Bleach

Other types of bleaches

beeswax, skin and in a number of other industries.

In the food industry, some flour bleaching and maturing agents.

oxygen are used in bleaching sequences in the pulp industry to produce totally chlorine free (TCF) paper.

Not all bleaches have to be of an oxidizing nature. reducing agent in some bleaching formulas. It is commonly used to bleach wood pulp used to make newsprint.

Hazards and concerns

Since bleaches are strong oxidizing agents, they can be quite hazardous, especially when reacted with other common household chemicals.

Mixing sodium hypochlorite with acids like vinegar or drain cleaners containing chlorine. Hypochlorite and chlorine are in equilibrium in water, the position of the equilibrium is pH dependant and low pH (acidic) favors chlorine,^[1]

Cl₂ + H₂O $\rightleftharpoons$ H⁺ + Cl^- + HClO

ppm (8-hour time-weighted average - 40 hour week) by OSHA in the US.^[2]

Sodium hypochlorite and nitrogen trichloride (NCl₃). These materials are very irritating to eyes and lungs and are toxic above certain concentrations.

NH₃ + NaOCl --> NaOH + NH₂Cl

NH₂Cl + NaOCl --> NaOH + NHCl₂

NHCl₂ + NaOCl --> NaOH + NCl₃

Additional reactions produce hydrazine, in a variation of the Olin Raschig process.

NH₃ + NH₂Cl + NaOH --> N₂H₄ + NaCl + H₂O

The hydrazine generated can further react with the monochloramine in an exothermic reaction:^[1]

2 NH₂Cl + N₂H₄ --> 2 NH₄Cl + N₂

Industrial bleaching agents can also be sources of concern. For example, the use of elemental chlorine in the bleaching of wood pulp produces organochlorines, persistent organic pollutants, including chlorine dioxide in these processes has reduced the dioxin generation to under detectable levels.^[4]

Chemistry

The process of bleaching can be summarised in the following set of chemical reactions:

Cl₂(aq) + H₂O(l) $\rightleftharpoons$ H⁺(aq) + Cl^-(aq) + HClO(aq)

The H⁺ ion of the hypochlorous acid then dissolves into solution, and so the final result is effectively:

Cl₂(aq) + H₂O(l) $\rightleftharpoons$ 2H⁺(aq) + Cl^-(aq) + ClO^-(aq)

How bleaches work

Color in most chromophores. Chemical bleaches work in one of two ways:

An oxidizing bleach works by breaking the visible light.

A reducing bleach works by converting single bonds. This eliminates the ability of the chromophore to absorb visible light.^[5]

Sunlight acts as a bleach through a process leading to similar results: high energy ultraviolet range, can disrupt the bonds in the chromophore, rendering the resulting substance colorless. Extended exposure often leads to massive discoloration usually reducing the colors to white and typically very faded blue spectrums.^[6]

References

^ ^a ^b Cotton, F.A; G. Wilkinson (1972). Advanced Inorganic Chemistry. John Wiley and Sons Inc. ISBN 0-471-17560-9.
^ Occupational Safety & Health Administration (2007). OSHA -- Chlorine. OSHA. Retrieved on 2007-08-26.
^ Rizk-Ouaini, Rosette & Ferriol, Michel; Gazet, Josette; Saugier-Cohen Adad, Marie Therese (1986), " ", Bulletin de la Societe Chimique de France 4: 512–21
^ ECF: The Sustainable Technology. Alliance for Environmental Technology. Retrieved on 2007-09-19.
^ Field, Simon Q (2006). Ingredients -- Bleach. Science Toys. Retrieved on 2006-03-02.
^ Bloomfield, Louis A (2006). Sunlight. How Things Work Home Page. Retrieved on 2006-03-02.