Hydrogen Peroxide

OtherH2O2CAS: 7722-84-1Shelf life: 12 mo
Hydrogen Peroxide
Image: Д.Ильин — CC0

Physical Properties

  • Molecular Weight: 34.01 g/mol
  • Solubility (20°C): 1000 g/L

Also known as: H2O2, Peroxide

Hydrogen peroxide (H₂O₂; CAS 7722-84-1) is a strong oxidizer used in photography primarily as a hypo eliminator — a chemical wash aid that accelerates the oxidation of residual sodium thiosulfate in prints, converting thiosulfate to harmless sulfate much faster than water washing alone achieves.[1] Modern workflows largely rely on sulfite-based hypo clears (which react with thiosulfate without the fire-safety concerns of peroxide), but peroxide remains in some specialty and historical formulas.

Photographic uses

  • Hypo eliminator (historical): A 1% hydrogen peroxide + 2% ammonia bath converts residual thiosulfate to sulfate in about 5 minutes, followed by a final water wash. Kodak HE-1 is the classical formula.
  • Silver-stain remover: Dilute peroxide can remove silver deposits from stainless-steel trays and tanks where ordinary acidic cleaners would etch the steel.
  • Reversal-processing bleach (historical): Some early B&W reversal kits used peroxide-based bleaches before the ferricyanide + dichromate bleaches became standard.
  • Print restoration: Carefully-controlled peroxide baths can remove yellow stains from archival silver prints, though the chemistry risks bleaching the image if overdone.

Practical notes

Supplied as aqueous solution at various concentrations: 3% (pharmacy OTC — the household "hydrogen peroxide"), 30% (laboratory grade), or 50%+ (industrial, rare in darkrooms). The 30% form is the typical photographic grade; 3% household peroxide requires correspondingly more volume for equivalent chemistry.

Solution stability: hydrogen peroxide slowly decomposes to water + oxygen, especially when catalyzed by metal ions or sunlight. Keep in amber bottles (brown peroxide bottles are standard) in a cool dark cabinet. Vented caps are used in some commercial products because O₂ pressure can build in sealed containers.

Never heat hydrogen peroxide — decomposition accelerates exponentially with temperature. Never mix with organic solvents, flammable materials, or metal shavings.

Related compounds

Sodium perborate and sodium percarbonate are peroxide-releasing compounds used in some "oxygen bleach" formulations. Hydroxylamine sulfate is a different oxidant/reductant-balance compound used in colour chemistry.

Safety Notes

Acute hazards

Hydrogen peroxide hazards scale sharply with concentration:

  • 3% (household): mild irritant. Brief skin contact produces transient whitening from oxygen microbubbles in the skin surface; rinses off without damage.
  • 30% (laboratory): severe skin and eye burns. Concentrated H₂O₂ causes painful chemical burns and can cause permanent eye damage with direct contact.
  • Oxidizer (H272): intensifies combustion of organic materials; contact with fabric, paper, or wood at 30%+ can initiate smoldering.

Fire & reactivity

  • Strong oxidizer: segregate from combustibles, organic solvents, reducing agents. Decomposition releases oxygen, which accelerates any nearby fire.
  • Decomposes violently when catalyzed by metal ions (iron, manganese, copper), organic solvents, or base. Never add metal shavings to H₂O₂; do not mix with organic liquids without understanding the compatibility.
  • Heat-sensitive: decomposition rate roughly doubles per 10 °C above 20 °C.

Storage & handling

  • Personal protective equipment: Nitrile or butyl gloves, safety goggles (not just glasses) for any handling of 30%+ solution. Long-sleeved clothing, closed-toe shoes.
  • Ventilation: Open 30% containers in well-ventilated area; O₂ evolution is normal and harmless but displaces air in closed spaces.
  • Storage: Amber glass or HDPE bottle in a cool dark cabinet. Separate from organic materials, reducing agents, and metals.

First aid

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

  • Skin contact: Flush with water for 15 minutes. For 30%+ splashes, seek medical attention.
  • Eye contact: Flush with running water for at least 20 minutes. Seek medical attention immediately for any exposure above 3%.
  • Ingestion: Rinse mouth; do NOT induce vomiting. 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. 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