Experiment 7
Potentiometric Titration
Valdosta State University
Purpose
To accurately determine the molar mass and pKa of an
unknown weak acid.
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Background
• In this experiment, the pH of a solution will be measured
directly as the sodium hydroxide is added.
• The pH will be measured using a glass combination
electrode.
• This device measures pH (or [H3O+]) as voltage.
Valdosta State University
Background
pH Combination Electrode
• combination refers to the ion sensing cell and the reference
cell.
Valdosta State University
Background
pH Combination Electrode
• combination refers to the ion sensing cell and the reference
cell.
Valdosta State University
Background
Consider the following graph:
13
12
11
10
pH
9
8
7
6
5
4
3
0
5
10
15
20
Volum e NaOH (m L)
Valdosta State University
25
30
35
Background
Consider the following graph:
13
12
11
10
pH
9
8
7
6
5
4
3
0
5
10
15
20
25
30
35
Volum e NaOH (m L)
In this region H+ dominates, the small change in pH is the result
of relatively small changes in H+ concentration.
Valdosta State University
Background
Consider the following graph:
13
12
11
10
pH
9
8
7
6
5
4
3
0
5
10
15
20
25
30
35
Volum e NaOH (m L)
In this region, relatively small changes in H+ concentration cause
large changes in pH, The midpoint of the vertical region is the
equivalence point.
Valdosta State University
Background
Consider the following graph:
13
12
11
10
pH
9
8
7
6
5
4
3
0
5
10
15
20
25
30
35
Volum e NaOH (m L)
In this region OH- dominates, the small change in pH is the result
of relatively small changes in OH- concentration.
Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)
• This experiment will use monoprotic acids.
• Therefore, at the equivalence point,
moles HA = moles NaOH
• Using the definition of moles, the molar mass can be
determined:
Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)
• This experiment will use monoprotic acids.
• Therefore, at the equivalence point,
moles HA = moles NaOH
• Using the definition of moles, the molar mass can be
determined:
sample weight
moles 
molar mass
Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)
• pKa can be determined by using the Henderson-Hasselbach
equation.
Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)
• pKa can be determined by using the Henderson-Hasselbach
equation.
 conjugatebase 

pH  pK a  log 
 weak acid  
Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)
• pKa can be determined by using the Henderson-Hasselbach equation.
• If the pH is measured at the point where
[conjugate base] = [weak acid]
 conjugatebase 

pH  pK a  log 
 weak acid  
Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)
• pKa can be determined by using the Henderson-Hasselbach
equation.
 conjugatebase 

pH  pK a  log 
 weak acid  
• If the pH is measured at the point where
[conjugate base] = [weak acid]
pH  pK a  log 1
pH  pK a
Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)
• This assumption is valid at the half-titration point.
• If the equivalence point is 20.00mL, the half-titration point is
at 10.00mL.
• The pH is then obtained directly from the graph.
Valdosta State University
Background
13
12
11
10
pH
9
8
7
6
5
4
3
0
5
10
15
20
Volum e NaOH (m L)
Valdosta State University
25
30
35
Background
13
12
11
10
pH
9
8
7
6
5
4
3
0
5
10
15
20
Volum e NaOH (m L)
Valdosta State University
25
30
35
Background
13
12
11
10
pH
9
8
7
6
5
4
3
0
5
10
15
20
Volum e NaOH (m L)
Valdosta State University
25
30
35
Procedure – Experiment 7
- For this experiment, work in pairs.
Technique - Titration
Clean The Buret
1. Rinse a buret twice with distilled water.
2. Rinse the buret twice with 10-mL portions of your
standardized NaOH solution.
3. Run some of the NaOH solution through the tip of the
buret.
4. Drain the buret, then fill it with the standardized NaOH
solution.
5. Let some NaOH solution run through the tip of the buret so
that all air bubbles are removed.
Technique - Titration
Clean The Buret
Technique - Titration
Prepare the Unknown
1. Weigh 0.300 – 0.400g of solid unknown or 0.120 – 0.180g
of liquid unknown to the nearest 0.001 g into a 250mL
beaker.
2. Add approximately 50 mL of distilled water to the beaker
and stir until all the unknown is dissolved.
Technique - Titration
Set-up the pH electrode – GLX Computer
Technique - Titration
Set-up the pH meter - Calibration
• Get a 400 mL beaker of distilled water to rinse the electrode between
readings. Rinse the electrode with distilled water.
Technique - Titration
Performing The Experiment
1. Put the electrode tip in the unknown acid solution. “Start” button on
2. Fill the buret to 0.00mL and record this initial pH and buret reading.
3. Slowly add NaOH with stirring until either the pH increases by 0.20
units or 2.0 mL of NaOH have been added, whichever occurs first. Stop
and record both the pH and the buret reading.
4. Repeat step 3 until the pH reaches 12, or until the titration has gone 15
mL past the equivalence point.
Data
Mass of unknown: 0.449g
mL NaOH
pH
mL NaOH
pH
0.00
4.02
18.80
10.08
2.30
4.36
19.00
10.60
3.00
4.47
19.40
10.98
4.80
4.67
19.92
11.24
6.10
4.81
21.25
11.46
8.48
5.03
23.95
11.69
10.61
5.21
25.10
11.84
12.80
5.42
27.28
11.94
15.08
5.72
28.12
11.94
16.90
6.09
29.80
12.00
17.82
6.47
Data
Experiment 7, trial 1
13.00
12.00
11.00
10.00
9.00
pH
8.00
7.00
6.00
5.00
4.00
3.00
2.00
0.00
5.00
10.00
15.00
20.00
Volum e NaOH (m L)
25.00
30.00
35.00
Data
Experiment 7, trial 1
13.00
12.00
11.00
10.00
9.00
pH
8.00
7.00
6.00
5.00
4.00
3.00
2.00
0.00
5.00
10.00
15.00
20.00
Volum e NaOH (m L)
25.00
30.00
35.00
Calculation – Molar Mass
18.50 mL NaOH 
1 L NaOH
0.116 mol NaOH

 0.00215 mol NaOH
1000 mL NaOH
1 L NaOH
Calculation – Molar Mass
18.50 mL NaOH 
1 L NaOH
0.116 mol NaOH

 0.00215 mol NaOH
1000 mL NaOH
1 L NaOH
 1 mol unknown 
  0.00215 mol unknown
0.00215 mol NaOH  
 1 mol NaOH 
Calculation – Molar Mass
18.50 mL NaOH 
1 L NaOH
0.116 mol NaOH

 0.00215 mol NaOH
1000 mL NaOH
1 L NaOH
 1 mol unknown 
  0.00215 mol unknown
0.00215 mol NaOH  
 1 mol NaOH 
 0.449 g unknown 

  209 g / mol unknown
 0.00215 mol unknown 
Calculation - pKa
Experiment 7, trial 1
13.00
12.00
11.00
10.00
9.00
pH
8.00
7.00
6.00
5.00
4.00
3.00
2.00
0.00
5.00
10.00
15.00
20.00
Volum e NaOH (m L)
25.00
30.00
35.00
Calculation - pKa
Experiment 7, trial 1
13.00
12.00
11.00
10.00
9.00
pH
8.00
7.00
6.00
5.00
4.00
3.00
2.00
0.00
5.00
10.00
15.00
20.00
Volum e NaOH (m L)
25.00
30.00
35.00
IMPORTANT REMINDER
Save the unused solution in a location identified by your
laboratory instructor.
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Safety
The NaOH solutions are corrosive. If you get some on your skin,
wash with water for at least five minutes. If any NaOH solution
gets in the eyes IMMEDIATELY wash with water for at least
fifteen minutes. Call for medical assistance!
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Waste Disposal
All wastes from this experiment can be flushed down the drain
with plenty of running water.
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