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  1. #151
    Photo Engineer's Avatar
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    Iron III salts + EDTA is claimed to cause more rapid oxidation of Ascorbic Acid than other mixtures such as Salicylic Acid.

    PE

  2. #152

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    The number of chelating agents that have been proven not to catalyse the Fenton reaction is rather small. Ryuji found very little data on the internet.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  3. #153
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    Way back when, he said he was going to get a patent on his special chelating agent, but I have seen nothing in the literature yet.

    PE

  4. #154

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    Quote Originally Posted by Photo Engineer View Post
    Way back when, he said he was going to get a patent on his special chelating agent, but I have seen nothing in the literature yet. PE
    He's been silent for months. I suspect he lost interest.

    For nearly a year, I thought that (1) DTPA would not chelate iron, and (2) Kodak avoided the problem by using iron-free chemicals that we ordinary folks can't obtain. I was wrong. DTPA chelates iron. True, it's harder to find, but I think it's obtainable with effort. For example, SpectrumChemical.com (in Los Angeles) sells 25 grams of pure DTPA (not a salt) for $17 here. I remember a posting where somebody mixed XTOL from the patent. He had a hard time finding DTPA, but finally located some in New York (I think). Also, it's worth calling Photo Formulary, as they stock chemicals not on their website.

    I mixed a new batch of XTOL a couple of days ago, storing it in five 1-litre bottles. And I thought of a way of making it last longer: Store the extra bottles in the refrigerator. Sulfite will precipitate out of solution, but when it's time to start a new bottle, warming and shaking it should bring the precipitate back into solution. Kirk Keys reminded me that chemical reactions proceed half as fast with each 10C temperature-drop, so refrigeration should make XTOL last around 1.5-2 years instead of 6 months. Is there any reason why this would not work?

    Mark Overton

  5. #155
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    Shaking entrains more oxygen.

    PE

  6. #156

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    Quote Originally Posted by Photo Engineer View Post
    Shaking entrains more oxygen. PE
    Hmm. And if there's no air in the bottle, shaking is far less effective. Everything is more complicated than first appearances. And that's mostly what keeps us engineers employed.

    Here's another question:
    We saw earlier in this thread that a concentrate consisting of PG+SodiumMetaborate+AscorbicAcid+DimezoneS crystallizes. That developer behaved exactly like XTOL, so I want to use it. So my question is: Can another solvent be used instead of PG? How about glycerine? Or ethylene glycol? Anything else?

    Mark Overton

  7. #157
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    Mark, check this out for an overview over common chelating agents. The document includes EDTA, PDTA, DTPA and others. Note that it is not just important to chelate Fe-III and Fe-II, also the difference in complex stability between the two has an impact.

    BTW: When you store your soup at cool temperatures, you also dissolve more air (and as a result oxygen) in the liquid in the long run. This could counteract the slowing down of the reactions you try to quench.
    Trying to be the best of whatever I am, even if what I am is no good.

  8. #158

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    Quote Originally Posted by albada View Post
    I mixed a new batch of XTOL a couple of days ago, storing it in five 1-litre bottles. And I thought of a way of making it last longer: Store the extra bottles in the refrigerator. Sulfite will precipitate out of solution, but when it's time to start a new bottle, warming and shaking it should bring the precipitate back into solution. Kirk Keys reminded me that chemical reactions proceed half as fast with each 10C temperature-drop, so refrigeration should make XTOL last around 1.5-2 years instead of 6 months. Is there any reason why this would not work?

    Mark Overton
    Mark,
    Xtol in full sealed glass bottles (no oxidation from air) is reported to last 7 1/2 years:
    http://www.apug.org/forums/forum37/1...ties-xtol.html
    The 6 months figure you quote is for part full bottles with air oxidation of ascorbate.The chelating agent is to slow down this oxidation.
    In full sealed bottles Xtol can only decompose by hydrolysis, not oxidation.Ascorbate hydrolysis is probably not significant ,Dimezone S is more resisistant to hydrolysis than phenidone.

  9. #159
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    Mark, I cannot suggest any other solvent. Sorry.

    I can suggest that you fill bottles with nitrogen "caps" and then shaking will not be a problem.

    PE

  10. #160

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    It's erroneous to believe that using a chelating agent forever stops the Fenton reaction. Actually it only slows the reaction down. How well a particular agent works depends on its stability constant for iron (III). There will always be some uncomplexed iron (III).
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery



 

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