Criminal Investigation Experimental: The luminol reaction?

Chemistry is closely related to our lives, and the basic necessities of life is inseparable from chemistry. Of course, it is also a special use that is used to solve the case. The Luminol reaction we introduced today is closely related to the scene of the murder. How does it help the police to bring criminals to justice?

What is the luminol reaction?

The luminol reaction is a classical chemiluminescence reaction. Unlike fluorescence, the source of luminescence is an excited-state intermediate produced in a chemical reaction, which releases blue light when it returns to the ground state. Luminol reacts with an oxidizing agent such as hydrogen peroxide in an alkaline environment to obtain an illuminating intermediate. This reaction usually requires catalysis of a transition metal salt, for example, the catalyst may be iron, copper or cobalt.

Illumination principle of Luminol

First, sodium hypochlorite oxidizes luminol to cause it to emit light;

Second, hydrogen peroxide reacts with sodium hypochlorite to form oxygen to oxidize luminol to cause it to emit light:

NaClO + H₂O₂ == NaCl + O₂ + H₂O

Then luminol reacts with hydroxide to form a double anion (Dianion) which is oxidized by oxygen decomposed by hydrogen peroxide and the product is an organic peroxide. The peroxide is very unstable and immediately decomposes nitrogen (Luminol is oxidized by an organic oxidant such as dimethyl sulfoxide, which does not generate nitrogen, but forms a nitrogen-containing organic substance) to form an excited state of 3-aminophthalic acid. In the transition from the excited state to the ground state, the released energy exists in the form of photons, and the wavelength is in the blue portion of the visible light.

How to prepare the luminol reagent?

1. Configure a mixed solution of alkaline luminol and hydrogen peroxide. The alkaline condition is provided by ammonium hydroxide.

Measure 420 mL of water, weigh 0.05 g of luminol and add luminol to 420 mL of water, and it will be found that luminol is difficult to dissolve. 20 mL of concentrated ammonium hydroxide was weighed and mixed with 0.05 g of luminol and 420 mL of water, and it was observed that luminol was gradually dissolved in the alkaline solution. After the luminol is completely dissolved, 10~20 drops of a 30% hydrogen peroxide solution are added thereto, and after mixing uniformly, the desired solution is obtained, which is referred to as solution A.

2. Configure sodium hypochlorite solution

20 mL of 10% sodium hypochlorite solution (based on the effective chlorine content) was weighed, 400 mL of water was weighed, and the desired solution was mixed and recorded as Solution B.

Ps: This solution can also be obtained by mixing 50 mL of 84 disinfectant (effective chlorine content of 4%) with 170 mL of water.

3. Mix solution A with B

Not only in criminal investigation, in all aspects of life, there is a chemical figure! Let us discover more interesting figures of chemistry in life!

Related links: Luminol

Edited by Suzhou Yacoo Science Co., Ltd.