The Standing Invitation

Posts Tagged ‘Smell

The Smell of Money

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There really is nothing quite like writing a doctoral thesis for increasing your interest in things other than your thesis. Today I wondered: why do some metals have a smell?

Go through your small change right now. The pennies have a distinct metallic smell. But how can this be?

In order to smell something, particles of it have to get to your nose. This is easily understandable with liquids, because all liquids are continuously shedding molecules to the air around them through the process of evaporation. The tendency of a liquid to release gas by evaporation is given by its vapour pressure, which varies with temperature: the higher the temperature, the higher the vapour pressure, faster the liquid evaporates.

Solids, on the other hand, do not evaporate in this way, although there is an analogous process called sublimation, in which particles leave the solid’s surface as gas without passing through a liquid state. This means that solids do have vapour pressures, but these are extremely low for things like metal coins, which do not have a noticeable tendency to evaporate when left on the pavement on a hot day.

So what is it about coins that gives them this smell of metal? Marvellously, there is a paper in Angewandte Chemie from 2006 that answers exactly this question. The authors focus on iron, which is often described as having a ‘musty’ aroma. What they find is that iron is in fact odourless, both as a solid lump of metal and as a solution.

They are unable to resist the pun: “Ironically, the iron odour on skin contact is a type of human body odour.”

The experiments involved the sweat and blood of researchers. Sweat is corrosive: it attacks the surface of the metal and partially dissolves it, forming small amount of the ion Fe2+. This is reacts within seconds with oxygen give to Fe3+, but also causes a reaction with the sweat itself. Lipidperoxides occurring naturally in sweat are  broken down into volatile carbonyl hydrocarbons that we are able to smell. Metal smells, but only because it has been touched by people.

The same mechanism explains the metallic smell of blood – one of the researchers’ own blood was used in the experiment, apparently. Blood contains iron, which decomposes lipidperoxides in the blood.

The typical “musty” metallic odor of iron metal touching skin (epidermis) is caused by volatile carbonyl compounds (aldehydes, ketones) produced through the reaction of skin peroxides with ferrous ions (Fe2+) that are formed in the sweat-mediated corrosion of iron. Fe2+ ion containing metal surfaces, rust, drinking water, blood etc., but also copper and brass, give rise to a similar odor on contact with the skin. The human ability to detect this odor is probably a result of the evolutionarily developed but largely dormant ability to smell blood (“blood scent”).

It’s a nice everyday example of the scientific problem of correlation implying causation. You might describe the smell as ‘metallic’, but that’s only because you only smell it around metals. In reality, the metal is not what you’re smelling; you’re smelling the decomposition of chemicals produced by your own skin.

In reality, the copper coins you’ve been sniffing have a smell we call ‘coppery’ because they are smeared with the chemically decomposed sweat and oils from the hundreds of greasy, sweaty fingers that have touched them. A lovely thought, isn’t it?



Written by The S I

October 31, 2011 at 11:59 pm