Bleaching
Classification of Bleaching
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Cause by physical property
Cause by oxidizing property
Cause by reducing property
Cause by synthesis(non redox)
physical property
Example 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 property
The 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 ==△== 2ClO2↑+ 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 :
peroxyacetic acid :
(Na4P2O7) 焦磷酸钠
(CH3COOOH)过氧乙酸
Reducing property
Not 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