What is the primary purpose of a stop bath in film processing?

The primary purpose of a stop bath in film processing is to halt the development process. After film has been exposed to a developer, the development of the image continues until it is neutralized or stopped. The stop bath, which typically contains a diluted solution of acetic acid or citric acid, is applied immediately after the developer to quickly stop the chemical reactions that lead to film development. This step is crucial because if the development is not halted, the image could become overdeveloped, leading to excessive density and unwanted contrasts. The use of a stop bath ensures that the film reaches the desired level of development before undergoing the fixing process, where unexposed silver halides are removed to make the image stable and light-resistant.