EDTA Disodium

Disodium EDTA (ethylenediaminetetraacetic acid, disodium salt; C₁₀H₁₄N₂Na₂O₈; CAS 139-33-3) is a chelating agent — a compound that binds dissolved metal ions (Ca²⁺, Mg²⁺, Fe²⁺/³⁺, Cu²⁺) into stable water-soluble complexes. In photographic solutions, it prevents hard-water scale from precipitating in developers and fixers, and sequesters trace iron and copper that would otherwise catalyze oxidation of developing agents.^[1]

Photographic uses

• Hard-water protection: 0.5–2 g/L EDTA in a developer stops calcium carbonate scum from forming on negatives and keeps mixed solutions clear. Essential in areas with water hardness > 150 mg/L CaCO₃.
• Oxidation inhibitor: EDTA extends working-solution life by binding trace iron and copper that would otherwise catalyze developer oxidation. Often paired with sodium sulfite in this role.
• Silver sequestration in wash aids: Removes residual silver-thiosulfate complexes during archival washing.
• Component in RA-4 and C-41 bleach formulas: EDTA-iron(III) complexes are the core of modern colour bleaches (a role served historically by potassium ferricyanide).

Practical notes

Supplied as a white free-flowing powder. Readily soluble in water at neutral pH; alkaline solutions dissolve it fastest. Working solutions are stable for months if kept free of metal contamination.

Related compounds

Tetrasodium EDTA (Na₄-EDTA) is the fully-deprotonated salt and is more strongly alkaline; the disodium form is preferred in photographic chemistry for its near-neutral pH.