Sodium thiosulfate or ammonium thiosulfate are used as fixing agents. An acid such as acetic acid stops the developing action, particularly when no stop bath is used. A preservative is included to prevent the disintegration of the fixer by the acid. A hardener is usually only used in film development to protect the emulsion from softening and scratches. Hardener used in print development makes spotting, toning and other manipulations difficult and is not advised.
Potassium alum is a common hardening agent.
A buffer , usually boric acid is used to maintain the acidity of the fixer when a hardener is used.