Ask A Scientist© Chemistry Archive

   
 >>    name         beth
 >>    status       other
 >>    age          40s

 >>    Question -   I am asking this for my daughter, as I did not want to
 >> put her personal info on the net.  She is 12 and doing a Science Fair
 >> project to see whether "chlorine" bleach is more effective in removing
 >> stains from cotton than "non-chlorine", or "color-safe", bleach.
 >>
 >>She is having difficulty (as am I) in understanding the many different
 >>explanations of how bleach works.  She would like to include how each
 >>product works, with some simple scientific information, as part of her
 >>research.
 >>
 >>The manufacturer told us that their "chlorine" bleach does not really
 >>contain chlorine, but uses its products (I think it was sodium chlorate)
 >>of the reaction of chlorine and caustic soda.  A reference material also
 >>talks about how "the oxidizing effect of chlorine breaks the carbon bonds
 >>of color".  There is other rather confusing information.  We have yet to
 >>get any info on the "color-safe" bleach.
 >>
 >>In summary, could you find a simple way to explain: 1) How "chlorine"
 >>bleach works in removing stains, and 2) How "color-safe" bleach works in
 >>removing stains.  Your help would be GREATLY appreciated!!
 >>

The mecahnism 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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