- Chlorine
Cl2 in aqueous solution forms hypochlorous acid or chlorate (I) acid [HOCl] which is a powerful oxidising agent.
Cl2(g) + H2O(l) Û HCl(aq) + HOCl(aq)
If a coloured, oxidisable material is presented, HOCl releases its oxygen to oxidise the material to a colourless compound. This process is known as bleaching.
HOCl(aq) + (dye) à HCl(aq) + (dye+O)[colourless]
Liquid bleach is usually aqueous sodium hypochorite (NaOCl), a sodium salt of HCl acid, and dry powdered bleach contains chloride of lime [calcium hypochlorite—Ca(OCl)2].
Chlorine can destroy silk and wool, therefore commercial hypochlorite bleaches should never be used on silk fibres. Instead, hydantoin and cyanurate bleaches, which release chlorine slowly in water, are used. Hence there is less chance of damaging the fabrics.
The process of bleaching by bromine is similar to that of chlorine.
SO2(g) in moist conditions is a bleaching agent.
à colourless compoundDye-oxygen
In the reducing action of SO2, oxygen is removed from coloured dye.
SO2 is used to bleach wool and silk. Salts of SO2 are also used in bleaching. Calcium hydrogen sulphite Ca(HSO3)2 is used to whiten wood pulp in the manufacturing of paper. 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, original colour slowly returns as oxygen in air oxidises the bleached substance.
- Others
KMnO4 is used to remove colour from fabrics that are able to tolerate strong oxidation due to its powerful oxidising property.
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 H2O2. Sodium peroxide, a compound of H2O2 is a pale yellow solid with oxidising and bleaching properties.
Primitive ways of Bleaching
The method of bleaching by exposure to the sun was
practised during ancient and medieval times in Egypt, China, Asia and Europe. In the 18th
century, bleaching solutions of potash and lye and of dilute sulphuric acid were
used in Holland and France, and in 1785 the powerful bleaching properties of chlorine were
discovered by the French chemist Claude Louis Berthollet. Bleaching powder Ca(OCl)2,
produced by the reaction of chlorine gas on slaked lime, with the same effect as chlorine,
was introduced in 1799 by the Scottish chemist Charles Tennant. Ca(OCl)2 was
used the standard bleaching agent until about 1925. Since that time the use of liquefied
bleach sodium hypochlorite (NaOCl), has largely replaced bleaching powder.
Powdered Bleach
Liquefied Bleach
