Question

What do you mean by Olfactory indicators?

Answer 1

Olfactory indicators develop a change in their smell when brought in contact with acidic and basic substances
For example :
Onion extract (naturally basic and hasstrong smell)
* when brought in contact with acidic substances its stingy smell disappears
* when brought in contact with basic substances its smell persists

Answer 2

olfactory indicators" are those indicators that help to identify whether the given solution is acidic or basic by changing their smell instead of colour as other indicators do.

olfactory indicator" implies that this is a compound whose can smell you can detect... vanilla, clove and onion are all olfactory indicators as these do not give their distinct odour in highly alkaline(basic) medium.