Kodak indicator stop bath dilution

Discussion in 'B&W: Film, Paper, Chemistry' started by michael_r, Aug 5, 2012.

  1. michael_r

    michael_r Subscriber

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    I believe the pH of the standard working solution (fresh) is somewhere between 2 and 3. If one were to mix it at half strength (say 8ml/l instead of 16ml/l), does that increase the pH significantly? I think total acidity would decrease, but not sure how much the pH would increase. Actually, is a half-strength mix of Kodak stop bath even still an effective stop bath? Working on some new experiments with PMK and I can't find all my old notes. Hutchings recommended the "ideal" stop bath pH of 4.5 in his book, but frankly I'm not sure how much of all that is still valid.

    Further, Wimberley always recommended a standard stop bath be used with his various WD2 developers even though they use a carbonate alkali. Which reminds me of another question - is it really necessary to avoid acid stop baths with carbonate-driven developers? Or does it really depend on the pH of the particular developer?
     
  2. cliveh

    cliveh Subscriber

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    If you are not a pyro dev user like me, then a standard indicator acidic stop bath is fine. I like to make mine to a dilution that is not harmful to dip your fingers in.
     
  3. wildbill

    wildbill Member

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    I can't answer any of those but I haven't used stop bath (just plain water) in 10 years. I use pmk, tmax, sometimes rodinal. Hell, I don't use it when printing either.
     
  4. JPD

    JPD Member

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    I use about 10 ml 24% acetic acid for a liter stop bath for film, less than recommended, and never had any problems.

    A plain water stop bath when using a developer with carbonate seems like a good suggestion. I don't remember which developer I used, but it contained Sodium Carbonate, and then an acid stop bath. The millions of small bubbles in the emulsion ruined the negatives...
     
  5. Leigh B

    Leigh B Member

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    I use Kodak Indicator SB per the instructions. No problems in over 55 years of doing so, with my bare hands.

    Some folks are more sensitive to chemicals than others.

    People write books to sell books. Repeating the common wisdom does not sell books.

    Ergo, anything you read must be taken with two or more grains of salt.

    Given the number of different developers available, I don't know how one could even define the 'ideal' stop bath.

    - Leigh
     
  6. Photo-gear

    Photo-gear Member

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    The commercial white vinegar is only 5% concentrated acetic acid. Where do you find the 24% solution?
     
  7. michael_r

    michael_r Subscriber

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    I mostly agree. I have always used Kodak SB per instructions except with developers using a carbonate alkali. With those developers I've followed the conventional wisdom and used a water stop. The exception to that rule has been Pyro, where some people say a stop bath is desirable to halt general dye formation. An exception to this, of course, is PMK which uses a Metaborate alkali, and Hutchings's book has never been entirely clear nor convincing to me. Some people say imagewise stain continues to form post-development. Others say it is just general stain that forms post-development. It could depend on the formula, but really?

    So I set out to test some of these things.
     
  8. JPD

    JPD Member

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    "Absolut Ren Ättika 24%" and "Ättikssprit 12%" is available in any grocery store here in Sweden. They are the standard white vinegars here. You can probably find something similar in North America, try a pharmacy, hardware store or a paint shop.
     
  9. Leigh B

    Leigh B Member

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    Food-grade glacial acetic acid is readily available at about $12 for 950ml here in the States.

    Dilute 1+3 with distilled water if you want 25%.

    Of course you can substitute citric acid, but peeling all those lemons and oranges is a royal PITA. :D

    - Leigh
     
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  10. Leigh B

    Leigh B Member

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    Not wanting to start an argument, but water is not stop bath, it's water.

    Any developer in the emulsion continues to do its job until fixation is complete and there's nothing left to develop.

    - Leigh
     
  11. michael_r

    michael_r Subscriber

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    I didn't mean it halts development like an acidic bath. Let's call it a water rinse, then. In any case, as long as a water rinse is used when testing and consistently thereafter, it should not be an issue.
     
  12. Leigh B

    Leigh B Member

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    As long as the process works and consistently yields the expected results, the details don't matter.

    My comment was just a matter of semantics, for those who might retrieve this thread from the archives.

    - Leigh
     
  13. Gerald C Koch

    Gerald C Koch Member

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    For more than 20 years I have used a stop bath only for FB papers. Film and RC papers are given a water rinse. Never had any problems even with high alkalinity developers such as Rodinal..
     
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  15. MattKing

    MattKing Subscriber

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    To rephrase Michael's question slightly ...

    If one dilutes Kodak Indicator Stop Bath more than recommended, does the indicator continue to be a reliable test for appropriate activity - i.e. is the stop bath still good if it hasn't turned blue?
     
  16. Gerald C Koch

    Gerald C Koch Member

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    Yes but its capacity is less.

    I consider indicator stop bath sort of a ripoff as plain acetic acid has its own indicator that is its smell. Sodium acetate has little or no smell. So if you can't easily smell the stopbath then it is time to replace it. However, stop bath should not be saved between printing sessions. By not using an indicator stop bath we keep one more dyestuff out of the environment!

    The indicator used in stop baths is bromcresol purple. It is yellow below a pH of 5.2 and purple at a pH of 6.8. It doesn't work all that well since the stop bath has really lost its effectiveness as soon as the yellow color begins to darken. If you wait for it to turn purple then it really has been exhausted for some time.

    Not all white vinegar is 5%. You really need to check the label as some are only 4%.
     
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  17. RattyMouse

    RattyMouse Subscriber

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    Stupid question from a noob here. Is wearing gloves while developing film a problem? I hear people here talking about dipping their bare hands in chemicals. Why not wear nitrile gloves or something similar?
     
  18. MattKing

    MattKing Subscriber

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    Wearing gloves is a really good idea.

    I almost never wear them, and I have been developing black and white film for 45+ years.

    Yes, those two sentences don't fit well together logically.

    I do, however, have running water at hand at all times when developing film.
     
  19. MattKing

    MattKing Subscriber

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    I asked my question, because I'm often developing just one or two rolls of film, which in the normal course of events would not come close to exhausting the capacity of something like 600 ml of stop bath.

    I always use stop bath at recommended dilution for paper.
     
  20. gordrob

    gordrob Subscriber

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    For film I have used only a water rinse or bath for film - anything from 35mm to 8x10 film. Never had any problems over the last 45 years.
    For paper I use the Kodak Indicator Stop Bath at 16ml/L. Again no problems with that mixture %.

    Gord
     
  21. michael_r

    michael_r Subscriber

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    Gerald, I equate total acidity to capacity. Is this reasonable? On the other hand the effect of dilution on pH is less clear to me. If the pH of a freshly mixed stop bath is 2.8, how does diluting it affect pH? If you mix Kodak Indicator SB to half strength, it's still yellow - although it is obviously a less intense yellow. The yellow color suggests the pH is still acidic enough to be effective (although with reduced capacity), but is that true?

    Sorry if I'm repeating Matt's question but I want to be clear on the impact on pH, not just capacity. I'm asking because if diluting Kodak SB can get you to a pH of around 4 it should work ok with Pyro and/or carbonate developers.

    This is really only an issue with Pyro where using a water rinse in place of an acidic stop bath may or may not be a problem. With non-staining carbonate developers I'd simply use a water rinse.
     
  22. Gerald C Koch

    Gerald C Koch Member

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    Its easy to calculate the pH of an acetic acid solution by using the following equation.

    [H+] [Ac-] = Ka = 1.8 x 10-5

    For a dilute acid and not a buffer [H+] = [Ac-]

    The pH is the log10[H+].

    An indicator cannot be used to determine a pH if the solution is outside of its range. So if bromcresol purple yields a yellow color we only know that the pH is equal or less than 5.2.

    If the pH of a stop bath is 2.8 and we double its volume with water the pH increases by the log102 = 0.3. So the resultant pH is 3.1. The capacity of such a bath is half what the original bath had.
     
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  23. michael_r

    michael_r Subscriber

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    What happens to the purple indicator dye at a pH somewhere between 5.2 and 6.8?

    So it seems dilution in this case is not going to raise the pH enough unless dilution is rather extreme, which could render the resultant stop bath useless from a capacity perspective. It might not even work for one-shot use.
     
  24. Gerald C Koch

    Gerald C Koch Member

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    Obviously the color changes from yellow to purple. At a pH of say 5.9 what you might see would be a tan or muddy color perhaps even a gray color. The color during the transition depends on the indicator. For this reason we want indicators that change color over as small a range as possible. Typically in the lab the color within the range is not very useful. You would have to have a color chart worked out using a set of buffers that are within the range. This is sometime done but not all that frequently.

    If you were titrating acetic acid with sodium hydroxide you would note the end point when the solution first turns purple.
     
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  25. michael_r

    michael_r Subscriber

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    Thanks. I wish I remembered more of my Chemistry. This is pretty sad.

    So I guess if in the end I wanted a normal capacity stop bath with a pH around 4 to 4.5 without resorting to scratch mixing I'm pretty much out of luck.
     
  26. Gerald C Koch

    Gerald C Koch Member

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    You could use a solution of boric acid which would get you in the range of a pH of 4 to 5. pH 5.1 (1.0% solution); 3.7 (4.7% solution) Its cheap and readily available. Powdered boric acid is hard to dissolve in cold water as water doesn't wet the powder and it tends to float. Use the crystalline form or the powder with hot water.