Isolation of noble gases

Isolation of noble gases :

Noble gases are present in the air. Isolation of Noble gases mainly occur in two steps.

  1. Separation of Noble gas mixture from the air.
  2. Separation of different inert gases from one another.

Separation of Noble gas mixture from the air : The first step involves the separation of Noble gaseous mixture from air. The following chemical methods adopted for this.

  1. Ramsay – Rayleigh’s first method
  2. Ramsay – Rayleigh’s second method
  3. Fischer – Ringe’s method.

Fischer – Ringe’s method :

Dry and pure air is passed over a mixture containing 90% calcium carbide and 10% \text {cacl}_{2} at 1073 K. \text O_{2} and \text N_{2} gases are removed by the following reactions.

\text {CaC}_{2}  +  \text N_{2}      \overset{1073 \;\text {K}}{\longrightarrow}           \text {CaCN}_{2} + \text {C}

Calcium carbide                    Calcium Cynamide

\text {C + O}_{2} \longrightarrow \text {CO}_{2}

\text {2 C + O}_{2} \longrightarrow \text{2 CO}

The carbon monoxide is removed by passing over cupric oxides where it turns to \text {CO}_{2}. The total \text {CO}_{2} is removed by passing through KOH solution.

\text {CuO + CO} \longrightarrow \text {CO}_{2} + \text {Cu}

\text {CO}_{2} + \text {2 KOH} \longrightarrow \text{K}_{2}\text{CO}_{3} + \text {H}_{2}\text {O}

The remaining mixture contains all the Noble gases.

Separation of different inert gases from one another :

Dewar’s method :

Principle involved in Dewar’s method :

  • Activated coconut charcoal adsorbs all the Noble gases except Helium.
  • The adsorption depends on temperature. Lower the atomic weight of the Noble gas, the lower is the temperature needed to adsorb it.

Procedure :

The mixture of inert gases is sent into a flask called Dewar’s flask. It is a double walled flask containing activated coconut charcoal in the middle.

At 173 K, Ar, Kr and Xe gases are adsorbed by the coconut charcoal while He and Ne remain unadsorbed.

The mixture of He and Ne is introduced into another Dewar’s flask and 93 K temperature is maintained. At 93 K, only Ne gets adsorbed leaving behind He.

The charcoal with Ar, Kr, Xe is cooled to 77K. Argon separates out.

The temperature of the charcoal raised to 183 K. At this temperature, Kr separates out. Xenon which remains on the first charcoal is released by warming.