“Luminol fights crime” experiment
How to make a solution that detects blood
Chemistry isn’t just interesting and impressive, it’s also a useful science. A knowledge of chemistry helps forensic scientists to find evidence of crimes that are not visible to the naked eye!
Reagents and equipment:
- 5% solution of iron(III) chloride;
- 5% solution of potassium thiocyanate;
- 3% hydrogen peroxide;
- 10% solution of sodium hydroxide;
- plastic knife;
Pour an alkaline solution of sodium hydroxide into the spray gun, and add luminol and hydrogen peroxide. Mix them together and start to spray. The luminol solution is ready. Before the experiment, rub the solution of potassium thiocyanate with your hand, immerse the knife in the solution of iron(III) chloride and run it across your hand. In the places where potassium thiocyanate and iron(III) chloride meet, a liquid resembling blood is formed. Rub off the bloodstains left on the table with a napkin. Turn off the light and spray the luminol solution on the surface of the table. In the places where the “blood stains” were wiped away, the luminol starts to glow.
When a solution of iron (III) chloride and potassium thiocyanate is mixed, iron(III) thiocyanate is formed, which has a red color, resembling blood. As this compound contains ions of trivalent iron, a blue glow will be emitted when it reacts with luminol. Real blood also contains iron ions, which are present in hemoglobin. Iron ions act as a catalyst for the breakdown of hydrogen peroxide, which in its turn oxidizes luminol in an alkaline medium, to an unstable particle, which breaks down with the emission of a blue glow. This property is used to detect bloodstains at crime scenes.
Wear protective gloves, mask and glasses.