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Basic question about Fe lasting in water

Discussion in 'Estimative Index' started by fablau, Nov 22, 2015.

  1. fablau

    fablau rotalabutterfly.com
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    Hello everyone.


    I have a question about Fe. I know that Fe ETDA and gluconate don't last long, whereas DTPA last longer in water... But in any case, any Fe source shouldn't last more than 48 hours. If that's correct, how is it possible that I can measure high concentrations of Fe (1-2ppm) with a colorimeter even after several days from the last dose?
     
  2. Solcielo lawrencia

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    This is something that I've wondered about, too. Does the test detect precipitated iron as well as free and chelated forms? Or does it only detect free and chelated? One way to test is to add iron and enough phosphate to precipitate them, then test again. If it detects iron, then precipitated forms are also measured.


    Another way to test is to simply add the Fe chelate and phosphate and just wait to see if precipitation occurs.
     
  3. fablau

    fablau rotalabutterfly.com
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    Good idea Solcielo, I can try your suggested test, but if the latter is true (the colorimeter detects just free and chelated Fe) that would contradict the idea that chelated Fe lasts just a few hours in water... Therefore, if that's a fact (that chelated Fe doesn't last long in water) the colorimeter must measure precipitated Fe. But if so, why can I detect and measure "plant intake" for days after the dose? I mean, plants shouldn't intake precipitated Fe right? And then, why do I see it decreasing daily? One idea contraddicts the other...


    I hope what I wrote makes sense...
     
    #3 fablau, Nov 22, 2015
    Last edited by a moderator: Nov 22, 2015
  4. Solcielo lawrencia

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    Since I had my water tested, I suspected as you did, that it really doesn't precipitate as assumed. Iron EDTA and DTPA are both stable for at least several weeks. If confirmed, then I wish I bought EDTA since it was cheaper and doesn't stain the water as much as DTPA does.


    Also, precipitation will result in obvious precipitates that settle to the bottom, appearing as a fluffy tan-brown gunk.
     
    #4 Solcielo lawrencia, Nov 23, 2015
    Last edited by a moderator: Nov 23, 2015
  5. burr740

    burr740 Micros Spiller
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    Also wondered about this. Ive read before that tests pick up both forms, no idea if that is accurate or not. Fablau's testing seems to suggest otherwise.

    I never see any of this in my tank, but the canister sponges sure do collect a lot.
     
  6. fablau

    fablau rotalabutterfly.com
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    Oh well, I didn't know that. I think that explains it then! Thanks!
     
  7. Pikez

    Pikez Rotala Killer!
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    Weeks?! I suspect hydroponics folks have addressed this. Any references?


    Interweb echo chamber says hours to days for EDTA and possibly a couple of days for DTPA.
     
  8. fablau

    fablau rotalabutterfly.com
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    Yes, that's what I read, and that's why I opened this thread... if it is really just hours and a couple of days, why I can still measure Fe after several days (7-10 or more) from dosing?
     
  9. UDGags

    UDGags Lifetime Charter Member
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    What type of colorimeter? They aren't that accurate for absolute values.
     
  10. fablau

    fablau rotalabutterfly.com
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  11. Solcielo lawrencia

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    One way to test is to add Fe in the tank and take a water sample after it's been thoroughly circulated. Then after a few hours, test both the tank water and the sample. If the tank has lower iron than the sample, then that indicates it may be used up, or possibly adsorbed to the substrate or wood.
     
  12. fablau

    fablau rotalabutterfly.com
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    Good idea Solcielo, I'll try, but from my measurements so far, I see a regular daily decay of Fe of about 0.05ppm. That could be plant intake or other kind of absorption.
     
  13. Solcielo lawrencia

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    For an experiment, I added KH2PO4 and FeDTPA into a test tube and mixed it. A day later, it has not precipitated.


    The water is stained slightly brown from the FeDTPA, though. Does Fe gluconate keep the water clear?
     
  14. fablau

    fablau rotalabutterfly.com
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    No idea about gluconate, but I can try. I have some Flourish Iron around.
     
  15. Solcielo lawrencia

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    More than two weeks later, FeDTPA has yet to precipitate.
     
  16. fablau

    fablau rotalabutterfly.com
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    Ok, therefore we can assume it doesn't precipitate with Po4. Thanks for the info.
     
  17. burr740

    burr740 Micros Spiller
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    But is it still in a form that can be used by plants at this point?
     
  18. Solcielo lawrencia

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    It will still be FeDTPA.
     
  19. fablau

    fablau rotalabutterfly.com
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    And this explains why I could still measure Fe in my tank with a colorimeter after 2 weeks of stop dosing any Fe.
     
  20. Tom Barr

    Tom Barr Founder
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    DTPA should last at least days, not 2, but more..........unless taken up by plants. Weeks for DTPA seems correct provided it does not get removed or if you dose leaner. Water changes would remove it.
     
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