Beer's Law Lab
CONTEXT
The absorption of light by a substance in a solution can be described mathematically by the
Beer-Lambert Law (usually abbreviated to Beer’s Law).
A=ebc
A = absorption at a given wavelength of light
e = molar absorptivity, unique to each molecule at a specific wavelength
b = path length through the solution that the light has to travel
c = concentration of the solution (M)
The more light that is absorbed by a solution, the more deeply colored the solution is in the
region of the spectrum that it is not absorbing. So if a solution is deeply colored, we would
expect it to have a large absorbance (A) in the wavelength region where it absorbs. It should
seem logical that the absorbance (A) would increase with increasing concentration (c). The
absorbance also increases with increasing path length. For example, if the light passes
through 10 cm of the solution, more light will be absorbed than if the light only passes through
1 cm of solution. If you place a colored solution in a large skinny container, like a salt shaker,
the solution will appear more deeply colored if you look up through it than if you look at it
through the side.
If absorption of a solution is measured in a series of solutions in which concentration is varied
systematically, a “Beer’s Law Plot” can be generated. This plot can act as a calibration curve
because it can be used to determine the concentration of an unknown solution.
Look at the graph below, what does it tell you about the relationship between absorbance and
concentration?
A straight line indicates a direct
proportion. Absorbance and
concentration are directly
proportional.
Colorimetry is based on a
particular application of Beer’s
Law, focusing on the
relationship between absorption
and path length. If you compare
two solutions of the same light
absorbing species, the molar
absorptivities (e) will be
identical. Thus any combination of path length and concentration that give the same product
(b x c) will absorb the same amount of light. Then, if two solutions with different
concentrations of a particular species appear to have the same depth of color when viewed
from above, their absorption will be the same.
A1 = A2
Since A = ebc
e1b1c1 = e2b2c2
In this lab, we will be comparing a solution made from food coloring of a known concentration
with a commercial product of the same color, but an unknown concentration. Since both the
known and unknown solutions are the same color, e1 = e2. This allows us to shorten the
equation above to:
b1c1 = b2c2
MATERIALS
Student Provided
You may choose to perform your experiment using
either a blue or yellow product. You do not need
both.
 blue food coloring (Blue #1) OR yellow food
coloring (Yellow #6)
 a commercial product that contains only Blue #1
or Yellow #6 (Examples: yellow or blue Fla-VorIce pops, various blue mouthwashs, yellow or blue
Gatorade, etc.)
 empty 2L soda bottle
 4 empty individual soda bottles with caps (to store
your solutions)
 water (tap water will be sufficient for this lab)
 items to label your containers (sharpie markers,
masking tape, sticky labels, etc.)
Equipment
Various equipment for
measuring volume
 Graduated cylinders
 Pipette, glass, 25 mL Mohr
 Pipette, glass, 5 mL Mohr
 Pipette filler
 Flask, volumetric, glass, 25
mL, with stopper
 6 test-tubes OR reaction
well plate
You may use any of the containers in your kit as needed.
PROCEDURE
Read through the procedure before you begin. Gather your materials, make your labels, and
prepare any data table necessary.
Preparation Instructions - standard dye solutions (food coloring)
Commercial food coloring solutions are quite concentrated compared to the products in which
they are used. You will need to make an initial dilution then make a series of standard
solutions of known concentrations to use for comparison to the product you chose. You will
be using a 2L soda bottle to make your initial solution. Before you begin preparing the
standard solutions of food coloring, you will need to determine exactly where the 2L mark is
on your bottle. Use items in your kit to determine this. Mark your empty soda bottle
accordingly.
Use a pipet to measure the 2.5 and 5.0 mL amounts below. Use the 25 mL volumetric flask
to make the dilutions after solution A is made.

If you do not know how to use a volumetric flask to make dilutions, watch the following
video http://www.youtube.com/watch?v=wiGUAh-_sUY.
 If you do not know how to use the pipet and pipet bulb, watch
http://www.youtube.com/watch?v=yBqTme0Oc-8
Once prepared, you will need to transfer each solution into a clean, labeled storage bottle
(individual plastic soda bottles with lids or other storage vessels you have on hand).
Solution Instructions
A
To make the initial dilution, measure 6.0 mL of blue OR yellow (to match
the product you have chosen) food coloring (using an appropriate
measuring device) into a clean, empty 2L soda bottle. Add water to reach
the 2L mark. Label this bottle Solution “A”.
B
Add water to 5.0 mL of A to make 25 mL of solution.
C
Add water to 2.5 mL of A to make 25 mL of solution.
D
Add water to 5.0 mL of B to make 25 mL of solution.
E
Add water to 2.5 mL of B to make 25 mL of solution.
Preparation Instructions – Product of unknown concentration
Dilute the commercial product your chose by a factor of 1:10. You may ultimately need to
dilute this solution further if it doesn’t match up properly with any of the standard solutions.
Colorimetric Procedure
Part A: Standard series method
Label 6 clean, dry test-tubes or wells in the reaction plate, A-E and “unknown”. Transfer 5
mL of each solution into the corresponding vial. Using a sheet of white paper placed flat
upon a surface such as a table or kitchen counter as a background, hold the vial of the
unknown above the paper as you look directly down into the vial. One by one, select a vial
containing one of your standards and compare the color intensity to that of your unknown by
looking down into the vials over the white paper. Continue this until you find the one that
matches most closely with the unknown solution. You should find that the color intensity of
your unknown solution lies between that of two successive standards, thus establishing a
concentration range for the unknown. If not, either dilute standard E (if E is more intense than
the unknown sample) or the unknown sample (if the sample is more intense than intense
than standard A). Do this by performing similar dilutions to those from the tables above. Be
sure to keep track of the dilutions you make so you can determine the final concentration of
the standard and the dye.
When you have found the successive standard solutions whose intensities span that of the
unknown, select the standard of lower concentration to continue with the balancing
method.
Part B: Balancing method
Now, you will use the balancing method to determine accurately the unknown concentration
of dye. Using the standard solution selected above, place it side by side with your unknown
solution against the white background. If you have selected an appropriate standard, you will
find that its color is slightly less intense than that of the unknown solution. Carefully use a
dropper to transfer small volumes of the unknown solution from the test-tube or well plate into
the container it occupied before. Do this until the color intensities of the unknown and
standard are matched very closely. Measure the heights of the columns of standard and
unknown solutions, using a ruler, and record your data in the appropriate space in your table.
Take a picture of your standards and product from this part lab to include in the data sheet.
DATA / CALCULATIONS
Using the following and the information provided in the context portion of the lab,
Concentration of
Molar Absorptivity
Dye
Commercial
Version
(M-1cm-1)
(M)
Blue #1
1.3x106
0.026
Yellow # 5
3.6x104
0.112
1) Calculate the concentration of each of your standard solutions. Show your work and
results in an organized manner.
2) Complete the chart below for your unknown.
Product Name
Identity of matching standard solution (A - E)
Matching standard solution concentration
Depth of standard solution
Depth of unknown solution
3) Include a picture of your comparison between the standards and your unknown.
4) Calculate the concentration of the product sample tested. Show your work.
Unknown concentration
Concentration in original product
LAB REPORT
A formal write-up is not required for this lab.
Your lab report should include:
 Your name (# of week schedule)
 Name of lab
 Data / Calculations
Submit to Dropbox folder: Beer’s Law Lab