Verify Lambert-Beer's law for solution CoCl 2.5H 2O (in water) or K2 Cr2 O7 (in water).

• Verify Lambert-Beer's law for solution CoCl 2.5H 2O  (in water) or K2 Cr2 O7 (in water). 

• Determine the concentration of CoCl 2.5H 2O  colorimetrically in the given solution of unknown concentration.

Theory:

If Beer's law is valid, the graph of optical density Vs concentration should be a straight line passing through the origin. This forms a calibrated curve. The optical density of the experimental solution of unknown concentration was determined with the help of a colorimeter. The concentration corresponding to this optical density can be determined from the calibrated curve.

Apparatus:

Calorimeter with accessories, set of filters, measuring flask, control flask. pipettes etc. 

Chemical Required: Cobalt chloride or potassium dichromate of unknown strength.

                                                  Plot of optical density Vs concentration.

Procedure:

• Before starting the experiment, carefully read the working operation of the colorimeter.
• Prepare 0.001 M solution of CoCl 2.5H 2O  in water by dissolving 0.22 g in 100 ml of solution. Also prepare 0.001 M solution of  K2 Cr2 O7 by dissolving 0.294 g in 1000 ml of solution.
• From the above standard solutions, prepare the following solutions with different dilutions:

Flask No.	Volume of 0.001 M cobalt chloride solution added	Volume of water added
1 2 3 4 5 6	10.0 ml 8.0 ml 6.0 mi 5.0 ml 4.0 ml 2.0 ml	0.0 ml 2.0 ml 4.0 ml 5.0 ml 6.0 ml 8.0 ml

• Turn on the colorimeter and wait for 10-15 minutes to warm up the instrument and to obtain temperature stability.
• Filter options Fill the cell with purified water (solvent). Select a filter color that complements the color for the test solution (1) i.e. the original color.

Solution colour	Complementary filter colour
Yellowish green Yellow Orange Red Purple Violet Blue Greenish blue Bluish green	Violet Blue Greenish blue Bluish green Green Yellowish green Yellow Orange Red

Insert the selected filter into the optical path. Adjust the shutter to obtain 100% solvent delivery. Remove the solvent from the cells. Cells are rinsed and filled with solution. Observe the light transmittance of the solution. Repeat these steps for each possible filter. Adjust 100% transmission for each filter. The solution presented showed the filter with the lowest transmittance to absorb it the most and was selected for further work.

• (vi) Collecting data for the calibration curve

Once the required filter has maximum absorption, set the shutter to 100% transmission with pure water and keep it undisturbed for the rest of the experiment. Fill the solution from 1 to 6 flasks one by one in the solution cell after rising, and determine optical density and % transmittance in each case. Also, determine the optical density with the unknown solution.

• Plot optical density Vs concentration when a straight line is obtained which shows the validity of the Lambert-Beer law. This straight line is the calibration curve.
• From the calibration curve note the concentration corresponding to the optical density of the given solution of unknown concentration. Repeat the experiment with K2 Cr2 O7 solution.

 Observations:

• Temperature before the experiment = ......... °C
• Temperature after the experiment =......... °C
• Colour of the solution = .......
• Filter used = ........
• Colour of the filter= ........

Solution No.	Concentration (C) moles/litre	Optical density D
1 2 3 4 5 6	…… …… …… …… …… ……	……. ……. ……. ……. ……. ……..
	d

 Now plot graph between D v/s C

                   lambert beer law graph

Find the concentration corresponding to optical density

d =.......... moles/litre

Result:

• The plot of optical density D vs concentration of C in moles/liter is a straight line that changes Beer's law. 
• The concentration of a solution whose concentration is unknown in moles/litre = .........

Questions & Answers

• What is Lambert-Beer's law?

Definition:  Lambert beer s law states that there is a linear relationship between the concentration and the absorbance of the solution.

• Lambert-Beer's law Equation?

            lambert beer law formula

  

A=εb C

A = absorbance

ε = molar absorptivity

b = length of light path

c = concentration

• Lambert beer law limitations

1. Lambert-Beer's law at high concentrations.

2. Light beam should not be scattered.

3. Electromagnetic radiation monochromatic should be used.

• Lambert beer law example

The nature and concentration of the poison or any substance can be determined.

• Beer lambert law is applicable for

The law is applied to the analysis of mixtures by spectrophotometry without extensive pre-processing of the sample.

Let me know if you have more questions or if there is a specific topic that you would like to know more about.