Potassium Alum Solution

HardenerKAl(SO4)2 (aq)Shelf life: 6 mo
Potassium Alum Solution
Image: Hoa112008 — Public domain

Physical Properties

Also known as: Alum Hardener, Chrome-Free Hardener

Potassium alum solution is the pre-mixed aqueous form of potassium alum — typically a 10% solution in dilute acid for pH stability. Supplied pre-mixed as a convenience for workers who use small quantities occasionally and prefer not to mix stocks from dry salt.[1]

Photographic uses

Functionally identical to potassium alum. The pre-mixed solution saves the mixing step for small-volume work. Typical working dilution is further 1:5 to 1:10 for a hardening pre-soak or post-fix bath.

Practical notes

Standard commercial potassium alum solution is 10% w/v in acidified water (pH 4–5 via acetic acid or citric acid). Keeps for a year in amber glass at room temperature. If the solution has developed a white cloudy precipitate, the pH has drifted above 5.5 and the solution should be discarded.

See potassium alum for the full photographic use discussion and formula references.

Related compounds

Potassium alum — the primary page. Chrome alum is the stronger hardener variant.

Safety Notes

Same hazard profile as potassium alum — low toxicity food-grade compound with mild irritant potential in concentrated form. Solution form is even less hazardous than dry salt (no dust inhalation route).

Consult the Sigma-Aldrich Safety Data Sheet for the active ingredient.[2] Rinse with water for any contact; no medical attention typically needed.

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 ↩

Last reviewed 2026-04-22