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  1. #21
    PeterB's Avatar
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    My Citric stop bath fails to indicate well

    The indicator in my Fotospeed (SB50) Citric Acid stop bath doesn't seem to be working. I poured heaps of bi-carb soda into a freshly made up batch and it barely turns blue. What is the indicator and where can I get some from ?
    The 1/2 full SB50 concentrate is probably about 3-4 years old.

    thanks
    Peter

    Quote Originally Posted by Bob F. View Post
    I generally use commercial citric acid stop with indicator (Fotospeed or Ilfostop). ...Cheers, Bob.

  2. #22
    Bob F.'s Avatar
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    Quote Originally Posted by PeterB View Post
    The indicator in my Fotospeed (SB50) Citric Acid stop bath doesn't seem to be working. I poured heaps of bi-carb soda into a freshly made up batch and it barely turns blue. What is the indicator and where can I get some from ?
    The 1/2 full SB50 concentrate is probably about 3-4 years old.

    thanks
    Peter
    The Darkroom Cookbook suggests 9.25g of bromcresol purple and 1g of sodium hydroxide with distilled water to make 250ml of indicator but where you would get bromcresol purple from, I have no idea...

    I suspect that it would take one of our resident chemists (of which I am definitely not one) to work out how much sodium bicarb it would take to neutralize the amount of citric acid in a batch of a particular stop bath given their respective pH (I suspect moles (or possibly some other small burrowing creatures) feature prominently in the equation, but that's as far as I dare venture)...

    My guess would be that if the stop is still yellow and does eventually turn blue/purple then it may be fine - some litmus paper which is cheap enough will confirm. Or not.

    Good luck, Bob.

  3. #23
    PeterB's Avatar
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    Calling any chemists out there !

    Quote Originally Posted by Bob F. View Post
    The Darkroom Cookbook suggests 9.25g of bromcresol purple and 1g of sodium hydroxide with distilled water to make 250ml of indicator but where you would get bromcresol purple from, I have no idea...

    I suspect that it would take one of our resident chemists (of which I am definitely not one) to work out how much sodium bicarb it would take to neutralize the amount of citric acid in a batch of a particular stop bath given their respective pH (I suspect moles (or possibly some other small burrowing creatures) feature prominently in the equation, but that's as far as I dare venture)...

    My guess would be that if the stop is still yellow and does eventually turn blue/purple then it may be fine - some litmus paper which is cheap enough will confirm. Or not.

    Good luck, Bob.
    Thanks Bob. I'll see if any chemists can help me work out the amount of sodium bicarb to add per mL of working solution of SB50 ODOURLESS STOP BATH to turn it blue. The MSDS doesn't list the indicator but does list Citric acid at 20-50% in the concentrate, and it is diluted 1+19 with water.

    regards
    Peter

  4. #24

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    Quote Originally Posted by PeterB View Post
    Thanks Bob. I'll see if any chemists can help me work out the amount of sodium bicarb to add per mL of working solution of SB50 ODOURLESS STOP BATH to turn it blue. The MSDS doesn't list the indicator but does list Citric acid at 20-50% in the concentrate, and it is diluted 1+19 with water.

    regards
    Peter
    IMO that is Good advice from Bob F.

    Bromocresol Purple is a colorometric pH indicator. Visual transition from yellow to purple occurs between pH5.3 (acid) and pH 6.8 (weakly basic).

    A replacement colorometric pH indicator like Litmus Paper will work just fine to show if the stop bath solution is still acidic.
    Tom Hoskinson
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  5. #25

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    I'm not really sure what the purpose is of an acid stop bath. Of course, the acid does stop development, but it seems to me that a plain water stop bath will do the job (somewhat more gradually) by dilution of the developer, particularly if the water treatment is done with adequate reproducibility and agitation, and provided that the overall development process is calibrated to include the more gradual effect of a plain water stop bath, which could include extending the development by a few seconds.

    Please clue me in if there is something absolutely essential that the acid provides. I may be missing something.

  6. #26
    MattKing's Avatar
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    Quote Originally Posted by alanrockwood View Post
    I'm not really sure what the purpose is of an acid stop bath. Of course, the acid does stop development, but it seems to me that a plain water stop bath will do the job (somewhat more gradually) by dilution of the developer, particularly if the water treatment is done with adequate reproducibility and agitation, and provided that the overall development process is calibrated to include the more gradual effect of a plain water stop bath, which could include extending the development by a few seconds.

    Please clue me in if there is something absolutely essential that the acid provides. I may be missing something.
    Alan:

    This is subject to correction by the chemists here, but I believe that the two basic advantages of acid stop bath are:

    1) dependable, quick and consistent cessation of development; and
    2) protection for acidic fixers.

    Clearly, point 2 does not apply for alkaline or neutral fixes. Point 1 is much more meaningful for production printing than more deliberate, 1 at a time printing workflows. It can also be useful in group darkrooms, because it can serve as a reminder to check whether fixer capacity has been exhausted. Finally, it is useful if water is at a premium, and a running water stop bath is impractical.

    In my case, the last time I used acid stop was when I printed my postcards for the postcard exchange. The time before that was when I was working with a group in a school darkroom. Other than that, I use a water rinse.

    Matt

  7. #27
    Ole
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    Way back in the dawn of photography, one of the "benefits" of acid stop was that it removed the "unsightly stain" formed by some developers...
    -- Ole Tjugen, Luddite Elitist
    Norway

  8. #28
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    Quote Originally Posted by Ole View Post
    ...it removed the "unsightly stain" formed by some developers...


    Barry

  9. #29

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    Quote Originally Posted by Tom Hoskinson View Post
    IMO that is Good advice from Bob F.

    Bromocresol Purple is a colorometric pH indicator. Visual transition from yellow to purple occurs between pH5.3 (acid) and pH 6.8 (weakly basic).

    A replacement colorometric pH indicator like Litmus Paper will work just fine to show if the stop bath solution is still acidic.
    pH 6.8 is weakly basic?
    How come?

  10. #30

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    Quote Originally Posted by alanrockwood View Post
    I'm not really sure what the purpose is of an acid stop bath.
    Of course, the acid does stop development, but it seems to
    me that a plain water stop bath will do the job (somewhat
    more gradually) by dilution of the developer, ...
    I side with those who maintain that it's purpose is to assure
    the acidity of an acid fix. The as little as 10 seconds in acid
    stop recommended by one supplier will give the film or paper
    a superficial acidic character; good for an acid fix.

    So why be concerned. Every one knows an alkaline fix will
    work. Why not just go directly from developer to fixer; at
    least for those not worried by the few additional seconds
    of lingering development. Besides, working with one-shot
    chemistry one may toss the fix in with the developer.
    I don't do that myself but Mr. Gainer has and may
    do; others as well. Dan

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