Verify Lambert’s Beer’s Law & Determine KMnO4 Concentration

APPARATUS REQUIRED:

i) Burettes (2)

ii) Test tubes (8)

iii) Pipettes

iv) UV-visible spectrophotometer

CHEMICALS REQUIRED:

i) Potassium permanganate (KMnO₄)

ii) 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.