Hydrogen peroxide

Structure and dimensions of H₂O₂ in the gas phase

Structure and dimensions of H₂O₂ in the solid (crystalline) phase

Hydrogen peroxide (H₂O₂) is a nonplanar molecule with (twisted) C₂ symmetry; this was first shown by Paul-Antoine Giguère in 1950 using infrared spectroscopy.[9][10] Although the O−O bond is a single bond, the molecule has a relatively high rotational barrier of 386 cm⁻¹ (4.62 kJ/mol) for rotation between enantiomers via the trans configuration, and 2460 cm⁻¹ (29.4 kJ/mol) via the cis configuration.[11] These barriers are proposed to be due to repulsion between the lone pairs of the adjacent oxygen atoms and dipolar effects between the two O–H bonds. For comparison, the rotational barrier for ethane is 1040 cm⁻¹ (12.4 kJ/mol).

The approximately 100° dihedral angle between the two O–H bonds makes the molecule chiral. It is the smallest and simplest molecule to exhibit enantiomerism. It has been proposed that the enantiospecific interactions of one rather than the other may have led to amplification of one enantiomeric form of ribonucleic acids and therefore an origin of homochirality in an RNA world.[12]

The molecular structures of gaseous and crystalline H₂O₂ are significantly different. This difference is attributed to the effects of hydrogen bonding, which is absent in the gaseous state.[13] Crystals of H₂O₂ are tetragonal with the space group D⁴
₄ or P4₁2₁2.[14]

In aqueous solutions, hydrogen peroxide forms a eutectic mixture, exhibiting freezing-point depression down as low as -56 °C; pure water has a freezing point of 0 °C and pure hydrogen peroxide of -0.43 °C. The boiling point of the same mixtures is also depressed in relation with the mean of both boiling points (125.1 °C). It occurs at 114 °C. This boiling point is 14 °C greater than that of pure water and 36.2 °C less than that of pure hydrogen peroxide.[15]

Phase diagram of H₂O₂ and water: Area above blue line is liquid. Dotted lines separate solid–liquid phases from solid–solid phases.

Hydrogen peroxide is most commonly available as a solution in water. For consumers, it is usually available from pharmacies at 3 and 6 wt% concentrations. The concentrations are sometimes described in terms of the volume of oxygen gas generated; one milliliter of a 20-volume solution generates twenty milliliters of oxygen gas when completely decomposed. For laboratory use, 30 wt% solutions are most common. Commercial grades from 70% to 98% are also available, but due to the potential of solutions of more than 68% hydrogen peroxide to be converted entirely to steam and oxygen (with the temperature of the steam increasing as the concentration increases above 68%) these grades are potentially far more hazardous and require special care in dedicated storage areas. Buyers must typically allow inspection by commercial manufacturers.

Hydrogen peroxide has several structural analogues with H_mX−XH_n bonding arrangements (water also shown for comparison). It has the highest (theoretical) boiling point of this series (X = O, S, N, P). Its melting point is also fairly high, being comparable to that of hydrazine and water, with only hydroxylamine crystallising significantly more readily, indicative of particularly strong hydrogen bonding. Diphosphane and hydrogen disulfide exhibit only weak hydrogen bonding and have little chemical similarity to hydrogen peroxide. Structurally, the analogues all adopt similar skewed structures, due to repulsion between adjacent lone pairs.

Hydrogen peroxide was once prepared industrially by hydrolysis of ammonium persulfate:

[NH₄]₂S₂O₈ + 2 H₂O → 2 [NH₄]HSO₄ + H₂O₂

[NH₄]₂S₂O₄ was itself obtained by the electrolysis of a solution of ammonium bisulfate ([NH₄]HSO₄) in sulfuric acid.[32]

Small amounts are formed by electrolysis, photochemistry, and electric arc, and related methods.[33]

A commercially viable rouote hydrogen peroxide by the reaction of hydrogen with oxygen favours production of water but can be stopped at the peroxide stage.[34][35] One economic obstacle has been that direct processes give a dilute solution uneconomic for transportation. None of these has yet reached a point where it can be used for industrial-scale synthesis.

Hydrogen peroxide is about 1000x stronger acid than water.[36]

H₂O₂ ⇌ H⁺ + HO−2 pK = 11.65

Hydrogen peroxide disproportionates to form water and oxygen with a ΔH^o of –2884.5 kJ/kg[37] and a ΔS of 70.5 J/(mol·K):

${\displaystyle {\ce {2 H2O2 -> 2 H2O + O2}}}$

The rate of decomposition increases with rise in temperature, concentration, and pH. H₂O₂ is unstable under alkaline conditions). Decomposition is catalysed by various redox-active ions or compounds, including most transition metals and their compounds (e.g. manganese dioxide (MnO₂), silver, and platinum).[38]

The redox properties of hydrogen peroxide depend on pH. In acidic solutions, H₂O₂ is a powerful oxidizer.

Sulfite (SO2−3) is oxidized to sulfate (SO2−4).

Under alkaline conditions, hydrogen peroxide is a reductant. When H₂O₂ acts as a reducing agent, oxygen gas is also produced. For example, hydrogen peroxide will reduce sodium hypochlorite and potassium permanganate, which is a convenient method for preparing oxygen in the laboratory:

NaOCl + H₂O₂ → O₂ + NaCl + H₂O

2 KMnO₄ + 3 H₂O₂ → 2 MnO₂ + 2 KOH + 2 H₂O + 3 O₂

Although usually a reductant, alkaline hydrogen peroxide converts Mn(II) to the dioxide:

H₂O₂ + Mn²⁺ + 2 OH⁻ → MnO₂ + 2 H₂O

In a related reaction, potassium permanganate is reduced to Mn²⁺ by acidic H₂O₂:[5]

2 MnO−4 + 5 H₂O₂ + 6 H⁺ → 2 Mn^+2 + 8 H₂O + 5 O₂

The reduction of hydrogen peroxide by sodium hypochlorite yields singlet oxygen.

Hydrogen peroxide is frequently used as an oxidizing agent. Illustrative is oxidation of thioethers to sulfoxides:[39][40]

${\displaystyle {\ce {Ph-S-CH3 + H2O2 -> Ph-S(O)-CH3 + H2O}}}$

Alkaline hydrogen peroxide is used for epoxidation of electron-deficient alkenes such as acrylic acid derivatives,[41] and for the oxidation of alkylboranes to alcohols, the second step of hydroboration-oxidation. It is also the principal reagent in the Dakin oxidation process.

Hydrogen peroxide is a weak acid, forming hydroperoxide or peroxide salts with many metals.

It also converts metal oxides into the corresponding peroxides. For example, upon treatment with hydrogen peroxide, chromic acid (CrO₃ and H₂SO₄) forms a blue peroxide CrO(O₂)₂.

The aerobic oxidation of glucose in the presence of the enzyme glucose oxidase produces hydrogen peroxide. The conversion affords gluconolactone:[42]

C₆H₁₂O₇ + O₂ → C₆H₁₀O₇ + H₂O₂

Superoxide dismutases (SOD)s are enzymes that promote the disproportionation of superoxide into oxygen and hydrogen peroxide.[43]

${\displaystyle {\ce {2 O2- + 2 H+ -> O2 + H2O2}}}$

${\displaystyle {\ce {2 H2O2 -> O2 + 2 H2O}}}$

Peroxisomes are organelles found in virtually all eukaryotic cells.[44] They are involved in the catabolism of very long chain fatty acids, branched chain fatty acids, D-amino acids, polyamines, and biosynthesis of plasmalogens, ether phospholipids, which are found in mammalian brains and lungs.[45] They produce hydrogen peroxide in s process catalyzed by flavin adenine dinucleotide (FAD):[46]

${\displaystyle {\ce {R-CH2-CH2-CO-SCoA + O2 ->[{\ce {FAD}}] R-CH=CH-CO-SCoA + H2O2}}}$

Hydrogen peroxide arises by the degradation of adenosine monophosphate, which yields hypoxanthine. Hypoxanthine is then oxidatively catabolized first to xanthine and then to uric acid, and the reaction is catalyzed by the enzyme xanthine oxidase:[47]

Hypoxanthine

Xanthine oxidase

H₂O, O₂

H₂O₂

Xanthine

Xanthine oxidase

H₂O, O₂

H₂O₂

Uric acid

Degradation of hypoxanthine through xanthine to uric acid to form hydrogen peroxide.

Australian bombardier beetle

The degradation of guanosine monophosphate yields xanthine as an intermediate product which is then converted in the same way to uric acid with the formation of hydrogen peroxide.[47]

Catalase, another peroxisomal enzyme, uses this H₂O₂ to oxidize other substrates, including phenols, formic acid, formaldehyde, and alcohol, by means of a peroxidation reaction:

${\displaystyle {\ce {H2O2 + R'H2 -> R' + 2 H2O}}}$

thus eliminating the poisonous hydrogen peroxide in the process.

This reaction is important in liver and kidney cells, where the peroxisomes neutralize various toxic substances that enter the blood. Some of the ethanol humans drink is oxidized to acetaldehyde in this way.[48] In addition, when excess H₂O₂ accumulates in the cell, catalase converts it to H₂O through this reaction:

H₂O₂ → 0.5 O₂ + H₂O

Glutathione peroxidase, a selenoenzyme, also catalyzes the disproportionation of hydrogen peroxide.

The reaction of Fe²⁺ and hydrogen peroxide is the basis of the Fenton reaction, which generates hydroxyl radicals, which are of significance in biology:

Fe(II) + H₂O₂ → Fe(III)OH + HO·

The fenton reaction explains the toxicity of hydrogen peroxides because the hydroxyl radicals rapidly and irreversibly oxidize all organic compounds including proteins, membrane lipids, and DNA.[49] Hydrogen peroxide is a significant source of oxidative DNA damage in living cells. DNA damage includes formation of 8-Oxo-2'-deoxyguanosine among many other altered bases, as well as strand breaks, inter-strand crosslinks, and deoxyribose damage.[50] By interacting with Cl¯ hydrogen peroxide also lead to chlorinated DNA bases.[50] Hydroxyl radicals readily damage vital cellular components, especially those of the mitochondria.[51][52][53] The compound is a major factor implicated in the free-radical theory of aging, based on its ready conversion into a hydroxyl radical.

Eggs of sea urchin, shortly after fertilization by a sperm, produce hydrogen peroxide. It is then converted to hydroxyl radicals (HO•), which initiate radical polymerization, which surrounds the eggs with a protective layer of polymer.

The bombardier beetle combine hydroquinone and hydrogen peroxide, leading to a violent exothermic chemical reaction to produce boiling, foul-smelling liquid partially becomes a gas (flash evaporation) and is expelled through an outlet valve with a loud popping sound.[54][55][56]

As a proposed signaling molecule, hydrogen peroxide may regulate of a wide variety of biological processes.[57][58]

At least one study has also tried to link hydrogen peroxide production to cancer.[59]

About 60% of the world's production of hydrogen peroxide is used for pulp- and paper-bleaching.[29] The second major industrial application is the manufacture of sodium percarbonate and sodium perborate, which are used as mild bleaches in laundry detergents. A representative conversion is:

Na₂B₄O₇ + 4 H₂O₂ + 2 NaOH → 2 Na₂B₂O₄(OH)₄ + H₂O

Sodium percarbonate, which is an adduct of sodium carbonate and hydrogen peroxide, is the active ingredient in such laundry products as OxiClean and Tide laundry detergent. When dissolved in water, it releases hydrogen peroxide and sodium carbonate.[23] By themselves these bleaching agents are only effective at wash temperatures of 60 °C (140 °F) or above and so, often are used in conjunction with bleach activators, which facilitate cleaning at lower temperatures.

Hydrogen peroxide has also been used as a flour bleaching agent and a tooth and bone whitening agent.

It is used in the production of various organic peroxides with dibenzoyl peroxide being a high volume example.[60] Peroxy acids, such as peracetic acid and meta-chloroperoxybenzoic acid also are produced using hydrogen peroxide. Hydrogen peroxide has been used for creating organic peroxide-based explosives, such as acetone peroxide. It is used as an initiator in polymerizations. Hydrogen peroxide reacts with certain di-esters, such as phenyl oxalate ester (cyalume), to produce chemiluminescence; this application is most commonly encountered in the form of glow sticks.

The reaction with borax leads to sodium perborate, a bleach used in laundry detergents:

${\displaystyle {\ce {Na2B4O7 + 4 H2O2 + 2 NaOH -> 2 Na2B2O4(OH)4 + H2O}}}$

Hydrogen peroxide is used in certain waste-water treatment processes to remove organic impurities. In advanced oxidation processing, the Fenton reaction[61][62] gives the highly reactive hydroxyl radical (•OH). This degrades organic compounds, including those that are ordinarily robust, such as aromatic or halogenated compounds.[63] It can also oxidize sulfur-based compounds present in the waste; which is beneficial as it generally reduces their odour.[64]

Hydrogen peroxide may be used for the sterilization of various surfaces,[65] including surgical tools,[66] and may be deployed as a vapour (VHP) for room sterilization.[67] H₂O₂ demonstrates broad-spectrum efficacy against viruses, bacteria, yeasts, and bacterial spores.[68][69] In general, greater activity is seen against Gram-positive than Gram-negative bacteria; however, the presence of catalase or other peroxidases in these organisms may increase tolerance in the presence of lower concentrations.[70] Lower levels of concentration (3%) will work against most spores; higher concentrations (7 to 30%) and longer contact times will improve sporicidal activity.[69][71]

Hydrogen peroxide is seen as an environmentally safe alternative to chlorine-based bleaches, as it degrades to form oxygen and water and it is generally recognized as safe as an antimicrobial agent by the U.S. Food and Drug Administration (FDA).[72]

Rocket-belt hydrogen-peroxide propulsion system used in a jet pack

High-concentration H₂O₂ is referred to as "high-test peroxide" (HTP). It can be used either as a monopropellant (not mixed with fuel) or as the oxidizer component of a bipropellant rocket. Use as a monopropellant takes advantage of the decomposition of 70–98% concentration hydrogen peroxide into steam and oxygen. The propellant is pumped into a reaction chamber, where a catalyst, usually a silver or platinum screen, triggers decomposition, producing steam at over 600 °C (1,100 °F), which is expelled through a nozzle, generating thrust. H₂O₂ monopropellant produces a maximal specific impulse (I_sp) of 161 s (1.6 kN·s/kg). Peroxide was the first major monopropellant adopted for use in rocket applications. Hydrazine eventually replaced hydrogen-peroxide monopropellant thruster applications primarily because of a 25% increase in the vacuum specific impulse.[73] Hydrazine (toxic) and hydrogen peroxide (less-toxic [ACGIH TLV 0.01 and 1 ppm respectively]) are the only two monopropellants (other than cold gases) to have been widely adopted and utilized for propulsion and power applications. The Bell Rocket Belt, reaction control systems for X-1, X-15, Centaur, Mercury, Little Joe, as well as the turbo-pump gas generators for X-1, X-15, Jupiter, Redstone and Viking used hydrogen peroxide as a monopropellant.[74] The RD-107 engines (used from 1957 to present) in the R-7 series of rockets decompose hydrogen peroxide to power the turbopumps.

As a bipropellant, H₂O₂ is decomposed to burn a fuel as an oxidizer. Specific impulses as high as 350 s (3.5 kN·s/kg) can be achieved, depending on the fuel. Peroxide used as an oxidizer gives a somewhat lower I_sp than liquid oxygen, but is dense, storable, non-cryogenic and can be more easily used to drive gas turbines to give high pressures using an efficient closed cycle. It may also be used for regenerative cooling of rocket engines. Peroxide was used very successfully as an oxidizer in World War II German rocket motors (e.g. T-Stoff, containing oxyquinoline stabilizer, for both the Walter HWK 109-500 Starthilfe RATO externally podded monopropellant booster system, and for the Walter HWK 109-509 rocket motor series used for the Me 163B), most often used with C-Stoff in a self-igniting hypergolic combination, and for the low-cost British Black Knight and Black Arrow launchers. Presently, HTP is used on ILR-33 AMBER[75] and Nucleus[76] suborbital rockets.

In the 1940s and 1950s, the Hellmuth Walter KG–conceived turbine used hydrogen peroxide for use in submarines while submerged; it was found to be too noisy and require too much maintenance compared to diesel-electric power systems. Some torpedoes used hydrogen peroxide as oxidizer or propellant. Operator error in the use of hydrogen-peroxide torpedoes was named as possible causes for the sinking of HMS Sidon and the Russian submarine Kursk.[77] SAAB Underwater Systems is manufacturing the Torpedo 2000. This torpedo, used by the Swedish Navy, is powered by a piston engine propelled by HTP as an oxidizer and kerosene as a fuel in a bipropellant system.[78][79]

Contact lenses soaking in a 3% hydrogen peroxide-based solution. The case includes a catalytic disc which neutralises the hydrogen peroxide over time.

Hydrogen peroxide has various domestic uses, primarily as a cleaning and disinfecting agent.

Hair bleaching

Diluted H₂O₂ (between 1.9% and 12%) mixed with aqueous ammonia has been used to bleach human hair. The chemical's bleaching property lends its name to the phrase "peroxide blonde".[80] Hydrogen peroxide is also used for tooth whitening. It may be found in most whitening toothpastes. Hydrogen peroxide has shown positive results involving teeth lightness and chroma shade parameters.[81] It works by oxidizing colored pigments onto the enamel where the shade of the tooth may become lighter. Hydrogen peroxide may be mixed with baking soda and salt to make a homemade toothpaste.[82]

Removal of blood stains

Hydrogen peroxide reacts with blood as a bleaching agent, and so if a blood stain is fresh, or not too old, liberal application of hydrogen peroxide, if necessary in more than single application, will bleach the stain fully out. After about two minutes of the application, the blood should be firmly blotted out.[83][84]

Acne treatment

Hydrogen peroxide may be used to treat acne,[85] although benzoyl peroxide is a more common treatment.

Oral cleaning agent

The use of dilute hydrogen peroxide as a oral cleansing agent has been reviewed academically to determine its usefulness in treating gingivitis and plaque. Although there is a positive effect when compared with a placebo, it was concluded that chlorhexidine is a much more effective treatment.[86]

Chemiluminescence of cyalume, as found in a glow stick

Horticulture

Some horticulturists and users of hydroponics advocate the use of weak hydrogen peroxide solution in watering solutions. Its spontaneous decomposition releases oxygen that enhances a plant's root development and helps to treat root rot (cellular root death due to lack of oxygen) and a variety of other pests.[87][88]

For general watering concentrations around 0.1% is in use and this can be increased up to one percent for anti-fungal actions.[89] Tests show that plant foliage can safely tolerate concentrations up to 3%.[90]

Fishkeeping

Hydrogen peroxide is used in aquaculture for controlling mortality caused by various microbes. In 2019, the U.S. FDA approved it for control of Saprolegniasis in all coldwater finfish and all fingerling and adult coolwater and warmwater finfish, for control of external columnaris disease in warm-water finfish, and for control of Gyrodactylus spp. in freshwater-reared salmonids.[91] Laboratory tests conducted by fish culturists have demonstrated that common household hydrogen peroxide may be used safely to provide oxygen for small fish. The hydrogen peroxide releases oxygen by decomposition when it is exposed to catalysts such as manganese dioxide.

Removing yellowing from aged plastics

Hydrogen peroxide may be used in combination with a UV-light source to remove yellowing from white or light grey acrylonitrile butadiene styrene (ABS) plastics to partially or fully restore the original color. In the retrocomputing scene, this process is commonly referred to as retrobright.

Historically, hydrogen peroxide was used for disinfecting wounds, partly because of its low cost and prompt availability compared to other antiseptics.[103]

There is conflicting evidence on hydrogen peroxide's effect on wound healing. Some research finds benefit, while other research find delays and healing inhibition.[104] Its use for home treatment of wounds is generally not recommended.[105] 1.5–3% Hydrogen peroxide is used as a disinfectant in dentistry, especially in endodotic treatments together with hypochlorite and chlorhexidin and 1–1.5% is also useful for treatment of inflammation of third molars (wisdom teeth).[106]

Practitioners of alternative medicine have advocated the use of hydrogen peroxide for various conditions, including emphysema, influenza, AIDS, and in particular cancer.[107] There is no evidence of effectiveness and in some cases it has proved fatal.[108][109][110][111]

Both the effectiveness and safety of hydrogen peroxide therapy is scientifically questionable. Hydrogen peroxide is produced by the immune system, but in a carefully controlled manner. Cells called phagocytes engulf pathogens and then use hydrogen peroxide to destroy them. The peroxide is toxic to both the cell and the pathogen and so is kept within a special compartment, called a phagosome. Free hydrogen peroxide will damage any tissue it encounters via oxidative stress, a process that also has been proposed as a cause of cancer.[112] Claims that hydrogen peroxide therapy increases cellular levels of oxygen have not been supported. The quantities administered would be expected to provide very little additional oxygen compared to that available from normal respiration. It is also difficult to raise the level of oxygen around cancer cells within a tumour, as the blood supply tends to be poor, a situation known as tumor hypoxia.

Large oral doses of hydrogen peroxide at a 3% concentration may cause irritation and blistering to the mouth, throat, and abdomen as well as abdominal pain, vomiting, and diarrhea.[108] Ingestion of hydrogen peroxide at concentrations of 35% or higher has been implicated as the cause of numerous gas embolism events resulting in hospitalisation. In these cases, hyperbaric oxygen therapy was used to treat the embolisms.[113]

Intravenous injection of hydrogen peroxide has been linked to several deaths.[114][110][111] The American Cancer Society states that "there is no scientific evidence that hydrogen peroxide is a safe, effective, or useful cancer treatment."[109] Furthermore, the therapy is not approved by the U.S. FDA.

• On 16 July 1934, in Kummersdorf, Germany, a propellant tank containing an experimental monopropellant mixture consisting of hydrogen peroxide and ethanol exploded during a test, killing three people.[115]
• During the Second World War, doctors in German concentration camps experimented with the use of hydrogen peroxide injections in the killing of human subjects.[116]
• In December 1943, the pilot Josef Pöhs died after being exposed to the T-Stoff of his Messerschmitt Me 163.
• In April 1992, an explosion occurred at the hydrogen peroxide plant at Jarrie in France, due to technical failure of the computerised control system and resulting in one fatality and wide destruction of the plant.[117]
• Several people received minor injuries after a hydrogen peroxide spill on board a flight between the U.S. cities of Orlando and Memphis on 28 October 1998.[118]
• The Russian submarine K-141 Kursk sailed to perform an exercise of firing dummy torpedoes at the Pyotr Velikiy, a Kirov-class battlecruiser. On 12 August 2000, at 11:28 local time (07:28 UTC), there was an explosion while preparing to fire the torpedoes. The only credible report to date is that this was due to the failure and explosion of one of the Kursk's hydrogen peroxide-fueled torpedoes. It is believed that HTP, a form of highly concentrated hydrogen peroxide used as propellant for the torpedo, seeped through its container, damaged either by rust or in the loading procedure back on land where an incident involving one of the torpedoes accidentally touching ground went unreported. The vessel was lost with all hands. A similar incident was responsible for the loss of HMS Sidon in 1955.[119]
• On 15 August 2010, a spill of about 30 US gallons (110 L) of cleaning fluid occurred on the 54th floor of 1515 Broadway, in Times Square, New York City. The spill, which a spokesperson for the New York City fire department said was of hydrogen peroxide, shut down Broadway between West 42nd and West 48th streets as fire engines responded to the hazmat situation. There were no reported injuries.[120]