“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;
  • lu­mi­nol;
  • 10% so­lu­tion of sodi­um hy­drox­ide;
  • sprayer;
  • plas­tic knife;
  • beaker.

Step-by-step in­struc­tions

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.

Pro­cess­es de­scrip­tion

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.

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves, mask and glass­es.