 Further purification is done by a chemical transport reaction developed by Anton Eduard van Arkel and Jan Hendrik de Boer . In a closed vessel , the metal reacts with iodine at temperatures of above 500 ° C forming metal ( IV ) iodide ; at a tungsten filament of nearly 2000 ° C the reverse reaction happens and the iodine and metal are set free . The metal forms a solid coating at the tungsten filament and the iodine can react with additional metal resulting in a steady turn over . 
