Polyprotic Acids — Definition & Examples

What are Polyprotic Acids?

Remember that acids lose protons or hydrogens. Polyprotic acids are able to lose more than one proton on a single molecule. The loss of the protons do not happen at the same time, but instead occurs one at a time.

Dissociation Constants

Each loss or dissociation of a proton is has a value associated with it known as a dissociation constant ($K_a$). For polyprotic acids there would be at least two dissociation constants, $K_{a1}$ and $K_{a2}$. The larger the $K_{a}$, the easier the proton will dissociate.

Let's consider an acid with three dissociable protons. Generally, it is easiest to remove the first proton from a neutral acid species. The now anionic species with a negative chrage will be harder to remove the next proton. The doubly negative species will be even harder to remove a proton from. Due to the increased difficulty of proton dissociation as the acid becomes more negative the dissociation constant value are typically ordered as $K_{a1}$>$K_{a2}$>$K_{a3}$.

Examples of Polyprotic Acids

(Video) 16.2 Binary Acids, Oxoacids, and Polyprotic Acids | General Chemistry

by Chad's Prep

In this section of the video, Chad explains polyprotic acid. He starts by defining what they are. Then he describes how to compare the concentrations of the acid and its conjugates. This is an excellent introduction to polyprotic acids.