# Is it correct to say that the pKa is the pH at equilibrium?

I've heard this said when looking at the charge of certain groups on a molecule (with different $$\mathrm{p}K_\mathrm{a}$$ values) in a solution of certain $$\mathrm{pH}.$$ But is that really correct to say?

Based on speciation diagrams, I would have thought that the $$\mathrm{p}K_\mathrm{a}$$ is always constant and that $$\mathrm{p}K_\mathrm{a}$$ is simply the $$\mathrm{pH}$$ at the point where the concentrations of the acid and its conjugate based are equal.

• To say that "The pKa is the pH at equilibrium" isn't really true. Acid/Base reactions in aqueous solutions happen very rapidly and thus the solution reaches some equilibrium quickly. The pKa is a particular equilibrium where the protonated form of the acid and it unprotonated form are present in equal concentrations in the solution.
– MaxW
Apr 16, 2020 at 3:08
• $\ce{}$No$\ce{}$ Apr 16, 2020 at 3:44

$$\mathrm{p}K_\mathrm{a}$$ and $$\mathrm{pH}$$ are apples and oranges.* We can't equate them on a graph. The correct wording should rather be that the $$\mathrm{pH}$$ at which the concentration ratio of an acid and its conjugate base is equal to $$1,$$ then the $$\mathrm{pH}$$ is numerically equal to $$\mathrm{p}K_\mathrm{a}.$$
* It will be another painful story if one begins to argue on the units of $$\mathrm{pH}$$ and $$\mathrm{p}K_\mathrm{a}.$$ They are dimensionless. Just a subtle point $$\mathrm{p}K_\mathrm{a}$$ is constant as long as the temperature/pressure is constant. It is not always a constant.