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Isolation of Noble Gases
Except radon which is obtained from radioactive element radium all the other members of zero group can be isolated from air. Two different approaches are used for the isolation of noble gases from air. They include the fractionation of liquid air which is a physical method and the isolation of noble gases using chemical methods.
Let us see each of them in detail.
- Fractionation of liquid air:
Helium and neon with low boiling points remain as gases and all the other constituents change to liquid.
- Argon and oxygen have the same boiling point and separate as a single fraction.
- Krypton and xenon with very high boiling points get collected in the less volatile portion of the liquid air.
- Isolation of gases by chemical method:
- Removal of nitrogen and oxygen from air:
- Separation of noble gases:
- What is major component used in the chemical separation of noble gases?
- Coconut charcoal
- Carbon monoxide
- Nitrogen and oxygen present in the air are removed with the help of
- 90% calcium carbide and 10 % calcium chloride
- Fractional distillation
- Which of the inert gases has the least ability to bind to coconut charcoal
This method is based on the difference in the boiling point of different constituents of liquid air at atmospheric pressure. When liquid air is fractionally distilled three different components are obtained.
Now from these three different components, inert gases are separated using various approaches.
In the first portion there is a mixture of three gases, helium, neon and nitrogen. This mixture is passed in a steam of evaporating liquid nitrogen. During this process, most of the nitrogen condenses leaving the mixture with inert gases and very little amount of nitrogen. The residual gas mixture is then passed over heated calcium carbide to remove the traces of nitrogen as calcium cyanamide. As a result of this a gaseous mixture containing 75% of neon and 25% of helium is obtained. Separation of these two gases is achieved by passing the mixture through tubes cooled by liquid nitrogen. This allows neon to solidify leaving behind the uncondensed helium.
In the second fraction there is a mixture of argon and oxygen. Argon is separated from this mixture by passing it through a tube cooled by liquid nitrogen. This enables argon to condense leaving behind the uncondensed argon.
The third portion contains xenon, krypton and oxygen. Oxygen is removed from this portion by evaporation. Finally xenon and krypton are separated either by fractional distillation or selective adsorption on coconut charcoal.
This process of separation of noble gases is based on the difference in adsorption on coconut charcoal at different temperatures. This process is done in two steps. Nitrogen and oxygen are removed first followed by separation of noble gases.
This is done by passing dry air over a mixture of 90% calcium carbide and 10 % calcium chloride at 1073 K. This causes oxygen and nitrogen to undergo the following reactions.
CaC2+ N2 --> CaCN2 + C
C +O2 -- > CO2
2C + O2 -- > 2CO
The above formed carbon monoxide and carbon dioxide are oxidized by passing over heated copper oxide.
CuO +CO -- > Cu + CO2
The so formed carbon dioxide is removed by passing the gaseous mixture through potash bulbs that absorb CO2.
2KOH + CO2 -- > K2CO3 + H2O
The gaseous mixture that is free of oxygen and nitrogen is used for separating the noble gases. This is done using coconut charcoal which adsorbs different gases at different temperatures. The property of adsorption to coconut charcoal increases with atomic weight of the noble gases. Thus, helium which has the least atomic weight adsorbs the least while xenon with maximum atomic weight adsorbs strongly. The temperature at which the adsorption takes place increases with the increase in the atomic weight.
The gaseous mixture is passed into a double walled bulb containing coconut charcoal. The mixture is retained in the bulb for about an hour. During this period the gases argon, krypton and xenon get adsorbed while the helium and neon remain unadsorbed and are pumped out.
The mixture of helium and neon are held separately in another bulb that is maintained at a temperature of 93 K. At this temperature neon completely adsorbs to the coconut charcoal leaving behind helium. Neon can be recovered from the charcoal by warming.
Now, the charcoal containing argon, krypton and xenon is brought in contact with another charcoal held at room temperature of 80 K. Argon gets adsorbed onto the charcoal that is held at room temperature.
Finally, the separation of xenon and krypton is achieved by heating the charcoal to a temperature of 183 K. At this temperature krypton is set free while xenon remains adsorbed to the charcoal.
As can be seen the chemical method of separation of noble gases can easily be done with the help of coconut charcoal maintained at various temperatures.
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