Stop bath

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The stop bath (also called interrupter bath ) is an acidic solution which is used to interrupt the development process in photographic films or papers. This is achieved by neutralizing the basic developer with the acid in the stop bath .

Because of the chemical properties of the subsequent fixer bath, only acidic solutions with a pH value of 4 to 6 are suitable for stop baths . The most commonly used bath is a 2% acetic acid . For this, 20 ml of acetic acid (glacial acetic acid) are made up to 1000 ml with water. Care should be taken when handling glacial acetic acid. It strongly etches the skin and solidifies at +12 ° C. Skin contact with rigid glacial acetic acid should be avoided at all costs.

A weak citric acid is just as effective, but without the odor typical of acetic acid.

The use of an acid stop bath is only required if an acid fixer is to be used to neutralize carried-over basic developer and thus to extend the service life of the fixer. Otherwise, a short intermediate soak is sufficient to end the development process.

Films and papers should not be treated in the stop bath for longer than 60 seconds, otherwise they will become acidic. During the final wash, acidification makes it difficult to wash out the silver salts .