Stop baths
From Phot-O-Vergne Wiki
To stop development abruptly a stopbath after the first developer bath is needed. Furthermore, alkaline residues can cause staining in combination with an acidic fixer. So, most photographers use some kind of stopbath, or at least several water rinses.
In fact many weak acid solutions can be used as a stop bath. But the stopbath chemical has to be compatibile with the following fixing bath, since a lot of acids and acid salts can cause the fixing bath to partly decompose and so stain the film or paper.
Most commonly acetic and citric acid are used. But Sodium bisulphite, Potassium or Sodium Metabisulphite and some other weak acids are perfectly safe at a strength of about 2 to 5 grams per liter of water. These chemicals form a solution of Sulphurous acid, and they're the most common chemicals used to acidify fixing baths anyway.
