Stop Bath.. How important?

[snusmumriken]

It's a general recommendation to not share use of stop and fixer between film and paper. But the most practical reason for it is to keep paper fibres from sticking to film. Film stop and fixer should be as clean as possible.

I read somewhere (perhaps earlier in this thread?!) that the main reason to keep them separate was that paper contains a lot more silver than film, so used paper fixer will contain a lot more breakdown products. I don’t know whether film is more sensitive to those than paper is, but it makes sense not to risk the negative, which is irreplaceable whereas the print is not.

 

[Ben Hutcherson]

I have heard that stop bath and in particular Kodak Stop Bath with Indicator is buffered to keep the pH in the proper range. That and a $50US bill will get you a cup of StarBucks coffee at the airport.

I'd be really curious on that one. The SDS for Kodak Indicating Stop Bath indicates 85-90% Acetic Acid. Glacial Acetic Acid is ~100% acetic acid, so there's at least some water(presumably) present in indicating stop. Acetic acid is the only thing listed on the SDS, but of course SDSs don't have to list everything present, just the things that are of concern for safe handling.

It COULD be that the water is used to dissolve the indicator(bromocresol purple from most references I can find). I'm not sure if some source of acetate ions would need to be listed out separately, especially considering that acetic acid in the presence of water will always dissociate to give acetate ions to some degree, so if they'd just happened to add extra it wouldn't necessarily need to be listed on the SDS.

I did mix some up the other day that I have yet to try. I started with ~100mL of water, then added ~50mL of glacial acetic. From there, I added in sodium acetate until I had the pH up to 5.25. At pH 5.5, I added the indicator, bromocresol purple, which is already starting to show a color change at that pH. I then added glacial acetic acid to bring the pH back down to 4.25, where I had a nice orange color, and bottled it all up to dilute for later use.

My thinking on doing it this way is this:

If I was going to go to the trouble of buffering, I wanted to get a lot of buffering capacity, especially since it will be used fairly dilute. PE told us that 4.5 is sort of an optimum value, if I understood his posts correctly. As I said, the usual rule of thumb is that the "range" of a buffer is +/-1 of its pKa, but if you graph out a titration curve for example you find that pH REALLY starts to stabilize around +/-.5 of the pKa, and evne more so at around +/-.25. Buffers really are "best" where pH=pKa, but 4.5 is still a very workable value for an acetate buffer(pKa=4.76, or 4.74 depending on who you ask). Of course pKas are usually given at 25ºC, and do change somewhat at different temperatures, but this should be close enough to typical B&W processing temperatures to work out close enough.

In any case, I have yet to use my stop to see how it works. I should probably buy a bottle of the real Kodak indicating stop and measure its buffering capacity, although that's more work than I can reasonably do at the moment...

 

[koraks]

I'd be really curious on that one

You've thought about it, so surely you've also come to the conclusion that even if you start with an unbuffered acetic acid stop bath, it'll turn into a buffer by simply using it, as developer is being carried over into the stop bath. Starting out with a buffer in fact reduces the capacity of the stop bath because you essentially 'contaminate' it in order to get within the buffer range right from the start. The advantage of this, however, would be that you don't risk the evolution of sulfur dioxide due to dissociation of sulfite at a low pH (<4.0). AFAIK this is also the main reason why pH~4.5 has been suggested as a suitable pH for a stop bath. It's acidic enough to stop development and not quite enough so that it starts stinking up the place if some dektol ends up in it.

 

[Milpool]

I'd be really curious on that one. The SDS for Kodak Indicating Stop Bath indicates 85-90% Acetic Acid. Glacial Acetic Acid is ~100% acetic acid, so there's at least some water(presumably) present in indicating stop. Acetic acid is the only thing listed on the SDS, but of course SDSs don't have to list everything present, just the things that are of concern for safe handling.

It COULD be that the water is used to dissolve the indicator(bromocresol purple from most references I can find). I'm not sure if some source of acetate ions would need to be listed out separately, especially considering that acetic acid in the presence of water will always dissociate to give acetate ions to some degree, so if they'd just happened to add extra it wouldn't necessarily need to be listed on the SDS.

I did mix some up the other day that I have yet to try. I started with ~100mL of water, then added ~50mL of glacial acetic. From there, I added in sodium acetate until I had the pH up to 5.25. At pH 5.5, I added the indicator, bromocresol purple, which is already starting to show a color change at that pH. I then added glacial acetic acid to bring the pH back down to 4.25, where I had a nice orange color, and bottled it all up to dilute for later use.

My thinking on doing it this way is this:

If I was going to go to the trouble of buffering, I wanted to get a lot of buffering capacity, especially since it will be used fairly dilute. PE told us that 4.5 is sort of an optimum value, if I understood his posts correctly. As I said, the usual rule of thumb is that the "range" of a buffer is +/-1 of its pKa, but if you graph out a titration curve for example you find that pH REALLY starts to stabilize around +/-.5 of the pKa, and evne more so at around +/-.25. Buffers really are "best" where pH=pKa, but 4.5 is still a very workable value for an acetate buffer(pKa=4.76, or 4.74 depending on who you ask). Of course pKas are usually given at 25ºC, and do change somewhat at different temperatures, but this should be close enough to typical B&W processing temperatures to work out close enough.

In any case, I have yet to use my stop to see how it works. I should probably buy a bottle of the real Kodak indicating stop and measure its buffering capacity, although that's more work than I can reasonably do at the moment...

There is discussion about the “ideal” acetic acid-sodium acetate buffered stop bath in Haist and other literature. It should be both very fast and have high capacity. This is probably something of more use to high volume processing than, although the lower odour of a pH 4-4.5 buffered acetic acid/acetate stop bath can be a nice thing for a home darkroom.

As far as I know Sprint Systems Block Stop Bath is the only commercially available stop bath of this type (acetic acid/sodium acetate mixture, working pH ~4.0).

 

[chuckroast]

You've thought about it, so surely you've also come to the conclusion that even if you start with an unbuffered acetic acid stop bath, it'll turn into a buffer by simply using it, as developer is being carried over into the stop bath. Starting out with a buffer in fact reduces the capacity of the stop bath because you essentially 'contaminate' it in order to get within the buffer range right from the start. The advantage of this, however, would be that you don't risk the evolution of sulfur dioxide due to dissociation of sulfite at a low pH (<4.0). AFAIK this is also the main reason why pH~4.5 has been suggested as a suitable pH for a stop bath. It's acidic enough to stop development and not quite enough so that it starts stinking up the place if some dektol ends up in it.

I buy glacial acetic acid by the gallon an make 28% stock mixing 3+8 with water. I then add 6oz/gal of this to make a working solution. Alternatively, I mix plain white vinegar 1+4 with water.

Both of these are dirt cheap (though, interestingly, the vinegar works out to be slightly more expensive). So my sort of meta question about all this why bother with buffering at all? My working strength stop bath is single shot and done. Even with very long printing sessions (I never use stop with film), the stuff is cheap enough that I can toss it and create a new tray full midway through the session.

What am I missing? This sort of feels like a non-problem but I am well aware that I may not fully understand the context.

P.S. Inexpensive roll-your-own stop bath was the reason I stopped buying both Kodak Indicator Stop Bath and 28% pre-packaged acetic acid years ago.

 

[koraks]

What am I missing?

Nothing. What you're doing works perfectly fine, as you've found. I do the same and mix stop bath from either cleaning vinegar or citric acid crystals. A pinch/dash of either; I never really measure it. But some find the sulfur dioxide smell that can sometimes occur objectionable; for those, some kind of buffer system can be worthwhile.

 

[Ben Hutcherson]

You've thought about it, so surely you've also come to the conclusion that even if you start with an unbuffered acetic acid stop bath, it'll turn into a buffer by simply using it, as developer is being carried over into the stop bath. Starting out with a buffer in fact reduces the capacity of the stop bath because you essentially 'contaminate' it in order to get within the buffer range right from the start. The advantage of this, however, would be that you don't risk the evolution of sulfur dioxide due to dissociation of sulfite at a low pH (<4.0). AFAIK this is also the main reason why pH~4.5 has been suggested as a suitable pH for a stop bath. It's acidic enough to stop development and not quite enough so that it starts stinking up the place if some dektol ends up in it.

Yes, of course, that's the whole "principle" of how buffers work in the first place(i.e. adding a base converts to the conjugate acid, that could then in turn react with further base, etc-I'm sure you know all this but just stating it).

As a couple of thoughts though-as you mentioned, by keeping the pH higher you do avoid or at least minimize the amount of SO2 released, and acetic acid(unbuffered) would need to be at a fairly low concentration, something I'm too lazy to work out now, to get to that pH. Granted that's irrelevant if you use one shot(and it's cheap enough to make that you certainly can!) As an idle thought too, though, I wonder if some of the reports of pinholes, found in this thread and elsewhere, are a result of the evolution of SO2...

As @Milpool said too, I like the lower odor of the buffered bath, and the one I made up has little smell even as the concentrate. I find the odor of acetic acid repulsive(probably too much working with acetic anhydride in graduate school) so that's definitely a good thing for my liking. In fact when I last had a proper darkroom set up(put in storage when I moved in 2020, have yet to get it out) I used citric acid for my prints primarily to avoid the smell of acetic acid, although it could still definitely give off SO2. Open trays are a bit different than a tank, though. Dektol is the only paper developer I've ever used, and FB paper in particular can carry a lot over. Somewhere along the way I was given a few thousand sheets of 8x10 and I went through a phase of doing a WHOLE lot of portrait vignettes from 6x6 negs...I think Azo may well be worse about it than modern Ilford FB paper.

 

[jejes]

Nothing. What you're doing works perfectly fine, as you've found. I do the same and mix stop bath from either cleaning vinegar or citric acid crystals. A pinch/dash of either; I never really measure it. But some find the sulfur dioxide smell that can sometimes occur objectionable; for those, some kind of buffer system can be worthwhile.

I'm starting to use C-41 Fixer for B&W (i think it's cheaper) or some alkali fixer (TF-4) and i would like to ask you if you use your stop bath plain water or with clenaning vinegar?

Thanks

 

[koraks]

The stop bath I usually use is a dash of cleaning vinegar in tap water.

 

[Donald Qualls]

The main (purported) advantage of alkaline fixer (C-41 fixer is barely alkaline, as I recall) is that the emulsion isn't subjected to large changes of pH -- which can lead to the emulsion, swelled in the alkaline developer, shrinking in the acidic stop and fix. You largely defeat this benefit (if you believe it exists at all, of course) if you use an acid stop bath with an alkaline fixer. Then again, if you want to keep your (for example) TF-4 alkaline, you'd need a water rinse between the acid stop bath and the fixer.

But with an alkaline fixer, you need something to keep development from continuing in the fixer -- either acidity to disable developing action, or enough water rinses to wash the developer out of the gelatin. If you believe in the benefits of alkaline fixer, a multi-change water stop is the only method that will both prevent development (and possible fogging) in the fixer and the pH changes you're trying to avoid.

If you're using C-41 fixer or TF-4 just to cut down on the vinegar odor in your darkroom, why would you then reintroduce that via an acetic acid stop bath? At the least, in that case, use a citric acid based stop.

 

[Augustus Caesar]

I read somewhere (perhaps earlier in this thread?!) that the main reason to keep them separate was that paper contains a lot more silver than film, so used paper fixer will contain a lot more breakdown products. I don’t know whether film is more sensitive to those than paper is, but it makes sense not to risk the negative, which is irreplaceable whereas the print is not.

No, film has more silver per unit of area.