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 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
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...
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.
What am I missing?
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.
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 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.
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