F.6/7 Chemistry Practical
Conductometric Titrations
Objective: To determine concentrations of acids/bases by conductometric titration
Group size: Two-students per group
Introduction
The conductance of a cell, containing an aqueous solution of an electrolyte, depends on:
(a) the number of free ions in the cell
(b) the number of charges on each ion and
(c) the speed at which the ions move
In the course of a titration, the number of free ions in the conductance cell will change. As a result,
the conductance of the cell containing the solution will change as the titration is performed. In any
cases, the rate of change of conductance as one solution is added, is different when reaction occurs
on addition, from the rate of change of conductance when excess of the solution is being added.
Molar conductivity of some ions at 25oC
Ion
Na+
H+
ClMolar conductivity 50.1
349.8 76.4
-1
2
-1
(ohm cm mol )
Ag+
61.9
K+
73.5
CH3COO40.9
NO3- OH71.5
198.3
Apparatus and chemicals:
Burette, 10 cm3 pipette, 100 cm3 measuring cylinder, 250 cm3 beaker, 150 cm3 beaker (3).
Conductance cell and circuit: a milli-ammeter, a dry cell, electric wire with crocodile clips, a
cylindrical cell with two graphite electrodes.
Solution of unknown concentration (0.10-0.15M): hydrochloric acid, ethanoic acid, potassium
chloride, iron(III) solution (0.4-0.7 M). Standard solution: 0.1 M sodium hydroxide solution, 0.1 M
silver nitrate, 0.05M EDTA
Pre-laboratory work
1. Which ions have extraordinarily high molar conductivity? Explain your answers.
2.
3.
What is the relation between the conductivity of a solution and the electric current passing this
particular cell?
Draw a circuit diagram for conductometric titration.
Theory
Give theory for this experiment.
Procedure
A. Titration of sodium hydroxide solution against hydrochloric acid.
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1. Pipette 10.00 cm3 hydrochloric acid of unknown concentration into a 250 cm3 beaker and
add approximately 100 cm3 of distilled water.
2. Set up the circuit as shown in the bottom part on page one. Read the current of the cell from
the ammeter.
3. Fill a burette with 0.1 M standard sodium hydroxide solution and run 2.0 cm3 of this
solution into the acid. Mix thoroughly and read the current of the cell from the ammeter.
4. Continuous adding 2.0 cm3 portions of alkali and determining the current of the cell
containing the solution until a total of 20 cm3 of alkali has been added.
B. Titration of sodium hydroxide solution against ethanoic acid.
1. Pipette 10.0 cm3 of ethanoic acid of unknown concentration into a 250 cm3 beaker and add
approximately 100 cm3 of distilled water.
2. Determine the current of the cell, containing this solution, and then add 2.0 cm3 portions
of 0.1 M standard sodium hydroxide solution, proceeding as in section A.
C. Titration of silver nitrate solution against potassium chloride solution
1. Pipette 10.0 cm3 of potassium chloride solution of unknown concentration into a 250 cm3
beaker and add approximately 100 cm3 of distilled water.
2. Determine the current of the cell containing this solution, and then add 2.0 cm3 portions of
0.10M standard silver nitrate solution, proceeding as before.
D. Titration of an aqueous solution of iron(III) ions against an aqueous solution of the disodium
salt of ethylenediamine tetra-acetic acid (EDTA)
1. Pipette 10.0 cm3 of iron(III) chloride solution of unknown concentrationvinto a 250 cm3
beaker and add approximately 100 cm3 of distilled water.
2. Determine the current of the cell, containing the solution, and then add 2.0 cm3 portions of
0.05 M standard EDTA solution proceeding as in previous experiments.
Data and result:
Procedure A
Tabulate your data and draw a graph in which electric current (in mA or arbitrary unit) is
plotted against volume of alkali added (in unit of cm3)
Procedure B
Tabulate your data and draw a graph in which electric current (in mA or arbitrary unit) is
plotted against volume of alkali added (in unit of cm3)
Procedure C:
F.6/7 Chem.Prac/AB4_condTitration/p.2(3)
Tabulate your data and draw a graph in which electric current (in mA or arbitrary unit) is
plotted against volume of alkali added (in unit of cm3)
Procedure D:
Tabulate your data and draw a graph in which electric current (in mA or arbitrary unit) is
plotted against volume of alkali added (in unit of cm 3 )
Determine the end-points in the above graphs and hence calculate the molarity of the solutions.
Questions for discussion
1.
Commonly five factors determine the speed at which the ion move; what are they?
2.
Alternate electric current is usually employed instead of direct current in conductometric
measurement. What is the advantage of using alternate current?
If the conductance of the cell is plotted against the volume of the solution added to a given
3.
4.
5.
6.
7.
volume of the other solution, how will the end-point be obtained from the graph?
Suggest two reasons to explain the addition of 100 cm3 of distilled water to the hydrochloric
acid in the beaker.
Explain the shape of all the graphs you plotted for the titrations in terms of molar conductivity
of the ions.
Can you determine the formula of the iron(III)/EDTA complex? Show your deduction.
What are the most useful applications of conductance measurements for the determination of
the end-point of titrations?
End
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