How are pH and pKa related?

pKa measures the strength of an acid in a solution while pH indicates whether a solution is acidic or basic. pH and pKa are related in that the pKa represents the pH at which a chemical species either accepts or donates a proton. A lower pKa indicates a stronger acid with higher capability to donate a proton in an aqueous solution. The Henderson-Hasselbalch equation establishes a relationship between pKa and pH.