Acid-Base Titrations
Strong Base to Weak Acid
• Consider adding a strong base (e.g. NaOH) to a solution of a
weak acid (e.g.KHP).
Acid-Base Titrations
Strong Acid-Base Titrations
• The plot of
pH versus
volume
during a
titration is a
titration
curve.
Acid-Base Titrations
Weak Acid-Strong Base Titrations
Problem
What is the pH of a solution prepared
by mixing exactly 30.0 ml of 0.200 M
HC2H3O2 with 15.0 ml of 0.400 M
KOH.
Strong acid: [H3O+] = concentration of acid
so: pH = -log [H3O+] = -log[acid]
Weak Acid: one must calculate the [H3O+]
from an equilibrium ionization expression.
HA + H2O  H3O+ + AKa = _ [H3O+][A-]
[HA]
These are equilibrium
concentrations.
25.0 mL of 0.10M Acetic acid Ka=1.8 x 10-5
Some base has been added
2
4
This is also the “buffer region”3
1. pKa = pH at this point (halfway point)
1.
Starting pH, only acid and water.
0.10 M NaOH
Approximations:
[H  ][ A  ]
Ka 
[HA ]
[H+][A-]
4
2
pOH=-Log[OH-]
3
1
[H  ][ A  ]
Ka 
[HA ]
[H+]=[A-]
[OH  ][HA ]
Kb 
[A  ]
[OH-]=[HA]
25.0 mL of 0.10M Acetic acid Ka=1.8 x 10-5
2.Some base has been added
This is also the “buffer region”
4
3
1.
pKa = pH at this point (halfway point)
0.10 M NaOH
The color change of the indicator bromthymol blue
basic
acidic
change occurs
over ~2pH units
Curve for a strong acid-strong
base titration
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Strong Acid-Strong base Titration
pH
starts low and increases gradually as acid
is neutralized by the added base
Close
to equivalence point pH rises steeply
Beyond
this, pH increases slowly with
addition of more base
Equivalence
point
– the mol OH1- = mol H1+
–The pH = 7
Titration of 40.00mL of 0.1000M HPr with
0.1000M NaOH
Curve for a
weak acidstrong base
titration
pH = 8.80 at
equivalence point
pKa of HPr
= 4.89
[HPr] = [Pr-]
methyl red
Titration of 40.00mL of 0.1000M NH3
with 0.1000M HCl
pKa of NH4+
= 9.25
Curve for a
weak basestrong acid
titration
pH = 5.27 at
equivalence
point
Curve for the titration of a
weak polyprotic acid.
pKa = 7.19
pKa = 1.85
Titration of 40.00mL of 0.1000M
H2SO3 with 0.1000M NaOH