INTRODUCTION TO HYDROGEN PEROXIDE

HYDROGEN PEROXIDE

INTRODUCTION

Hydrogen peroxide, a weakly acidic colorless liquid, was discovered by Thenard in 1818 and has been uses industrially since the mid-19th century. He coined the famous term”eau oxygene'e” to express his belief that it was oxygen diluted into water!

Its scale of manufacture and use have increased markedly since about 1925 when electrolytic processes were introduced to the United States and industrial bleach applications were developed. Now prepared primarily by anthraquinone autoxidation (where a molecule is oxidized by oxygen), hydrogen peroxide is used widely to prepare other peroxygen compounds and as a nonpolluting oxidizing agent. Its main applications are pulp bleaching (50% of demand), chemical synthesis, and textile bleaching.

APPLICATIONS

Hydrogen peroxide (H₂O₂) is a weakly acidic, colorless liquid, miscible with water in all proportions. It is the simplest peroxide (molecules containing two oxygen atoms covalently bonded to one another) and is commercially available in aqueous solution over a wide concentration range.

The main uses of hydrogen peroxide are in the preparation of other peroxides and as an oxidizing agent. It can however act as a reducing agent for strong oxidants. When it decomposes, it forms water and releases oxygen, which makes it an attractive “environmental friendly” product. It is a “clean oxidant”.

H₂O₂ → H₂O + ½ O₂

Its oxidizing action is used for:

·                     bleaching paper pulp,

·                     bleaching textile and plant fibers,

·                     manufacturing chemical compounds and preparing other oxidants,

·                     destroying pollutants and toxic substances,

·                     metal surface treatment, minerallurgy, and uranium hydrometallurgy.

It also features outstanding disinfectant and antiseptic properties which are exploited in many applications.

The major uses of peroxide in Pakistan utilize its strongly oxidizing nature to oxidize various chemical groups. These oxidizable groups primarily include lignins, cyanides, sulphides and phenols (benzyl alcohols). Hydrogen peroxide can react to form HO^- or HOO^- or other species depending of the conditions chosen. Pulp bleaching generally uses alkaline pH to generate HOO^-:

H₂O₂ + HO^- → HOO^- + H₂O

Phenol destruction normally uses peroxide to generate free radicals. This can be achieved by either UV light or metallic catalysis (Fenton's Chemistry).

Some industrial uses of hydrogen peroxide are shown in Table.

Industry	Application
Pulp and paper	Bleaching wood pulp
Mining	Detoxification of cyanide tailings
Textile bleaching	Bleaching of cotton fabrics
Wool scouring	Bleaching of wool
Waste water treatment	Measuring dissolved oxygen. Destroying soluble cyanides, sulfides and phenols
Packaging	Aseptic packaging of milk and fruit juice

 PHYSICO-CHEMICAL PROPERTIES

The physico-chemical properties of hydrogen peroxide solutions are given in the following tables.

Property	Value
freezing point / oC	-52
boiling point / oC	114
density at 25oC / g mL-1	1.4425
viscosity at 20oC / mPa s	1.17
surface tension at 20oC / mN m-1	80.4
specific conductance at 25oC / Ω.cm	4 x 10-7
enthalpy of fusion / J g-1	367.52
specific heat at 25oC / J g-1	2.628
heat of vaporization at 25oC / J g-1	1517
pKa at 20oC	11.75
heat of dissociation / kJ mol-1	34.3

  

CHEMICAL PROPERTIES

Hydrogen peroxide is best known for its use as an oxidizing agent.

Its strong oxidizing potential allows it to oxidize a large number of organic and inorganic compounds.

It can however also act as a reducing agent for strong oxidants.

This ability to act both as reductant and oxidant allows hydrogen peroxide to react in a wide range of applications. With water as it’s only by-product, hydrogen peroxide is ideal for chemical reactions or syntheses where by-products would be undesirable.

Some of the more significant reactions of hydrogen peroxide are listed below.

·                     Decomposition

Hydrogen peroxide decomposes, particularly under the influence of metal catalysts or in basic medium, into water and gaseous oxygen in an exothermic reaction:

2H₂O₂ → 2H₂O + O₂

·                     Oxidation

In this type of reaction, hydrogen peroxide can oxidize organic or inorganic compounds:

H₂O₂ + W → WO + H₂O

·                     Reduction

Hydrogen peroxide acts as a reducing agent for powerful oxidizing agents, in a reaction yielding gaseous oxygen.

H₂O₂ + Z → ZH₂ + O₂

·                     Transfer of Peroxide Group

The peroxide group in hydrogen peroxide is transferred to another molecule, for example in the case of the preparation of per-acids:

H₂O₂ + RCOOH → RCOOOH + H₂O

·                     Formation of Addition Products

Hydrogen peroxide can graft onto other molecules to form addition products which are similar to hydrates.

Sodium perborate:                                       NaBO₂, H₂O₂, 3 H₂O

Sodium percarbonate:                                2 Na₂CO₃, 3 H₂O₂

Urea peroxide:                                              CO(NH₂)₂, H₂O₂

·                     Molecular addition

H₂O₂ + Y → Y.H₂O₂

·                     Substitution

H₂O₂ + RX → ROOH + HX

H₂O₂ + 2RX → ROOR + 2HX

Where R, W, X, Y, Z represent various functional groups or molecules.