Copper Sulfate

TonerCuSO4·5H2OCAS: 7758-99-8
Copper Sulfate
Image: ERS ROHU — CC BY-SA 4.0

Physical Properties

  • Molecular Weight: 249.68 g/mol
  • Solubility (20°C): 317 g/L

Also known as: Blue Vitriol, Cupric Sulfate, CuSO4

Copper sulfate (pentahydrate, CuSO₄·5H₂O; CAS 7758-99-8) — the iconic bright blue crystalline salt of laboratory chemistry — is used in photographic copper toning to produce red-to-reddish-brown tones on silver gelatin prints, and as the copper source for copper intensification of weak negatives.[1] The toning chemistry is a bleach-and-redevelop two-bath process: a copper sulfate + potassium ferricyanide bleach converts the silver image to silver ferrocyanide + copper ferrocyanide; the combined salts produce a characteristic warm reddish-brown image.

Photographic uses

  • Copper toner: The classical formula: CuSO₄ + potassium ferricyanide + potassium bromide + ammonium carbonate or sodium acetate, brushed or tray-applied to a well-fixed silver print. Produces warm red-brown tones that shift toward deeper red with extended treatment.[2]
  • Copper intensification: Weak underexposed negatives gain density through copper ferrocyanide deposition on the silver image. Increases printing contrast without reprocessing.
  • Cyanotype variants: Trace copper additions to the sensitizer modify Prussian-blue formation kinetics.
  • Historical split-toning: Brief copper toning followed by gold or selenium toning produces classical split-tone effects (warm highlights, cool shadows).

Practical notes

Supplied as deep blue crystalline granules (the pentahydrate) or as fine pale-blue anhydrous powder. The pentahydrate is the common photographic grade; to substitute anhydrous, multiply formula weights by 0.64.

Highly soluble in water (30 g/100 mL at 20 °C) giving intense blue solutions. Solutions are stable indefinitely in closed glass. The pentahydrate slowly effloresces (loses water) at warm temperatures but the blue colour remains — weight accuracy drops as the efflorescence progresses.

Staining: copper sulfate solutions stain skin and porous surfaces a green-blue that persists for days.

Related compounds

Copper chloride (CuCl₂) is an alternative copper salt for specific toner formulas. Potassium ferricyanide is the partner reagent in most copper-toner formulas.

Safety Notes

Acute hazards

  • Harmful if ingested: moderate acute toxicity; substantial swallow causes nausea, vomiting, and copper accumulation. Medically significant.
  • Skin and eye irritant: concentrated solution causes redness and stinging.
  • Very toxic to aquatic life (H410): dispose of spent solutions as hazardous waste, not to drain.

Storage & handling

  • Personal protective equipment: Nitrile gloves, safety glasses; dust mask for weighing anhydrous powder.
  • Storage: Tightly closed container in a dry cabinet.

First aid

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

  • Skin contact: Flush with water. Blue-green staining fades over days.
  • Eye contact: Flush with running water for at least 15 minutes.
  • Ingestion: Rinse mouth; seek medical attention for substantial swallow.

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 2026-04-22