CHEMICAL NAME = hydrogen peroxide CAS NUMBER = 7722–84–1
MOLECULAR FORMULA = H2O2 MOLAR MASS = 34.0 g/mol
COMPOSITION = H (5.9%) O(94.1%) MELTING POINT = −0.43°C
BOILING POINT = 150.2°C DENSITY = 1.44 g/cm3
Hydrogen peroxide is a colorless liquid that is widely used as an oxidizer and bleaching agent.
Hydrogen peroxide was discovered in 1818 by the French chemist Louis-Jacques Th enard (1777–1857) who called the compound eau oxygene (oxygen water). Th enard burned bar-ium salts to form barbar-ium peroxide, BaO2, which was dissolved in water to produce hydrogen peroxide.
Th enard’s method was used for the initial commercial production of hydrogen peroxide start-ing in the 1870s. A popular use at this time was the bleachstart-ing of straw hats. From 1920 to 1950, the primary method of production was electrolysis. One process involved passing electric current through sulfuric acid to produce the peroxydisulfate ion (S2O82-), which was then hydrolyzed to H2O2: 2H2O + S2O82-(aq) → 2H2SO4-(aq) + H2O2(aq). Th e relatively high cost of electricity of this method encouraged a search for a more economical production process. Hydrogen peroxide is currently produced on a large scale using the anthraquinone autooxidation procedure, which was developed in the 1940s. In this process, an anthraquinone, typically 2-ethyl-anthraquinone, is hydrogenated to a hydroquinone (2-ethyl-anthrahydroquinone) then reoxidized back to the anthraquinone (2-ethyl-anthraquinone) while forming hydrogen peroxide (Figure 47.1). A metal palladium or nickel catalyst is used to convert the anthraquinone to the hydroquinone, fol-lowed by autooxidation in air to generate hydrogen peroxide. Th e anthraquinone and hydrogen peroxide are separated; the former is recycled to repeat the process while the hydrogen peroxide is purifi ed. Th ere are current attempts to commercialize the direct combination of oxygen and hydrogen to H2O2 using catalysts, and researchers are examining new electrochemical methods.
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Figure 47.1 Production of hydrogen peroxide.
Synthesized hydrogen peroxide is approximately 60% H2O2 by weight and is distilled to higher concentrations and diluted to lower concentrations for intended purposes. Diff erent grades of hydrogen peroxide are sold that contain stabilizers and additives dependent on the end use. Food grade hydrogen peroxide comes in 35% and 50% concentrations. It is used for disinfecting purposes and also as an ingredient in cosmetics, shampoos, and medications.
Reagent hydrogen peroxide for chemical and medical laboratories has a concentration of 30%.
Standard grades of 35%, 50%, 60%, and 70% are used for industrial bleaching. General household hydrogen peroxide is 3% H2O2 and 6% is used by beauticians for hair coloring.
Very high grades such as 90% are used as oxidizers in rocket propulsion.
Hydrogen peroxide decomposes over time to water and oxygen. Heat, ultraviolet light, and contaminants accelerate its decomposition, so it should be stored in cool, dark places. Th e foaming of H2O2 when a wound is cleaned is due to its catalytic decomposition from blood.
Low-grade hydrogen peroxide is typically sold in brown bottles to protect the contents from ultraviolet-light exposure. Higher concentrations are stored in expandable or vented contain-ers to accommodate the gases produced during decomposition.
World production of hydrogen peroxide is approximately 2.5 million tons, with approxi-mately 50% used for bleaching in the pulp and paper industry and another 10% for bleach-ing and desizbleach-ing textiles. Th e paper industry uses H2O2 in kraft bleaching. Kraft bleaching is the treatment of wood chips with sodium hydroxide and sodium sulfi de to dissolve the lignin and produce a strong fi brous pulp called kraft pulp (kraft means strong in German).
Kraft pulp is composed of fi bers with the color of brown paper bags or cardboard. Because a number of metals catalyze the decomposition of H2O2, chelating agents are added to pulp mixes to deactivate metals. Bleaching with hydrogen peroxide is due to the formation of the perhydroxyl anion, HOO- under alkaline conditions: H2O2 + H2O → H3O+ + HOO-. Th e use of hydrogen peroxide to augment and replace chlorine in the paper and (textile industry) has environmental advantages and is part of the green chemistry movement.
Hydrogen peroxide has a number of environmental uses. Th ese include water treatment, odor control, oxidation of pollutants, and corrosion control. Hydrogen peroxide is used to remove iron, manganese, and hydrogen sulfi de from water supplies and wastewater. Th e oxidation of substances such as hydrogen sulfi de reduces odors. Because H2O2 decomposes into oxygen and water, it has the added advantage of lowering the biological oxygen demand of wastewater.
Hydrogen peroxide is used in chemical synthesis and can function as both an oxidizing and reducing agent. Caro’s acid (H2SO5) is made using H2O2. Peracetic acid (C2H4O3) is produced by reacting acetic acid and hydrogen peroxide and is used as a disinfectant. Solid bleaching agents such as perborates and percarbonates are made using H2O2. It is used in epoxida tion and hydroxylation reactions. Epoxidation reactions involve the breaking of double bonds in alkenes, with the carbons then bonding to the same oxygen atom to form an epoxide ring
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(Figure 47.2). Hydroxylation introduces hydroxyl groups (-OH) into a compound. Hydrogen peroxide has been used since the 1930s as a propellant and oxidizer in rockets, torpedoes, submarines, and aircraft. Th e Germans initiated the use hydrogen peroxide for weapons and military applications. Th e German Messerschmitt rocket plane and V-2 rockets used H2O2 as a fuel. As a monopropellant (used individually), high concentration H2O2 (> 70%) is catalyti-cally decomposed with a metal in a chamber at high temperature to produce gaseous products for propulsion. Rocket-grade H2O2 has a concentration of 90%. Th e X-15 rocket, which ushered in the start of the Space Age with fl ights starting in 1959, used hydrogen peroxide as a fuel. Th e Mercury, Gemini, and Apollo programs all used hydrogen peroxide for rocket pro-pulsion of the space capsules. Th e lunar landing vehicle was powered and controlled by H2O2 rockets. As a monopropellant, H2O2 is much less effi cient than when used as an oxidizer mixed with other fuels. Th e use of hydrogen peroxide as a propellant was displaced in the 1980s by other more powerful fuels, but it is still used as an oxidizer in rocket systems.
Figure 47.2 Epoxidation process.