When it’s combined in an acidic or basic solution, olfactory markers are the smells or odors that alter. It can be used in the lab to determine whether a solution is acid or basic, as well as to do olfactory titration.
The olfactory indications are onion and vanilla extracts.
When a basic solution, such as sodium hydroxide, is put to a fabric strio that has been treated with onions, the scent is gone.