1. Using the absorbance of light to measure concentration
Beer’s Law Lab
Using the absorbance of light to measure concentration

2. Absorbance and Concentration
This lab is based on the principle that the “darker” a solution, the more concentrated it is
A concentrated solution is “darker” because the solute absorbs light

3. Absorbance and Concentration
You can QUANTITATIVELY measure the concentration of solution by measuring how much light is absorbed

4. Colorimeter
Measures numerically how much light is absorbed by a sample “Absorbance”

5. Colorimeter
A colour of light is used that is absorbed well by the solution
The colorimeter compares the intensity of light entering and leaving the sample.

6. Cuvette Used to hold the sample Handle by the ridges
Light enters and leaves on the smooth sides

7. Cuvette
make a reference mark on one smooth side to line up the cuvette consistently
Wipe fingerprints and dust away from smooth surface with a tissue

8. Calibration
When the cuvette contains only water, it should read 0.00 absorbance.
Calibration corrects the absorbance reading to take into account any light absorbed by the plastic cuvette or the water.

9. Making Standard Solutions
A Series of dilutions make a set of solutions of known (calculated ) concentrations
Absorbance of each solution is measured

10. Creating a calibration curve
Absorbance vs. Concentration is plotted

11. Determining the concentration of an unknown sample
Measure Absorbance
Use graph to determine Concentration of unknown sample

12. Determining the concentration of an unknown sample
Use the equation for the line:
Abs = (slope) Conc
and the slope value from the graph to calculate Conc. Of the unknown

13. Example
If your unknown is measured to have an absorbance of 0.250, what is its concentration?
Use the graph
Use the equation:
y = x or Abs = (Conc)
Rearrange: x = y/ or Conc = Abs/ x = Conc. = /1.209
x = Conc. = mol/L

14. Expectations You will be evaluated on the following:
A pre-lab quiz testing your knowledge of the theory and the procedure /8
Your dilution calculations for your standard solutions /3
Your graph /5
The accuracy of your unknown concentration /3
Total marks = /19