A stop bath is recommended between developer and fixer if the machine configuration permits. This has the following function: It stops post-development. It stops alkaline developer being carried over into the fixer. This prevents the formation of dichroic fog. It permits the use of neutral fixer. It lengthens the life of the fixer. The 2% acetic acid stop bath is mixed as follows : 1 parts acetic acid (60%) + 30 parts water.