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CaCl2 Macro Mix Problem

Discussion in 'Aquatic Plant Fertilization' started by Philosophos, Jul 17, 2009.

  1. Philosophos

    Philosophos Lifetime Charter Member
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    So I decided to add CaCl2 to my liquid fert mix yesterday, rather than the usual dry dose. I took out the MgSO4 and mixed it over with the CSM+B to avoid precipitation issues.

    The mix was standard EI stuff outside of the CaCl2; KNO3, KH2PO4 and K2SO4. Upon dumping the weighed fertilizers in to water, the mix began to bubble, and heat was produced enough to warm the glass measuring cup up. From there, a nice crusty, flaky precipitate was produced that seemed to be made from fused granules of CaCl2. Out of curiosity, I heated the mixture up and stirred it for a while; zero solubility at boiling point, though the precipitate broke down in to a fine silt.

    What just happened here?

    -Philosophos
     
  2. jonny_ftm

    jonny_ftm Guru Class Expert

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    CaCl2 solid, when diluted, can produce exothermic reactions that can even induce burns

    It should be used in a diluted state for safety

    Now, maybe the extreme heat released caused some Calcium salts precipitation or other salts maybe
     
  3. Philosophos

    Philosophos Lifetime Charter Member
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    Wow, missed that section of the wikipedia article on CaCl2. Perhaps I should try the mix again, starting with the CaCl2 and waiting until it cools off. Thanks for the tip.

    -Philosophos
     
  4. Philosophos

    Philosophos Lifetime Charter Member
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    I've now tested out the issue. Turns out the KH2PO4 is likely the cause of the issue.

    I added 8.3g CaCl2 to 100ml DI H2O and stirred until the water was clear, then left it sit for a few minutes.

    Next, 0.6g of KH2PO4 was added to the mixture. Immediately flakes measuring somewhere around 1-3mm in diameter formed, just as it did with the previous batch.

    As a pseudo-control I then mixed up a batch of the usual NPK, without calcium, and it's sitting on the table looking just fine.

    I haven't bothered to test the KNO3 or K2SO4 with the CaCl2 yet, so I can't say what would happen in the KH2PO4 were left out.

    For now, CaCl2 is staying in its own bottle.

    I'm going to be dosing CaCl2 on micro days from now on, or at least long after the NPK mix given the reaction it has to the potassium phosphate.

    -Philosophos
     
  5. jonny_ftm

    jonny_ftm Guru Class Expert

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    I think philosophous that the issue is in the solubility of components when mixed together in a part and the precipitation of calcium phosphate salts on another part

    Try mixing CaCl2 and KH2PO4 after having dissolved each one completely in a distilled water solution than mixing the solutions rather than adding the solid compound

    But, I still believe a part will precipitate as (Ca)3(PO4)2 with KCl formation

    Usually, mixing PO4 with other salts is a major issue and it is better added alone in a premixed solution (not even dry in the tank)
     
  6. Philosophos

    Philosophos Lifetime Charter Member
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    Thanks for the advice. I'll note down trying it out next time I mix a fert batch, if nothing else to see what happens.

    So what you're saying is that the KH2PO4 is only an issue when not as an aqueous solution? It's not the issue of hitting maximum solubility in the sense of physics, but a symptom of not having it as a solute yet?

    As an example, if I mixed 5.7g of KH2PO4 in to 100-200ml of DI H2O, then added it to 800ml of CaCl2 in a solution, it shouldn't be too much of an issue?

    The KH2PO4 is pretty much being dosed on its own now, outside of the K2SO4 and KNO3. This has been a pretty safe, stable bet for everyone doing EI in liquid form so far. All the same, you seem to know a bit about chemistry; can you see any issue with it?

    -Philosophos
     
  7. jonny_ftm

    jonny_ftm Guru Class Expert

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    Sadely my chemistry knowledge was good at school time, before I change target

    Mixing KH2PO4, K2SO4 and KNO3 won't cause issues. PO4, SO4 and NO3 are negative ions (anions), so they won't react. K is their common positive ion (cation), so no issue at all, you can mix them safely

    Ca salts is another issue, as Ca will react with other cations and could precipitate because of very low solubility.

    In your case, adding directly dry KH2PO4 and dry CaCl2 could cause a contact between the 2 solutes at very high concentration. The heat provided by CaCl2 could cathalize the (Ca)3(PO4)2 precipitation (that part, I'm not sure though).

    Now, adding both solutes separately pre-mixed with distilled will cause contact between solutes at a much lower concentration for both, thus yielding to lesser chance of precipitation. It's just like when you add KH2PO4 pre-mixed with DI water in your tank.

    But, personally, I won't keep a stock solution with a mix of both of them as the chances of precipitation are rather big
     
  8. cliffclof

    cliffclof Junior Poster

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    precipitation reaction


    CaCl2 + 2 KH2PO4 = Ca(H2PO4)2 + 2 KCl

    KCl is soluble. You must be seeing Ca(H2PO4)2 .

    You should keep PO4 away from Mg and Ca in high concentrations.
     
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