Analytical Titration Theory, Henderson-Hasselbalch, and Equivalence Points
A potentiometric acid-base titration is categorized into four distinct equilibrium zones as standard titrant volume (V) is systematically delivered from a calibrated burette into an analyte solution:
Governed purely by initial weak acid ionization: [H⁺] = √(Kₐ · Cₐ).
Modeled by Henderson-Hasselbalch: pH = pKₐ + log₁₀([A⁻] / [HA]). At V = Veq/2, pH = pKa.
Stoichiometric conversion: conjugate base hydrolyzes water giving an alkaline pH (> 7).
pH is governed by unreacted excess hydroxide [OH⁻] from the burette.
First-Derivative Peak Analytics (dpH / dV)
In analytical laboratories, experimental noise can obscure inflection points. Numerical central first derivatives pinpoint the exact equivalence volume without subjective visual guesswork:
Frequently Asked Questions
Why is the equivalence pH greater than 7 for weak acids?
All weak acid is converted to its conjugate base (A⁻). This base undergoes hydrolysis in water: A⁻ + H₂O ⇔ HA + OH⁻, creating hydroxide ions and raising the pH above 7.
What is the significance of the half-equivalence point?
At V = Veq / 2, exactly half of the weak acid has been neutralized, meaning [HA] = [A⁻]. In the Henderson-Hasselbalch equation, log([A⁻]/[HA]) becomes 0, so pH = pKa.