What is pKa?

• A. The pH value at which a chemical species will accept or donate a proton
• B. The pH value at which a base becomes neutral
• C. The pH at which water dissociates
• D. The pH value of pure water

Answer: (A)

pKa tells you the pH at which a molecule is half protonated and half deprotonated. In other words, it’s the pH where the acid (or conjugate acid) donates a proton as readily as it can accept one back, so [protonated form] ≈ [deprotonated form]. This arises from Ka = [H+][A-]/[HA], and pKa = -log Ka. Through the Henderson–Hasselbalch relationship, pH = pKa + log([A-]/[HA]); when pH equals pKa, the ratio [A-]/[HA] is 1, meaning equal amounts of the two forms.

So the statement describing a pH value at which a species can donate or accept a proton captures the essence of pKa: the point where proton transfer is equally favorable, i.e., half-dissociated. This is not the same as the pH where water simply dissociates or the pH of pure water, nor a general statement about a base becoming neutral.