Rochelle Salt

Rochelle salt (potassium sodium tartrate tetrahydrate; KNaC₄H₄O₆·4H₂O; CAS 6381-59-5) is a mild tartrate-buffer salt used in a few specialty developers, blue-toning formulas, and as a clearing-bath component in historical processes.^[1] It forms a tartrate buffer system with tartaric acid and acts as a weakly-complexing chelator for calcium and some transition-metal ions.

Photographic uses

• Developer buffer: Present in some fine-grain and low-contrast B&W developers as a secondary buffer alongside carbonate or borate systems.
• Blue toning formulas: A component of some iron-ferricyanide blue-toner variants where its mild chelation helps keep the toner bath clear.
• Farmer's Reducer variations: Used in some soft-working reducer formulas where the tartrate buffers the reducer action against runaway oxidation.
• Carbon printing (historical): An ingredient in some 19th-century carbon-tissue sensitizer clearing baths.

Practical notes

Supplied as large colorless crystalline prisms or as a white powder. Readily soluble in water. The hydrate form (tetrahydrate) is the common commercial grade — substitutions with anhydrous forms require a molar-weight adjustment.

Related compounds

Sodium potassium tartrate is the same compound under a different name. Tartaric acid is the free acid form.