Stop bath is a chemical bath usually used in processing traditional black-and-white photographic films, plates, and paper used after the material has finished developing. The purpose of the stop bath is to halt the development of the film, plate, or paper by either washing off the developing chemical or neutralizing it. With the former, a simple water rinse can be used between developer and fixer, but the development process continues (though possibly at a very low level) for an indefinite and uncontrolled period of time during the rinsing.
Where an immediate stop of development is desired, a stop bath will usually consist of some concentration of acetic acid, commonly around 1 to 2%. Since organic developers only work in alkaline solutions, stop bath halts the development process almost instantly and thus provides more precise control of the development time. It also cuts overall processing time, because the required immersion time in the stop bath—typically fifteen to thirty seconds—is much shorter than the time required for an adequate plain-water rinse. As well, by neutralizing the alkalinity of basic developers, it can help to preserve the strength of the fixer, making it last longer.
Stop bath accounts for the characteristic vinegar-like odor of the traditional darkroom. In its concentrated form it can cause chemical burns, but is harmless when diluted to a working solution. Stop bath becomes exhausted when bases carried over from the developer cause the solution to become alkaline. For indicator stop bath - a stop bath that changes colours to indicate when the stop bath is exhausted and no longer effective - a pH indicator like bromocresol purple is used to determine when the solution has become too alkaline to use. Low-odor stop baths use citric acid or sodium bisulfite in place of acetic acid.