Bleaching with Hydrogen per oxide (H2O2):

Hydrogen peroxide (H2O2) is a simplest peroxide (compound with oxygen-oxygen single bond). The pure hydrogen peroxide is a colourless liquid, slightly viscous than water. It is strong oxidizing agent and used as a bleaching agent and disinfectant. It was first used as bleaching agent in 1920's. Today, it is estimated that 90 to 95% of cotton and cotton/synthetic blends are bleached with hydrogen peroxide. For commercial use the the hydrogen peroxide is available 35 to 70% concentration. It is ecologically acceptable and economically feasible.  

Mechanism of Hydrogen per Oxide:

Hydrogen peroxide is weak acid and in water it is ionize to form a hydrogen ion and a perhydroxyl  ion   as per the given equation:

H2O2  +  H2O----------------H+         +       HOO- (Perhydroxyl ion) 

This perhydroxy ion is the active hydrogen peroxide and unstable and break down as per the following equation: 

HOO(-) ---------------  OH(-)    +   O (Active and Nascent oxygen)

The bleaching, most probably takes place via the reaction of HOO(-) or O with coloured material (Like grey cotton) where by they oxidized into colourless compounds. 

Since the amount of HOO(-) at pH 7 is extremely small, neutral solutions of hydrogen peroxide are quite stable.The catalytic effect of transition metal ions (e.g. Cu++, Fe+++) on hydrogen peroxide, accelerating its decomposition and may cause great damage to the fibre.

In order to activate hydrogen peroxide for the bleaching reaction to take place, a base is added to the bleaching bath. The base will shift the equilibrium dissociation to the right:

NaOH    +   H2O2 ----Equilibrium-----------------Na (+)      +   HOO (-)   + H2O

Working of  bleaching agents: Let us discuss working of following bleaching agents:


 i) Sodium hypochlorite (NaOCl)

ii) Hydrogen peroxide (H2O2)

iii) Sodium Chlorite (NaClO2)

      Before bleaching, cotton fabric should be prepared well i.e it should be desized (if woven and having sizing chemicals) and scoured. If  material is not prepared well then there are chances of uneven bleaching effect.

i) Sodium Hypochlorite (Manufacture, Reaction and Mechanism): It is a stronger oxidative bleaching chemicals used in textile industry. This manufacture by bubbling chlorine into a solution of sodium hydroxide.

2NaOH + Cl2 --------------   NaOCl  + NaCl

when acid is added to a hypochlorite   solution,   chlorine gas is liberated

NaOCl + HCl---------------- NaCl  +  Cl2    +  H2O

        Commercial NaOCl mostly contains 12-15% active chlorine while household bleach contain around 5% active chlorine. NaOCl is strong alkaline having pH around 12 and free caustic present in the solution act as a stablizer. To make it stable, it should be kept in dark room. 

NaOCl  +  H2O----------------- Na + +  OCl- (Hypochlorous ion)

OCl- + H2O-------------------- HOCl  +  OH-

Hypochlorous acid (HOCl) is a active bleaching agent.