Charge of predominant species

[Tony Chen 1F]

Why is it that when pH value is above the pKa values, the predominant species is charged, and when the pH values are below the pKa values, the predominant species is neutral?

[alexjung1A]

In terms of weak acids, what you said is correct, but it is the reverse for weak bases.

To start with, when the pKa is equal to the pH, there are equal amounts of a deprotonated species A- or B (acid or base, resp) and the protonated species HA or BH+. When the pH is lower than the pKa, the protonated species HA or BH+ will be the predominant species. At pH values higher than the pKa, the deprotonated species A- or B will be the predominant species.

When we write it out like this, we can already see that for weak acids, when the pH is higher than the pKa, the predominant, deprotonated species A- is charged. We can also see that for weak bases, when the pH is lower than the pKa, the predominant, protonated species BH+ is charged.

Basically, just remember that pH higher than pKa means deprotonation and pH lower than pKa means protonation. Then remember that protonated bases are charged and deprotonated acids are charged.