pKa Finder & Henderson-Hasselbalch Calculator
Interactive Titration Curve
Hover over the points to learn more.
Henderson-Hasselbalch Calculator
pH = pKa + log₁₀([A⁻]/[HA])
Calculation Result
Steps
Finding pKa from a Titration Curve
A titration curve graphically represents the pH change of a solution as a titrant (usually a strong base or acid) is added. For the titration of a weak acid with a strong base, the curve has a characteristic shape that reveals the acid's pKa.
- Identify the Equivalence Point: This is the point on the curve where the amount of titrant added is stoichiometrically equivalent to the amount of acid initially present. It's typically identified by the steepest slope (inflection point) on the curve, usually at a pH above 7 for a weak acid/strong base titration. Note the volume of titrant added at this point (Veq).
- Find the Half-Equivalence Point: Calculate half the volume of the equivalence point (Veq / 2). Locate this volume on the x-axis (Volume of Titrant).
- Determine the pKa: Read the corresponding pH value on the y-axis at the half-equivalence point volume. At this specific point, the concentration of the weak acid ([HA]) is equal to the concentration of its conjugate base ([A⁻]). According to the Henderson-Hasselbalch equation (
pH = pKa + log₁₀([A⁻]/[HA])), when [A⁻] = [HA], the log term becomes log₁₀(1) = 0. Therefore, at the half-equivalence point: pH = pKa.
Frequently Asked Questions
What is a buffer region?
The buffer region on a titration curve is the relatively flat portion surrounding the half-equivalence point. In this region, the solution contains significant amounts of both the weak acid (HA) and its conjugate base (A⁻). This mixture acts as a buffer, resisting large changes in pH upon addition of small amounts of strong acid or base. The buffering capacity is maximal exactly at the half-equivalence point (where pH = pKa).
How do you find pKa values for a polyprotic acid?
A polyprotic acid can donate more than one proton (e.g., H₂CO₃, H₃PO₄). Its titration curve will show multiple equivalence points and multiple half-equivalence points, corresponding to the stepwise removal of each proton. Each half-equivalence point corresponds to a different pKa value (pKa₁, pKa₂, pKa₃, etc.). For example, for H₃PO₄, you would find pKa₁ at the first half-equivalence point, pKa₂ at the second, and pKa₃ at the third.
Why use the Henderson-Hasselbalch equation?
The Henderson-Hasselbalch equation is a convenient rearrangement of the acid dissociation constant (Ka) expression. It allows for direct calculation of a buffer solution's pH if the pKa and the concentrations of the acid and conjugate base are known. It also mathematically demonstrates why pH = pKa when the concentrations of the acid and conjugate base are equal (at the half-equivalence point).
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
Mail- Sachin@pharmacyfreak.com