Hydrogen peroxide

• 1. A Deadly Torpedo

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  Hydrogen peroxide sank the Royal Navy submarine HMS Sidon in June 1955. Concentrated peroxide leaked from a torpedo and caused it to explode inside its launch tube. Twelve crew members died, and so did a member of the rescue party. The substance behind the disaster is the simplest peroxide, a compound defined by a single bond between two oxygen atoms. Yet the same compound is found in human bodies, used as an antiseptic, and relied on as a bleach. How can one plain molecule be a household helper, a rocket propellant, and a killer? Who first made it, and how is it produced by the million tonnes today? Why do living cells brew it on purpose? And why do medical authorities warn people so firmly against swallowing it?

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• 2. A Twisted Molecule

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  In 1950, the chemist Paul-Antoine Giguère used infrared spectroscopy to show that hydrogen peroxide is not a flat molecule. Its two halves are twisted against each other, with an angle of roughly 100 degrees between the two O-H bonds. That twist makes the molecule chiral, so it exists in two mirror-image forms. It is the smallest and simplest molecule known to do this. Some researchers have proposed that a preference for one form over the other may have nudged early RNA toward a single handedness.

  Switching between the two forms is harder than a single bond would suggest. Rotating through the anti arrangement costs 4.62 kJ per mole, and rotating through the eclipsed arrangement costs 29.4 kJ per mole. Ethane, by comparison, needs only 12.4 kJ per mole. The proposed cause is repulsion between lone electron pairs on neighbouring oxygen atoms, plus dipolar effects between the two O-H bonds.

  Pure hydrogen peroxide is a very pale blue liquid, though at lower concentrations the color is too faint to see. The molecule is strongly polarized and builds dense networks of hydrogen bonds, which make the liquid more viscous than water. Among water and its sulfur, nitrogen and phosphorus analogues, it has the highest theoretical boiling point. That extrapolated value is 150.2 degrees Celsius, about 50 degrees above water. Heated to that temperature, it can decompose explosively, so it can be distilled safely only at lower temperatures under reduced pressure.

  Mixed with water, it forms a eutectic mixture that stays liquid down to minus 56 degrees Celsius. Pure water freezes at 0 degrees, and pure peroxide at minus 0.43 degrees. Pure peroxide at 100 percent has a density of 1.45 grams per cubic centimeter.

  Pharmacies sell consumer solutions at 3 and 6 percent by weight. Some labels describe strength by volume instead: one milliliter of a 20-volume solution releases twenty milliliters of oxygen gas when it fully decomposes. Laboratories most often use 30 percent. Commercial grades run from 70 to 98 percent, and above 68 percent a solution can convert entirely into steam and oxygen. Those grades need dedicated storage areas, and buyers must typically allow inspection by the commercial manufacturers.

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• 3. Discovery and Scale

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  Alexander von Humboldt is sometimes credited with reporting the first synthetic peroxide, barium peroxide, in 1799. He obtained it as a by-product of his attempts to decompose air. The credit is disputed, because his wording was ambiguous. Nineteen years later, Louis Jacques Thénard saw that this compound could be used to prepare a previously unknown substance. He called it eau oxygénée, or oxygenated water, and it later became known as hydrogen peroxide.

  Thénard's recipe was later improved with hydrochloric acid, followed by sulfuric acid to precipitate barium sulfate. That method served from the end of the 19th century until the middle of the 20th. Thénard's experiments in the 1820s had shown that peroxides bleach natural dyes, but early industrial attempts failed. The first plant to make hydrogen peroxide was built in Berlin in 1873.

  Electrolysis with sulfuric acid offered a more efficient route. It was first commercialized in 1908 in Weißenstein, in Carinthia, Austria. During the 1930s, the German manufacturer IG Farben developed the anthraquinone process in Ludwigshafen, and that process is still used today. Annual output rose from 35,000 tonnes in 1950 to over 100,000 tonnes in 1960, and then to 300,000 tonnes by 1970. By 1998 it had reached 2.7 million tonnes.

  Making the pure liquid was a separate struggle. Early attempts to obtain neat peroxide failed, and the anhydrous form was first obtained by vacuum distillation. Working out the molecule was just as hard. In 1892, the Italian physical chemist Giacomo Carrara, who lived from 1864 to 1925, measured its molecular mass by freezing-point depression and confirmed its formula. The arrangement of the atoms stayed uncertain, and as late as the middle of the 20th century at least half a dozen hypothetical isomeric variants seemed consistent with the evidence. In 1934, the English mathematical physicist William Penney and the Scottish physicist Gordon Sutherland proposed a structure very close to the one accepted today.

  The anthraquinone process starts with an anthraquinone such as 2-ethylanthraquinone. Hydrogenation over a palladium catalyst converts it to an anthrahydroquinone. Compressed air is then passed through the solution. Oxygen takes hydrogen atoms from the anthrahydroquinone, which yields hydrogen peroxide and restores the original anthraquinone. The peroxide is extracted, and the quinone goes back for another cycle. The economics depend heavily on recycling the solvents, the catalyst and the expensive quinone.

  In 2006, bulk 30 percent solution sold for around 0.54 US dollars per kilogram. That is equivalent to US$1.50 per kilogram on a 100 percent basis. Reacting hydrogen directly with oxygen favors water, though the reaction can be stopped at the peroxide stage. The catch is that direct routes give a dilute solution that is uneconomic to ship, and none has reached industrial scale. The electrochemical reduction of oxygen is another route being explored, generating peroxide from oxygen and water.

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• 4. Chemistry Inside Cells

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  Sea water holds between 0.5 and 14 micrograms of hydrogen peroxide per liter, and fresh water holds 1-30 micrograms per liter. Sunlight on water can form it, and it has also been detected in groundwater and in the atmosphere. Air typically carries 0.4-4 micrograms per cubic meter, varying with season, altitude, daylight and water vapor. In rural nighttime air it drops below 0.014, while moderate photochemical smog pushes it to 14-42.

  Living cells make it as well. The aerobic oxidation of glucose by the enzyme glucose oxidase produces it, and so do superoxide dismutases, which convert superoxide into oxygen and peroxide. Peroxisomes, organelles found in virtually all eukaryotic cells, generate it while breaking down very long chain fatty acids and other molecules. The degradation of adenosine monophosphate and guanosine monophosphate ends in uric acid, with peroxide formed along the way.

  The enzyme catalase, another resident of the peroxisome, puts the peroxide to work. It uses the compound to oxidize phenols, formic acid, formaldehyde and alcohol, destroying the poison as it does so. This matters most in liver and kidney cells, where peroxisomes neutralize toxic substances that enter the blood. Some of the ethanol humans drink is oxidized to acetaldehyde this way. When peroxide builds up in excess, catalase converts it to water and oxygen. Glutathione peroxidase, a selenoenzyme, also breaks it down.

  The danger lies in the Fenton reaction, which turns hydrogen peroxide into hydroxyl radicals. These radicals rapidly and irreversibly oxidize all organic compounds, including proteins, membrane lipids and DNA. Peroxide is a significant source of oxidative DNA damage, including the altered base 8-oxo-2'-deoxyguanosine, strand breaks and inter-strand crosslinks. Mitochondria are especially vulnerable, and the ease of conversion to hydroxyl radicals makes peroxide a major factor in the free-radical theory of aging.

  Nature has found uses for the same hazard. Sea urchin eggs produce peroxide shortly after fertilization, and it becomes hydroxyl radicals that start a polymerization, wrapping each egg in a protective layer. The bombardier beetle mixes hydroquinone with hydrogen peroxide in a violent exothermic reaction. The result is a boiling, foul-smelling liquid that partly flashes to gas and shoots out of an outlet valve with a loud pop. Researchers also propose that peroxide acts as a signaling molecule regulating many biological processes, and at least one study has tried to link its production to cancer.

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• 5. Everyday Uses

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  About 60 percent of the world's hydrogen peroxide goes into bleaching pulp and paper. The second major industrial use is making sodium percarbonate and sodium perborate, mild bleaches for laundry detergents. Sodium percarbonate is the active ingredient in products such as OxiClean and Tide. On its own it works only at wash temperatures of 60 degrees Celsius or above, so detergents pair it with bleach activators for cooler washes. Peroxide has also been used to bleach flour and to whiten teeth and bones.

  The phrase "peroxide blonde" comes from hair care. Diluted peroxide, at 1.9-12 percent, mixed with aqueous ammonia, aniline and a coupler has been used to color human hair. Bleaching compounds do not remove the pigment-producing cells. They drive wedges between color molecules so that more visible light passes through the hair shaft. Whitening toothpastes often contain peroxide, which oxidizes colored pigments on the enamel.

  A fresh blood stain can be bleached fully away with liberal applications of peroxide. After about two minutes the spot should be blotted firmly. Peroxide may treat acne, though benzoyl peroxide is more common. One academic review found a positive effect on gingivitis and plaque compared with a placebo, but concluded that chlorhexidine is much more effective.

  In sewage treatment, peroxide helps remove organic impurities. Through the Fenton reaction it yields reactive hydroxyl radicals that degrade even robust aromatic or halogenated compounds. It also oxidizes sulfur compounds in waste, which generally reduces odour.

  As a disinfectant, peroxide sterilizes surfaces such as surgical instruments, and as a vapour it can sterilize whole rooms. It works against viruses, bacteria, yeasts and bacterial spores. A 3 percent solution works against most spores, while 7-30 percent with longer contact times improves sporicidal activity. Gram-positive bacteria are generally more vulnerable than Gram-negative ones, although catalase or other peroxidases in these organisms may increase tolerance at lower concentrations. Because it degrades to oxygen and water, peroxide is seen as an environmentally safe alternative to chlorine-based bleaches. The U.S. Food and Drug Administration generally recognizes it as safe as an antimicrobial agent.

  Peroxide also lights up glow sticks. It reacts with certain di-esters, such as phenyl oxalate ester, to produce chemiluminescence.

  Some horticulturists and hydroponic growers add about 0.1 percent peroxide to watering solutions, hoping its oxygen will boost root development and help treat root rot. The strength can rise to one percent for antifungal effect, and plant foliage tolerates up to 3 percent. In 2019, the U.S. FDA approved it for controlling Saprolegniasis in coldwater finfish and certain other fish diseases. Fish keepers also use sodium percarbonate to oxygenate water that is acutely short of oxygen. Combined with a UV-light source, peroxide can remove yellowing from white or light grey ABS plastics, a process the retrocomputing scene calls retrobright.

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• 6. Rocket Fuel

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  Steam at over 600 degrees Celsius is what a peroxide rocket thruster produces. At 70-98 percent concentration, hydrogen peroxide is called high-test peroxide, or HTP. Pumped into a reaction chamber, it meets a catalyst, usually a silver or platinum screen, and decomposes into steam and oxygen. The hot gas leaves through a nozzle and creates thrust. Used alone as a monopropellant, it reaches a maximal specific impulse of 161 seconds.

  Peroxide was the first major monopropellant adopted for rockets. Hydrazine eventually replaced it in thrusters, mainly because it offers a 25 percent increase in vacuum specific impulse. Peroxide has its own advantage, since it is less toxic. The Bell Rocket Belt used it, as did the reaction control systems of the X-1, X-15, Centaur, Mercury and Little Joe. It also drove the turbopump gas generators on the X-1, X-15, Jupiter, Redstone and Viking. The RD-107 engines of the R-7 rocket series have used it to power their turbopumps from 1957 to the present.

  As the oxidizer in a bipropellant rocket, peroxide can deliver specific impulses as high as 350 seconds, depending on the fuel. That is somewhat lower than liquid oxygen. But peroxide is dense, storable and non-cryogenic, and it can drive gas turbines efficiently in a closed cycle. It can also cool rocket engines by regenerative cooling.

  German engineers used it very successfully in World War II. T-Stoff, a peroxide mixture with an oxyquinoline stabilizer, powered the Walter HWK 109-509 rocket motors of the Me 163B and the Walter HWK 109-500 Starthilfe booster. Most often it was paired with C-Stoff in a self-igniting hypergolic combination. Later the British used it for the low-cost Black Knight and Black Arrow launchers. Today, HTP flies on the ILR-33 AMBER and Nucleus suborbital rockets.

  In the 1940s and 1950s, a turbine conceived by Hellmuth Walter KG used peroxide to power submarines while submerged. It proved too noisy and needed too much maintenance compared with diesel-electric systems. Some torpedoes also used peroxide as oxidizer or propellant. Operator error in handling it has been named as a possible cause of the sinking of the Russian submarine Kursk. Another torpedo is still built today. SAAB Underwater Systems makes the Torpedo 2000 for the Swedish Navy, a piston-engine design that burns kerosene with HTP as the oxidizer.

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• 7. Hazards and Accidents

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  Concentrations up to 35 percent leave only white oxygen bubbles in the skin, along with some biting pain. These disappear with the blood within 30-45 minutes. A 98 percent solution dissolves paper. Even a 3 percent solution can endanger the eye, because oxygen forms inside it.

  Swallowing peroxide solutions is especially dangerous. Decomposition in the stomach releases large quantities of gas, ten times the volume of a 3 percent solution, and causes internal bloating. Inhaling concentrations over 10 percent can cause severe pulmonary irritation. The vapour is hazardous too. The U.S. NIOSH sets the immediately dangerous to life and health limit at only 75 ppm. OSHA's permissible exposure limit is 1.0 ppm as an 8-hour time-weighted average. The American Conference of Governmental Industrial Hygienists classes peroxide as a "known animal carcinogen, with unknown relevance on humans".

  Laboratory chemists worry about pressure buildup in sealed vessels, trace metal ions that speed decomposition, and self-accelerating thermal runaway. They also keep the oxidizer well away from organic solvents and flammable materials. Pharmaceutical peroxide comes in brown bottles, because light breaks it down quickly. Streams above 40 percent count as hazardous under U.S. rules, and the EPA reportable quantity is 100 pounds, or roughly 10 US gallons.

  On the 16th of July 1934, a propellant tank holding an experimental mixture of hydrogen peroxide and ethanol exploded during a test in Kummersdorf, Germany. Three people died. In December 1943, the pilot Josef Pöhs died after exposure to the T-Stoff in his Messerschmitt Me 163. During the Second World War, doctors in German concentration camps also experimented with peroxide injections as a way of killing human subjects.

  In April 1992, the control system failed at the peroxide plant at Jarrie in France. The explosion killed one person and destroyed much of the plant. On the 28th of October 1998, a spill aboard a Northwest Airlines flight from Orlando, Florida to Memphis, Tennessee caused minor injuries. On the 15th of August 2010, about 30 US gallons of cleaning fluid spilled on the 54th floor of 1515 Broadway in Times Square. The fire department said it was peroxide, and Broadway was closed between West 42nd and West 48th streets. No injuries were reported.

  In August 2024, a railcar holding 70 percent peroxide suffered overpressure at the Evonik Industries chemical facility in Theodore, Alabama. The explosion happened around 2:00 PM, and only minor first aid was needed on site. Evonik began a phased restart after safety inspections.

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• 8. Wound Care and False Cures

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  For years, people poured hydrogen peroxide on wounds, partly because it was cheap and always at hand. Evidence on its effect on healing is conflicting. Some research finds benefit, while other studies find delayed healing. Its use for home treatment of wounds is generally not recommended.

  Dentistry still uses it. A 1.5-3 percent solution serves as a disinfectant in root canal treatment alongside hypochlorite and chlorhexidine, and 1-1.5 percent helps treat inflammation around wisdom teeth.

  Practitioners of alternative medicine have advocated peroxide for emphysema, influenza, AIDS and, above all, cancer. Medical authorities and regulators warn that these uses lack scientific support and can be dangerous. The immune system does make peroxide, but in a carefully controlled way. Cells called phagocytes engulf pathogens and kill them with it inside a compartment called a phagosome. The peroxide is toxic to both the cell and the pathogen. Free peroxide damages any tissue it meets through oxidative stress, a process also proposed as a cause of cancer.

  Claims that peroxide therapy raises cellular oxygen have not been supported. The doses given would add very little oxygen beyond normal breathing. Raising oxygen around cancer cells is also hard, since a tumour's blood supply tends to be poor, a condition known as tumor hypoxia.

  Large oral doses of a 3 percent solution may cause irritation and blistering of the mouth, throat and abdomen, plus abdominal pain, vomiting and diarrhea. Swallowing concentrations of 35 percent or higher has caused numerous gas embolisms that sent people to hospital, where hyperbaric oxygen therapy was used. Injecting peroxide into the bloodstream has been linked to several deaths. Even "food-grade" concentrations have led to serious adverse effects and fatalities.

  The American Cancer Society states that "there is no scientific evidence that hydrogen peroxide is a safe, effective, or useful cancer treatment." The therapy is not approved by the U.S. FDA.

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