Beer’s Law Lab
Using the absorbance of light to
measure concentration
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
Absorbance and Concentration
• You can QUANTITATIVELY measure the
concentration of solution by measuring
how much light is absorbed
Colorimeter
• Measures
numerically how
much light is
absorbed by a
sample
“Absorbance”
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.
Cuvette
• Used to hold the
sample
• Handle by the ridges
• Light enters and
leaves on the smooth
sides
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
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.
Making Standard Solutions
• A Series of dilutions make a set of
solutions of known (calculated )
concentrations
• Absorbance of each solution is
measured
Creating a calibration curve
• Absorbance vs. Concentration is plotted
Determining the concentration
of an unknown sample
• Measure
Absorbance
• Use graph to
determine
Concentration of
unknown sample
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
Example
• If your unknown is measured to have an
absorbance of 0.250, what is its concentration?
• Use the graph
• Use the equation:
y = 1.2109 x
or
Abs = 1.2109 (Conc)
Rearrange:
x = y/1.2109 or
Conc = Abs/ 1.209
x = Conc. = 0.250 /1.209
x = Conc. = 0.206 mol/L
Expectations
•
You will be evaluated on the following:
1. A pre-lab quiz testing your knowledge of the
theory and the procedure /8
2. Your dilution calculations for your standard
solutions /3
3. Your graph /5
4. The accuracy of your unknown concentration /3
Total marks = /19