Olfactory indicators are compounds that emit a distinct odor in an acidic or basic medium. They are used in laboratories to determine whether a solution is acidic or basic. An olfactory indicator is defined as a substance whose smell varies when it is mixed with an acidic or basic solution. When an olfactory indicator is mixed with an acidic or basic solution, its smell changes. For example, onion and vanilla extracts are two olfactory indicators. When sodium hydroxide solution is added to an olfactory solution like onion juice, the smell of onion cannot be detected.