Experiment:
Aim: Prepare standard solutions
of KMnO4 record their absorbance and verify Beers Law and determine unknown
concentration.
Apparatus: i) Burettes
(2) ii) Test tubes (8) iii) Pipettes iv) UV-visible spectrophotometer
Chemicals: 0.005 M Potassium permanganate (KMnO4), Water (H2O)
Theory:
Lambert’s
Beer’s law states that the absorbance of a substance in a solution is directly
proportional to its concentration and the path length of the light passing
through the solution.
Mathematically, it is expressed as,
A=
ϵLC
where, A = absorbance
ϵ = Molar absorptivity
L=
path length
C
= concentration
Absorbance
of a known concentration of substance is measured by a uv-visible
spectrophotometer of a specific wavelength. The principle of uv-visible
spectrophotometer is based on the interaction between light and matter. By plotting a graph of absorbance against
concentration, we can verify Lambert’s beer’s law and determine the
concentration of unknown solution.
Procedure:
i) Prepared a
0.0005 M stock solution of potassium permanganate by dissolving 0.0079 gm of
KMnO4 in 100ml of water.
ii) Eight test
tubes were taken.
iii) 1 test tube
(0.00005 M solution): 1 ml of KMnO4 stock
solution and 9 ml of water were added to the test tube by pipette.
iv) 2nd test tube
(0.0001 M solution); To 2 ml of KMnO4 stock
solution, 8 ml of water was added.
v) 3rd test tube
(0.00015 M solution); To 3 ml of KMnO4 stock
solution, 7 ml of water was added.
vi) 4th test tube
(0.0002 M solution ): To 4 ml of kMnO4 stock
solution, 6 ml of water was added.
vii) 5th test tube
(0.00025M solution); To 5 ml of KMnO4 stock
solution, 5 ml of water was added.
viii) 6th test tube
(XM solution); A solution of unknown concentration was made by mixing unknown
volume of water and volume was made upto 10ml.
ix) The solutions
should be thoroughly mixed to ensure the KMnO4 was
completely dissolved in the water.
x) 7th test tube :
used water as a blank.
xi) 8th test tube :
used pure kMnO4 stock solution as preference.
xii)The maximum
wavelength of KMnO4 was determined using a
spectrum or graph plotting in graph paper. This wavelength was used for maximum
sensitivity in absorbance measurement.
xiii) A graph with
absorbance on x-axis and concentration on Y-axis was plotted. It was ensured
that the graph shows a linear relationship.
xiv) The concn of
unknown solution was determined by comparing its absorbance value to the graph.
Observation:
Graph was plotted for maximum wavelength
between wavelength and absorbance.
|
Wavelength(nm) |
Absorbance |
|
470 |
0.070 |
|
485 |
0.119 |
|
500 |
0.173 |
|
515 |
0.209 |
|
530 |
0.249 |
|
545 |
0.240 |
|
560 |
0.133 |
|
575 |
0.108 |
Now, at 530 nm kMnO4 have maximum absorbance 0.249
Graph was plotted
between concentration and absorbance.
|
Concentration
(M) |
Absorbance |
|
0.00005 |
0.098 |
|
0.00010 |
0.267 |
|
0.00015 |
0.442 |
|
0.00020 |
0.613 |
|
0.00025 |
0.782 |
|
Unknown |
0.457 |
From the graph, the
unknown concentration of KMnO4 was
0.000165 M.
Result:
This experiment was valid for Lambert’s
Beer’s law and the unknown concentration at KMnO4 was found to be 0.000165 M.
