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  1. #21
    markbarendt's Avatar
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    Quote Originally Posted by RattyMouse View Post
    I'm not over thinking this. You guys are!

    I'm looking for a number. At what pH is stop bath considered exhausted? Give me a number. That's all I want.
    Okay, we might be overthinking, but if that's true then you are under thinking it.

    There isn't an exact answer.

    The stop doesn't just quit working at a given PH. The first film or paper that is run through the stop is going to stop really fast, next one a bit slower, slower again for the next:it's a progressive change. If you are using indicator stop it is my understanding that when it turns color it has essentially reached a point where "it is as effective as water" and some of us think water is effective.

    So you need to define what "exhausted" means.
    Mark Barendt, Ignacio, CO

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

  2. #22

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    As the function is to lower the alkaline pH of the developer and stop it's actions, then one also needs to consider the developer and the size of the print and trays, as well as the pH of the second-bath (with water being more-or-less neutral, pH7).

    With a small print it is relatively easy to give evenly fast submersion and vigorous agitation to the whole print surface, while with a larger print this becomes much harder to to achieve perfectly -- in water, working by massive dilution of the carried-over developer, that could mean a visible difference from one area to another. An acidic stop-bath does it's job chemically and hence much more quickly than the dilution action of water.

    Whether this is significant depends on the print, the developer, the critical standards of the printer and the use to which a particular print will be put. So, as usual, the correct answer to the question is "it depends".

  3. #23
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    The classic stop bath is acetic acid diluted in water to 0.7 percent. This is a very cheap liquid that you could reuse a few times and then toss and replace.

    The benefit over water is that you stop the dvelopment instantly and uniformly across the negative, which provides greater control and consistency.

  4. #24

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    Quote Originally Posted by markbarendt View Post
    Okay, we might be overthinking, but if that's true then you are under thinking it.

    There isn't an exact answer.

    The stop doesn't just quit working at a given PH. The first film or paper that is run through the stop is going to stop really fast, next one a bit slower, slower again for the next:it's a progressive change. If you are using indicator stop it is my understanding that when it turns color it has essentially reached a point where "it is as effective as water" and some of us think water is effective.

    So you need to define what "exhausted" means.
    Does the indicator flip at 7 or is it higher?

  5. #25

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    Quote Originally Posted by RattyMouse View Post
    Does the indicator flip at 7 or is it higher?
    The indicator is bromocresol purple and it doesn't "flip", it's a gradual transition. Have a look here. I toss it when it has that dirty colour, about pH 6. It's still acidic.

  6. #26
    markbarendt's Avatar
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    Quote Originally Posted by Anon Ymous View Post
    The indicator is bromocresol purple and it doesn't "flip", it's a gradual transition. Have a look here. I toss it when it has that dirty colour, about pH 6. It's still acidic.
    Great info Anon.

    Quote Originally Posted by RattyMouse View Post
    Does the indicator flip at 7 or is it higher?
    It's not a light switch.
    Mark Barendt, Ignacio, CO

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

  7. #27
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    Water does not "stop" development, but it does seriously retard (slow) it. There is still some developer carry over when you use water between developing and fixing. This will shorten the life span of whatever fixer you use. If you use fixer as a single shot and dump, this is not a problem. If you reuse fixer, as many do, you will need to change fixer more often, using a two bath method is highly recommended.
    Rick A
    Argentum aevum

  8. #28

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    The pH of a fresh acetic acid stop bath is usually in the 2.5-3.0 range. The question of exhaustion is more complicated than just pH because there is also total acidity to consider. The "ideal" stop bath would be higher in pH (~4.5) but buffered. Waiting for your indicator stop bath to go purple may not be the best idea because by that time the pH is high enough that significantly longer stop times would have been required for some time. There is a "transition" range for bromcresol purple between pH values of ~5.2-6.8. Without overcomplicating things, just follow Kodak's directions.

    A standard stop bath does not harden the emulsion.

    As for water stop baths (which are not stop baths but just dilution/washing), it takes longer than a few extra seconds. A water stop must be thorough, and at least for paper it should be a running water bath. This is less critical when a typical acidic fixer is being used, but if you are using an alkaline fix, development can actually restart in the fixer if the intermediate stop/wash is incomplete.

    All this to say there is not a simple answer with respect to a specific pH value below 7 at which the stop bath is exhausted. It depends on the stop bath, the materials involved, the fixer etc. Follow the manufacturer's directions.

  9. #29

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    Thank you. I'll toss my home made brew at pH 6.0 then.

  10. #30
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    I think an indicator stop bath is a good idea. If you are a visual person, as most photographers are, the change from yellow to purple means you need to replace it.

    “The contemplation of things as they are, without error or confusion, without substitution or imposture, is in itself a nobler thing than a whole harvest of invention”

    Francis Bacon



 

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