Stop Bath question

[Steve Goldstein]

Two stop bath formulae in The Darkroom Cookbook 4th edition use 28% Acetic acid. The SB-1 formula calls for 48ml/l while non-swelling SB-5 uses 32ml/l plus 45ml (I assume that should be grams) Sodium sulfate anhydrous.

I was just looking through my new copy of The Film Developing Cookbook 2nd edition and spotted TS-7, billed as a buffered non-swelling stop bath. It uses 120ml/l of 28% Acetic acid plus 80g/l Sodium acetate. The accompanying text doesn't say anything about this being a concentrate or stock solution so I infer it must be working strength. 120ml/l seems like an awful lot of 28% Acetic acid. Can any of you chemists out there comment? I have vague memories from AP chemistry about how buffers work, but let's just say it's been a few decades since I acquired that knowledge....

 

[pentaxuser]

I don't think it needs a chemist to comment. I use Kodak stop at 28% and it has to be diluted at 1:63 so that equals about 15ml of stop at 28% 120 m/L of 28% acetic acid would make over 7.5L of usable stop

Is there something different about the Cookbook's stop in terms of use?

pentaxuser

 

[JPD]

I use 24% Acetic acid, and only about 20 ml per liter. I use it "one shot" for film and for making a few prints, and have never had any problems with it. I think that if you are printing a lot or reusing the stop bath for film, then a commercial one with indicator would be better. But Acetic acid is cheap, so you can change the stop bath after ten or so prints anyway.

 

[Steve Goldstein]

JPD, thank you, but my question is very specifically about the formulation in the new edition of The Darkroom Cookbook - the high proportion of 28% Acetic acid caught my eye and I'm curious. I use 15 g/l Citric acid stop for printing and half-strength SB-1 Acetic acid stop for film, both one-shot, and have no plans to change that practice.

Pentaxuser, the Cookbook's stop bath, which I believe is a Bill Troop formulation, claims a capacity of 40 8x10 per liter, which is a lot more than a non-buffered stop could do. There's no mention of dilution for use so the implication is to use as-formulated. It's pH is stated to be higher than a simple acetic acid stop (though still low enough to be effective), and because of the buffering it holds pretty close to that pH until nearly exhausted. He also claims it's very quick-acting, as little as 3 seconds, which he says is faster than something like the standard indicator stop (the SB-1 formula with indicator). The same amount of 28% Acetic acid will make 2.5 liters of SB-1 and give approximately the same capacity, so I also wonder what the real benefits of the buffering would be. If my film spends 10-15 minutes being developed, what does it matter if it stops in 3 seconds versus 10?

I'm hoping one of the chemists on Photrio can enlighten me.

 

[Donald Qualls]

I'm not a chemist, either, but I'd point out that books sometimes contain errors. In this case, I suspect that the formula was supposed to 20ml/L of 28% rather than 120. That, combined with the sodium acetate buffering agent, ought to produce a long-lasting working solution, rather than a concentrate at an odd 6:1 or higher ratio of normal working strength.

What's supposed to happen here is that the sodium acetate provides a reserve of acetate in solution without all of it being in the acidic form; as carryover of developer neutralizes the acid, the acetate in solution "steps up" to restore it, while the sodium ions (which form sodium hydroxide in solution, sort of) stabilize the pH against excessive drop (as would be the case if you use acetic acid for all of the acetate). As such, you should have enough 28% (or whatever your starting strength) to produce a 1.5% final strength, and then sodium acetate is added to prolong the solution life.

 

[john_s]

A post I copied from the puresilver (iirc) site years ago:

"buffered stop bath (replenishable)
Written by Ryuji Posted: 06:09 AM - 01-19-2005

acetic acid, 90% 20ml
sodium acetate 80g
water to make 1.0 liter
target pH 5.0 plus/minus 0.5

same thing can be made this way:
acetic acid, 90% 60ml
sodium hydroxide 25g
water to make 1.0 liter
target pH 5.0 p/m 0.5

Either bath can be replenished at the rate of 30 ml glacial acetic acid per each 30 of 80 sqi processed. Buffered stop bath may appear more expensive but lasts MUCH longer than 1.7% acetic acid bath. The replenishing capability is very useful for Nova slot processor because changing the entire bath is a real pain in that processor."

 

[john_s]

And if you're really interested, try this:

https://www.photo.net/discuss/threads/what-is-stop-bath-for.37251/

 

[Steve Goldstein]

A followup...

I asked Bill Troop about the TS-7 formula. He says the FDC2 formula assumed Sodium acetate trihydrate, although this wasn't stated. If I've done my maths correctly, 48.23g of the Sodium acetate anhydrous can be used instead of 80g of the trihydrate. Mike at Artcraft told me a couple of days ago that he sells the anhydrous form.