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Sulfides are molecules with one or more isolated sulfur atoms, each having a surplus of two electrons. These make up the negative portion, or "anion," of the sulfide — the corresponding positive portion being the "cation." One simple example is potassium sulfide, chemical formula K2S; the two potassium atoms each carry a +1 charge, the one sulfur atom, a -2 charge. A somewhat similar molecule, called potassium hydrogen sulfide or potassium bisulfide, chemical formula KHS, has for its anion portion the two-atom unit (HS)-1, which carries only one added electron. It should be noted that the words bisulfide and disulfide are not synonymous, as is illustrated by the common automotive lubricant, molybdenum disulfide, chemical formula MoS2, which contains no hydrogen in its structure.
Some compounds may be erroneously identified as bisulfides — among these is the solvent carbon disulfide, CS2. Those compounds more closely similar in structure to bisulfide yet not bisulfide are the sulfur-containing alcohol analogues called thiols or mercaptans; these are sometimes used as artificial odorants for natural gas, to prevent accidental asphyxiation. Of commercial importance among the correctly identified bisulfides are sodium and ammonium bisulfide. Sodium bisulfide is used in large quantities in the Kraft paper manufacturing process. Ammonium bisulfide, on the other hand, is a major source of corrosion damage, particularly to the petroleum industry’s desulfurization hydrotreaters.
The prefix "bi-" dates back more than a century, and suggests the neutralization of only one of two protons — hydrogen ions — of a "diprotic" acid. For example, sulfuric acid (H2SO4) possesses two protons or hydrogen atoms, capable of being replaced. If not one but both are replaced by sodium hydroxide, for example, the result is the sulfate, Na2SO4. Reducing the amount of sodium hydroxide, however, results in the replacement of only one hydrogen atom to form sodium bisulfate, NaHSO4. In this latter instance, even though the product is a salt, since one hydrogen atom remains, it is also an acid — sodium bisulfate is an acid-salt.
Hydrogen sulfide gas, H2S, behaves in similar fashion. Although weak, it too is a diprotic acid, which dissolved in water ionizes in two steps. The first step produces hydrosulfide ions: H2O + H2S → (H3O)+1 + (HS)-1. Step two is the further breakdown, H2O + (HS)-1 → (H3O)+1 + S-2. If desired, two products may be produced — partial neutralization gives the bisulfide: NaOH + H2S → NaHS + H2O, while complete neutralization gives the sulfide: 2 NaOH + H2S → 2 NaS + 2 H2O.
Additional, well known diprotic acids are carbonic and tartartic acids — H2CO3 and H2(C4H4O6), respectively. These can be neutralized either partially or completely to produce sodium bicarbonate or sodium carbonate; or, in the case of tartaric acid, sodium bitartrate or sodium tartrate. If preferred, the neutralization can be carried out in stages. Using a different base in step two then gives a "double-salt," one notable example being the common food additive historically known as Rochelle salt, potassium sodium tartrate.
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