were widely used in the late 17th through the early 20th century, and one of them, calomel, was, for a long time, the only treatment for syphilis.
Calomel is "inorganic" mercury strictly speaking, but ingesting it is not usually fatal.
Calomel is an extremely insoluble compound but it is mercur
ous chloride, which is mercury in the +1 oxidation state, aka Hg(I). Actually this ion is a dimer, and the ion is Hg
2+2. Under the right conditions, mercurous Hg(I) can disproportionate, with the mercurous chloride disproportionating into the free metal and the very, very, very, very, very, very toxic Hg
+2 ion a
monomeric species. This ion parent ion found in the most common mercury ore, cinnabar, used since antiquity as an orange pigment, sometimes in tableware. This is HgS, a very insoluble compound, and the mineral form of most of the mercury on earth. (Ironically another comment orange pigment, used until the mid 20th century was an oxide of uranium.) When "roasted" in air, cinnabar reacts to give metallic mercury, which was known since antiquity as "quicksilver."
Mercury metal is not particularly toxic, except as the vapor, since a way to keep mercury reduced to the mercurous state is to keep it in the presence of the metal. It is safe enough that many barometers have been used in laboratories around the world without too much toxicity observed. Thus one can eat mercury metal and often not experience overall immediate toxic symptoms, since much will be excreted as calomel. However, over a long period of time, some Hg
+2 is formed by oxidation of the metal, which accounts for the fact that mad hatters did not immediately go mad as soon as the took the job, but did go mad after decades of work.
The toxicity of mercury is, in fact, related to its affinity for sulfur. It forms irreversible complexes with certain chemicals involved in the Krebs cycle that are important for the catalysis of the oxidation of sugars. A similar effect accounts for the toxicity of cadmium, of solar cell fame, which has very similar chemistry to mercury and forms toxic thiolate complexes in living systems.
Since nerve cells are the most dependent on access to sugar metabolism and oxidation, the toxicity of mercury and cadmium is observed in these cells before other cells, although mercury is toxic to all cells, not just nerve cells.
Mercury also bonds with proteins containing cysteine, often replacing physiologically important magnesium, zinc and copper complexes and inactivating the proteins. Methyl mercury also works in this way.
Environmental sources of mercury are dominated by (roughly in this order): coal, medical and scientific devices, and sewage. The latter source is derived from slow leaching, often as calomel or other halides, from dental amalgams. Although the calomel leachate is usually not especially toxic, once it is passed into the environment, the calomel can and does oxidize to the toxic state.
There has been much discussion of replacing dental amalgams with synthetic polymers of various types, some of which are inorganic polymers.
Mercury
is dangerous fossil fuel waste.
Scrubbing mercury from smokestacks simply creates a mercury
disposal problem. Mercury has no half-life. It stays toxic
forever. There are no permanent mercury repositories that are known to be safe for billions of years. I once proposed a chemical idea for one, but I was more or less joking, although there is no reason to assume it wouldn't work:
http://www.dailykos.com/story/2007/1/3/152128/3838">Radioactive Isotopes from French Commercial Nuclear Fuel Found In Mississippi River.
Interestingly enough, the presence of heavy metals in sewage resulted in the prevention of corrosion in sewer systems in previous times, particularly when it was
legal to dump mercury, lead, and other toxic heavy metal solutions into sewer systems. The malodorous condition of sewage is connected to toxic hydrogen sulfide, which often precipitated mercury in septic systems forming synthetic minerals like cinnabar. (Ore quality deposits often form near sewage outfall pipes in oceans and rivers.) (Iron, copper, lead, zinc and other elements also do this.) The reduction in heavy metal pollution in sewers allowed more sulfur to remain unbonded, whereupon it slowly oxidized to sulfuric acid, causing the accelerated corrosion of metal and concrete pipes.
I am not sure what is being referred to in this accounts as "inorganic mercury." Cinnabar and calomel are not all that toxic, although they certainly qualify as "inorganic mercury."
The acidification of the atmosphere is certainly causing mercury to oxidize more readily than would otherwise be the case. Mercury metal dissolves in common acids to give the +2 ion, although HCl can result under some conditions in passivated mercury with a thin layer of insoluble calomel protecting it from further oxidation. There are lots of common acids around in the environment, particularly nitric acid, which easily oxidizes mercury, and sulfuric acid.
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