What Are Olfactory Indicators?

Answer:

Olfactory indicators are compounds whose smell (or aroma) changes in acidic or basic solutions.

Olfactory markers include onion and vanilla extracts.

When a basic solution, such as sodium hydroxide, is put to a textile strip that has been treated with onions (or onion extract), the onion odour goes away.

When sodium hydroxide is applied to vanilla extract, the scent of vanilla is completely destroyed.