Titration Curves of Polyprotic Acids (College Board AP Chemistry)

Polyprotic Acids

• Polyprotic acids contain more than one ionising hydrogen atoms
  • E.g. H₂SO₄, H₃PO₄
• These acids ionize and stepwise

Ionization steps for phosphoric acid, H₃PO₄

Step	Equation
Step 1	H3PO4  H2PO4– + H+
Step 2	H2PO4– HPO42–  + H+
Step 3	HPO4–  PO43–  + H+

 

• Titrations of polyprotic acids have more than one equivalence point due to the series of ionization steps
• If H₃PO₄ is titrated against a strong base such as NaOH
• At each equivalence point, the acid is neutralised to H₂PO₄, HPO₄^2– and PO₄^3– respectively
• The pH at these points can be determined by using
  • K_a = 
• Between the equivalence points, there are mixtures of a conjugate acid and its conjugate base and are known as buffer regions
• At the midpoint of these regions, the pH = pK_a of the acid and the concentration of the conjugate acid and base are equal 
  •  E.g. [H₂PO₄] = [HPO₄^2–]
• This is the half-equivalence point

pH curve for addition of NaOH to H₃PO₄

At the midpoints of each ionization step, the pH of the acid is equal to the pK_a