Stop bath
Encyclopedia
Stop bath is a chemical bath usually used in processing traditional black-and-white photograph
ic film
s, plate
s, 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 alkali
ne 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 burn
s, 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 -- the pH indicator
bromothymol violet 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.
Photograph
A photograph is an image created by light falling on a light-sensitive surface, usually photographic film or an electronic imager such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of...
ic film
Photographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
s, plate
Photographic plate
Photographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...
s, and paper
Photographic paper
Photographic paper is paper coated with light-sensitive chemicals, used for making photographic prints.Photographic paper is exposed to light in a controlled manner, either by placing a negative in contact with the paper directly to produce a contact print, by using an enlarger in order to create a...
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
Photographic developer
In the processing of photographic films, plates or papers, the photographic developer is a chemical that makes the latent image on the film or print visible. It does this by reducing the silver halides that have been exposed to light to elemental silver in the gelatine matrix...
and fixer
Photographic fixer
Photographic fixer is a chemical or a mix of chemicals used in the final step in the photographic processing of film or paper. The fixer stabilises the image, removing the unexposed silver halide remaining on the photographic film or photographic paper, leaving behind the reduced metallic silver...
, 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
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
, commonly around 1 to 2%. Since organic developers only work in alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
ne 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
Darkroom
A darkroom is a room that can be made completely dark to allow the processing of light sensitive photographic materials, including photographic film and photographic paper. Darkrooms have been created and used since the inception of photography in the early 19th century...
. In its concentrated form it can cause chemical burn
Chemical burn
A chemical burn occurs when living tissue is exposed to a corrosive substance such as a strong acid or base. Chemical burns follow standard burn classification and may cause extensive tissue damage. The main types of irritant and/or corrosive products are: acids, bases, oxidizers, solvents,...
s, 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 -- the pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...
bromothymol violet 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.