BLEACHING

Bleaching agents are compounds which are used to remove colour from substances such as textiles, skin, teeth, hair etc. In earlier times textiles were bleached by exposure to the sun and air.

Today most commercial bleaches are oxidizing agents, such as sodium hypochlorite (NaOCl) or hydrogen peroxide (H2O2) which are quite effective in "decolourising" substances via oxidation.

The decolourising action of bleaches is due in part to their ability to remove these electrons which are activated by visible light to produce the various colours.

The hypochlorite ion (OCl-), found in many commercial preparations, is reduced to chloride ions and hydroxide ions forming a basic solution as it accepts electrons from the coloured material as shown on the next slide.

Bleaches are often combined with "optical brighteners". They are capable of absorbing wavelengths of ultraviolet light invisible to the human eye, and converting these wavelengths to blue or blue-green light.

Chlorine, Cl2, reacts with water to produce hypochlorite, OCl-. The position of the equilibrium depends very much upon the pH of the solution.

Cl2(g) + H2O(l) OCl- (aq) + 2H+(aq) +

Cl-(aq)

Carotene with chromophores

Colour in organic materials is the result of light absorption by certain chemical configurations called chromophores in molecules. C=C and C=O bonds are examples of chromophores.

A chromophore, eg C=C and C=O, is a part of a molecule that is able to absorb UV or visible light and producing colour in organic compounds.

Oxidising bleaches break up these double bonds

Reducing bleaches convert double bonds to single bonds

Chlorine water chlorine bleach

• 1.Species responsible for the bleaching

Aqueous sulfur dioxide

HOCl(aq) SO2(aq)

Comparing bleaching actions

Chlorine water chlorine bleach

Aqueous sulfur dioxide

• 2. Is the bleaching by oxidation or reduction?

By oxidation By reduction

Comparing bleaching actions

Chlorine water chlorine bleach

Aqueous sulfur dioxide

• 3. Is the bleaching effect permanent?

Last for a long time

Original colour will be restored later

Comparing bleaching actions

Chlorine water chlorine bleach

Aqueous sulfur dioxide

• 4. Rate of bleaching?

Faster and more vigorous

Slower and milder

Comparing bleaching actions

Chlorine water chlorine bleach

Aqueous sulfur dioxide

• 5. What does it bleach?

Wool, silk, paper, some food

Cotton, linen, some food stains

Comparing bleaching actions

•Production of safe drinking water the world over. Even the smallest water supplies are now usually chlorinated.

•extensively used in the production of paper products, dye stuffs, textiles, petroleum products, medicines, antiseptics, insecticides, foodstuffs, solvents, paints, plastics, and many other consumer products

•most chlorine is used in the manufacture of chlorinated cleaning compounds, pulp bleaching, disinfectants, and textile processing

•manufacture of chlorates, chloroform, carbon tetrachloride

•used for the extraction of bromine

•PVC pipe used to provide safe drinking water

Dioxin enters the environment in several different ways:

•The manufacture of certain herbicides

•The manufacture of bleached paper

The burning of PVC plastics