What do you mean by all …

An Olfactory indicator is defined as a substance whose smell varies when it is mixed with an acidic or basic solution.Such substances can be used in the laboratory to test whether a solution is a base or an acid, and this process is called olfactory titration. All onion, clove oil and vanilla extract are examples.
An olfactory indicator works on the principle that when an acid or base is added to it, then different odour can be detected in bases whereas odour remains same in acids. Certain substances changes odour in acidic or basic medium.
1.Onion:- Paste or juice of onion loses its smell when added with base.It does not change its smell with acid.
2.Vanilla:- The smell of vanilla vanishes with base,but it’s smell does not vanishes with an acid.
3.Clove oil- In bases its characteristis smell cannot be detected.