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Old Nov 6th 2017, 06:03 AM   #3
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Join Date: Oct 2017
Location: Texas
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Answered my own question - - easier than expected

Well, MgO is typically dead burnt magnesium oxide at very highrefractory temps.

I stored the MgO free samples in outdoors in humidity for 10 years, so it's likely absorbed moisture and even Co2 converting it to magnesium carbonate, precise stochiometry isn't much use here.

Fact is MgO and the carbonate are not very soluble in water, whereas MAP is quite soluble.

So, just combine an excess amount of Mg carb. based on it molar stochiometry with the MAP in hot stirred solution (assuming I now have carbonate), allow to react overnight, then decant liquid, through fine filter paper, boil off water to obtain crystals of ammonium magnesium phosphate.

Throw out sludge residue that does not react.

Bingo !
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