This is Ruhemann’s purple, and you can probably figure out, from the picture, the legal reasons it will ruin your life. Now let’s talk about the chemistry behind that.

It’s easy to see a fingerprint on the side of a glass of water, or in blood on a wall. But what if a killer is neat and well-hydrated? Then you bring in a little substance called ninhydrin. It was first invented all the way back in 1911 by Siegfried Ruhemann.

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It reacts with amino acids. When it encounters an amino acid, it tears the acid apart, producing three things: ammonia, aldehydes, and carbon dioxide. That isn’t very useful.

In the process, ninhydrin takes a hit itself. Some of it is reduced to a substance called hydrindantin. Also not useful.

But hydrindantin doesn’t stay isolated. Over about 24 to 48 hours, it combines with some unreduced ninhydrin and some ammonia, and turns a vivid purple. This purple, called Ruhemann’s purple, has sent a lot of people to jail.

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It’s tough picking up a fingerprint from a porous surface, like paper or fabric, but everything we touch has the remains of our amino acids on it. The tiniest traces of them stay in the ridges on our fingerprints. Spray a piece of paper with ninhydrin, and over the next two days, if it has any fingerprints on it, the tell-tale Ruhemmann’s purple shows up. Then it’s up the river for you.

Image: Ninhydrin Staining, shared by Horoporo (CC BY-SA 3.0).

[Source: What is a Simple Test for the Presence of Amino Acids? General Chemistry Online.]

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