Sodium Sulfate

Sodium sulfate (Na₂SO₄; CAS 7757-82-6) is a neutral salt used as a fixer restrainer, a controlled-diffusion developer component, and the active ingredient in Ilford's archival washing method.^[1]

Photographic uses

• Ilford washing sequence: A 1-minute bath in ~2% sodium sulfate between fixer and final wash accelerates the displacement of residual thiosulfate from fibre-based print emulsions. The full Ilford sequence (fix → rinse → 1-minute sulfate wash aid → 5-minute final wash) dramatically reduces total wash water compared to the traditional 1-hour running-water wash, with equivalent archival permanence.
• Fixer restrainer: Present in some Kodak and Ilford fixer formulations (not the standard F-5 or rapid fixers) to slow the dissolution of undeveloped silver halides and give finer grain in high-definition developers.
• Controlled-diffusion developers: Acts as an osmotic agent in some fine-grain formulas (Kodak D-23 variants) to retard the diffusion of developing agents into the emulsion, producing very-fine-grain results at the cost of film speed.

Practical notes

Supplied as white crystalline powder (anhydrous form Na₂SO₄) or as Glauber's salt (the decahydrate Na₂SO₄·10H₂O, which loses its water of crystallisation near 32 °C). The anhydrous form is photographically preferred — recipes assume the anhydrous mass unless specified otherwise. Working solutions are stable indefinitely.

Related compounds

Sodium sulfite is a different compound — it contains sulfur in the +4 oxidation state (a reducer) whereas sulfate is +6 (non-reducing). Do not substitute.