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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: versaigo on October 18, 2008, 04:23:00 PM

Title: Third Equivalence of H3PO4
Post by: versaigo 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?

Title: Re: Third Equivalence of H3PO4
Post by: Astrokel on October 18, 2008, 04:47:57 PM

yes, since like you mention the K_a3 is so small and that it has to compete with ionization of water for proton dissociation, making it hard to see during titration.

Title: Re: Third Equivalence of H3PO4
Post by: Borek 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.