compounds get dissolved in water. If chloride, carbonate & sulphate are separated in crystallized form by fractional crystallization from the solution, then sodium & potassium iodide remain as residue. If cone. H2SO4 & MnO2 are mixed with the soln. & heated then iodine is produced.
2NaI + 3H2SO4 + MnO2 = 2NaHSO4 + MnSO4 + 2H20 + I2
2KI + 3H2SO4 + MnO2 = 2KHSO4 + MnSO4 + 2H2O+ I2
Description : The production of iodine from sea weed goes through following stages :
- Preparation of kelp : Sea-weeds are obtained & dried in open space at sea- This dried weed is burnt carefully so that iodide salt is not decomposed. The ash found from this burnt weed is called kelp.
- Separation of iodide from kelp : When kelp is heated in water, sodium & potassium chloride, sulphate, carbonate & iodide salt present in it get dissolved. Thus by filtration (transparent) clear soln is obtained. This soln is heated & concentrated & then gradually cooled when less soluble chloride, sulphate & carbonate salt get separated from the soln as crystals. But iodide being highly soluble remains in the After separating the crystallized material by filtration the mother-liquor of iodide salt is obtained.
- Manufacture of iodine from iodide : The salt soln of Na & K iodide is heated along with cone. H2SO4 & MnO2 in a steel made retort. So iodine is separated vaporized form and is passed through the exit tube, when they deposit as crystals in the aludels (earthen vessels) Which are arranged in series attached to the exit tube Later the aludels are broken up to collect the crystal of iodine.
- Purification : The iodine collected from aludels is sublimated for pure iodine.
Manufacture of iodine from sea weeds