Thiols ("mercaptans") are organic
sulfur compounds similar to phenols, but
with a sulfur atom in place of the oxygen
in phenol. Besides the four thiols that combine
to put the stink in skunk, thiols flavor
garlic or chopped onions. Traces of thiols
are part of the aroma of coffee, but other
thiols are added to natural gas, which may
be quite odorless, to give it the warning
odor of cooking gas.
Thiols are pretty transient in a filtered
aquarium with some water movement, but they
are so powerfully skunky that it doesn't
take more than a few molecules to give a
tank a stale, skanky, vaguely foxy or "gassy"
odor. Methyl mercaptan is the thiol responsible
for low-tide "swamp gas" odors
of exposed mud. Aquarists smelling thiols
are apt to think they are smelling hydrogen sulfide.
Thiols are so easily oxidized that a little
extra splash from the filter return or slight
venting of the glass tank cover may be enough
to eliminate them. Hydrogen peroxide and
other oxidants convert thiols to non-odorous
disulfides and then to mild sulfonic acids.
Thiols readily bind to heavy metal ions in
the water. Their alternative name "mercaptans"
inherited from the early days of chemistry,
registers that thiols are mercurium captans-- capable of "siezing mercury."
