pH = pKa + log₁₀([A⁻]/[HA]); pI = ½(pKa1 + pKa2) for amino acid

Henderson-Hasselbalch equation (Henderson 1908, Hasselbalch 1917): logarithmic rearrangement of the weak-acid equilibrium K_a = [A⁻][H⁺]/[HA]. Taking −log₁₀ of both sides and rearranging: pH = pK_a + log([A⁻]/[HA]). At equimolar…