“Iodine handprint” experiment

How to reveal a handprint with iodine vapors

Ev­ery de­tec­tive needs to be a bit of a chemist, since crim­i­nals rarely leave no­tice­able tracks. In this sim­ple ex­per­i­ment, you’ll see how to re­veal a hand­print on pa­per in 20 min­utes.

Safe­ty pre­cau­tions

Work in pro­tec­tive gloves, a mask and glass­es in a well-ven­ti­lat­ed room. Neu­tral­ize the re­mains of io­dine with the sodi­um thio­sul­fate so­lu­tion.

Reagents and equip­ment:

  • sodi­um thio­sul­fate so­lu­tion (10 g/l);
  • cit­ric acid so­lu­tion (10 g/l);
  • potas­si­um io­dide so­lu­tion (10 g/l);
  • 3% hy­dro­gen per­ox­ide so­lu­tion;
  • crys­tal­liz­er;
  • white pa­per.

Step-by-step in­struc­tions

Pour the so­lu­tions of potas­si­um io­dide, cit­ric acid and hy­dro­gen per­ox­ide into the crys­tal­liz­er. Stir. Ob­serve the change of col­or from the re­lease of free io­dine. On a sheet of white pa­per, leave a hand­print and cov­er the crys­tal­liz­er with the pa­per, on the side with the hand­print on it. Leave for 20 min­utes. The print ap­pears. Neu­tral­ize the re­mains of the io­dine with the sodi­um thio­sul­fate so­lu­tion.

Pro­cess­es de­scrip­tion

In an acidic medi­um — which is cre­at­ed by the cit­ric acid so­lu­tion — hy­dro­gen per­ox­ide ox­i­dizes io­dide ions to free io­dine. Io­dine trans­forms to va­por, which dis­solves in the sweat and grease de­posits left on the pa­per. In this way, the hand­print ap­pears:

6KI + 3H₂O₂ + 2C₆H₈O₇ → 3I₂(sol­id) + 2K₃C₆H₅O₇ + 6H₂O