Sodium Carbonate Monohydrate

Sodium carbonate monohydrate (Na₂CO₃·H₂O; CAS 5968-11-6) is the hydrated form of sodium carbonate and the canonical photographic grade of the compound. The water of crystallization makes the monohydrate less hygroscopic than the anhydrous form, stabilizing its weight across varying humidity and making it the accurate-weighing form preferred by formulas that specify a carbonate concentration.^[1]

Photographic uses

Photographically identical to anhydrous sodium carbonate — same activator role, same working pH, same formulas. The monohydrate is simply the form most classical published recipes intend when they say "sodium carbonate" (particularly recipes from the Kodak and Ilford formula handbooks, where the monohydrate was the supplied form).

Practical notes

Weight conversion: Monohydrate contains roughly 85% sodium carbonate by weight (MW ratio 106/124). Multiply the monohydrate weight by 0.85 to get the equivalent anhydrous weight, or multiply the anhydrous weight by 1.17 to get the equivalent monohydrate. Example: Dektol calls for 80 g/L of the monohydrate, which is 68 g/L of the anhydrous form.

Getting this wrong produces under- or over-alkalized developer, which shifts contrast and film speed noticeably (15% error in developer alkali typically means 1/3-stop speed shift and 0.05–0.10 change in contrast index).^[2] If a formula is ambiguous about which form it specifies, compare the total carbonate mass to known recipes (D-72 uses 80 g/L monohydrate = 68 g/L anhydrous) to infer the intended form.

Storage and stability are the same as the anhydrous form.

Related compounds

Sodium carbonate (anhydrous) — see the primary page for the full photographic-use context and chemistry. Potassium carbonate is the more-soluble potassium analog.