Monopotassium phosphate question?

Discussion in 'Reef Chemistry by Randy Holmes-Farley' started by TbyZ, Feb 17, 2017.

  1. TbyZ

    TbyZ Well-Known Member

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    I have some Monopotassium Phosphate powder (K2HPO4).

    I mixed a few t-spoons into 150ml RODI water. Its very soluble.

    When I tested the solution with my salifert PO4 test kit the reading was zero?

    Why wouldn't I get a reading for phosphate?

    o_O
     
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  2. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    What happened? It may not work in that solution, but it might also be so high that it doesn't show a recognizable color. The concentration will be on the order of 100,000 ppm.
     
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  3. TbyZ

    TbyZ Well-Known Member

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    I checked it after 1 t-spoon & there was no reading. No blue color at all. The same after 2 more spoons.

    What do you mean by it not working in "that solution"?
     
  4. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    The problem is likely the pH.

    Chemical tests often require a certain pH for them to work. Many kits acidify the solution during the test. Phosphate is one of these.

    For example, this is the standard ascorbic acid method for phosphate::

    http://public.iorodeo.com/docs/phosphate/ascorbic_acid_method.html

    "Ammonium molybdate and potassium antimonyl tartrate react in acid medium with orthophosphate to form a heteropoly acid - phosphomolybdic acid - that is reduced to intensely colored molybdenum blue by ascorbic acid. This method for measuring phosphate is based on APHA Standard Method 4500-P E"


    That much phosphate is a very potent pH buffer. The kit likely cannot overcome the pH buffering of the phosphate to bring the ph low enough for the kit to work.
     
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  5. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    A better way to test, if you have the capability, is to spike a tank water sample with 0.1 or 0.5 ppm phosphate and detect that with the kit.
     
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  6. TbyZ

    TbyZ Well-Known Member

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    Thanks Randy for the info, very much appreciated.
     
  7. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    You're welcome.

    Good luck. :)
     
  8. Alfrareef

    Alfrareef Well-Known Member

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    Thks Randy for another important contribute for the better understanding about how our tests work.
    What we can estimate to be the highest PH that a PO4 test can handle?
     
  9. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    Thanks. :) Its not necessarily the pH at the start, but the inability of the acid in the test kit to lower the pH enough. A phosphate buffer can hold the pH very strongly at pH 7, even without starting higher than tank water pH.
     
  10. Alfrareef

    Alfrareef Well-Known Member

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    Understand. Thanks Randy.
     
  11. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    You're welcome.

    Happy reefing. :)
     
  12. TbyZ

    TbyZ Well-Known Member

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    Ok, so I added just a few specs of phosphate to 10ml of aquarium water, waited a few hours, & got some blue color on a test.
    I estimate my PO4 to be somewhere below 0.01ppm.

    Randy, you reckon the solution I made up is is in the order of 100,000 ppm.
    If I wanted to raise PO4 by 0.04ppm in 180L (47gallons) how much solution would I add.
    I get something like one drop, but that seems unlikely?
     
  13. TbyZ

    TbyZ Well-Known Member

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    I'm starting to wonder if i was supplied K2HPO4 or maybe cocain?

    I made a new solution using James' planted aquarium calc. I dosed an amount to my tank that should have raised PO4 by 0.14ppm, but measured no reading. I turned off the return pump before dosing & waited 5 minutes before measuring with salifert
    ?????????
     
  14. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    It may have rapidly bound to calcium carbonate surfaces (5 min seems too short, however, maybe it wasn't even mixed in well). Or the kit may not be adequately sensitive/accurate.
     
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