Bleaching

Classification of Bleaching

• Cause by physical property

• Cause by oxidizing property

• Cause by reducing property

• Cause by synthesis(non redox)

physical propertyExample 1 : Al(OH)3. the Aluminum hydroxide can be regared as colloid and the Aluminum hydroxide colliod is particles with possitive charge. We know the particles with posstive chaege can adsorb the particles with negative charge. So the Aluminum hydroxide colliod is able to clean the pigment with negative charge.

Example 2 : activated carbon. One gram of activated carbon can have a surface area in excess of 500 m2. So the activated carbon can adsorb other particles as many as possible. In history seaman use it to clean the honey and alcohol. It also used to eliminate the the big molecular in solution

oxidizing propertyThe oxidizing property of compound can can make some organic pigment bleached.

First example : Cl2

The chlorine can bleach the organic pigment, because the reaction water in solution. Cl2 + H2O == HCl + HClO . This is a disproportionation reaction which means the chlorine is both oxidizing agent and reducing agent. The oxidizing property of chlorine element is expressed by HClO. Since the chlorine usually gain electrons in reaction. But in HClO the oxidation number is 1+ so the chlorine have a tendency to gain electrons to become Cl-, the stable structure of chlorine atom.

Second example : HClO

The hypochlorous acid is weak acid, so it is not very stable. Usually we use its salt NaClO or Ca(ClO)2, when need to use it , just put it in air. It will soon change into hypochlorous acid .

2NaClO + CO2 + H2O == Na2CO3 + 2HClO

Ca(ClO)2 + CO2 + H2O == CaCO3↓ + 2HClO

And, the hypochlorous acid has a strong oxidizing property, we use it to bleach organic pigment. And the decomposition of hypochlorous acid .

2HClO == 2HCl + O2↑ (见光死 )

Third example : ClO2

The chlorine dioxide has irritant smell. It is so sensitive to heat and friction that it can easily exploded. Its solubility in water is 5 to 8 times lager than chlorine. The chlorine dioxide major has several way to produce.

2NaClO2 + Cl2 == 2NaCl + 2ClO2↑

2NaClO3 + 4HCl（浓） == 2NaCl + Cl2↑+ 2ClO2↑+ 2H2O

2NaClO3 + Na2SO3 + H2SO4 == == 2ClO△ 2↑+ 2Na2SO4 + H2O

Then its oxidizing also express in state of HClO when it react with water.

4ClO2 + H2O == 4HClO + 3O2↑

This process produce the hypochlorous acid which has the strong oxodizing property.

+ === +

Fourth example : O3

Ozone is the allotropy of oxygen, the ozone’s structure is three oxygen atoms bind to each other, each two make up a single covalent bound, and the angle of the particlej is 116.79 degrees. The ozone is also unstable because the single bound is not strong enough. The ozone is so active so active that its oxidizing property is just a little bit weaker than florine(F2), it can easily oxidize the matter that oxygen can not oxidize.

2Ag + 2O3 == Ag2O2 + 2O2

3PbS + 4O3 == 3PbSO4

In industry ozone is used to bleach the flour and pulp, because ozone docompose into O2 which is no damage to human body. Ozone is also have the ablity to blaech organic pigment such as indigotin(靛蓝 )。

indigotin

Fifth example : H2O2

Hydrogen peroxide is an example of the “peroxide” , all peroxides have strong oxidizing property. Peroxide all have -O-O- base. They can be divided into 4 groups : metal peroxide, hydrogen peroxide, peroxide salt and organic peroxide. The -O-O- base can easily change to water, so the hydrogen peroxide can bleach the clothes easily. Here are equations showing the property of hydrogen peroxide.

H2O2 + MnO2 == H2MnO4 H2MnO4 + H2O2 == MnO2 + O2 + 2H2O

H2O2 + SO3 == H2SO4

H2O2 + 2KI + 2HCl == 2H2O + I2↓ + 2KCl

Other oxidizing agent

The mater that strong oxidizing property is usually a good bleaching agent. Other compound like peroxyacetic acid, sodium perborate, sodium thiosulphate, sodium pyrophosphate and sodium Percarbonate also can be use as bleaching agent.

sodium perborate : NaBO3 + HCl + H2O == NaCl + H2O2 + HBO2

sodium thiosulphate : Na2S2O3 + HNO3 + NaOH == S↓+ Na2SO3 + NaNO3 + H2O

Na2S2O3 + H2SO4 == Na2SO4 + SO2↑ +S↓+ H2O

sodium pyrophosphate : (Na4P2O7) 焦磷酸钠

peroxyacetic acid : (CH3COOOH)过氧乙酸

Reducing propertyNot as oxidizing property, the compounds use reducing property are only small a amount.

Example one : SO2

Sulfur dioxide can react with some pigment to produce colorless compound, but this production is not stable, so when it is heated, the origin color will come back, a common example is the bleaching of megenta solution. This process is not a redox reaction.

Sulfur dioxide also use its reducing property to bleach other compound. Like clorine, bromine and potassium permanganate.

SO2 + Cl2 + 2H2O == H2SO4 + 2HCl

SO2 + Br2 + 2H2O == H2SO4 + 2HBr

5SO2 + 2KMnO4 + 2H2O == K2SO4 + 2MnSO4 + 2H2SO4

Example two : C2H2

Unsaturated hydrocarbon can use addition reaction to absorb HR or halogen, such as Cl2, Br2. not only ethylene, other organic compund also have this ablity.

C2H2 + Br2 → CH2BrCH2Br

C2H2 + HCl → CH3CH2Cl

C2H2 + HBr → CH3CH2Br