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Stop bath

Acid Foot Soak (Acetic or Citric Acid)

A simple acid solution that instantly stops development and extends the life of the fixer.

About the foot bath

A standard acid stop bath is the most common method of stopping development between the developer and the fixer. Unlike simply rinsing with water, the acid instantly neutralizes the alkali remaining in the emulsion after development and prevents it from carrying over into the fixer, thereby extending the fixer’s life. The working concentration is about 2% acetic acid; citric acid is used as a less corrosive and nearly odorless alternative. Both options are equally suitable for film and photographic paper.

How to Prepare and Use

  1. 1Dilute 30 ml of 70% acetic essence (or 6 ml of glacial acetic acid, 99–100%) in 1 liter of water—this will yield a working solution with a concentration of about 2%. Always pour the acid into the water, not the other way around.
  2. 2An option without a strong odor: Dissolve 15 g of citric acid (powder) in 1 liter of water instead of vinegar.
  3. 3Immediately after the developer (without an intermediate rinse or with a short rinse), immerse the film or paper in the stop bath.
  4. 4Gently swirl the vial or cuvette for 20–30 seconds.
  5. 5Pour the solution back into the storage container—the working solution can be reused multiple times—and move on to the fixing step.

Pros

  • It stops the development more precisely and reliably than rinsing with water
  • Extends the service life of the fastener
  • Inexpensive and readily available components
  • Citric acid—a mild-smelling option

Cons

  • Acetic acid has a pungent odor and irritates the respiratory tract when the vapors of the concentrate are inhaled
  • Requires ventilation when mixing with essential oils or glacial acetic acid
  • Does not replace a tanning stop bath for damaged or "delicate" emulsions

Recipe for 1 liter

Acetic acid 70% (essence)
30 ml
Acetic acid 70% (essence)
up to 1 liter
Water

Pour the acid into the water in a thin stream while stirring—never the other way around. Instead of the essence, you can use 6 ml of glacial (99–100%) acetic acid per 1 liter of water. For a milder scent, replace the acetic acid with 15 g of citric acid (powder) per 1 liter of water; it’s slightly less effective but easier on the nose.

Chemical structures: PubChem (public domain)