Luminol - Detecting Blood

Luminol is a common reagent used to detect blood stains and other body fluids at crime scenes because it reacts with iron in blood to produce a blue glow.
Its IUPAC name is 5-amino-2,3-dihydro-1,4-phthalazinedione.

The luminol solution used by crime scene investigators is a solution of luminol and an activator, an oxidant such as hydrogen peroxide and a hydroxide salt in water. In the presence of a catalyst, such as the iron in haemoglobin, the hydrogen peroxide decomposes to form oxygen and water and the luminol reacts with the hydroxide salt to form a dianion. The oxygen produced during the decomposition reaction reacts with luminol dianion producing an organic peroxide which is very unstable and immediately decomposes to produce 3-aminophthalic acid with electrons in the excited state. As the excited state electrons relax to the ground state, energy is released as visible blue light.

Unfortunately, luminol reacts with other substances besides the iron in haemoglobin such as copper, bleaches and horseradish.

University of South Carolina Chemists are using a new thermal infrared technology to illuminate blood stained objects with pulses of invisible infrared waves, using filters to block out particular wavelengths, allowing certain chemicals to stand out from their surroundings. The technique can detect blood diluted to as little as one part blood in 100 parts water. It can also distinguish between blood, bleach, rust and coffee.

Journal References:

1. Heather Brooke, Megan R. Baranowski, Jessica N. McCutcheon, Stephen L. Morgan, Michael L. Myrick. Multimode Imaging in the Thermal Infrared for Chemical Contrast Enhancement. Part 3: Visualizing Blood on Fabrics. Analytical Chemistry, 2010; 82 (20): 8427 DOI: 10.1021/ac101107v
2. Heather Brooke, Megan R. Baranowski, Jessica N. McCutcheon, Stephen L. Morgan, Michael L. Myrick. Multimode Imaging in the Thermal Infrared for Chemical Contrast Enhancement. Part 2: Simulation Driven Design. Analytical Chemistry, 2010; 82 (20): 8421 DOI: 10.1021/ac101108z
3. Heather Brooke, Megan R. Baranowski, Jessica N. McCutcheon, Stephen L. Morgan, Michael L. Myrick. Multimode Imaging in the Thermal Infrared for Chemical Contrast Enhancement. Part 1: Methodology. Analytical Chemistry, 2010; 82 (20): 8412 DOI: 10.1021/ac101109w

Further Reading
Functional Groups
Molecular Mass (Formula Weight)
Percent Composition (percentage composition)
Oxidation and Reduction
Energy Profiles and Catalysts
Parts per million concentration

Study Questions

1. Write the molecular formula for luminol given the structural formula shown in the article above.
2. Calculate the molecular mass (formula weight) of luminol.
3. Calculate the percentage of each element present in luminol.
4. Write a balanced chemical equation for the decomposition of hydrogen peroxide as described in the article.
5. The new thermal infrared technique can detect 1 part blood in 100 parts of water. Express this as a concentration in parts per million.
6. On the structure of luminol locate the following functional groups:
   • amine groups
   • double bond
   • carbonyl group
7. Luminol reacts with the hydroxide salt to form a dianion. Explain what is meant by the term dianion.
   
8. Explain how luminol could produce a dianion.
9. Do you think that crime scene investigators should use luminol to detect blood in commercial laundry? Explain your answer.