Acid-Base Titrations (Titration Curve)
The following sample data models the titration of a weak acid with a strong base.
Volume Of Base (mL) pH Volume Of Base (mL) pH 1.00 3.15 15.00 4.90 2.00 3.24 16.00 5.20 3.00 3.39 16.50 5.40 4.00 3.54 17.00 5.60 5.00 3.63 17.50 5.95 6.00 3.78 18.00 6.60 7.00 3.85 18.50 7.30 8.00 3.98 18.70 7.60 9.00 4.11 18.90 8.15 10.00 4.20 19.10 9.95 10.50 4.26 19.30 10.50 11.00 4.31 20.50 10.90 11.50 4.39 21.00 11.80 12.00 4.47 21.50 12.20 13.00 4.60 22.50 12.30 14.00 4.75 23.50 12.35
The titration curve below, pH vs Titrant Volume, represents the sample titration data.
This method estimates the equivalence point from the inflection point of the titration curve. To determine the equivalence point, draw two best-fit lines along the portions of the curve where the pH values are fairly constant. A third best-fit vertical line is drawn for the portion of the curve where the pH changes rapidly. The midpoint of this vertical line represents the equivalence point.
Once the volume at the equivalence point is determined, then one-half that volume represents the volume of strong base needed to neutralize one-half the weak acid. The equilibrium constant expression for the weak acid is given by:
Ka = [H+][A-]/[HA]
When the acid is half neutralized, [HA] = [A-], and the concentration terms cancel leaving
Ka = [H+]
Taking -log of both sides and rearranging results in
-log[H+] = -log Ka or pH = pKa.
This is not the only method to determine the equivalence point, so be sure to check out Acid-Base Titrations (First Derivative), Acid-Base Titrations (Second Derivative), and Acid-Base Titrations (Gran Plot).
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