# Dissociation degree of polyprotic acids

1. Mar 6, 2012

### ComptonFett

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. Mar 6, 2012

### 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).

3. Mar 7, 2012

Cheers!