I want to prepare a 1M NH4H2PO4 solution with pH10.
I used NH4OH solution to adjust the pH.
However, when the pH is over 9.0, precipitation occurred.
How can I solve this problem?
Hello B.S. Kim thanks for posting this interesting technical question. First of all, it should be noted that ammonium dihydrogenphosphate (NH4H2PO4) is an acidic salt of phosphoric acid. Thus an aqueous solution of the salt is acidic (pH 4.7 at 0.1% concentration, 4.2 at 5% according to Wikipedia). When you add ammonia solution, you neutralize the acidic hydrogens and convert the dihydrogenphosphate into the monohydrogenphosphate and finally the phosphate (NH4)3PO4. tThis means that when you have pH 10 you don't have any more NH4H2PO4 in your solution. However, all these compounds are easily soluble in water. In my opinion the formation of a precipitate can only be explained by the presence of impurities such as Ca2+. I hope this helps. Good luck with your work!
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