“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.
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;
- white paper.
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.
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