UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III

Q1(e). Purge gases in thermogravimetric analysis

Question

Name three gases used for purging in thermogravimetric analysis (TGA). Justify your answer.

Nitrogen, argon (inert: pyrolysis only, no oxidation) and air or oxygen (oxidising: burns off carbon, tests oxidative stability).

prevents oxidation and sweeps out volatile products so they do not recondense on the balance; most used for decomposition studies (polymers, hydroxides, carbonates).
inert at very high temperature (above 1200°C, where N₂ can form nitrides with Ti, Al, Li, Mg); denser than N₂ and gives a protective blanket.
oxidising atmosphere for combustion, ash and fixed-carbon content (proximate analysis of coal) and for oxidative stability. A switch from N₂ to air at the end of a run gives carbon black or ash.
(Also: CO₂ for carbonate decomposition equilibria; He for fast heat transfer; H₂ for reduction studies.)

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

N₂ (cheap, inert to 1000°C): prevents oxidation and sweeps out volatile products so they do not recondense on the balance; most used for decomposition studies (polymers, hydroxides, carbonates).
Ar: inert at very high temperature (above 1200°C, where N₂ can form nitrides with Ti, Al, Li, Mg); denser than N₂ and gives a protective blanket.
Air or O₂: oxidising atmosphere for combustion, ash and fixed-carbon content (proximate analysis of coal) and for oxidative stability. A switch from N₂ to air at the end of a run gives carbon black or ash.
(Also: CO₂ for carbonate decomposition equilibria; He for fast heat transfer; H₂ for reduction studies.)

CoversTGA atmospherePurge gases in thermogravimetric analysis