March 28, 2024, 05:45:30 AM
Forum Rules: Read This Before Posting


Topic: Third Equivalence of H3PO4  (Read 20676 times)

0 Members and 1 Guest are viewing this topic.

Offline versaigo

  • Very New Member
  • *
  • Posts: 1
  • Mole Snacks: +0/-0
Third Equivalence of H3PO4
« on: October 18, 2008, 04:23:00 PM »
Does anyone know why the third equivalence point of H3PO4 is hard to see during titration? I think it has something to do with the disassociation constant of HPO4 being so small that the release of H3O+ is also very small so the eventual change of pH is too small to see. Are there any other reasons?

Offline Astrokel

  • Full Member
  • ****
  • Posts: 989
  • Mole Snacks: +65/-10
  • Gender: Male
Re: Third Equivalence of H3PO4
« Reply #1 on: October 18, 2008, 04:47:57 PM »
yes, since like you mention the Ka3 is so small and that it has to compete with ionization of water for proton dissociation, making it hard to see during titration.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Third Equivalence of H3PO4
« Reply #2 on: October 18, 2008, 05:44:45 PM »
Very weak acids in general will not have visible equivalence points in water. See upper plot - that's how the monoprotic acid of the same strength as third proton of phosphoric acid behaves. Initial pH is 6.63, barely below 7.00.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links