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.

  • 5
    $\begingroup$ 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. $\endgroup$
    – MaxW
    Apr 16, 2020 at 3:08
  • 3
    $\begingroup$ $\ce{}$No$\ce{}$ $\endgroup$ Apr 16, 2020 at 3:44

1 Answer 1


$\mathrm{p}K_\mathrm{a}$ 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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.