Teaching Modules, Outreach, Chemical Engineering, University of Utah
Colin Young, NSF Mentors | Created: February 15, 2011 | Last Modified: March 29, 2012
Luminol is oxidized by Hydrogen Peroxide in the presence of an an Iron catalyst which excites electrons and causes a blue light to be given off. The light is given off when the electrons return to their normal state in what is called chemiluminescence.
IMPORTANT!!! Under no circumstances should an unsupervised minor perform the procedures described herein. All the following described experiments and methods should be supervised by an adult who is completely familiar with and takes full responsibility for all possible hazards.
|Main Curriculum Tie:||Chemistry, Physics|
|Additional Curriculum Ties:||Algebra|
|Career Connections:||Luminol is used to detect iron containing compounds in forensic investigations. Most commonly, this is demonstrated in crime drama, where the investigators spray a luminol solution around a room to detect blood. Luminol is also an excellent demonstration of chemiluminescence.|
|Mean Time Frame:||
Setup: 10-15 minutes Experiment: 5 minutes Discussion: 10-15 minutes
|Group Size:||Large Groups|
|Student Prior Knowledge:||Conservation of energy, electron orbitals|
- What causes the solution to light up?
- What is a catalyst?
- What is the relationship between wavelength and energy?
- What is the electromagnetic spectrum?
- Young, C., Luminol Classroom Demonstration. http://outreach.chemeng.utah.edu/Modules/module.php?p_id=9
- Wikipedia, Chemiluminescence., http://en.wikipedia.org/wiki/Chemiluminescence
- New World Encyclopedia., Electromagnetic Spectrum., http://www.newworldencyclopedia.org/entry/Electromagnetic_spectrum
- 0.5 grams of Luminol
- 15-30 mL of 1 M Sodium Hydroxide
- 2-3 mL of 3% Hydrogen Peroxide
- 1.25 grams Potassium Ferricyanide
- 1 gram of Fluorescein
- 400 mL of distilled water
- Ring Stand
- 4 Beakers or Flasks
For the mixing of the solutions develop a simple apparatus that will allow you to pour in the two solutions and have them mix. For our demonstration we used a length of tubing weaved together and a pump to pump solution through. Some distance into the tubing, fluorescein was added to turn the solution green. This experiment can simply be done in a beaker, however, by mixing the luminol and iron compound. A short time later, add the fluorescein to make the solution turn green.
In the reaction, luminol, is oxidized by the hydrogen peroxide in the presence of an iron catalyst. The products of the reaction areaminophthalic acid, nitrogen gas, water, and light.
Luminol + 2H2O2 + 2OH- ? Areaminophthalic Acid + N2 + 4H2O +hv
The reaction is a chemiluminescent reaction. Chemiluminescence is a result of a chemical reaction causing electrons to become excited and "jump" to a higher energy state. Chemiluminescence is similar to fluorescence, but, instead of the adsorption of electromagnetic radiation (light) causing the electrons to be excited, the chemical reaction excites the electrons.
According to quantum mechanics, electrons can only gain specific energies of light called "quanta." The energies an electron can absorb relate to the energy required to move up in energy level. If an electron does not gain the right amount of energy, it can't jump up in level and will remain at an unexcited state.
As the electrons return to the lower energy ground states, they must release the energy they had gained. This energy is released in the form of light. The energy of this light, due to conservation of energy, must be exactly the same amount of energy the electron gained to jump to a higher orbital.
The energy of the light is given by the product of Planck's constant, h and the frequency of the light, v. Since frequency is the inverse of wavelength we can also say that the energy is equal to hc/?, where c is the speed of light and ? is the wavelength of the light.
This is an important result because it tells us that the shorter wavelength (also higher frequency) of light, the more energy that light has. Below is the Electromagnetic Spectrum, which characterizes the different kind of light.
We can see that visible light (the light we can see using our eyes) is only a relatively small range of the total spectrum. Only wavelengths between about 350 to 750 nanometers (10-9 meters) are able to processed by our eyes. Other ranges refer to the highest energy (gamma rays) to the lowest energy (radio waves).
In the luminol reaction we see a pale blue light given off. Using the spectrum we can estimate the wavelength of the light and calculate its energy. The energy of the light is equal to the energy gained by the electrons. When Fluorescein is added, a similar process happens and a brighter, but lower energy green light is given off.
The luminol reaction is what forensics teams use to detect the presence of blood on walls, carpets, etc. Blood contains a lot of iron, so when a solution of luminol and hydrogen peroxide is sprayed onto an area containing iron it will glow. However, some copper compounds and other iron containing compounds can cause it to light up so it doesn't always mean blood is present.
- Gain an understanding of the planetary/orbital model of the atom
- Develop an understanding of the relationship between light and energy.
- Develop an understanding of relationship between the wavelength of the light and the energy.
- Make a 1 molar solution of sodium hydroxide. For example 40 grams of solid NaOH in 1 liter, 20 grams in 500 mL, etc.
- In a separate beaker add 0.5 grams of luminol in 200 mL of water. The luminol will not go into solution initially.
- Add in enough of the 1 M sodium hydroxide solution to force the luminol to dissolve, usually about 10-30 mL.
- Dissolve 1.25 grams of the potassium ferrocyanide in 200 mL of water. Potassium ferrocyanide is relatively harmless unless mixed with acid. If mixed with acid it can release cyanide gas. Be very careful when mixing this compound. It should dissolve quickly and be a deep yellow color.
- Add 2-3 mL of hydrogen peroxide to the ferrocyanide solution.
- Mix the two solutions, either with an assembly device or in a beaker, and watch them glow.
- Add a small amount of fluorescein to make the solution turn green.
- Give students small samples of both solutions and have them mix them
- Have the students time how long it takes for the reaction to stop glowing
- Test difference in ability to see the glowing with the lights on and off.
If you have a question regarding this teaching module or any other,
please feel free to contact Professor Butterfield, email@example.com.