The noble gases are isolated from air by removing oxygen and nitrogen from air free from carbon-di-oxide, water vapour, dust particles, etc., This can be accomplished by either chemical methods or physical methods. In the chemical method, the unwanted gases are removed by means of compound formation while in the physical method, these are removed by the fractional evaporation of liquid air.
The first step in this method is to isolate the noble gases mixed together, from the atmosphere by passing repeated electric sparks in air so as to remove nitrogen and oxygen as nitrogen dioxide (N2 + 2O2→ 2NO2 ). The second step is to separate the various constituents from one another taking advantage of the fact that they can be adsorbed on activated charcoal at different temperatures.
Ramsay – Raleigh’s method:- A mixture of air and oxygen is constantly admitted into a glass globe of about 50 litres capacity. Two platinum electrodes are introduced and a discharge from a transformer of about 6000 – 8000 volts is passed by the action of which nitrogen and oxygen rapidly combine to form oxides of nitrogen. The oxides are dissolved out in a solution of sodium hydroxide continuously circulated through the flask.
N2 + O2 → 2 NO
2 NO + O2 → 2NO2
2NO2 + 2NaOH → NaNO3 + NaNO2 + H2O
Oxygen if any is removed by introducing alkaline pyrogallol in the globe.
The supply of air and electric discharge is shut after some time and the remaining mixture of noble gases is pumped out.
Fig: Chemical method for isolation of noble gases