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Dissociation degree of polyprotic acids

  1. Mar 6, 2012 #1
    I can quickly calculate the dissociation degree of a monoprotic acid with the formula below (derived from the Hendersson-Hasselbach equation). I don't see any reason why this would not work also for polyprotic acids but I would like to confirm that it does. I would appreciate it if someone could confirm/disconfirm this. Thanks.

    \begin{align}
    & pH=pKa+\log \frac{A^{-}}{HA} \\
    & \log \frac{A^{-}}{HA}=pH-pKa \\
    & \frac{A^{-}}{HA}=10^{pH-pKa} \\
    \end{align}
     
  2. jcsd
  3. Mar 6, 2012 #2

    Borek

    User Avatar

    Staff: Mentor

    Yes it will work - pH and pKa are enough to calculate ratio of acid and conjugate base concentrations.

    As Henderson-Hasselbalch equation is just a rearranged form of the dissociation constant definition, formula you derived is also just a rearranged form of the dissociation constant definition. It is quite useful in the analysis of the endpoint detection accuracy (see discussion of acid/base titration indicators where the same formula is used).
     
  4. Mar 7, 2012 #3
    Cheers!
     
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