Sodium Hydroxide

AcceleratorNaOHCAS: 1310-73-2
Sodium Hydroxide
Image: Claudio Pistilli — CC BY-SA 4.0

Physical Properties

  • Molecular Weight: 40 g/mol
  • Solubility (20°C): 1090 g/L

Also known as: Caustic Soda, Lye, NaOH

Sodium hydroxide (NaOH; CAS 1310-73-2), also called lye or caustic soda, is a very strong alkali used where maximum developer activity or minimum induction time is needed. It drives developer pH to 12–13 — the range where phenolic developing agents are fully ionized and where even small additional amounts produce chemical fog.[1] This is the active ingredient in high-contrast lithographic developers and rapid-access processors, and it shows up in small supplemental quantities in some general-purpose formulas where fine pH adjustment matters.

Photographic uses

  • Kodak D-8 and related high-contrast developers: Lithographic formulas use 60–100 g/L of sodium hydroxide with hydroquinone as the sole developing agent to produce extreme contrast grades.
  • Rapid-access film developers: Formulas designed for 30-second development use high concentrations of NaOH to maximize developing-agent activity.[2]
  • pH adjustment: Small additions (fractions of a gram per litre) bring a borax or carbonate developer up to target pH precisely.
  • Emulsion stripping / paper base cleaning: Very dilute NaOH removes residual emulsion and stains; industrial use only.

Practical notes

Supplied as white pellets, flakes, or fine granular powder. Extremely hygroscopic and carbon-dioxide-reactive — fresh pellets quickly develop a slick surface of sodium carbonate + moisture on contact with ambient air. Use a precision scale in a draft-free space and minimize the time between opening the container and closing it. Degraded (carbonated) NaOH contains mixed NaOH + Na₂CO₃ with reduced alkali strength.

Dissolution is violently exothermic: adding NaOH pellets to water can boil a small volume of water within seconds. The textbook rule is absolute: always add NaOH slowly to excess cool water, stirring continuously. Never pour water onto NaOH — the reaction can eject boiling caustic solution explosively.

Related compounds

Potassium hydroxide is the potassium analog; essentially interchangeable for photographic use, preferred in liquid concentrates for solubility. Sodium carbonate is the next-milder alkali for lower-activity formulas. Sodium hydroxide solution is this same chemical stocked as a pre-mixed aqueous solution.

Safety Notes

Acute hazards

Sodium hydroxide is highly corrosive and the most dangerous chemical in this cluster alongside potassium hydroxide:

  • Severe skin burns on direct contact with pellets or concentrated solution. Saponification of skin fats produces a slippery feel that masks the injury; contact with dilute solution may feel like soap before the burn becomes evident.
  • Severe eye damage: concentrated solution causes permanent corneal damage within seconds of contact.
  • Inhalation risk: fine dust from fresh pellets is strongly irritating to the upper respiratory tract.
  • Violent water reaction: dissolution releases enough heat to eject boiling solution if added incorrectly.

Fire & reactivity

  • Water-reactive in bulk. Always add pellets slowly to excess cool water.
  • Incompatibilities: Strong acids (violent neutralization with splash risk), aluminum / zinc / tin metals (hydrogen gas evolution; real explosion hazard with confined solution), nitromethane and some organic solvents.

Storage & handling

  • Personal protective equipment: Butyl or neoprene gloves; face shield (not just safety glasses) for pellet handling; long-sleeved apron; closed-toe shoes.
  • Ventilation: Mix in well-ventilated area; use local exhaust for bulk pellet handling.
  • Storage: Tightly sealed in a cool dry cabinet with desiccant. Keep physically separated from acids and reactive metals.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[3] In brief:

  • Skin contact: Remove contaminated clothing. Flush affected skin with copious running water for at least 20 minutes. Seek medical attention for any visible burn or prolonged contact.
  • Eye contact: Flush eyes with running water for at least 30 minutes, lifting the lids. Seek medical attention immediately; NaOH eye contact is a medical emergency with permanent-blindness risk.
  • Inhalation: Move to fresh air; seek medical attention if breathing is difficult.
  • Ingestion: Do NOT induce vomiting. Rinse mouth with water. Seek medical attention immediately.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. BOOK Anchell, Steve. The Darkroom Cookbook 4th ed. Focal Press, 2016. ISBN 9781138959170. ↩
  3. WEB Sigma-Aldrich (Merck KGaA). Sigma-Aldrich Safety Data Sheets Sigma-Aldrich. https://www.sigmaaldrich.com/US/en/search/safety-data-sheets ↩

Reference databases

Last reviewed 2026-04-21