Stop Bath

Overview

Development is a chemical reaction with momentum. When the timer ends, developer still trapped in the emulsion keeps working. Stop bath ends that immediately by dropping the pH, and it does it in about ten seconds.

Its second job is protecting the fixer. Fixer is mildly acidic and buffers its own pH; carry alkaline developer into it on the film and you slowly exhaust that buffer, shortening the fixer's life and risking incomplete fixation.

A two percent acetic acid solution is the classic. Indicator stop bath adds a dye that turns purple as the acid is neutralised, so you can see when to replace it. For people who dislike the smell — acetic acid is pungent — a plain water rinse at the same temperature is an acceptable alternative, though it is less effective.

What it does well

• Stops development in about ten seconds
• Protects the fixer from alkaline carry-over
• Indicator versions show exhaustion by colour
• Also removes developer staining from trays and hands
• Cheap enough to mix fresh for every session

Safety notes

Working notes

• Always dilute acid into water, never water into acid.
• Keep the stop bath within 2 °C of the developer; a cold stop causes reticulation in thin emulsions.
• For prints, a 10-second stop is enough — longer just wastes chemistry.
• If you use a water rinse instead, change the water at least twice and agitate continuously.

Compatible with