Name
Period
Date
16. Colorimetric Analysis*
Driving Question
How can you determine the concentration of a colorful solution?
Pre-Lab Activity
Setting the stage for the activity
When light interacts with light-absorbing particles, some of the light is removed. Consequently, objects with many
light-absorbing particles appear darker than objects with fewer light-absorbing particles. The ability to absorb color,
as well as what color is to be absorbed, depends on the type of particle present.
One can vary the number of particles with which light interacts in two ways. If you compare two solutions of the
same colored substance with different concentrations, you see that the one with the higher concentration appears
darker because it has more absorbing particles and it absorbs more of the incident light. Also, if you pour the same
solution into a test tube and a 100-mL beaker, the solution in the beaker appears darker. Even though the
concentration in the two solutions is the same, the light has to travel a longer path in the beaker. Therefore, photons
have a higher probability of being absorbed so less light will leave the beaker and the solution appears darker.
Next, if you pour a little bit of the 1 M copper(II) sulfate solution into a 1-L beaker nearly full with water, why is the
original beaker so much darker? It is darker because the "light catching" particles in the more diluted solution are
much farther apart. From this we see that concentration is another variable to consider when looking at absorbed
light.
To measure the amount of light absorbed, we use a colorimeter. Like all electronic measuring devices, the
colorimeter produces a voltage based on the amount of light that hits it. The voltage is converted to an absorbance
level in optical density units (o.d.).
Example calculation to try
In an experiment, four calibrating solutions are used to determine the concentration of a CuSO 4 solution with
unknown concentration. Given the following data, create a calibration graph, find the equation, and use the
equation to solve for the concentration of the unknown solution.
Table 1: Concentration and absorbance calibration data
Sample
Concentration
(M)
Absorbance
1
0.20
0.062
2
0.50
0.156
3
0.70
0.218
4
0.90
0.281
The graph of these calibrating solutions:
*
This is an AP Chemistry course recommended experiment.
Student Inquiry Worksheet
y = (3.119  10-1)x
If a solution of unknown concentration has a absorbance A of .200, the concentration c of the unknown can be found
on the graph, which looks to be between 0.63 M and 0.64 M or calculated from the obtained equation:
The equation of the line is
y  0.3119x
Where y = absorbance A, and x = concentration
Solving for the concentration:


A  0.3119 M1 c


0.200  0.3119 M1 c
c 
0.200
0.3119 M1
 0.641 M
It is worth mentioning that the equation provides a more accurate estimate of the unknown concentration, since the
graph-based estimate can be subjective.
Student Inquiry Worksheet
16. Colorimetric Analysis*
Driving Question
How can you determine the concentration of a colorful solution?
Materials and Equipment
For each student or group:
 Data collection system
 Graduated cylinder, 50-mL
 Colorimeter
 Pipet with pump or bulb, 10-mL
 Cuvette
 Glass stirring rod
 Sensor extension cable
 0.40 M copper(II) sulfate (CuSO4), 30 mL
 Beakers (2), 100-mL
 Distilled water, 30 mL
 Test tubes (6), large
 Marking pen
 Test tube rack
 Wash bottle with distilled water
Safety
Follow all standard laboratory procedures.
Procedure
After you complete a step (or answer a question), place a check mark in the box () next to that step.
Set Up
1.  Connect the colorimeter to the data collection system using the extension cable.
2.  Start a new experiment on the data collection system. Click on Sparkvue and then Build.
3.  Configure the data collection system to manually collect absorbance of orange (610 nm) light
and the concentration in a table. To do this scroll down and click on “User entered numeral data”. For
measurement type in “concentration” and for unit name type in “molarity”.
4.  Next click on both absorbance orange and concentration. Then click on the table button and
ok. On the bottom by the clock, click on it and change to manual.
5.  Calibrate the colorimeter with the blank solution (use distilled water for the blank). To do this rinse a
cuvette with a small amount of “blank” solution & empty. Next fill the cuvette. Wipe clean and dry and place
it into the colorimeter. Press the green button and the light will go on. Leave in until the light goes off.
*
This is an AP Chemistry course recommended experiment.
1
Student Inquiry Worksheet
Light -tight cover
Calibration button
Colorimeter
Extension cable (optional)
2
3
4
Unknown
1
Stock
6.  Water absorbs a small amount of light.
7.  Label four clean, dry test tubes “1” through “4” and place them into a test tube rack.
8.  Pipet 2.0, 4.0, 6.0, and 8.0 mL of the 0.40 M copper(II) sulfate solution into test tubes 1 through 4,
respectively.
9.  Deliver 8.0, 6.0, 4.0, and 2.0 mL of distilled water into test tubes 1 through 4 so that each test tube has
10.0 mL of solution.
10.  Thoroughly mix each solution with a stirring rod.
Note: Clean and dry the stirring rod before stirring a different solution.
Table 2: Volumes and concentrations for the calibration solutions
Trial #
0.40 M CuSO4 (mL)
H2O (mL)
Concentration
(M)
1
2.0
8.0
0.08
2
4.0
6.0
0.16
3
6.0
4.0
0.24
4
8.0
2.0
0.32
5
10.0
0
0.40
Student Inquiry Worksheet
Collect Data
11.  Start a new, manually sampled data set. Click on
and then the arrow button. Click on the
concentration trials and enter the concentrations above for the first 5 test tubes.
12.  Measure the absorbance of the five known solutions following the steps below.
a. Rinse the cuvette twice with a small portion of the first solution and then fill the cuvette two-thirds full.
Wipe the cuvette clean and dry and place it into the colorimeter.
b. “Turn on” the colorimeter by clicking the “on” button.
After the reading stabilizes, record a data
point by clicking on the green
button again.
c. Dispose of the solution appropriately and rinse the cell thoroughly with water.
d. Each time you place a new cuvette in, wait for it to stabilize and again click to record. When you have
recorded all of your data, stop the data set by clicking on:
Note: The data for test tube 6 is not yet recorded because it has an unknown molarity and it will not be part of the
standardization curve.
13.  Save your experiment. To do this click on
and then save as….
3
Data Analysis
1.  Create a data table to record the concentration an absorbance of the 5 solutions. Also include a place to
record the unknown (to be done later in the lab)
2.  To graph your data start a new page by clicking on
. Select the absorbance orange and
concentration again, but now click on the graph button and ok.
3.  Adjust the scale of the graph to show all data, if needed.
4.  Find the slope of the best-fit line. To do this click on
ok.
and then
. Select “Linear Fit” an
5.  Print the graph. Click on the camera and then
. Click on Journal and then print journal.
6.  Attach your graph of Concentration versus Absorbance.
3
Student Inquiry Worksheet
Testing the Unknown
1.  Monitor Orange (610 nm) Absorbance data in a digits display. Start a new page, click on orange
absorbance and then
2.  Measure the absorbance of the sixth (unknown) solution. Record the Orange (610 nm) Absorbance in
the data table and clean up according to your teacher's instructions.
3.  Use the equation for the line to calculate the concentration of the unknown. Show all of your work.
4.  Use the calibration curve to estimate the concentration of the unknown.
4