Why does copper smell so bad

Press release from October 20, 2006

Leipzig / Blacksburg (USA). The typical metallic smell when touching iron-containing coins, tools and other objects is not caused by vaporized metal, as previously assumed, but by organic molecules from the human skin. With this type of human body odor, iron (similar to copper) only acts as a catalyst for splitting off and evaporation of odorous organic skin components.

"Pecunia non olet" (money doesn't stink) - this saying is passed down from the Roman emperor Vespasian when he introduced a latrine tax.
Graphic: Dr. Dietmar Glindemann

Left: Detection of bivalent iron ions from iron dust corroded by sweat through a violet color reaction with "FerroZine"
Center: Detection of monovalent copper ions from a copper coin corroded by sweat, by an ocher color reaction with bathocuproin
Right: Detection of monovalent copper ions from a brass key corroded by sweat, by an ocher color reaction with bathocuproin.

It should be said that bivalent iron and monovalent copper are not stable because these very reactive meta-stable ions are quickly oxidized in air to ordinary trivalent iron (as in ordinary rust) and bivalent copper (as in verdigris).

Photo: Dr. Dietmar Glindemann

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Another well-known garlic-like odor when etching phosphorus-containing cast iron and steel with acids is not caused by the gas phosphorus hydrogen, as previously assumed, but by organophosphorus compounds.
A German-American research team led by the chemist Dietmar Glindemann succeeded in refuting previous errors. The research results, in which scientists from the Virginia Polytechnic Institute and State University (Virginia Tech), the University of Leipzig and the Environmental Research Center Leipzig-Halle (UFZ) were involved, have now been published in the international edition of the renowned specialist journal "Angewandte Chemie".

Iron causes "metallic" human body odor

Of all the human senses, the sense of smell is the most difficult to explain scientifically. It was only two years ago that Linda Buck and Richard Axel were honored with the Nobel Prize for Medicine. You were the first to decipher the more than 1,000 genes that determine the sense of smell. Scientists from Blacksburg (VA, USA) and Leipzig are now working to unravel the complex chemistry of odors. "We were the first to demonstrate that what people describe as the 'metallic' smell of iron is not the evaporating iron atoms. The smells that people perceive as 'metallic' when they touch ferrous metal are actually volatile components of the skin." says Prof. Andrea Dietrich from Virgina State University. These odorous aldehydes and ketones split off from a kind of rancid skin fat (lipid peroxide) as a result of decomposition by "divalent" iron ions. This reactive form of iron (also called green rust) is caused by partial dissolution (corrosion) of the ferrous metal with the "sour sweat" on the skin. Since sebum adheres to metal, a coin or door handle with iron or copper, for example, smells like the decomposed lipid peroxides of all those users who have touched it recently.

From the smell of iron to the smell of blood - a trace of the early days

The researchers also showed that the same iron odor occurs when blood comes into contact with skin, since fresh blood contains "divalent" iron. They concluded that the very sensitive perception of the "iron smell" corresponds to the well-known pronounced sense of animals and humans for traces of blood. The ability to smell blood nice was an evolutionary asset as it allowed primitive humans like their animal relatives to track and find wounded bleeding prey like a track dog.

The completely different metallic smell of iron containing phosphorus

The researchers' publication also provides data on another more "garlic-like" metallic odor of iron that can be felt when cast iron or steel dissolves in acid. This odor is the result of the formation of organophosphines from carbon and phosphorus impurities in iron. Dr. Dietmar Glindemann recommends a simple smell test to track down these organophosphines: "Cast iron filings heated in a test tube with aqueous hydrochloric acid result in foaming gas bubbles that smell of garlic or carbide. Cast iron is used because it contains a lot of phosphorus, it dissolves easily and therefore has a clear smell." . "The difficulty was in the laboratory to maximize the formation of these air-sensitive organophosphines from iron in such a way that they could be safely analyzed with modern equipment, since they were still far less sensitive than the human nose," says Dr. Hans-Joachim Stark from the UFZ presented the analytical challenge. Because these organophosphines oxidize rapidly in the air, they have never been discovered in the environment, although high-tech laboratories produce organophosphines to treat semiconductors. The publication also shows that the everyday corrosion of cast iron leads to organophosphorus compounds, which are on the watch list of the Chemical Weapons Convention because they are a "fingerprint" for the production or use of nerve gas. The scientists fear that ubiquitous organophosphorus corrosion products could lead to confusion in the monitoring of the convention.

New applications

"Our research results confirm the evidence from the human nose that the ferrous metal, which is widespread in our everyday lives, tends to react with the human skin and to form very odorous and reactive organic compounds. We wanted to investigate whether iron can influence health in this way ", suggests Glindemann. With their publication, the scientists want to encourage medical professionals to further develop an analytical iron test for measuring peroxidation of the skin, blood and body tissue of patients. Individual body odor, oxidative stress and various diseases could thus be identified using their specific chemical "fingerprint" from volatile odorous substances. Just recently, a report about dogs that can smell cancer cells caused a stir. In addition, the elucidation of the metallic skin odor could help water engineers to clarify customer complaints about the "metallic" odor of drinking water. The environmental researchers hope that these results will help to detect organic and metallic contaminants in the soil through their odorous reaction products or to eliminate such odor scenarios in landfills.

Dietmar Glindemann, Andrea Dietrich, Hans-Joachim Staerk and Peter Kuschk:
The two smells of iron when touched and when exposed to acids - (skin) carbonyl compounds and organophosphines.
Angewandte Chemie International Edition, Early View
www3.interscience.wiley.com

Dr. Hans-Joachim Stark / Dr. Peter Kuschk
Environmental Research Center Leipzig-Halle (UFZ)
Phone: 0341-235-2289 / 0341-235-2821

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