thumb|A black and white photographic print in a tray while being processed after exposure to light under a photographic enlarger. Typically three trays are used containing either developer, stop bath, or fixer, in that order. The print must then be rinsed in water to remove the fixer.
A stop bath is an acidic solution used for processing black-and-white photographic film, plates, and paper. It is used to neutralize the alkaline developer, thus halting development.
Stop baths account for the vinegar-like odor of the darkroom. In its concentrated form it can cause chemical burns, but is harmless when diluted to a working solution. The stop bath gets exhausted when developer solution carried over on the film or paper is transferred into the bath, raising its pH and making the solution alkaline, meaning it loses its acidity and no longer stops the development process.
For indicator stop baths, which changes color to indicate when the bath is exhausted and no longer effective, a pH indicator dye like bromocresol purple is used.