I physics graduate student, but I teach an interdisciplinary lab for the university which incorporates biology, physics, and chemistry.
We recently added a weak-acid titration lab to the course and I got a result which surprised me.
The procedure was to neutralize 100 mL of 0.010 M Phosphoric acid by adding 0.10 M NaOH.
We have 0.001 mole of phosphoric acid in solution, but this acid is triprotic which means that a maximum of 0.003 moles of hydronium can be released into the solution. My understanding of the equillibrium reactions was that as I lowered the ambient hydronium ion concentration more protons would be released by the acid.
So my expectation was that I would need to add 30 mL of the NaOH solution to reach a pH of 7. In practice it only took between 12-15 mL.
I talked this over with a Chemist I work with and she wasn't sure why this discrepancy happened either. One possiblity is that the person who prepared the acid screwed up, but we don't have more stock for me to test that theory.
So my question is do these results sound normal or did I have the right idea? If I didn't have the right idea then what am I missing theoretically speaking?