Electrochemistry
LEC 06
06.20 Amperometric equivalent point determination with the dead stop method
What you can learn about
Amperometric titration
Electrode polarisation
Maximum limiting diffusion
current
Overvoltage
Polarography
Principle and tasks
In amperometric titration, the current intensity is measured as a function of the added titrant. Here the
content of an aqueous sodium thiosulphate solution is determined by
titrating it with an iodine-potassium
iodide solution. The equivalence
point can be determined amperometrically with platinum electrodes.
What you need:
Power supply, universal
13500.93
1
Multirange meter with amplifier
07042.00
1
Digital multimeter
07128.00
1
Rheostat, 100 ⍀, 1.8 A
06114.02
1
Platinum electrode, protective tube, d = 8 mm
45206.00
2
Connecting cord, l = 250 mm, red
07360.01
1
Connecting cord, l = 250 mm, blue
07360.04
3
Connecting cord, l = 500 mm, red
07361.01
1
Connecting cord, l = 500 mm, blue
07361.04
1
Connecting cord, l = 750 mm, red
07362.01
1
Retort stand, h = 750 mm
37694.00
1
Right angle clamp
37697.00
1
Holder for 2 electrodes
45284.01
1
Spring balance holder
03065.20
2
Burette clamp, roller mounting
37720.00
1
Magnetic stirrer, mini
47334.93
1
Magnetic stirrer bar, l = 30 mm
46299.02
1
Burette, 50 ml, with Schellbach line
36513.01
1
Precision balance CPA 623S (620 g/0.001 g),
set with software
49224.88
1
Weighing dishes, 805014 mm
45019.25
1
Volumetric flask, 1000 ml
36552.00
2
Course of the current in the titration of thiosulfate solution with iodine solution.
Volumetric pipette, 1 ml
36575.00
1
Volumetric pipette, 10 ml
36578.00
1
Pipettor
36592.00
1
Wash bottle, 500 ml
33931.00
1
1
Sodium thiosulphate solution, 0.1 M, 1000 ml
48345.70
1
Funnel, glass, do = 55 mm
34457.00
Funnel, glass, do = 80 mm
34459.00
1
Iodine, 25 g
30093.04
1
Glass beaker, 150 ml, tall
36003.00
1
Potassium iodide, 50 g
30104.05
1
Pasteur pipettes
36590.00
1
Water, distilled, 5 l
31246.81
1
Rubber bulbs
39275.03
1
Microspoon
33393.00
1
Amperometric equivalent point determination
with the dead stop method
90 Laboratory Experiments Chemistry
PHYWE Systeme GmbH & Co. KG · D - 37070 Göttingen
P3062001
LEC
06.20
Amperometric equivalent point determination
with the dead stop method
Related concepts
Amperometric titration, electrode polarisation, maximum limiting diffusion current, overvoltage, polarography.
Principle
In amperometric titration, the current is measured as a function
of added titrant. The equivalence point is shown by a drastic
increase in the current.
Tasks
Determine the content of an aqueous sodium thiosulphate solution by titrating it against iodine-potassium iodide solution.
Equipment
Power supply, universal
Multirange meter with amplifier
Digital multimeter
Rheostat, 100 Ohm, 1.8 A
Platinum electrode, protective tube, d = 8 mm
Connecting cord, l = 250 mm, red
Connecting cord, l = 250 mm, blue
Connecting cord, l = 500 mm, red
Connecting cord, l = 500 mm, blue
Connecting cord, l = 750 mm, red
Retort stand, h = 750 mm
Right angle clamp
Holder for 2 electrodes
13500.93
07042.00
07128.00
06114.02
45206.00
07360.01
07360.04
07361.01
07361.04
07362.01
37694.00
37697.00
45284.01
1
1
1
1
2
1
3
1
1
1
1
1
1
Spring balance holder
Burette clamp, roller mounting
Magnetic stirrer, mini
Magnetic stirrer bar, l = 30 mm
Burette, 50 ml, with Schellbach line
Set of precision balance Sartorius CPA 623S
and measure software
Weighing dishes, 80 x 50 x 14 mm
Volumetric flask, 1000 ml
Volumetric pipette, 1 ml
Volumetric pipette, 10 ml
Pipettor
Funnel, glass, do = 55 mm
Funnel, glass, do = 80 mm
Beaker, 150 ml, tall
Pasteur pipettes
Rubber bulbs
Microspoon
Wash bottle, 500 ml
Sodium thiosulphate solution, 0.1 M, 1000 ml
Iodine, 25 g
Potassium iodide, 50 g
Water, distilled, 5 l
03065.20
37720.00
47334.93
46299.02
36513.01
2
1
1
1
1
49224.88
45019.25
36552.00
36575.00
36578.00
36592.00
34457.00
34459.00
36003.00
36590.00
39275.03
33393.00
33931.00
48345.70
30093.04
30104.05
31246.81
1
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Set-up and procedure
Set up the experiment as shown in Fig. 1.
Fig. 1. Experimental set-up.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3062001
1
LEC
06.20
Amperometric equivalent point determination
with the dead stop method
Prepare the solutions required for the experiment as follows:
– 0.001 molar Na2S2O3 solution: Pipette 10 ml of 0.1 molar
sodium thiosulphate solution into a 1000 ml volumetric flask,
and make up to the mark with distilled water.
– Iodine / potassium iodide solution, 0.001 molar for I2: Weigh
0.746 g of potassium iodide into a 1000 ml volumetric flask
and dissolve it in approximately 60 ml of distilled water. Add
0.254 g of iodine, shake until it has completely dissolved,
then make up to the mark with distilled water.
Connect the 100 Ω rheostat to the direct current outlet of the
power supply as a potential divider. Connect the voltmeter in
parallel and the ammeter in series to the two bare platinum electrodes as shown in Fig. 2. Choose a current range of 1 µA on the
ammeter. Place a 150 ml beaker on the magnetic stirrer and put
in a magnetic stirrer bar. Pour 60 ml of distilled water into the
beaker and add 10 ml of the 0.001 M thiosulphate solution.
Immerse the electrodes to a depth sufficient to ensure that the
vent holes are completely covered by water. Fill the burette with
the iodine solution. Adjust the magnetic stirrer to a medium stirring speed. Set the adjustment knob of the power supply to
between 0 and 1 V, and adjust the voltage on the rheostat to
exactly 40 mV. Record the initial current value. Perform the titration in steps of 0.5 ml. Wait at least 30 seconds between the
additions. Record the current corresponding to the volume of
iodine solution added. When the equivalence point has been
reached, switch the measuring range to 3 µA. Terminate the
experiment after a total of 15 ml of iodine solution has been
added.
The dead stop method allows measurement to be made with
very small overvoltages and below the maximum diffusion current. Virtually no current flows across the two bare platinum electrodes, which are polarised with 40 mV, virtually no current flows,
as both the evolution of hydrogen and thiosulphate oxidation
require a higher overvoltage to occur. As long as there are still
thiosulphate ions in the solution, the addition of iodine does not
change the current, because the iodine is immediately reduced
to iodide with tetrathionate formation:
I2 + 2 S2O32-
2 I- + S4O62-
When the equivalence point is exceeded, a drastic current
increase is to be observed, because of the redox pair iodine /
iodide in the solution.
I2 + 2 e-
2 I-
When current is plotted against the consumption of iodine solution, as shown in Fig. 3, the equivalence point can be exactly
determined as the intersection point of a straight line through the
region of the large current increase and the zero line.
Fig. 3:
Course of the current in the titration of thiosulfate solution with iodine solution
Theory and evaluation
End-point indication given by amperometric titration is inherently more accurate than that in titrations with visual indicators.
Amperometric titrations possess greater sensitivity than conductometric and potentiometric titrations. In amperometry, the
potential of the working electrode is held at a constant value,
and the resulting current is measured. Analytical determinations
are made from the current, which is proportional to the concentration of the electroactive species. Usually, the potential is held
in its limiting current region. During titration, the current flowing
through the system is recorded as a function of the volume of
titrant.
Fig. 2:
Potential divider circuit
2
P3062001
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen