Ingredients --
Bleach
We bleach our clothes. We bleach our hair.
We bleach our teeth. We bleach our skin.
We bleach our food.
We use bleaches to disinfect and deodorize.
Some bleaches smell awful. Some, like lemon juice
and sunshine, are refreshing.
Color and bleach
Colored substances contain molecules with
chromophores,
areas of the molecule that have double bonds between carbon
atoms or oxygen atoms. A good example is
beta carotene, and that
section goes into more detail on how molecules become
colored.
Bleaches attack these chromophores in one of two ways.
Oxidizing bleaches like
sodium hypochlorite
break the molecules at the double
bond. This results in either a shorter molecule that
does not absorb visible light, or a molecule whose
chromophore is either shorter or non-existant. A
shorter chromophore will absorb light of a shorter
wavelength than visible light (such as ultraviolet light),
and so does not appear colored.
Reducing bleaches such as lemon juice (in combination with
sunlight) or sulfur dioxide, convert the double bonds
in the chromophore into single
bonds, eliminating its ability to absorb visible light.
Sometimes the reaction is reversible, where oxygen in the
air reacts with the molecule to repair the chromophore, and
the stain returns.
Bleaching clothing
Laundry bleaches fall into two categories. The first is
what are called "chlorine" bleaches. The second are "oxygen"
bleaches.
While pure chlorine gas will bleach colors, in laundry bleaches,
sodium hypochlorite
or
calcium hypochlorite
are actually used, and they work by releasing oxygen, not
chlorine. The chlorine remains in solution, either as
sodium chloride (table salt), or calcium chloride.
These bleaches are made by bubbling chlorine gas through a
solution of sodium hydroxide (lye) or calcium hydroxide (quicklime).
Chlorine gas can be released if the bleach is mixed with an acid.
To prevent this from happening, commercial bleaches leave extra
alkalies in the solution to keep the pH very high (pH 12). This
small amount of extra lye in the solution, along with the caustic
nature of the hypochlorite itself, is what eats away the cloth
if undiluted bleach is spilled on the clothing.
Another chlorine bleach often used is
sodium dichloroisocyanurate.
Oxygen bleaches also work by releasing oxygen.
Hydrogen peroxide
is the active ingredient, either as itself, or as a product
of reacting another ingredient with water to release hydrogen
peroxide.
Oxygen bleaches such as
sodium carbonate peroxide
(also called sodium percarbonate),
sodium peroxide,
or
sodium perborate
are made by reacting molecules with hydrogen peroxide. When the
result is added to water, the hydrogen peroxide is released.
Borax also works by releasing hydrogen
peroxide into the water.
Most oxygen bleaches work best in hot water.
Additives such as
tetra acetyl ethylene diamine
allow the hydrogen peroxide to work in warm water (50° C).
Bleaching hair
Ultraviolet light from the sun is the most common hair-bleaching
agent. Lemon juice is sometimes added to speed up the process of
reducing the double bonds in hair pigments to single bonds.
However, the most famous hair bleach is
hydrogen peroxide
of
peroxide blonde fame. Unlike sunlight and lemon juice,
peroxide is an oxidizing bleach, and its effects are less easily
undone.
The
calcium hypochlorite
or
sodium dichloroisocyanurate used to disinfect swimming pools
also bleaches hair, although (contrary to popular belief) it does
not turn the hair green. It bleaches the hair, allowing the
green copper sulfate in the water to show in the hair. The copper
sulfate comes from the reaction of the copper pipes in the plumbing
to the sulfuric acid used to neutralize the alkalies in the
chlorination chemicals.
Bleaching teeth
Dental bleaches are found in whitening toothpastes and in whitening
gels or strips applied to the teeth.
In toothpastes,
sodium carbonate peroxide
is generally used.
In gels and strips,
carbamide peroxide
is used, often with
tetra acetyl ethylene diamine
as a bleach activator.
All of these products owe their bleaching action to the
hydrogen peroxide
that is liberated when they are applied.
Bleaching skin
Skin lighteners, freckle and age spot removers, and other
remedies for hyperpigmentation are not actually bleaches like
the others listed so far.
The active ingredient is
hydroquinone,
which inhibits melanin formation when applied to the skin.
Since the effect is easily reversed by exposure to sunlight
or ultraviolet light, a
sunscreen
is usually included in the
formula.
Bleaching food
Wheat flour normally becomes white by normal oxidation in air
when stored for a few weeks. To speed up the process,
benzoyl peroxide is used
as a bleaching agent.
Sulfur dioxide is a reducing bleaching agent that is used to
preserve dried fruits.
Disinfecting
Oxidizing bleaches kill microbes by reacting with cell membranes
and cell proteins. The most widely used is
sodium hypochlorite
for household and hospital uses, and
calcium hypochlorite
for drinking water and swimming pool disinfecting.
By Simon Quellen Field