Sulfuric Acid

OtherH2SO4CAS: 7664-93-9
Sulfuric Acid
Image: Jü — Public domain

Physical Properties

  • Molecular Weight: 98.08 g/mol
  • Solubility (20°C): 1000 g/L

Also known as: Oil of Vitriol, Battery Acid, H2SO4

Sulfuric acid (H₂SO₄; CAS 7664-93-9) is the strong mineral acid of photographic chemistry — a viscous, dense, colourless liquid used in specialty bleaches, intensifier formulas, and as the acid component in some silver-recovery cells.[1] Most routine darkroom work uses the much safer weak organic acids (acetic acid, citric acid, tartaric acid), but a handful of formulas specifically require strong mineral acid chemistry that only sulfuric can provide.

Photographic uses

  • Chrome intensifier / chromium intensification: A historical intensifier formula uses dilute sulfuric acid + potassium dichromate to convert silver image to chromium-silver complex for increased density. Rare today; dichromate handling concerns have largely retired this technique.
  • Permanganate bleach acidifier: Potassium permanganate-based reversal bleaches require acidic pH for the bleach chemistry to work; dilute sulfuric acid (1–2%) is the usual acidifier.
  • Silver recovery (electrolytic): Electrolytic silver recovery cells use sulfuric acid as the electrolyte for the electrochemistry that deposits silver on a cathode for commercial recovery.
  • Ferrous sulfate stabilization: A few drops of dilute sulfuric acid per litre of ferrous sulfate developer stock extends shelf life by suppressing Fe(II)→Fe(III) oxidation.

Practical notes

Supplied as concentrated sulfuric acid (96–98% w/w), a viscous colourless oily liquid, or as various dilute solutions (10%, 25%, "battery acid" at ~37%). Concentrated sulfuric acid is dangerously reactive with water — dissolution is strongly exothermic, and adding water to concentrated acid can boil the water explosively, spraying hot acid out of the container.

The absolute rule: always add acid to water, slowly, with continuous stirring. The water must be in a large vessel capable of absorbing the dissolution heat without overheating. Never reverse this — the reverse is a classical laboratory injury scenario.

Dilute stock solutions (10–37%) are much safer to handle but still corrosive. Store in glass; concentrated sulfuric attacks most plastics over time.

Related compounds

Acetic acid is the weak organic acid of photographic stop baths. Nitric acid and hydrochloric acid are alternative strong mineral acids used in some alt-process formulas.

Safety Notes

Acute hazards

Concentrated sulfuric acid is among the most dangerous common laboratory chemicals:

  • Causes severe skin burns and eye damage (H314): direct contact produces deep chemical burns. Concentrated sulfuric dehydrates skin and organic tissue violently, producing characteristic charred-black burns.
  • Corrosive to metals (H290): degrades iron, aluminum, zinc, and most reactive metals.
  • Exothermic water reaction: dissolution releases enough heat to boil a small volume of water. Splash hazard during dilution.
  • Inhalation: acid mist is strongly irritating to respiratory tract.

Fire & reactivity

  • Violent water reaction — always add acid to water, slowly, with cooling.
  • Incompatibilities: Reactive metals (hydrogen gas evolution, explosion hazard with finely-divided aluminum or zinc), strong bases (violent neutralization), organic materials (charring, heat, potential ignition of cellulose on prolonged contact).
  • Chlorate / permanganate contact: produces manganese heptoxide (explosive) or chlorine dioxide; do not mix strong oxidizing salts with sulfuric acid.

Storage & handling

  • Personal protective equipment: Butyl or neoprene gloves (nitrile degrades quickly in concentrated H₂SO₄), splash-rated safety goggles with face shield for concentrated-acid work, long-sleeved clothing with acid-resistant apron, closed-toe shoes.
  • Ventilation: Use a fume hood for all work with concentrated sulfuric acid.
  • Storage: Glass container (never plastic) with tight acid-resistant cap, in a cool dry cabinet segregated from bases, organic materials, and reactive metals. Do not store above other chemicals — a spill splashing downward is a serious hazard.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[2]

  • Skin contact: Remove contaminated clothing. Flush affected skin with copious running water for at least 30 minutes. Seek medical attention immediately for any visible burn.
  • Eye contact: Flush eyes with running water for at least 30 minutes, lifting the lids. Seek medical attention immediately; eye exposure to concentrated sulfuric is a medical emergency.
  • Inhalation: Move to fresh air. Seek medical attention for respiratory symptoms.
  • Ingestion: Do NOT induce vomiting. Rinse mouth with water. Seek medical attention immediately; sulfuric ingestion is a medical emergency.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. 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 2026-04-22