I want to end up with a solution or anhydrous sample of potassium phosphate with which I can grow larger crystals. I want a challenge and something to do so I don't want to buy potassium phosphate, I would rather make it myself.

I don't have access to a lot of chemicals but I do have a fair amount of monoammonium phosphate, (or ammonium dihydrogen phosphate), from a crystal growing kit. I plan to heat this until the ammonia is released to leave behind phosphoric acid to which I can then add water to have an acidic solution. I will then dissolve potassium carbonate from corroded alkali batteries into water and add this to the phosphoric acid. The products will then be carbon dioxide, water and potassium phosphate.

My question is will these reactions work and how easy/difficult will it be. I am sorry if this makes no sense at all because I have done my research for this on Wikipedia and it isn't exactly a 100% reliable source. Thanks.


1 Answer 1


Getting phosphoric acid by heating ammonium phosphate probably won't work since it requires temperatures above 200°C where phosphoric acid will start to vaporize and decompose.

A better approach might be the reaction of ammonium phosphate with potassium carbonate. The ammonium carbonate formed by this reaction decomposes at much lower temperatures (> 60°C). However, heating will produce corrosive, irritant and toxic products (ammonia, phosphoric acid, phosphorus oxide), so I do not recommend doing such experiments.

  • $\begingroup$ So monoammonium phosphate will react at room temperature with potassium carbonate to form ammonium carbonate and potassium phosphate but the products will decompose at temperatures above roughly 60 degrees Celsius. Also what will the phosphoric acid decompose into if I heat it beyond 60 degrees? $\endgroup$ Commented Apr 28, 2016 at 21:15
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    $\begingroup$ The idea is tho get rid of the ammonia by decomposition of ammonium carbonate which starts at 60°C (in dry state). But you have to test before if this actually will work. I would try to mix the ammonium phosphate with potassium carbonate and a small amount of water and then cautiously heat the mixture until it's dry. Phosphoric acid begins to decompose at its boiling point of 213°C. It loses water and forms species like pyrophosphoric acid and higher condensated products (not actually phosphorus oxide as I said before). $\endgroup$
    – aventurin
    Commented Apr 29, 2016 at 16:12

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