Editing Stop bath

{{short description|Acidic solution for processing black-and-white photos}}
[[File:Self portrait in the darkroom JPG01.jpg|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]], [[Photographic plate|plates]], and [[Photographic paper|paper]]. It is used to neutralize the alkaline developer, thus halting development.<ref name="Sowerby-1961" />

A stop bath is commonly a 2% dilution of [[acetic acid]] in water, though a 2.5% solution of [[potassium metabisulfite|potassium]] or [[sodium metabisulfite]] works just as well.<ref name="Sowerby-1961" /> Because organic developers only work in alkaline solutions, stop baths halt the development process almost immediately and provides precise control of development time. Neutralizing the alkalinity of basic developers also helps to preserve the strength of the fixer, making it last longer.{{cn|date=March 2020}}

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. Stop bath becomes exhausted when carried over developer causes the solution to become alkaline.{{Incomprehensible inline|date=April 2025}} 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.<ref name="Anchell 2016" /> Low-odor stop baths use [[citric acid]] or [[sodium bisulfite]] in place of acetic acid.{{cn|date=March 2020}}

==References==
{{reflist|
<ref name="Anchell 2016">{{cite book |url=https://books.google.com/books?id=u67OCwAAQBAJ&pg=PT622 |title=The Darkroom Cookbook |first=Steve |last=Anchell |via=Google Books |year=2016 |edition=4 |publisher=Routledge |isbn=9781317337607}}</ref>
<ref name="Sowerby-1961">{{cite book |title=Dictionary of Photography: A Reference Book for Amateur and Professional Photographers |editor=Sowerby, A.L.M. |publisher=Iliffe Books Ltd. |pages=627–628 |location=London |edition=19th |year=1961}}</ref>
}}

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[[Category:Photographic processes]]