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Thread: Pyro PMK fixer

  1. #11
    Ole
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    [QUOTE=dancqu]
    Quote Originally Posted by Ole
    ... It is in fact the thiosulfate, be it
    sodium or ammonium, which complexes well with the halogens of
    silver. While the ammonium ion will complex well with the chloride and
    some what with the bromide, it has practically no affinity for the iodide.
    Sorry, but...

    The point is not the halogens - thiosulfate complexes very well with silver, not halogens. Same for ammonium, another silver complexer - but not as efficient. Ammonium also forms complexes with halogens, but to a much lesser extent. This increases the solubility of the silver halogens, of which sliver iodide is the least soluble. In this way ammonium thiosulfate (or ammonium and thiosulfate) dissolves silver iodide in a fraction of the time it would take without the ammonium. This lets silver iodide be fixed out in a reasonable time, which it won't without ammonium.

    I'm afraid you've got the fixing process backwards: The halogen ions are very soluble, the point of fixer is to make the silver unavailable to them by wrapping the silver in tight complexes. And the solubility of the silver halogen compounds play a role in the speed with which this happens - (silver) chloride is more soluble than the bromide which is more soluble than the iodide. About a factor of 10 difference with each step.
    -- Ole Tjugen, Luddite Elitist
    Norway

  2. #12

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    [QUOTE=Ole]
    - thiosulfate complexes very well with silver -

    In reading your post I don't see any disagreement. I should have been
    more clear on one matter. Better put I might have said; "Thiosulfate
    complexes very well with silver in the presence of chloride ion,
    less well in the presence of the bromide ion, and least well
    in the presence of the iodide ion.

    As you pointed out ammonium does less well in the presence of all three
    of the halogens. In fact it's ability to complex silver in the presence
    of iodide ion is almost nill. Of course if the amount of silver iodide
    is extremly low the entire matter becomes academic.

    I've not tested for "Rapid". From reading I've the impression it
    applies much more so to papers than silver iodided emulsions.
    I'd say as one would expect. Dan

  3. #13
    Ole
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    What I tried to say is that it's not so much the complexing that's affected by the type of halogen as the solubility of the silver halide. The effect of ammonium can be easily seen with a "clearing test" with a small piece of film. I have done this while mixing my own fixer (see "Chemistry Recipes" section), by putting in small pieces of film before and after adding the ammonium chloride. The acceleration of the fixing action is obvious, and remarkable.

    Many emulsions today contain significant amounts of iodide, with the "special grain" films being the prime suspects (T-Max, Delta etc). One paper that contains significant amounts of iodide is Bergger Art Contact, which has a chloroiodide emulsion. "Normal" fixing in a "normal" fixer is not enough for this paper - which I rather like, by the way. After fixing in a "rapid" fixer the image is stable, and will not fog with exposure to light.
    -- Ole Tjugen, Luddite Elitist
    Norway

  4. #14
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    Is the formulary TF-4 sufficient to clear the high iodide materials?

  5. #15
    Ole
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    Quote Originally Posted by mobtown_4x5
    Is the formulary TF-4 sufficient to clear the high iodide materials?
    Yes.

    It doesn't take much ammonium to accelerate the action to where iodide will be fixed out during normal fixing times (q.v. the recommendation for three times clearing time for many films), and TF-4 contains lots of ammonium. Much more than mine.
    -- Ole Tjugen, Luddite Elitist
    Norway

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