TITRATION AND INDICATOR [ANALYTICAL CHEMISTRY]

[chuong]

Hi all,
i've carried out the titration of H₃PO₄ with NaOH followed by the indicator pT 5.1 and 9.9. What i've recorded is the V(Naoh) for pT 5.1 is 8.500 ml, V(NaOH) for pT 9.9 is 19,00 ml. In my opinion, i think the value using pT 9.9 is better because the pH at equivalence is 1/2(pKa₂ + pKa₃) ~ 9.77 so it might be better. Therefore, the V(NaOH) = 19,00 ml is more reliable. Am i right ? Could someone help me !!!
Thanks !

[ChenBeier]

I think it doesn't matter.
In one case you titrate NaOH + H₃PO₄ => NaH₂PO₄ + H₂O consumption 8.5 ml
In the other case you got 2 NaOH + H₃PO₄ => Na₂HPO₄ + 2 H₂O consumption 19 ml

What is meant with pT 5.1 and 9.9?

Normaly the consumption should be only 17 ml or the first value 9.5 ml.

[kunzejea]

I think the V(NaOH) = 19.00 ml is a more reliable measure of the concentration of H₃PO₄ because it is closer to the true equivalence point, right?