Stop Bath
A mildly acidic solution used after development to halt the process immediately.
Measured in
seconds
In depth
Development is alkaline, so an acid bath neutralises it instantly rather than waiting for a water rinse to dilute it away. This gives precise control over development time and prevents developer being carried into the fixer, which would exhaust it prematurely. Many photographers use plain water for film and reserve acid stop for paper.
Examples in practice
Thirty seconds in an acetic acid stop bath halts development to the second, making a ten per cent time change meaningful.