Buffers of polyprotic acids

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

2
Complex Acid - Base Systems

• Buffers of polyprotic acids
• Example Calculate the pH of a solution that is
  0.1000 M H3PO4 and 0.1000 M H2PO4-
• H3PO4 H2O H3O H2PO4-

3
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

• C2H4(COOH)2
• Kas
• H2M H HM- Ka11.2x10-2
• HM- H M2- Ka25.96x10-7
• Some typical assumptions cannot be made
• Work through the example in the text

4
Complex Acid - Base Systems

• Calculation of titration curves for polyprotic
  acids and bases
• Titration curves for H2SO4, H2C2O4 and H3PO4
• Figure 11-4, FAC7 p.238

Using 0.1000 M NaOH to titrate 25.00 mL A 25.00
mL 0.1000 M H3PO4 B 25.00 mL 0.1000 M oxalic
acid C 25.00 mL 0.1000 M H2SO4
5
Complex Acid - Base Systems

• Calculation of titration curves for polyprotic
  acids and bases
• Titration of CO32-
• FAC7, p. 240
• Titration of amphiprotic substances
• FAC7 p. 241

6
Complex Acid - Base Systems

• 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 ions form
  and negative ion form
• NH2CH2COO- NH3CH2COOH

7
Applications of Neutralization Titrations

• Standard acid solutions
• HCl(aq) is most common acid reagent
• 0.1 M can be boiled under reflux for 1 hour
  without acid loss
• 0.5 M can be refluxed for 10 min without acid
  loss
• Constant boiling HCl can be weighed and diluted
  to a known volume to prepare standard solutions
  of HCl
• for P between 670 torr and 760 torr
• H2SO4 of HClO4 are usable, but not HNO3
• Standardize using Na2CO3 or (HOCH2)3CNH2 using
  bromocresol green indicator or add measured
  excess of acid and back titrate with OH- with
  known acidbase ratio using phenolphthalein

8
Applications of Neutralization Titrations

• Standard base solutions
• NaOH most common
• Reasonably stable if protected from air and glass
• Store in polyethylene or parafin coated glass
• Effect of CO2
• CO2 2OH- CO32- H2O
• 1 mol CO2 ? 2 mol OH- ? 2 mol H
• if an indicator with acid color transition is
  used
• CO32 2H H2CO3 no error in base
  consumed
• if an indicator with a base color transition
  is used
• CO32- H HCO3- introduce a
  determinate error
• Preparation of NaOH standard solutions
• Purchase 50 NaOH in which Na2CO3 is insoluble
• Pre-prepared solutions are available - Acculute
• Boil the distilled water to remove CO2
• Standardization
• Use potassium biphthalate - molar mass 204
• KH(IO3)2 - a strong acid with molar mass 390

9
Applications of Neutralization Titrations
Kjeldahl determination of N Carbonate mixtures