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

[Vaughn]

The citric acid route does sound a good alternative if keeping concentrated acid solutions is not acceptable. Citric acid is found in fruit, and the kids might enjoy the fact that citric acid was produced on an industrial scale using mold shit. Instead of yeast beasties producing alcohol from sugar, we have/had molds produced citric acid from sugar.

Hopefully all chemicals, except those in use under supervision, are stored where they are not accessible by the students.

[Michael R]

Hi Doremus,

The concentration of the bottle of Kodak Indicator stop bath is 85-90% acetic acid. This is why you only need 16ml/l to make the working solution which is more or less SB-1 (with indicator dye).


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

[Joe O'Hara]

Steve's procedure is what I do. There is practically no problem with youngsters hurting themselves with citric acid. It is used in canning food, after all, and tasting the powder will cure you forever of the urge to do so again. The five pounds of food grade from A-zon in my darkroom will last me the rest of my working life.

Capacity? If the print still feels "slippery" after 30 seconds with constant agitation, the stop bath is dead. I have never had this happen, but I do not invite throngs of teenagers into my darkroom, either. (You do plan to equip these kids with nitrile gloves, don't you? Harbor Freight is your friend, in that connection.)

I use 15g per liter (1.5%) but as others note it is not critical. The only reason for stop bath when printing, anyway, is to prevent brown stains from oxidized developer that might persist in the fiber backing. I used to do just a water stop bath but got burned a couple of times by staining, so I "stopped" doing that, so to speak.

BTW if you are using RC paper (good for people with short attention spans ;) ) you could probably just use running water instead, if cost is a critical issue.

[Doremus Scudder]

Hi Doremus,

The concentration of the bottle of Kodak Indicator stop bath is 85-90% acetic acid. This is why you only need 16ml/l to make the working solution which is more or less SB-1 (with indicator dye).

Thanks for setting me straight, Michael. I should know that, of course, but wasn't thinking.

Doremus

[FotoD]

There is practically no problem with youngsters hurting themselves with citric acid.

It's really not a good idea to get it in the eyes. That goes for either acid. Flush the eye with water for many minutes.

[Rod Klukas]

Note: IndicatorStop can stain if used at bottle indicated dilution. For film, You need to be aware that with some water, it can cause a sort of blow out between the emulsion and the Estar backing, creating pinholes. So we always used it at 1/2 strength, especially for prints. The print won't look definitely yellow, but the very slight stain cuts the contrast of the print. Sort of a slight veiling of the overall printed image.

Just some notes.

Rod