Somebody actually asked me how luminol reacts with blood, and as my close friends know, a single question is enough to set me on a massive lecture. Many thanks to the Anthro Med Library for their wonderful picture of the oxidation (electron loss) of luminol, which, if anyone is curious, is used in an article on a “Chemiluminescence Assay of H2O2 Production by Neuroblastoma Cells Stimulated with High Potencies of the Cytotoxin TNFalpha”, of which I have only a vague concept of what that means.
But what we’re concerned with is luminol. Essentially, a testing kit for blood is a solution of luminol, a strong base like potassium hydroxide, and hydrogen peroxide. What I mean by “strong base” is that in water it completely breaks up into a positive potassium ion (K+), and a negative hydroxide ion (OH-). Luminol reacts with hydroxide in an equilibrium reaction to remove the H from the two -NH groups, making two -N- groups. There is an equilibrium with this molecule (called a dianion because it has two negative charges) and another, where the negative charges are on the two oxygens.
Hydrogen peroxide decomposes slowly into water and oxygen, which means that over time the luminol solution will go bad, because oxygen reacts with the second dianion to make the “amino phthalat” seen in the opening image, which releases nitrogen gas and a photon (which is what the hv means). When the solution is sprayed onto blood, however, the iron in hemoglobin (the molecule in blood that moves oxygen) catalyzes the decomposition of hydrogen peroxide, so suddenly there’s a lot of oxygen that can react with the dianion. Lots of oxygen means lots of the “amino phthalat”, meaning lots of photon. All the CSI guys have to do now is close the shutters and find the glow.
It’s not perfect, mind you. Many other metals catalyze the reaction, copper, manganese dioxide, to think of two, and certain chemicals (like bleach) can make the reaction go as well.
This post is brought to you by M. Alice, who probably now knows that I’m much better at chemistry than the history of Renaissance authors.