Calculating Ka Values of H3PO4 from Titration Data

[chadwip]

Homework Statement

I need to determine the Ka values of H3PO4 having titrated a solution of H3PO4 and HCl with 0.1142 M NaOH. The concentrations of H3PO4 and HCl were found to be 0.106 M and 0.003 M respectively. The first EP was reached at 4.76 mL with a pH of 4.65 and the second EP was reached at 9.41 mL and a pH of 9.23.

Homework Equations

Kw=Ka*Kb
pH=pKa+log([A-]/[HA])
Ka=([H+][H2PO4-])/[H3PO4]

The Attempt at a Solution

Struggled with this for a while now and I feel like I am missing something obvious.

Attempted to use the fact that pKa = pH at 1/2 the EP for the second EP received an answer of 1.51*10^(-7), which is about an order of magnitude off of the accepted pKa2 of 6.8*10^(-8). Not sure how this could be - all of the HCl has been neutralized and you are just converting H2PO4- to HPO4 2-, however this is all based off data I collected. I'm not even sure how to determine the first Ka value or even where to start. I don't believe you can say that pKa=pH at 1/2 the first EP because you still have the HCl in solution. Not sure how else you could go about solving it. Any help would be great appreciated.

 

[Borek]

Have you taken into account thermodynamic corrections?