3. Bleaching with Sodium Chlorite (NaClO₂):

Bleaching process with Sodium Chlorite is carried out in acidic conditions. During the process yellow-brownish ClO₂ gas is released which is toxic in nature. The bleaching action is supposed to be carried out by this gas. When a solution of NaClO₂ is acidified, ClO₂, hypochlrous acid (HClO₂), sodium chlorate (NaClO₃) and NaCl are formed. ClO₂ and HClO₂ are used as bleaching species. 

The optimum pH for bleaching with sodium chlorite is 3 to 5. Which can be achieved using formic, acetic or phosphoric acid.  Below pH 3, there may be severe damage to the cotton fibre. Bleaching rate increases if the temperature is around 90^oC. Below 50 ^oC, there is a negligible bleaching effect.

This bleaching agent can be used for acrylic, polyester and rayon. It removes all kitties, wax etc from cotton and also the feel of the fabric is very soft. It gives better whiteness than hydrogen peroxide. 

There are some disadvantages also associated with this bleaching agent. It creates pollution as ClO₂ liberate during process. It also corrodes equipment. This bleaching agent cannot be used for continuous process as H₂O₂. 

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.