The HEI-Group

Page 4.3: HYDROGEN PEROXIDE - General Information

Background History and Properties of Hydrogen Peroxide

Hydrogen peroxide was discovered in 1818 by the French chemist Louis Jacques Thenard.

A peroxide is a chemical compound characterized by the presence of two linked oxygen atoms. There are several hundred different peroxides but only a few have commercial value.

The best known and most widely used peroxide is hydrogen peroxide. The chemical formula is H2O2. It has a molecular weight of approximately 34. It is a colourless liquid that freezes at about the same temperature as water. Very pure solutions are stable but trace contaminants cause this compound to decompose and in most cases evolve oxygen. The following is a summary of its properties:

Description	Value
mp (oC)	-0.41
bp (oC)	150.20
S.G. (at 25 oC & 1 atmos.)	1.4425
viscosity (@ 20 oC - cP)	1.245
specific heat (J/g)	2.628

Hydrogen peroxide is normally sold as an aqueous solution in the range of 3 to 90% by weight. A trace of a stabilizing agent such as sodium stannate is usually added to inhibit decomposition. This compound has been used as a rocket fuel (90 wt% +), bleaching agent, deinking compound, household antiseptic (in dilute solutions), and as an environmentally friendly pollution control agent. It also has applications in the metals and mining industry and the organic and inorganic manufacturing industry (eg. PVC plasticisers and stabilizers). With regards to its environmental applications it has been used for water disinfection; the reduction of H2S gas, BOD , COD, nitrites, cyanides, NOx, SO2, amines, phenols, reduced sulphur compounds, and free chlorine; the control of odours and water colour; solids flotation assistant; arsenic recovery; activated sludge conditioning, denitrification control in secondary clarifiers. The majority of the commercially produced hydrogen peroxide is to bleach natural and synthetic fibers and paper.

The following is a summary of the properties of aqueous hydrogen peroxide:

wt% H2O2 in liquid	Melting Point (oC)	Boiling Point (oC)	wt% H2O2 in vapour	Specific Gravity
10	- 6.4	101.7	0.9	1.0324
30	-25.7	106.2	4.2	1.1081
50	-52.2	113.8	13.0	1.1914
70	-40.3	125.5	33.4	1.2839
90	-11.5	141.3	75.0	1.3867

The hydrogen peroxide is usually stored and/or transported in aluminium or polyethylene lined drums and aluminium tank cars and trucks are used to ship grades containing up to 52% H2O2. Grades containing over 52% H2O2 are shipped in special double-headed aluminium drums, or aluminium tank trucks or tank cars.

An interesting property of peroxides is their ability to form radicals when they decompose and hence follow a free radical chemical addition and substitution reactions. The reactions of hydrogen peroxide are generally exothermic. For example:

Reaction	dH
HOOH -> H. + .OOH	dH=380 kJ/mol
HOOH -> 2.OH	dH=210 kJ/mol

All the alkali metals and alkali earth metals, as well as many others can form peroxides. The most important is sodium peroxide, a pale yellow solid used as an oxidizing and bleaching agent.

Hydrogen peroxide can act as both an oxidising and reducing agent. The oxygen in this compound is in an intermediate state between free oxygen and metal oxides. In the decomposition of hydrogen peroxide, both water and molecular oxygen are formed. This process is known as auto-oxidation and is catalyzed by impurities in the sample. The following is a summary of the chemical properties:

Description	Reaction(s)
Decomposition (if contaminants or UV light present)	2H2O2 -> 2H2O + O2
Molecular Additions	H2O2 + Y -> YH2O2
Substitutions	H2O2 +RX -> ROOH + HX H2O2 + 2RX -> ROOR + 2HX
Oxidations	H2O2 + W -> WO + H2O
Reductions	H2O2 + Z -> ZH2 + O2 H2O2 + Q -> QH + O2
Dissociation	H2O2 -> H+ + HO2-
Peroxone Process	H2O2 + O3 -> 2OH. + 3O2

where:

Y could be metal peroxides, nitrogen compounds, etc.
RX could be a halogenated organic molecule
W could be a metal
Z could be a group 6 radical or compound
Q could be group 7 compounds, hypochlorites, etc.
OH. is the hydroxyl free radical

Manufacture of Hydrogen Peroxide

Hydrogen peroxide is prepared primarily by anthraquinone auto-oxidation processes. The process is derived from the Riedl-Pfleiderer process. The following are the reaction equations:

[to be added next week]

A 2-alkylanthraquinone dissolved in a suitable solvent is reduced catalytically to the corresponding anthrahydroquinone. The anthraquinone is commonly called the reaction carrier or working material, whereas the anthraquinone-solvent mixture is called the working solution. The working solution containing the anthrahydroquinone is separated from the hydrogenation catalyst and aerated with an oxygen containing gas (such as air) to reform the anthraquinone and simultaneously produce hydrogen peroxide. The hydrogen peroxide is extracted with water and the aqueous solution purified and concentrated to the required degree. The extraction solution is recycled. Although the chemical yield of hydrogen peroxide is very high, secondary reactions necessitate regeneration of the solution and the hydrogenation catalyst and removal of organic material from the extracted hydrogen peroxide. Usually the catalyst that is used is made of palladium, however, less effective catalysts such as nickel can also be used. Figure 1.0 shows the manufacturing process schematic.

[Figure 1.0 to be addded next week]

Other manufacturing processes are as follows:

Nitrogen compound auto-oxidation.
Direct combination of hydrogen and oxygen (electrolysis).
Metal oxides plus sulphuric acid and roasted in hot air (ie. barium + O2 + H2SO4).
Distillation of hydrogen peroxide from ammonium persulphate solutions.
Organic based processes (isopropanol + O2 -> H2O2 + acetone).
Photochemical reaction of ozone with water.

Health and Safety Concerns

Hydrogen peroxide is a high energy material and a strong oxidising agent.

It is irritating to skin, eyes, and mucous membranes. Use excess water to wash down areas of contact.

Hydrogen peroxide decomposes with the generation of heat and oxygen. Decomposition is promoted by catalytic impurities. Containers and equipment should be constructed of suitable materials, be vented, and maintained free of contaminants. Decomposition hazards increase with increasing concentration. Concentrations of greater than 86% can detonate, but only with a high energy ignition source. Temperatures should not exceed 115 oC. The lower explosive limit is 26 mol%. The hazard of explosion is usually present in organic oxidations with high strength hydrogen peroxide. Contact of hydrogen peroxide with many inorganic reagents should be avoided because of the potential for an explosive reaction (eg. mercurous oxide).

If you have any questions please send e-mail to John at: jahibberd@hei-group.com

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Revised on July 23, 2006