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References

How does it WORK?

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

Reducing bleaches convert double bonds to single bonds

The normal electronic configuration of a molecule is known as the ground state (which is the most stable state of an electron). When a molecule absorbs light of a particular wavelength, it can be promoted from its highest occupied molecular orbital () to its lowest unoccupied molecular orbital (LUMO), i.e. from its original orbital in the ground state to a higher orbital. Such a transition is called an electronic transition. The molecule is then in an excited state.

E = hc/l

Where E is energy of radiation

Double bonds that are separated by just one single bond are said to be conjugated double bonds. On the other hand, isolated double bonds occur when 2 or more single bonds are found between the ends of the double bonds.

    conjugated double bonds

                      (more stable than isolated double bonds)

(isolated double bond)

Conjugated double bonds interact with each other and systems containing conjugated double bonds tend to be more stable than similar systems containing isolated double bonds.

Because conjugation raises the energy of the and lowers the energy of the LUMO, electronic transitions are easier for conjugated systems than non-conjugated systems. The more conjugated double bonds there are in a compound, the less energy is required for the electronic transition and therefore the longer the wavelength at which the transition takes place. This corresponds to the region of the visible spectrum of light. Hence making such conjugated system highly coloured.

If a compound has enough conjugated double bonds, it will absorb visible light. White light is a mixture of all possible wavelengths of visible light. If any colour is removed from white light, the remaining light appears coloured. So, if a compound absorbs any visible light, it will appear coloured. Its colour depends on the color transmitted to the eye. In other words, it depends on the colour produced from the wavelengths of light that are not absorbed.

b-Carotene, a precursor of vitamin A, is an orange substance found in carrots, apricots and sweet potatoes. Lycopene is red and is found in tomatoes, watermelon and pink grapefruit.

b-Carotene

Lycopene

There are a few common examples of chemical agents used as bleaches.

They are (click for more detail):

1. Chlorine

2. Sulphur  dioxide

7. Sodium perborate

1) CHLORINE- Containing bleaches

Cl-Cl

The green coloured spheres are the individual chlorine atoms.

Manufacture:

Nearly dry slaked lime readily absorbs chlorine to form bleaching powder.

2CaO+Cl₂   2CaClO

Chemical Aspects of chlorine

Fully chlorinated bleaching powder is a complex mixture of a probably basic calcium hypochlorite and a non-deliquescent basic calcium chloride:

3Ca(ClO)₂.2Ca(OH)₂.2H₂O

Ca(ClO)₂.2Ca(OH)₂

CaCl₂.Ca(OH)₂.H₂O

Ca(ClO)₂ + CaCl₂.Ca(OH)₂.H₂O

Bleaching powder is partially soluble in water, the hypochlorite dissolving and being responsible for oxidising and bleaching powers. On boiling the aqueous solution, calcium chlorate(I) and calcium chloride are formed. One method of assaying bleaching powder involves the measurement of the volume of oxygen liberated from a known mass of bleaching powder acting in a suspension of approximately 5-volume hydrogen peroxide:

OCl^- +H₂O_{2 ---->} Cl^- + H₂O+ O₂

Available chlorine is a measure of the oxidising power of the hypochlorite ion, determined by titrimetric analysis. Acidification of bleaching powder leads to the following changes:

Ca(ClO)₂ + CaCl₂.Ca(OH)₂.H₂O + 6H⁺

3Ca²⁺ + 5H₂O+ 2Cl₂

Cl₂ + H₂O -> HCl + HClO

                          (a)                                                            (b)

Taken from "Qualitative Analysis—Handbook for O-level Chemistry" by Peter S.P. Lim & B.H. Yeap

Picture (b) shows both litmus papers are decolourised (bleached) when held at the mouth of the test tube containing heated Clorox solution.

The process of bleaching by bromine is similar to that of chlorine. It is less oxidising but more stable than chlorine water.

Iodine is only very slightly soluble in water and its oxidising and bleaching powers are feeble.

2) Sulphur dioxide gas, SO₂

The yellow coloured sphere is the sulphur atom whereas the red atoms are the oxygen atoms.

Other than SO₂, sodium hydrosulphite (Na₂S₂O₄) is also used to bleach wool. However, bleaching using SO₂ is not permanent. On exposure to air and light, the original colour slowly returns as oxygen in air oxidises the bleached substance.

3)Potassium Manganate(VII), KMnO₄

4)Hydrogen Peroxide, H₂O₂

70% of all H₂O₂ manufactured is used to bleach fibres and paper. However H₂O₂ is most widely used for bleaching cotton cloth due to the strong oxidising properties of H₂O_2. On the other hand, H₂O₂ can also be used as an antiseptic and a bleaching agent for hair. It is sold as a solution labeled ’20 volumes’ (as an antiseptic) or ’30 volumes’ (for bleaching hair). One advantage of using hydrogen peroxide for bleaching is that the only waste product is water.

5)Sodium Peroxide, Na₂O₂

Sodium peroxide, a compound of H₂O₂ is a pale yellow solid with oxidising and bleaching properties. This type of bleach is called the oxygen-releasing bleach. They are not as active as chlorine bleach but they are better for synthetic fibers. More bleach, hotter water and a more alkaline solution is required than using a chlorine bleach but they work just as well when used properly.

6)Borax Na₂B₄O₇

Borax, also known as sodium pyroborate, Na₂B₄O₇, is often added to oxygen-releasing bleaches because of their ability to clean. Its pH is about 9.5, so it produces a basic solution, which makes the bleach work better.

It is the most common bleaching chemical added to powdered detergents. It works by converting some water molecules into H₂O₂, an oxidising agent in warm water. Activators are added to make perborates work better at lower temperatures. They are less harmful than chlorine-containing bleaches.