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Crime Scene Chemistry—The Cool Blue Light of Luminol

Difficulty
Time Required Average (6-10 days)
Prerequisites None
Material Availability You will need to order a special kit for this science fair project. You also need access to a digital camera and a tripod. See the Materials and Equipment list for details.
Cost Low ($20 - $50)
Safety Wear latex gloves when handling the chemicals.

Abstract

You may have seen police investigators on TV spraying a crime scene with a liquid that glows blue if there is any blood present. The chemical that glows in the liquid is called luminol. In this chemistry science fair project, you will investigate what factors make this interesting molecule "light up."

Objective

In this chemistry science fair project, you will investigate how temperature affects the eerie blue glow created by the chemical luminol.

Credits

David Whyte, PhD, Science Buddies

Edited by Jamie Furneisen, Schering Plough

  • StyrofoamTM is a registered trademark of The Dow Chemical Company.
  • Adobe® and Photoshop® are registered trademarks of Adobe Systems Incorporated in the United States and/or other countries.

Cite This Page

MLA Style

Science Buddies Staff. "Crime Scene Chemistry—The Cool Blue Light of Luminol" Science Buddies. Science Buddies, 8 Oct. 2013. Web. 23 July 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p078.shtml>

APA Style

Science Buddies Staff. (2013, October 8). Crime Scene Chemistry—The Cool Blue Light of Luminol. Retrieved July 23, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p078.shtml

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Last edit date: 2013-10-08

Introduction

Luminol is a chemical that has the special property of emitting light when it reacts with certain other chemicals. The luminol reaction is an example of chemiluminescence. It is used by crime scene investigators to discover spots of blood, even spots that may have been washed. The luminol actually detects the iron that is present in the blood. The iron functions as a catalyst in the light-producing chemical reaction, shown in Figure 1.

Luminol chemical reaction
Figure 1. This is the reaction scheme for the production of light by luminol. The luminol molecule reacts with hydroxide molecules (OH-) to form the dianion (two negative charges). The dianion, which exists in two forms (the two-way arrow), reacts with oxygen to form 5-aminophthalic acid and nitrogen gas (N2). The oxygen is formed from the hydrogen peroxide, with the iron functioning as a catalyst. The electrons in the 5-aminophthalic acid are in an unstable, high-energy state. When the electrons return to their normal ground state, they release a photon of blue light. (Modified from Wikipedia, 2009.)

The crime scene investigator prepares a solution of luminol, sodium hydroxide, and hydrogen peroxide and sprays it throughout the area under investigation. The iron present in any blood in the area catalyzes the chemical reaction, which leads to the luminescence, revealing the location of the blood. The amount of catalyst necessary for the reaction to occur is very small, relative to the amount of luminol, allowing the detection of even trace amounts of blood. The glow lasts for about 30 seconds and is blue. Detecting the glow requires a fairly dark room. Any detected glow may be documented by a long-exposure photograph or by videotape.

To exhibit its luminescence, the luminol must first be activated. As mentioned above, usually the activator is a solution of hydrogen peroxide (H2O2) and sodium hydroxide (NaOH) in water. In the presence of iron, the hydrogen peroxide is decomposed to form oxygen and water. In the detection of blood, the catalyst is the iron present in hemoglobin. When luminol reacts with the hydroxide ion (OH-), a dianion is formed. A charged molecule or atom is called an ion. A molecule that has a negative charge is called an anion. And a dianion is an anion with two negative charges. The oxygen produced from the hydrogen peroxide then reacts with the luminol dianion. The product of this reaction is very unstable and immediately decomposes with the loss of nitrogen to produce 5-aminophthalic acid. The electrons in the 5-aminophthalic acid produced by this reaction are in an excited state. Electrons in an excited state are in a higher energy level. As the excited state relaxes to the ground state, the excess energy is liberated as a photon, which is then visible as blue light.

In this chemistry science fair project, you will investigate how temperature affects the amount of light produced by the luminol reaction. You will use a kit called the "Cool Blue Light Experiment Kit" that you can order online. The kit contains luminol, perborate (an oxidizer that is safer to use than sodium hydroxide), and copper sulfate (copper, like iron, is a catalyst), as well as containers for mixing, and a pamphlet with experimental procedures.

Terms and Concepts

  • Luminol
  • Chemiluminescence
  • Catalyst
  • Hydroxide molecule
  • 5-aminophthalic acid
  • Electron
  • High-energy state
  • Ground state
  • Photon
  • Sodium hydroxide
  • Hydrogen peroxide
  • Chemical decomposition
  • Hemoglobin
  • Ion
  • Anion
  • Dianion
  • Excited state
  • Energy level
  • Oxidizer

Questions

  • What other chemicals, besides iron and copper, can act as catalysts for the luminol reaction?
  • Why does the light reaction stop after a certain amount of time?
  • What are the roles of each of the ingredients in the "Cool Blue Light Experiment Kit"?
  • How would you expect temperature to affect the amount of light produced in the luminol reaction?
  • How would you expect temperature to affect how long the blue light is produced?
  • What are the chemical formulas for perborate (the oxidizer you will use in the experimental procedure) and copper sulfate (the catalyst)?

Bibliography

Materials and Equipment

  • "Cool Blue Light Experiment Kit" (item # 3053439) from Edmund Scientific at scientificsonline.com
  • Paper plate
  • Digital camera that can take pictures in dim light (should allow for long exposures; for example, 5 sec)
  • Tripod
  • Lab notebook
  • Ice
  • StyrofoamTM cups, 12-oz (2)
  • Liquid measuring cups, 1/3-cup capacity (2)
  • Kitchen thermometer
  • Helper
  • Metal spoons (2)
  • Stopwatch or timer
  • Optional: Image-analysis software

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Experimental Procedure

Setting Up Your Materials

Note: The blue light made by the luminol reaction is best viewed in a dimly lit room. You might want to perform the experiments on a bathroom counter so you can shut out surrounding light by closing the door.

  1. Read the information pamphlet that accompanies the Cool Blue Light Experiment Kit.
  2. Work through the recommended experiments that come with the kit so you are familiar with the ingredients and the reaction setup.
  3. When you are ready to begin investigating how temperature affects the luminol reaction, set out the two clear plastic cups that came with the kit.
  4. Add one scoop each of luminol and perborate mixtures and a few copper sulfate crystals to each cup.
    1. Add a consistent amount of each chemical.
    2. Record all data in your lab notebook.
    3. Try to count the grains of copper sulfate so that you are adding close to the same amount to each reaction.
      • For example, try adding 10 grains of copper sulfate. Experiment with different amounts.
      • If the reaction is going to completion (that is, no more light is produced) too fast, lower the amount of copper sulfate added to the reaction.
    4. As an option, premix the dry ingredients in a clean, dry container. Add enough luminol, perborate, and copper sulfate for 10 reactions. This will minimize variation that could result if you add them separately.
  5. Put the cups with the ingredients on the paper plate on your work surface. The work surface should be waterproof.
  6. Make sure you know how to work your camera and how to set the exposure time. Set up the camera and tripod so that the camera is focused on the two cups. See Figure 2.


Set-up to take picture of luminol reaction
Figure 2. Setup for taking pictures of the luminol reaction. The digital camera on a tripod is set to take pictures with a 15-sec exposure time (with the lights off). The two clear plastic cups contain the chemicals for the luminol reaction. The reaction is started by adding water.


  1. Experiment with taking pictures of the cups in dim light. For example, try 5-, 10-, and 15-sec exposures in dim light. See Figure 3. You want a picture that clearly shows the relative brightness of the two cups.


Figure 3. Two luminol reactions at different temperatures. The picture was taken with an f-stop of 8 and an exposure time 10 sec. The reaction was started 20 sec prior to taking the picture.


Running the Experiment

Note: Now that the ingredients are ready and the camera is set up, you can begin the experiment. The procedure below is for a digital camera. As an option, develop a method using a video camera to record the changing brightness of the reactions.

  1. Add 1/3 cup of ice-cold water to a Styrofoam cup.
  2. Add 1/3 cup of hot tap water (about 50°C) to a second Styrofoam cup.
  3. Determine the temperature of the water in each Styrofoam cup and record it in your lab notebook.
  4. Now add the cold water to one of the plastic cups containing the luminol, perborate, and copper sulfate.
  5. Add the hot water to the other plastic cup.
    1. Have your helper pour water into one of the containers so that the reactions start at the exact same time.
  6. You and your helper should each mix a solution with a clean spoon.
  7. Start the stopwatch or timer.
  8. Dim the lights and observe the light produced by each cup.
  9. Take a picture of the two cups. Record the time on the stopwatch or timer at which the picture was taken.
  10. Continue taking pictures, recording the time at which each picture is taken.
    1. The number you take will depend on the length of the exposure.
  11. It is important not to vary the conditions for the pictures once you have settled on an exposure time that works well.
    1. A good exposure time should give you a clear picture of the two cups so you can compare their brightness.
    2. You will want to compare all of the pictures later, so the conditions should be as consistent as possible.
  12. Repeat steps 3–11 two more times, with clean and fresh materials.
    1. Be sure to use the same starting temperatures of the water.
    2. Take pictures at the same time intervals for each trial and using the same exposure length every time.

Analyzing Your Results

In order to graph your results, create a scale for the brightness of the light in the cups.

  1. Pick five pictures of the blue light in the cups that form a series from the brightest (5) to the dimmest that is still visible (1). Which reaction produced the brightest light?
  2. Make a figure based on these five. This will be your standard for assigning a brightness level to each cup.
  3. Using the scale you have made, record the brightness of all of the cups, with 1 being the dimmest and 5 being the brightest.
    1. As an option, use an image-analysis software, such as Adobe® Photoshop®, to determine the brightness of the reactions.
  4. Make a data table that contains the brightness for each cup, the starting temperature, and the time at which the picture was taken.
  5. Convert the temperatures to Kelvin. Add 273 degrees to the temperature in Celsius to get the temperature in degrees Kelvin. Using the Kelvin scale allows you to compare the temperatures accurately.
  6. Average the results of the three trials for each cup.
  7. Graph the average brightness (1 through 5) of each reaction vs time.
    1. Graph the results for the hot and the cold cups on the same chart.
    2. Graph the time on the x-axis and the brightness of the reaction (1–5) on the y-axis.
    3. Add a note on the graph indicating the starting temperature.
  8. How do the curves for the different starting temperatures differ in their maximum brightness and in the length of time the reaction proceeded?

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Variations

  • Repeat the experiment at various other starting temperatures. How does the maximum brightness vary with temperature?
  • Devise a way to mix the ingredients in a way that will allow you to detect blood (from a meat tray, for example). The Cool Blue Light Experiment Kit instructions will be helpful for this.
  • Experiment with a different oxidizer, such as hydrogen peroxide. How does this compare with the perborate? The Cool Blue Light Experiment Kit instructions will be helpful for this.
  • If you have access to a laboratory that uses X-ray film, devise a way to record the brightness of the luminol reaction on the X-ray film.
  • Try modifying the light meter in the Science Buddies science fair project What is in this Water? Experiments with a Homemade Turbidity Meter to measure the light produced by the luminol. You may need to devise a more-sensitive light detector to measure the light from the luminol reaction.

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