How Chemistry Helps Detectives

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How Chemistry Helps Detectives

There is a special substance called Luminol used by forensic scientists at crime scenes to detect blood. In the movies you see some detective spray a crime scene with a wet substance and then shine a black light to reveal blood. There is some true chemistry behind this fictional scene.

Luminol reacts to iron with a short blue fluorescent glow. At real crime scenes, luminol in a mixture of sodium hydroxide solution with hydrogen peroxide is sprayed from a spray bottle on to the crime area. The luminol reacts with the iron which is a major component of hemoglobin in blood and glows. Luminol is useful because can react with tiny amounts of iron, even after the visible red blood spill has been cleaned off of a crime scene.


Luminol reveals that blood was wiped off this floor (above).


Footprint revealed by luminol reaction.

To luminesce (glow), the luminol is activated by mixing it with hydrogen peroxide and a hydroxide salt in water. When hydrogen peroxide meets an iron compound (like the iron in hemoglobin in blood) the hydrogen peroxide breaks down into water and oxygen.

 2 H2O2 → O2 + 2 H2O 

The luminol reacts with the hydroxide salt and then the oxygen to form an unstable compound that decays quickly but releases a flash of blue light as it decays (see the reaction below). The flash of blue does not last very long, not the way you see it in the movies. You have to be ready with a video camera to capture the momentary flash of blue.


550px-Luminol_chemiluminescence_molecular_representation.svg.png

Luminol reacting with the products of hydrogen peroxide and then decaying to produce a  flash of blue light.


There may be other future uses for this chemical test such as home test kits to see if you have iron in your drinking water or backyard streams. Cities like Philadelphia have a lot of old pipes that water travels through before it gets to your home. Luminol might be a useful way to test for iron in your household water pipes.

https://chem.libretexts.org/Ancillary_Materials/Demos%2C_Techniques%2C_and_Experiments/Lecture_Demonstrations/Luminol


https://www.youtube.com/watch?v=w9TAs6jjykA



https://dps.mn.gov/divisions/bca/bca-divisions/forensic-science/Pages/forensic-programs-crime-scene-luminol.aspx

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