Lecture Seven covers the following points:-


- Reaction of polyprotic acid
- Titration curves of diprotic and triprotic acid
- Displacement titration
- Titration of mixtures

Diprotic Acids

H2A + 2 OH- → A2- + 2 H2O

H2A + OH- → HA- + H2O
HA- + OH- → A2- + H2O
If K1 / K2 > 10000
The solution behaves like a mixture of two acids
pH = ½ ( pK1 + pK2 )

Triprotic Acids


• H3PO4 + OH- = H2PO4- + H2O K1= 7.5x10-3

• H2PO4- + OH- = HPO42- + H2O K2=6.2x10-8

• HPO42- + OH- = PO43- + H2O K3=5.0x10-13

• K1/K2> 10000 K2/K3 > 10000
• Phosphoric acid can be titrated as monoprotic and diprotic acid
• pH = ½ (pK1 + pK2)
• = ½ (2.23 +7.21) = 4.66

• pH = ½ ( pK2 + pK3 )
• = ½ ( 7.21 + 12) = 9.6

Analysis of Mixtures

Na2CO3 and NaHCO3

Na2CO3 and NaOH

HCl and CH3COOH