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In a proton-transfer reaction, an acid donates H⁺ to a base, producing a conjugate base and conjugate acid. The acid dissociation constant Ka and its logarithmic form, pKa, quantify acid strength; conjugate acid and base strengths are related through Ka × Kb = Kw. Solvent auto-protolysis establishes the solvent’s acid-base limits, while the leveling effect and solvent properties shape how acidity is expressed. The Henderson–Hasselbalch equation relates pH, pKa, and the ratio of conjugate base to acid, helping explain buffer behavior and capacity. Polyprotic acids dissociate in stages, and titration curves can reveal their equilibria. The topic also introduces common-ion shifts, activity corrections, temperature dependence, and the isoelectric point of ampholytes.