What is an olfactory indicator? Name two olfactory indicators. What is the effect of adding sodium hydroxide solution to these olfactory indicators?

Olfactory indicators are those smell or odour changes when it is mixed in an acidic or basic solution. It can be used in laboratory to test whether the solution is base or acid and can perform olfactory titration Onion and vanilla extracts are the olfactory indicators

When a basic solution like sodium hydroxide solution is added to a cloth strio treated with onions the smell cannot be detected