Complex Acid Base Systems

1

• Complex Acid - Base Systems
• Buffers of polyprotic acids
• For H3PO4 there are 3 possible buffer systems
• H3PO4 / H2PO4- H3PO4 H2O
  H3O H2PO4-
• H2PO4- / HPO42- H2PO4- H2O
  H3O HPO42-
• HPO42- / PO43- HPO42- H2O
  H3O PO43-
• If ratio of successive Kas lt 10-3, and solutions
  not too dilute, use the theory of simple,
  monoprotic acid buffers
• Example Calculate the pH of a solution that is
  0.1000 M H3PO4 and 0.1000 M H2PO4-
• H3PO4 H2O H3O H2PO4-

2

• Complex Acid - Base Systems
• Calculation of titration curves for polyprotic
  acids and bases
• Example titration of 0.1000 M maleic acid with
  0.1000 M NaOH - Example 11-8, FAC7 p. 235
• Titration curves for H2SO4, H2C2O4 and H3PO4
  Figure 11-4, FAC7 p.238
• Titration of CO32- FAC7, p. 240
• Titration of amphiprotic substances FAC7 p. 241
• Amino acids are important amphiprotic substances
• Example glycine - NH2CH2COOH
• NH2CH2COOH NH3CH2COO-
• NH3CH2COO- H2O NH2CH2COO- H3O
  Ka2x10-10
• NH3CH2COO- H2O NH3CH2COOH OH-
  Kb2 x 10-12
• Electrophoresis allows characterization of amino
  acids
• Measure the migration of charged molecules in an
  electric field
• Isoelectric point is the pH at which no migration
  occurs because equal number of positive ion form
  and negative ion form
• NH2CH2COO- NH3CH2COOH