Question

Which method of purification is represented by the equations:
$\begin{matrix}
   \text{Ti} & \text{+} & \text{2}{{\text{I}}_{\text{2}}} & \xrightarrow{\text{500K}} & \text{Ti}{{\text{I}}_{\text{4}}} & \xrightarrow{\text{1675K}} & \text{Ti} & \text{+} & \text{2}{{\text{I}}_{\text{2}}} \\
   \text{(Impure)} & {} & {} & {} & {} & {} & \text{(Pure)} & {} & {} \\
\end{matrix}$
A) Cupellation
B) Poling
C) Van Arkel
D) Zone refining

Answer 1

Hint: The titanium is converted into the volatile substance in presence of halogens. The metal halide vapours are then decomposed such that the metal deposits on the filament. The obtained metal ultra-pure. This method is called the crystal bar process also called the Van Arkel method.

Complete answer:
The Van Arkel method is used to get the ultra-pure metal. In this method, the impure metal is converted into a volatile unstable compound in presence of halogen on taking care that the impurities are not affected during the compound formation. The compound obtained is decomposed to get pure metal. This method is used for the purification of metals like titanium and zirconium.
Let’s understand the Van Arkel method,
This method is a thermal decomposition of metal compounds. The Van Arkel method depends on the process of decomposition of compounds into different substances by heat. It is also called a crystal bar process.
Titanium or zirconium is purified by the Van Arkel method. The impure titanium $\text{ (Ti) }$ is heated at the temperature of $\text{ 25}{{\text{0}}^{\text{0}}}\text{C }$ the iodine forms a volatile titanium tetraiodide $\text{ (Ti}{{\text{I}}_{\text{4}}}\text{) }$ compound. The impurities which are formed during the process are left behind as they do not interact with the iodine.
 \[\begin{matrix}
   \text{ Ti} & \text{+} & \text{2}{{\text{I}}_{\text{2}}} & \xrightarrow{\text{500K}} & \text{Ti}{{\text{I}}_{\text{4}}} \\
   {} & {} & {} & {} & \text{(Titanium tetra-Chloride)} \\
\end{matrix}\]
The vapours of titanium tetraiodide $\text{ (Ti}{{\text{I}}_{\text{4}}}\text{) }$are passed over the hot tungsten filament at the temperature of about $\text{ 140}{{\text{0}}^{\text{0}}}\text{C }$. The vapour formed is then decomposed and the pure titanium then deposits on the filament and then removed to get the ultra-pure titanium. The iodine liberated at the filament is reused for the purification of metal.
  \[\begin{matrix}
   \text{Ti}{{\text{I}}_{\text{4}}} & \xrightarrow{\text{1675K}} & \text{Ti} & \text{+} & \text{2}{{\text{I}}_{\text{2}}} \\
   {} & \text{Tungsten Filament} & \text{Pure Metal} & {} & {} \\
\end{matrix}\]
 The metal deposited on the filament conducts the electricity and thus the high current is required to maintain the temperature of the filament. This process can be performed in hours or can take several weeks. It depends on the setup used for the van Arkel process.
The Van Arkel process for the zirconium is given as follows:

\[\begin{matrix}
   \text{Zr} & \text{+} & \text{2}{{\text{I}}_{\text{2}}} & \xrightarrow{\text{870K}} & \text{Zr}{{\text{I}}_{\text{4}}} & \xrightarrow{\text{2075K}} & \text{Zr} & \text{+} & \text{2}{{\text{I}}_{\text{2}}} \\
   \text{(Impure)} & {} & \text{(Vapour)} & {} & \text{(Vapour)} & {} & \text{(Pure)} & {} & {} \\
\end{matrix}\]

Hence, (C) is the correct option.

Note:
The Van Arkel method is based on the fact of vapour phase refining. The impurities are not affected during the compound formation. The nickel can be refined by the vapour phase refining method called the Mond’s process. But the Van Arkel method is applied only for titanium, zirconium.