PREPARATION OF ELEMENTS OF GROUP VII
Preparation of Fluorine.
Because of its extremely reactive nature, fluorine does not occur free in nature. In combined state, it is fairly and widely distributed. It constitutes nearly 0.08 per cent of lithosphere.
It occurs in fluorspar (CaF2), cryolite (AIF3.3NAF) and apatite [CaF2.3Ca3(PO4)2].
In small quantities, it occurs in some of the micas. Traces of fluorine occur in sea water, some mineral springs, bones, teeth, blood, milk, plants, etc.
Production :
Production by the electrolysis of aqueous solution of fluorides was negated. It attacked glass, carbon and almost all metals including platinum and hence the apparatus could not be made of these materials. The problem was, however, finally solved by Moissan, in 1886, who prepared fluorine by the electrolysis of a solution of potassium Hydrogen Fluoride (KHF2) in anhydrous hydrogen fluoride using an apparatus made of platinum-iridium alloy using electrodes also of the same alloy.
In later years, Moissan found that platinum-iridium alloy could be safely replaced by copper. This metal, though attacked by fluorine, gets covered with a thin impervious layer of copper fluoride which resists further attack.
For laboratory preparation of fluorine, Dennis, Veeder and Rochow electrolysed perfectly dry potassium hydrogen fluoride in a heavy V-shaped tube (about 5 cm in diameter) made of copper, using graphite electrodes .The ends of the tube are closed by copper caps through which pass the graphite electrodes. The latter are sealed and insulated by bakelite cement which is found to resist the attack of fluorine. The tube is heated in an electric resistance furnace to fuse the potassium hydrogen fluoride.
For electrolysis, a current of 5 amperes at 12 volts is passed. The fluorine liberated at the anode is passed through copper U-tubes containing sodium fluoride to free it from hydrogen fluoride
(NaF + HF→ NAHF2).
Commercial Preparation.
Today even the commercial preparation of fluorine is no problem. Fluorine gas stored in steel tanks under pressure is offered for sale in America and other countries .
It consists of a rectangular steel tank surrounded by a steam jacket. The tank is fitted with air-tight removable cover which carries a carbon anode, two 'slitted' copper or steel cathodes and a steel gas barrier. The cathode is 'slitted' to permit circulation of the electrolyte. The anode and the barrier are insulated from each other and also from the cathode with a special material, teflon or P.T.F.E. [polytetrafluoroethylene, (C2F4)n] which is not attacked by fluorine. The gas barrier surrounds the anode and extends below the surface of the eletrolyte with a view to separating the products of electrolysis.
The electrolyte in this case is a 90 per cent solution of KHF, in anhydrous hydrogen fluoride. This is kept in the molten state by heating it to about 95°C with the help of steam passing through the steam jacket. But once the electrolysis starts, heating is no longer necessary because sufficient heat is produced during the process of electrolysis itself. On the other hand, cooling may sometimes be needed to maintain the electrolyte at about 95°C. The loss of hydrogen fluoride from the electrolyte is made good by passing in hydrogen fluoride periodically.
The fluorine obtained in this process is almost pure and is compressed into steel cylinders at a pressure of about 26 atm. If required, the small contamination of hydrogen fluoride is removed as usual by passing the gas over sodium fluoride.
Preparation of Chlorine.
Chlorine constitutes nearly 0.19 per cent of lithosphere. It occurs mostly as chlorides of sodium and other alkali and alkaline earth metals. It is prepared mainly from sodium chloride which is crystallised from brine.
In the laboratory, it is obtained by the oxidation of chloride ion in acid medium by using MnO2 or permanganate ion as the oxidising agent.
2Cl + 4H+ + MnO2 = Mn2++ 2H2O + Cl2
Bulk of chlorine required for commercial purposes is prepared by electrolysis of brine solution. The electrolysis is carried out primarily for the purpose of manufacturing caustic soda and chlorine is an important by-product of this process.
Preparation of Bromine.
Bromine, which constitutes nearly 0.01 per cent of lithosphere, occurs in combined state in sea water. In laboratory, it can be prepared by oxidation of Br - ion in the same way as that of Cl - ion for the preparation of chlorine.
On a large scale, it is obtained from sea water by passing chlorine into sea water which contains bromides in sufficient amounts.
2Br - + Cl2 = 2Cl - + Br2
The free bromine thus liberated is blown out with air and dissolved in sodium carbonate solution to get sodium bromide and sodium bromate.
3CO3 2- + 3Br2 = 5Br -+ BrO3- + 3CO2
3CO3 2- + 3Br2 = 5Br -+ BrO3- + 3CO2
Bromine is recovered from this solution by acidification.
6H++ 5Br - + BrO3 - = 3H2O + 3Br2
Preparation of Iodine.
Iodine is the rarest of all the halogens. It constitutes merely 10-4 per cent of lithosphere. Its main source is kelp', viz., the ash obtained on burning sea-weeds in which it occurs as iodides and chile saltpetre in which it occurs as iodates.
The kelp containing about 0.1 percent of iodine as iodides is extracted with water. The extract is concentrated when sulphates and chlorides which are less soluble separate out while the iodides remain in solution (mother liquor).
Iodine is recovered by passing chlorine through the mother liquor.
2I-+ Cl2 = 2Cl - + I2
It is purified by sublimation.
