Acid / Base Titrations

1
Acid / Base Titrations

• Changes in Acid-Base Reaction Systems

2
Titration Terminology

• Titration the progressive transfer of a
  solution from a buret (called the titrant) into a
  measured volume of another solution (called the
  sample).
• Equivalence point the volume of titrant
  required to neutralize the sample ( mol acid
  mol base).
• Endpoint the pH at the equivalence point of a
  titration.
• Indicator a chemical which is added to the
  sample that changes colour at the equivalence
  point of a titration.
• Buffering region a horizontal region of the pH
  curve where pH is not changing significantly.

3
pH curves

• A pH curve is a graph showing the continuous
  change in pH during a titration (an acid-base
  reaction)
• Titration of NaOH and Na2CO3 with
  HCl
• 14
• pH
• 7
• 0
• vol. of titrant (HCl) added (mL)

4
Interpreting pH curves

• Titration of NaOH with HCl
• 14
• buffer region
  titrant
• pH endpoint equivalence
  point
• 7 X
• 
  
• equivalence
  point volume
• 
  buffer region
• 0
• 0 10 20
  30 40
• volume of titrant added (mL)

5
Which is the titrant?

• Take 2 titrations involving NaOH and HCl
• pH pH
• vol. of titrant
  vol. of titrant
• HCl is titrant
  NaOH is titrant
• (pH high to low)
  (pH low to high)

6
Choosing an Indicator

• pH curves are used to determine which
  indicator(s) are suitable for a particular
  titration
• The goal is to get an indicator that changes
  colour in the same region as the endpoint of your
  titration
• Titration of NaOH with
  HCl
• 14
  unsuitable alizarin yellow (too early)
• pH
  good indicators bromothymol blue,
• 7
  litmus and phenol red
• 
  
• 
  unsuitable orange IV (too late)
• 0
• vol. of titrant (HCl) added (mL)

7
Multiple Endpoints

• Titration of Na2CO3 with HCl
• 14
• endpoint 1
• 
  
• 
  
• 8 X
• pH endpoint 2
• 
  equivalence point volume 1
  
• 4 X
  equivalence point volume 2
• 
  
• 0
• 0 10 20
  30 40
• volume of titrant added (mL)

8
Choosing Indicators for Multiple Endpoints

• Titration of Na2CO3 with HCl
• 14
• endpoint 1 - metacresol purple
  (7.4-9.0)
• 
  
• 
  
• 8 X
• pH endpoint 2 methyl orange (3.2-4.4)
• 
  
• 4 X
• 
  
• 0
• 0 10 20
  30 40
• volume of titrant added (mL)

9
Polyprotic Acids and Bases

• Polyprotic acids are those which can donate more
  than one proton in most cases, it is equal to
  the number of hydrogens in the chemical formula
• E.g. H2SO4 can donate 2 protons
• E.g. H3PO4 can donate 3 protons
• Polyprotic bases are those which can accept more
  than one proton in most cases it is equal to the
  magnitude of the charge on the anion
• E.g. CO3 2- can accept 2 protons
• E.g. PO4 3- can accept 3 protons

10
Explaining Polyprotic Endpoints

• Titration of Na2CO3 with HCl
• 14
• 
  
• 
  
• 8 X endpoint 1
• pH
• 
  
• 4 X
  endpoint 2
• 
  
• 0
• 0 10 20
  30 40
• volume of titrant added (mL)

11
Explaining Polyprotic Endpoints

• Titration of Na2CO3 with HCl
• 14
• 
  
• 
  
• 8 X H3O CO3
  2- H2O HCO3 -
• pH
• 
  
• 4 X
  H3O HCO3 - H2O H2CO3
  
• 0
• 0 10 20
  30 40
• volume of titrant added (mL)

12
Other Polyprotic Systems

• Consider the titration of phosphoric acid, H3PO4,
  with sodium hydroxide, NaOH
• 11
• X
  endpoint 2
• pH
• X endpoint 1
• 0
• Vol. of NaOH (mL)

13
Other Polyprotic Systems

• Consider the titration of phosphoric acid, H3PO4,
  with sodium hydroxide, NaOH
• 11 HPO42- OH - H2O PO43-
  (not observed)
• H2PO4- OH- H2O HPO42- X
• pH
• X H3PO4 OH -
  H2O H2PO4 -
• 0
• Vol. of NaOH (mL)