ANSWERS: 1
  • The mechanism of action of both chlorine and non-chlorine bleaches is similar. Substances are colored because they contain electrons that can absorb visible light and jump to higher energies. The kinetic energy of the electrons in a molecule can only have certain very specific values, a phenomenon known as "quantization." An electron can absorb a photon of light only if the energy of the photon (which depends on its wavelength, or color) is the difference between the electron's final and initial energies. Many, but not all, colored molecules absorb visible light because their electrons are bound in a system of linked carbon-carbon double bonds. Oxidation of such molecules breaks these bonds, ruining the light-absorbing properties of the molecule. The bleaches are oxidizing agents. This action does not work for some stains, such as blood. The red substance in blood, hemoglobin, is not decolorized by oxidation; in fact, it is red only when oxidized. But oxidizing agents destroy enough colored compounds that they make good bleaches. To say that chlorine bleach does not contain chlorine is true, but somewhat misleading. True, it does not contain diatomic chlorine gas, CL2. It does contain sodium hypochlorite, NaOCl, which is one of the products produces when aqueous sodium hydroxide (caustic soda) reacts with Cl2. (The other product is sodium chloride, NaCl.) It is a very strong oxidizing agent. Some non-chlorine bleaches contain slightly weaker oxidizing agents, which will oxidize the colored molecules in many common stains, but not the robust pigments of commercial textile dyes. That's what makes them "color-safe." It's a trade-off: if the stain is a tough molecule (such as turmeric, the vegetable dye used to make mustard bright yellow), the strength of bleach required to oxidize it will also destroy the textile's color”. Richard Barrans Jr., Ph.D. Chemical Separations Group Chemistry Division CHM/200 Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439

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