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Topic: Third Equivalence of H3PO4  (Read 18237 times)

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Offline versaigo

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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

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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.
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Offline Borek

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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.
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