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Crime Scene Chemistry—The Cool Blue Light of Luminol

Difficulty
Time Required Very Short (≤ 1 day)
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. Luminol is the chemical which causes the glowing. 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.

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Credits

David Whyte, PhD, Science Buddies
Svenja Lohner, PhD, Science Buddies

Thank you to the volunteers on the Science Education Council at PPG Industries, for helpful feedback, advice and improvements on this science project.

  • 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.

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General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Whyte, David, and Svenja Lohner. "Crime Scene Chemistry—The Cool Blue Light of Luminol." Science Buddies, 30 Aug. 2018, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p078/chemistry/crime-scene-luminol-blood. Accessed 20 Nov. 2018.

APA Style

Whyte, D., & Lohner, S. (2018, August 30). Crime Scene Chemistry—The Cool Blue Light of Luminol. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p078/chemistry/crime-scene-luminol-blood


Last edit date: 2018-08-30

Introduction

Glow-in-the-dark objects are fascinating and mysterious. Why do they glow and where does the light come from? It seems like magic when the glow suddenly appears! There is no magic involved though, but a process called chemiluminescence. Chemiluminescent objects use chemical energy to produce light. For example, when you twist or bend a glow stick, you start a chemical reaction. One of the products of the reaction is light. The basic premise of the reaction is that the reaction between the involved chemicals releases enough energy to excite the electrons in one of the reaction partners from the ground state to the excited state, as shown in Figure 1. This causes the electrons to jump to a higher energy level and then fall back down (relax) and release light. Depending on the involved chemicals a variety of colors can be produced.

Diagram showing the electronic states of a molecule and the transitions between them.
Figure 1. Simplified diagram that shows the electronic states of a molecule and the transitions between them. In a chemical reaction, electrons can be excited from the ground state to an excited state. When they relax and return to the ground state, they release their energy in the form of light.

Did you know that you can use chemiluminescence not only in glow sticks but also for solving crimes? Forensic scientists, who use science for criminal investigations, make use of chemiluminescent reactions to detect blood at crime scenes—so they can make blood visible even when it has been washed away already! When treated with a chemical called luminol, all hidden blood traces literally glow up in the dark! A demonstration of this is shown in the video below:

Luminol is a chemical that has the special property of emitting light when it reacts with certain other chemicals. The luminol reaction, as shown in Figure 2, is another example of chemiluminescence.

reaction (simplified).
Figure 2. This is the simplified reaction mechanism 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 electrons in the 5-aminophthalic acid are in an unstable, excited state. When the electrons return to their normal ground state, they release a photon of blue light.

To exhibit its luminescence, the luminol must first be activated. Usually the activator is a solution of hydrogen peroxide (H2O2) and sodium hydroxide (NaOH) in water. 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 most important step in this reaction is the oxidation of luminol with oxygen, which results in the excited state dianion. The oxygen is usually formed from hydrogen peroxide (H2O2), that is added to the reaction solution. In an aqueous solution, hydrogen peroxide can decompose to produce oxygen according to the reaction below (Figure 3).

Decomposition reaction of hydrogen peroxide catalyzed with blood
Figure 3. Decomposition reaction of hydrogen peroxide catalyzed with blood

However, this reaction proceeds very slowly. Chemical reactions can be sped up by using catalysts, which are chemicals that increase the speed of a reaction but are not actually consumed. This is where the blood comes into play! The reason why the luminol reaction makes blood glow is that it detects the iron that is present in hemoglobin, an iron-containing oxygen-transport metalloprotein in our red blood cells. This iron functions as a catalyst to speed up the production of oxygen. You can replace the hydrogen peroxide in this reaction with other oxidizing chemicals such as sodium perborate, which will release hydrogen peroxide in water according to the reaction below (Figure 4).

Hydrogen peroxide production from sodium perborate.
Figure 4. Hydrogen peroxide production from sodium perborate.

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.

All that a crime scene investigator has to do is to prepare a solution of luminol, sodium hydroxide (which provides the hydroxide ions for the first step of the reaction), and hydrogen peroxide and spray 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.

In this chemistry science fair project, you will explore the luminol reaction and investigate how the solution temperature can affect the glow. In every chemical reaction, the temperature can affect how the reaction proceeds. Do you think a crime scene investigator will get the same glow from blood traces in a car during a snowy winter compared to the same car in the middle of a hot summer? You will find out in this science project with the help of a kit called the "Cool Blue Light Experiment Kit" that you can order online. Note that this kit uses perborate as the oxidizer instead of hydrogen peroxide. And don't worry, there will be no blood involved in this project— you will use copper sulfate (CuSO4) as an alternative catalyst that works just as well as real blood!

Terms and Concepts

  • Chemiluminescence
  • Electron
  • Ground state
  • Excited state
  • Forensic scientist
  • Luminol
  • Catalyst
  • (Sodium) hydroxide
  • 5-aminophthalic acid
  • Photon
  • Hydrogen peroxide
  • Ion
  • Anion
  • Dianion
  • Oxidation
  • Catalyst
  • Hemoglobin
  • Sodium perborate

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?

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Materials and Equipment

  • Cool Blue Light Experiment Kit available from Home Science Tools
  • Paper plate
  • StyrofoamTM cups, 12-oz (2)
  • Liquid measuring cups, 1/3-cup capacity (2)
  • Kitchen thermometer
  • Water
  • Ice
  • Metal spoons (2)
  • Lab notebook
  • If you measure the luminescent glow with a digital camera, you will need following materials:
    • Digital camera that can take pictures in dim light (should allow for long exposures; for example, 5 sec)
    • Tripod
    • Stopwatch or timer
    • Helper
    • Optional: Image-analysis software
  • If you measure the luminescent glow with Google's Science Journal app, you will need following materials:
    • Glass vial with cap that comes with the Cool Blue Light Kit
    • Pipette
    • A transparent, waterproof plastic bag that fits your phone or tablet inside
    • A smartphone or tablet to record your data
      science-journal-app-icon This project uses Google's Science Journal app, a free app that allows you to gather and record data with a cell phone or tablet. You can download the app from Google Play for Android devices (version 4.4 or newer) or from the App Store for iOS devices (iOS 9.3 or newer).
      Note: This project only works with an iPhone as the iOS version of the app uses the phone's camera to measure brightness whereas the Android version uses the ambient light sensor, which is not sensitive enough to pick up the luminol glow.

Remember Your Display Board Supplies

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Poster Making Kit

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ArtSkills Trifold with Header

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Poster Lights

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Crime Scene Chemistry—The Cool Blue Light of Luminol

www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p078/chemistry/crime-scene-luminol-blood

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 in a location where you can shut out surrounding light by closing the door. You can either use a digital camera to capture the luminol glow, which is described here, or use Google's Science Journal app, which is described here.

  1. Read the information pamphlet that accompanies the Cool Blue Light Experiment Kit.
  2. 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.
  3. Add one scoop each of luminol and perborate mixtures to each cup as shown in Figure 5.
    1. Add a consistent amount of each chemical.
    2. Record all data in your lab notebook.
Preparing the luminol/perborate mixture.
Figure 5. Preparing the luminol/perborate mixture.

Procedure For Using a Digital Camera

  1. Next, add a few copper sulfate crystals to each cup.
    1. 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 as shown in Figure 6. 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.
Copper sulfate crystals for the luminol reaction.
Figure 6. Use a defined amount of copper sulfate crystals for each reaction.
  1. Note: You can also 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.
  2. 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 7.
The camera is focused on the two cups.
Figure 7. Setup for taking pictures of the luminol reaction. The digital camera is on a tripod and focusing on the two cups. The camera 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 the water from the Styrofoam cups.
  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 8. You want a picture that clearly shows the relative brightness of the two cups.

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

Running the Experiment

  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 the provided popsicle sticks or a clean spoon as shown in Figure 9.
Stir the solution with a spoon or popsicle stick.
Figure 9. Once you added the water make sure to mix the solution.
  1. Start the stopwatch or timer.
  2. Dim the lights and observe the light produced by each cup.
  3. Take a picture of the two cups. Record the time on the stopwatch or timer at which the picture was taken.
  4. Continue taking pictures for about three minutes, recording the time at which each picture is taken.
    1. The number you take will depend on the length of the exposure.
  5. 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, as shown in Figure 8.
    2. You will want to compare all of the pictures later, so the conditions should be as consistent as possible.
  6. Repeat the whole experiment (steps 1–18) 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 images that shows the assigned brightness level for each number (1–5). 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?

Procedure For Using Google's Science Journal App

Science Journal is an app that lets you record data using sensors that are built into many smartphones, including a light sensor which measures light levels (normally this sensor is used to automatically adjust the brightness of your phone's screen). To learn how to use the Science Journal app and how to use the light sensor, you can review the relevant tutorials on this Science Journal tutorial page. In this project, you can use the app to record the brightness of the luminol glow. This procedure only works with an iPhone, as the iOS version of the app measures brightness with the phone's camera, whereas the Android version uses the ambient light sensor which is not sensitive enough to pick up the chemiluminescent glow.

  1. Next, prepare a copper sulfate solution by adding 2-3 scoops of copper sulfate crystals to the provided vial in the kit and adding about 10–20 mL of water (Figure 10). Mix the solution until all the copper sulfate is dissolved. Add more water if not all crystals dissolve.
Preparing the copper sulfate solution.
Figure 10. Preparing a copper sulfate solution.
  1. Protect your phone from any potential spills by putting it into a waterproof plastic bag. Make sure the bag is transparent, so light can get through.
  2. Make sure you know the location of the light sensor in your phone and test if it works as expected. The light sensor tutorial on the you Science Journal tutorial page explains how to do that.
  3. Cut a piece of paper that covers the display part of the phone as shown in Figure 11. The light from the display can interfere with the measurement of the chemiluminescence.
  4. Prepare one Styrofoam cup with 1/3 cup of ice-cold water and a second Styrofoam cup with 1/3 cup of hot tap water (about 50°C).
  5. Add the hot water to one of the prepared cups with the perborate/luminol mixture and mix with a clean spoon or popsicle stick until all the chemicals are dissolved.
  6. Add the ice-cold water to the other cup with the perborate/luminol mixture and again stir until all the chemicals are dissolved.
  7. Determine the temperature of the solutions in each Styrofoam cup and record it in your lab notebook.

Running the Experiment

  1. Once you have all your solutions and your phone ready, move to a dark location. If there is too much light, the camera sensor cannot pick up the luminol glow.
  2. Lay your phone flat on the table with the display facing upwards. Then open the Science Journal app and start a new experiment. Make sure to label it appropriately. Choose the light sensor and check that your readings are constant as long as the light is not changing.
  3. Place your first cup with the perborate/luminol solution directly on top of the light sensor and cover the rest of the display with the cut piece of paper as shown in Figure 11. You can fold the lower part up to be able to press the record button.
Experimental setup for measuring the luminol glow.
Figure 11. Setup for measuring the luminol glow with the Science Journal app.
  1. Make sure you have closed the door and covered all windows to minimize the surrounding light. Then press the record button and add 2 mL of your prepared copper sulfate solution to the cup. Make sure to mix the solution for at least 30 seconds. Note: You can vary the amount of copper sulfate solution to change the duration of the luminol glow. 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.
  2. Stop the recording after about 1–2 minutes.
  3. Then repeat steps 14–16 with the second cup. Make sure to use exactly the same amount of copper sulfate.
  4. Repeat the whole experiment (steps 1–17) two more times, with clean and fresh materials and record the brightness of your luminol reaction each time.

Analyzing Your Results

  1. After you have finished three trials for each temperature, look at the graph for one trial. It should look something like the graph in Figure 12. You can clearly see how the brightness increases once you add the copper sulfate solution.
Example data from the Science Journal app - luminol results graph.
Figure 12. Example data from the Science Journal app. The x-axis of the graph shows the time in minute:seconds [min:s] and the y-axis is brightness in EV.
  1. Drag the cursor along the graph to the sensor reading just before you added the catalyst and to the highest sensor reading at the top of the peak. In Figure 12, the readings jumped from -5.13 EV in the beginning to -3.02 EV at the highest point of the curve. The difference of these brightness levels is a measure of the intensity of your luminol reaction. In Figure 12, the brightness changed by 2.11 EV.
  2. Repeat the measurements and calculations from the previous step for the other trials with the same temperature.
  3. Make a data table that contains the initial and highest brightness levels, the brightness changes due to the luminol glow, as well as the starting temperature for each trial.
  4. Average the change in brightness levels for all your three trials of one temperature and record the result in your lab notebook.
  5. Repeat steps 20–23 for the other water temperature.
  6. Make a bar graph in which you show how the intensity of the luminol reaction changes depending on the temperature.
    1. Graph the results for the hot and the cold cups on the same chart.
    2. Graph the temperature on the x-axis and the average brightness change of the reaction the y-axis.
  7. Which temperature resulted in a more intense luminol glow? Can you explain why?
  8. From your graph recorded with the Science Journal app you can also find out how long the luminol glowed. Is there a correlation between the water temperature and how long the reaction glows?

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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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