stopbaths

A stopbath is a solution used in photographic processing to halt the action of the developer and prevent overdevelopment. Typically, it is an acidic solution, commonly made from acetic acid diluted in water. Its primary function is to neutralize the alkaline developer remaining on the film or photographic paper after the development stage. By lowering the pH, the stopbath quickly stops the chemical reactions that are converting exposed silver halide crystals into metallic silver.

The use of a stopbath is a crucial step in many photographic processes, especially for black and white photography. After developing the film or paper, it is briefly immersed in the stopbath for a period ranging from 30 seconds to a minute. This ensures that the developer is thoroughly neutralized and removed from the emulsion. Failing to use a stopbath or using a depleted one can lead to uneven development, staining, or a reduction in image quality as development continues inappropriately.

After the stopbath, the photographic material is typically moved to the fixing bath, which removes the unexposed silver halide crystals, making the image permanent and insensitive to light. While water can sometimes be used as a stopbath, especially for certain paper processes, an acidic stopbath is generally more effective and efficient at halting development. Some photographic processes, particularly those using color developers, may use specific formulations for their stopbaths to ensure optimal results.