DEFINITION
Van arkel process is also known as crystal bar process or iodide process or van arkel-De Boer process. This process was developed by Anton Eduard van arkel and John Hendrik de Boer in 1925.
Van arkel method is the process of metals are converted into other metal-compounds like metal-iodides. Metal-iodides are decomposed easily and highly pure metals are then obtained.
Van arkel method is based on the thermal decomposition of metal compounds which lead to the formation of pure metals.
This method is used to purify the metals are on titanium, zirconium, hafnium, thorium, and protactinium
For example
In a pure block of Ti, Fe can be an impurity. This block is heated in iodine at a temperature of 250°C. Ti converts into TiI4 but the impurities do not. TiI4 is passed over hot tungsten filament so that TiI4 is decomposed and ultra-pure Ti is obtained.
The Titanium and zirconium can be an impurity. This block can be purified by using this method. The impure titanium metal is heated in an evacuated vessel with iodine at a temperature of 550 K to form the volatile titanium tetra-iodide (TiI4). The impurities are left behind, as they do not react with iodine.
Ti (s) + 2I2 (s) →TiI4 (vapour)
The volatile titanium tetraiodide vapour is passed over a tungsten filament at a temperature around 1800 K. The titanium tetraiodide is decomposed and pure titanium is deposited on the filament. The iodine is reused.
Ti (s) + 2I2 →TiI (s)
This method is as same as to the removal of impurity from the zirconium metal,
Zr (s) + 2I2 (g) → ZrI4 (g)
ZrI4 (g) → Zr (s) + 2I2 (g)
