Iodine Vapor Fingerprints

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Forensics

Alternative titles: Revealing Fingerprints with Iodine Fuming

Summary

This demonstration shows how iodine vapor can reveal fingerprints by binding to the fatty residues left behind on surfaces.

Procedure

Place a sheet of paper with a fingerprint inside a sealable container such as a jar or plastic box.
Add a few solid iodine crystals to the bottom of the container (do not let them touch the paper).
Seal the container and wait a few minutes for iodine vapors to fill the space.
Observe the fingerprint as it gradually appears brown where iodine binds to the oils.
Remove the paper when the fingerprint is clear.

Links

The Home Scientist 006 - Forensic Fingerprinting I: Iodine Fuming:

Fingerprint Fixing with Iodine - RACInational:

📄 Iodine fingerprint - MEL Science (uses copper sulfate and potassium iodide instead of solid iodine): https://melscience.com/AU-en/chemistry/experiments/diffusion-iodine-fingers/?srsltid=AfmBOopdRexxLJsB66vaYzwTo4NRhco_DSCZ05F8UynAJFxTpqRPYMWn

Variations

Try developing fingerprints on different porous surfaces such as cardboard or fabric.
Place the container on a warm (not hot) surface to increase sublimation and speed up fingerprint development.
Use starch solution after iodine development; the starch-iodine complex produces a dark blue color, making the fingerprint more visible.

Safety Precautions

Wear safety goggles and protective gloves.
Handle iodine crystals with care; avoid direct skin contact.
Perform the experiment in a well-ventilated area or under a fume hood to avoid inhaling vapors.
Keep iodine crystals sealed when not in use, as vapors can irritate eyes and lungs.
Dispose of iodine residues safely according to local chemical disposal rules.

Questions to Consider

Why does iodine make the fingerprint visible? (Because iodine vapors bind to fats and oils present in fingerprints.)
Why does heating the iodine speed up the fingerprint appearance? (Heating increases sublimation, releasing vapors faster.)
Why do the fingerprints fade over time? (Iodine sublimes back into the air, leaving less bound to the fingerprint.)
What is the advantage of using iodine fuming over ink or powder methods? (It works well on porous surfaces like paper where powders are less effective.)