[Rayan]

hello.
when titrating H3PO4 against NaOH, at the first half equivalence point, we get pH=pka1.
but how do we calculate pka2 and pka3?
thank you very much.

[Borek]

That's not so easy.

Ka1 is just too large. At 50% neutralization important part of the acid left dissociates and this dissociation controls the pH. Paradoxically the pH is higher then pKa1.

To find out Ka1 you should measure pH of pure acid and assume that Ka2 and Ka3 are so small that they can be neglected.

Ka2 is just OK - and when the phosphoric acid is neutralized 150% pH = pKa2.

Ka3 is so small that the situation is reverse of that at the beginning of titration. PO4(3-) hydrolizes and pH at the 250% neutralization is much lower than expected.

The best way to find out Ka3 will be to dissolve sodium phosphate, measure pH, and calculate Kb1 (pKb1 + pKa3 = pKw) assuming that the effects of Kb2 and Kb3 are neglectable.

Play with BATE to get some experience.

[GCT]

Hi, you'll need to be more clear in your question.  I believe that for this case, you can use the henderson hasselbach equation for all three cases, due to the relative Ka values.