Indicator Stop Bath
Indicator Stop Bath is acetic-acid stop bath with a bromcresol-purple pH indicator dye added — the standard commercial stop bath for B&W photography for the past 50+ years. The indicator dye turns from clear-yellow (fresh, working pH ~3-4) to purple-blue (exhausted, pH > 5), giving the printer a visible cue to discard or replenish without pH-strip testing or arbitrary capacity counting.
Key features
- Acetic acid stop chemistry with bromcresol-purple pH indicator
- Visual exhaustion cue: clear-yellow → purple-blue color shift
- Sold by Kodak, Ilford, Photographer's Formulary in liquid concentrate form
- Standard dilution: 1.5% acetic acid working solution (typically 1+47 from concentrate)
- Capacity per liter: ~25 8x10 fiber prints or ~30 RC prints before indicator color shift
Workflow
Standard procedure:
- Pour working solution from concentrate (typically 1+47 dilution to working strength)
- Tray at room temperature
- Use as the second tray after development (5-15 sec for prints, 30 sec for film)
- Watch for color shift — when bath turns purple, discard and mix fresh
Practical notes
- The indicator dye is harmless to prints — it does not stain or affect tonal rendering
- Choose Indicator Stop over plain acetic stop when you don't want to track capacity manually
- Choose Citric Acid Stop Bath over Indicator Stop when odor matters (acetic smells like vinegar; citric is odorless) or when fiber-paper blistering risk matters
- Concentrate keeps 1+ year sealed; opened concentrate keeps 6+ months
- Working solution lifetime: visual indicator is the authoritative signal
- PPE: nitrile gloves; acetic acid smell during mixing of fresh working solution
Related recipes
- [[recipe-citric-acid-stop-bath|Citric Acid Stop Bath]] — odorless modern alternative; preferred for fiber paper printing
- [[recipe-water-stop-bath|Water Stop Bath]] — minimalist alternative for film
- [[recipe-kodak-fixer-f-5|Kodak Fixer (F-5)]] — the standard next step after stop bath
- [[recipe-rapid-fixer|Rapid Fixer]] — modern fast-fixing alternative
Mixing Instructions
For working solution: add 48ml of 28% acetic acid per liter of water at 20°C. If using glacial acetic acid (99%), dilute to 28% first: slowly add 3 parts glacial acetic acid to 8 parts water (ALWAYS add acid to water, never water to acid). The indicator dye is included in commercial preparations. The solution is ready to use immediately — no waiting required. Replace when the yellow color shifts toward purple-blue, indicating exhaustion.
Ingredients for 1L of Concentrate
| # | Chemical | Role | Qty (1L) | Unit | Note |
|---|---|---|---|---|---|
| 1 | Acetic Acid | Other | 15 | ml |