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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.

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    $\begingroup$ 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
    Commented Apr 16, 2020 at 3:08
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    $\begingroup$ $\ce{}$No$\ce{}$ $\endgroup$
    – Karsten ♦
    Commented Apr 16, 2020 at 3:44

1 Answer 1

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

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