STUDY OF POTENTIOMETRIC TITRATION CURVES
E3
This exercise does not require a report.
Two practical sessions are allocated to this exercise.
PURPOSE
1.
To identify the equipment and apparatus required to perform potentiometric titrations of acidbase, redox and precipitation reactions
2.
To perform simple examples of each class of reaction
3.
To observe the factors affecting the shape of potentiometric titration curves
4.
To use the first-derivative method to determine the endpoint of the titrations
PROCEDURE
Each pair of students will be required to perform three titrations – one from each reaction class. A
schedule will be posted that will tell you which combination of reactants you are to analyse.
In each case, use 25 mL of sample.
Acid-base reactions
1
2
3
4
5
6
7
Sample (in beaker)
0.1 M HCl
0.001 M HCl
0.1 M NaOH
0.001 M NaOH
0.1 M CH3COOH
0.05 M Na2CO3
0.05 M HCl/0.1 M CH3COOH
Titrant (in burette)
0.1 M NaOH
0.001 M NaOH
0.1 M HCl
0.001 M HCl
0.1 M NaOH
0.1 M HCl
0.1 M NaOH
Apparatus
pH meter
pH
electrode
Sample
Redox reactions
8
9
10
11
Sample (in beaker)
0.01 M KMnO4 *
0.01 M K2Cr2O7 *
0.1 M Fe (II) *
0.05 M I2
Titrant (in burette)
0.1 M Fe (II)
0.1 M Fe (II)
0.01 M K2Cr2O7
0.05 M Na2S2O3
* add 50 mL of 2.5 M H2SO4
Apparatus
multimeter
2V DC
saturated
calomel
electrode
Pt
electrode
Sample
Precipitation reactions
12
13
14
15
Sample (in beaker)
0.01 M NaCl
0.01 M NaBr
0.01 M NaI
0.005 M NaI/0.01 M NaCl
Titrant (in burette)
0.01 M AgNO3
0.01 M AgNO3
0.01 M AgNO3
0.01 M AgNO3
Apparatus
multimeter
2V DC
Ag
electrode
saturated
calomel
electrode
Sample
NH4NO3
salt bridge
(paper soaked in NH4NO3)
General procedure for performing the titration
1.
Set up the apparatus and solutions, as above, including a magnetic stirrer bar in the sample
solution. The sample beaker sits on the stirrer motor. The electrode(s) in the sample beaker
must be clamped so that they are above the level of the stirrer bar.
2.
Add the titrant in 5 mL volumes initially. Note the pH or mV reading after each addition.
3.
When the readings between successive additions begin to change faster than before (0.5 pH units
or 10 mV as a general rule), reduce the next addition volume as in the scheme below.
4.
After endpoint, when the change in readings begin to slow down, increase the addition volumes
as below. Generally you will need to have added around 10 mL after endpoint (one 5 mL
addition).
5 mL
→
1 mL
→
0.5 mL
Start
→
0.1 mL
→
0.5 mL
→
1 mL
→
5 mL
Endpoint
Your titrations
Reaction
Number
Titrant molarity
Acid-base
Redox
Precipitation
CALCULATIONS
•
using the first derivative method, determine the endpoint for each of your three titrations
•
for your acid-base titration, graph the data and determine the pH at the endpoint
•
for each titration, calculate the concentration of analyte in the sample using the following steps:
⇒ calculate the moles of titrant (molarity x volume of titrant in L)
⇒ determine the reaction ratio
⇒ calculate the moles of analyte (moles of titrant x reaction ratio)
⇒ calculate the molarity of analyte (moles ÷ sample volume in L)
⇒ calculate the g/L of analyte (molarity x FW of analyte)
QUESTIONS
These will require you to examine the set of class data.
1.
2.
3.
4.
5.
6.
Which of the acid-base titrations has a pH of 7 at endpoint? Explain why this is so, and why the
others don’t.
In acid-base titration 7, which endpoint has the larger titrant volume? Which acid has the higher
concentration? What does this tell you about the order of titration versus the strength of acid?
Choose an acid-base indicator from the list in your notes suitable to detect the endpoint of each
of the titrations 1-6.
Which of the redox titrations has the largest endpoint break?
Which of the halide ions gives the largest endpoint break in the precipitation titrations 12-14?
In precipitation titration 15, which endpoint has the larger titrant volume? Which ion has the
higher concentration? What does this tell you about the order of titration?