Formula for Citric Acid Stop Bath?

[neil poulsen]

What's a formula for a decent citric acid stop bath?

I'm volunteering for a non-profit that supports youth, and I'm part of their photography program. Of course, Kodak stop bath is a standard; but, I don't like the concentrated form being around the kids.

Just to check, I've heard that a citric acid based stop bath is less caustic. Is this the case?

[paulbarden]

Yes, it's less hazardous to handle and the fumes aren't as corrosive.
2 tsp per liter of water.

[Tin Can]

Using food grade

Long Time

[neil poulsen]

Oh dear . . . I think I meant acetic acid stop bath, not citric acid. So, what would be a recipe for using acetic acid?

How long lasting is an acetic acid stop bath? That is to say, how would one know that it's spent?

Thanks for your comments.

[Mark Sampson]

The classical method was to use Kodak Indicator Stop Bath, which turns from orange to purple when the pH gets too high. I don't know if you can still buy that, or something equivalent.
i've used a citric acid stop bath for some years, and am very happy. It doesn't stink!
And I have always thrown out the stop bath at the end of the darkroom session. Easy, cheap, and no worries,

[jnantz]

it's about 1-2% using white household vinegar ,,

[Michael R]

1 part white vinegar to 1 part water gets you to approximately the concentration of working strength Kodak stop bath. The precise dilution is not critical. A litre of working solution is probably good for at least say 30 8x10 prints? Not sure as I’ve never had it exhaust on me in a printing session. Be conservative since it’s cheap.

Oh dear . . . I think I meant acetic acid stop bath, not citric acid. So, what would be a recipe for using acetic acid?

How long lasting is an acetic acid stop bath? That is to say, how would one know that it's spent?

Thanks for your comments.

[Duolab123]

Kodak SB-1 stop bath is 48mL of 28% acetic acid in 1 L water

[Steve Goldstein]

Kodak SB-1 stop bath is 48mL of 28% acetic acid in 1 L water

And household vinegar is 3-5% as I recall, so diluting 1:1 with water as Michael R suggests is fine.

Personally I use food-grade citric acid, 15 grams per liter. No smell. Capacity is somewhere around 15-18 8x10 per liter, it was still reasonably acidic after 18 sheets the one time I tested using pH strips.

Vinegar is probably cheaper. But a bottle of indicator stop bath goes a long way and will be more reliable in the environment you're working in. And besides, "vinegar" sounds way less threatening that "citric acid" to helicopter parents.

[Doremus Scudder]

A citric-acid stop:

Kodak SB-7a
Water 750 ml
Citric Acid 37.5g
Water to make 1 liter
An odourless, mild stop bath.


An acetic-acid stop:

Kodak SB-1
Water 750 ml
Acetic Acid (28%) 48 ml
Water to make 1 liter
For film or paper.


Acetic-acid stop baths are typically around 1.5-2% acetic acid in water. You can dilute distilled vinegar to that easily. It's usually around 5%. The 28% acetic acid mentioned in the above formula is the good old Kodak acetic acid concentrate they sold (glacial acetic acid mixed 3+8 with water - probably the strongest practical dilution that was safe to ship without hazmat precautions). I believe that the Indicator Stop is 28% acetic acid plus the indicator dye.

I still buy and like Kodak Indicator Stop. A bottle lasts me for a long time.

Doremus