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- Mar 2025
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chem.libretexts.org chem.libretexts.org
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Figure 44\PageIndex{4}: Titration of a Weak Polyprotic Acid. Another 10 mL, or a total of 20 mL, of the titrant is added to the weak polyprotic acid to reach the second equivalence point. (CC BY; Heather Yee via LibreTexts)
This figure is misleading. The Equivalence Point is where the moles of acid and conjugate base are equal. The Midpoint to ionization is NOT where the moles are equal. The midpoint is where the pKa is determined... by sliding left from the midpoint dot to the Y axis and reading the pH. The pKa = pH at the Midpoint (which is found by taking the 1/2 Equivalence Point (or half the distance required (on the X axis) for the ionization) and moving up to the graphed line). Presumably the next diagram also has the same error.
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