Applications of Aqueous Equilibria Part 2

Overview

• Focus on calculating the pH at different points on a titration curve.
• Points labeled from A to H on the curve.
• Strategy for calculating pH at each point.
• Species in solution are analyzed at each point, considering both the analyte and titrant.

Initial Analysis

• Point A: Mostly oxalic acid, which is a weak diprotic acid.
  • pH < 7 due to ionization.
• Point D (First equivalence point): Formation of hydrogen oxalate after deprotonation of one proton.
  • Hydrogen oxalate present between Points A and D.
• Point G (Second equivalence point): Formation of oxalate after deprotonation of the second proton.
  • Oxalate present after Point G.
• Sodium Ions: Present after the addition of titrant but are neutral in terms of pH.
• Hydroxide Ions: Accumulate after the second equivalence point.

Calculation of pH

Point A (Initial pH)

• Is it a buffer? No.
• Strategy: Use an ICE table and the ionization constant (Ka1) for oxalic acid.
• Important: Neglect X initially but confirm it’s not negligible (24% ionization).
• Solve using the quadratic equation to find pH = 0.67.

Point B

• Buffer Analysis: Oxalic acid and hydrogen oxalate present.
• Use Henderson-Hasselbalch equation with the ratio of conjugate base to conjugate acid.
• Resulting pH = 0.75.

Point C (First Half Equivalence Point)

• Buffer Analysis: Ideal buffer with equal amounts of oxalic acid and hydrogen oxalate.
• Henderson-Hasselbalch equation simplifies; pH = pKa1.
• Resulting pH = 1.23.

Point D (First Equivalence Point)

• No Buffer Present
• Use ICE table and Ka2 for hydrogen oxalate ionization.
• Account for dilution due to titrant.
• Resulting pH = 2.37.

Point E

• Buffer Analysis: Ideal buffer with equal amounts of hydrogen oxalate and oxalate.
• Henderson-Hasselbalch equation where pH = pKa2.
• Resulting pH = 4.19.

Point F

• Buffer Analysis: Use Henderson-Hasselbalch equation.
• Ratio of oxalate to hydrogen oxalate = 3.
• Resulting pH = 4.67.

Point G (Second Equivalence Point)

• No Buffer Present
• Oxalate acts as a weak base.
• Use ICE table and Kb1 for oxalate ionization.
• Resulting pH = 8.71.

Point H (After Second Equivalence Point)

• No Buffer: Excess hydroxide present from titrant.
• Calculate pH based on excess hydroxide (dilution considered).
• Resulting pH = 12.11.

Conclusion

• Each point on the titration curve requires a different approach based on the species present and whether the solution is buffered.
• Use of ICE tables, Henderson-Hasselbalch equation, and consideration of dilution are key in calculating accurate pH values at each stage.