How does peroxide, baking soda, and soap get rid of skunk smell
?
(by Paul Krebaum, Chemical Engineer)
To get one ingredient out of the way,
the soap's just there
to help wet the fur and get the oily skunk spray into solution
where it can react with the baking soda and peroxide. What happens next
is all chemistry, but I'll try to keep it very simple.
First we need a basic understanding of skunk spray. It is mostly
composed of chemical compounds called
thiols, and acetate
derivatives of same. Chemists represent thiols (
or
mercaptans as they used to be called) with the letters
R-S-H. "R" represents the rest of the carbon and hydrogen atoms in the
molecule, "S" is a sulfur atom, and "H" is hydrogen. I've put in dashes
to show the bonds between them.
The human nose is very sensitive to low molecular weight thiols, a few
parts per billion in some cases. Thiols are often found in wastes, and
in food which is contaminated or spoiled. Your nose is right above your
mouth. So if something you're about to eat doesn't "smell right",
little alarm bells should be going off in your brain saying "whoa !
don't eat that or you'll get sick !".
This ability to sense/warn us of very trace amounts of thiols has been
exploited by your local natural gas utility. Natural gas is naturally
odorless after refining. Just imagine what would happen if you couldn't
smell a natural gas leak. A lot of explosions, not to mention the gas
that would be wasted. So they put thiols in the gas to make it stink.
On purpose. Methane thiol is the simplest organic thiol. The simplest
thiol of all is hydrogen sulfide, which is inorganic. Hydrogen sulfide
gives rotten eggs their smell.
Methane thiol has one carbon atom and three hydrogens in the "R".
Ethane thiol has two carbons and five hydrogens in the "R". Skunk spray
has (mostly) 4-6 carbons in the "R".
H3C-CH2-S-H Ethane thiol
H3C-CH2-S-O-C(=O)-CH3 Ethane thioacetate
"Notepad" really isn't good for showing chemical structures, and
Deltapoint's HTML editor butchers Notepad even further, so the more
complex ones in skunk spray won't be shown here. Actually, all we're
concerned about is the S-H part of the molecule, nothing happens to the
"R" group ! Don't let the thioacetate confuse you, it's really a thiol
in disguise. If you add water to a thioacetate, it splits into a thiol
and acetic acid.
Many theorize that this explains the observed
mysterious "return to stinkiness" of a rain-soaked pet after the
traditional tomato juice treatment.
Now that you know all this, let's move on to the baking soda and
hydrogen peroxide.
The baking soda is there to raise the pH of
the brew, that is, to make it more alkaline. I should say
"just alkaline enough". Baking soda is a very mild alkali and won't eat
holes in your skin like a strong alkali (e.g. lye) would. Raising the
pH does three things:
1) It rapidly splits thioacetates into thiols and
acetate.
2) It accelerates the reaction between thiol and peroxide.
3)
It neutalizes the sulfonic acid produced by 2), above.
Peroxide reacts with thiols in a number of steps,
gradually going through the multiple oxidation states of sulfur
chemistry until the end product, a sulfonic acid, is produced. This is
neutralized to the sodium salt by baking soda. I'm not going to go
through all of them in Notepad, but will try to illustrate with a few
well- known compounds, using methane thiol (R=CH3) as an example.
The first step in the oxidation is actually a coupling of the thiol to
a disulfide.
2 { RSH } + 1/2 O2 ----} RSSR + H2O (notepad does not do arrows,
subscripts, etc...)
Whether the S-S bond is broken next or one of the sulfur atoms is
further directly oxidized is unclear.
It is known that if one gets to a sulfenic acid RSOH that these are
extremely unstable and rapidly oxidize to a sulfinic acid, R-S(=O)-OH
and these are stable enough to be isolated. The oxidation then
progresses to the sulfonic acid R-S(=O)(=O)-O-H, and these are very
stable indeed. The only reaction after that is a simple neutralization
of the acid with the baking soda to form the sodium salt.
Some other simple sulfur compounds in various oxidation states:
H3C-S(=O)-CH3 dimethyl sulfoxide (topical analgesic)
H3C-S(=O)(=O)-CH3 Dimethyl sulfone (in brocolli)
H3C-O-S(=O)(=O)-O-CH3 Dimethyl sulfate (toxic alkylating agent)
CH2=CH-CH2S(=O)-S-CH2CH=CH2 Allicin (in garlic), antibacterial
CH3SO3H Methanesulfonic acid (catalyst, electroplating chemical)