Anonymous asked in Science & MathematicsChemistry · 1 month ago

What is an alternate way of preparing a sodium hydrogen phosphate/sodium dihydrogen phosphate buffer?

So my friend and I are stumped. This is in the context of achieving the sodium hydrogen phosphate/sodium

dihydrogen phosphate buffer by titrating the acid with a strong base. 

My friend says the answer would be H3PO4 + Na3PO4. In my case, I'd think that sodium hydrogen phosphate would be the acid that we'd titrate in that scenario and all we'd need then is a strong base (although I honestly don't know how to figure out what strong base in particular you must use.) 

What's correct? Or what would be the correct way?

2 Answers

  • 1 month ago

    H3PO4 + Na3PO4 ---> Mixing these two would get a mess that I don't even want to think about. I think you'd get a mixture of lots of H3PO4, lots of Na3PO4 and small amounts of NaH2PO4 and Na2HPO4.

    You mention using a strong base. The strong base to use is NaOH. How might you make the buffer you desire?

    1) Titrate H3PO4 with one equivalent of NaOH. result = solution of NaH2PO4 (ignore tiny bits of ionization to other stuff). Titrate with 0.5 equivalent of NaOH to get a 50/50 mix of NaH2PO4 and Na2HPO4.

    2) Titrate NaH2PO4 (easily obtained from the stockroom) with 0.5 equivalent of NaOH. Result is a 50/50 mix of NaH2PO4 and Na2HPO4.

    "I'd think that sodium hydrogen phosphate would be the acid that we'd titrate in that scenario"

    If you titrate Na2HPO4 with NaOH (but not to the equivalence point, you get Na2HPO4 and Na3PO4. That's a buffer, but not the one you want.

  • Dr W
    Lv 7
    1 month ago

    buffer = weak acid + salt of conjugate base  (or weak base + salt of conjugate acid)

    in this problem

    .. H3PO4 .. .<--> H+ + H2PO4(-).... Ka1 = 7.25x10^-3

    .. H2PO4(+) <--> H+ + HPO4(2-)... Ka2 = 6.31x10^-8

    .. HPO4(2-) <--> H+ + PO4(3-).. .. . Ka3 = 3.98x10^-13

    since Ka1 >>> Ka2 >>>Ka3... for all practical purposes, you only need to consider the first dissociation.. H3PO4 <-> H2PO4(-)

    same thing goes for the base 

    .. Na3PO4 --> 3Na+ + 1 PO4(3-)... the salt completely dissociates

    then considering those Ka values, for all practical purposes

    .. PO4(3-) + H ---> HPO4(2-)

    .. HPO4(2-) + H --> H2PO4(-)

    and we're back to 

    .. H2PO4(-) + H+ <--> H3PO4.. Ka'1

    so if you add add Na3PO4 you'll end up with an H2PO4(-) conjugate base in solution

    likewise if you add Na2HPO4 or NaH2PO4, you'll end up with the same conjugate base H2PO4(-)

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