Sulfuric acid has its two hydrogen atoms bonded to oxygen, ionizes in two stages, and is a strong diprotic acid. In aqueous solution, loss of the first hydrogen (as a hydrogen ion, H+) is essentially 100 percent. The second ionization takes place to an extent of about 25 percent, but HSO4− is nonetheless considered a moderately strong acid. Because it is a diprotic acid, H2SO4 forms two series of salts: hydrogen sulfates, HSO4−, and sulfates, SO42−. The sulfates of the alkaline-earth metals—calcium (Ca), strontium (Sr), and barium (Ba)—as well as that of lead (Pb) are virtually insoluble, and these salts are found as naturally occurring minerals. These important minerals include gypsum (CaSO4 · 2H2O), celestine (SrSO4), barite (BaSO4), and anglesite (PbSO4). These insoluble salts can be prepared in the laboratory by metathesis reactions. A metathesis reaction is one in which compounds exchange anion-cation partners. For example, if a solution of barium nitrate, Ba(NO3)2, is added to a solution of sodium sulfate, Na2SO4, a precipitation of barium sulfate, BaSO4, occurs. This is an important reaction because it can be used as both a qualitative and quantitative test for the sulfate ion and the barium ion. (Qualitative tests are used to determine the presence or absence of a substance, whereas quantitative tests are used to measure the amount of a constituent.) In addition to metathesis reactions, sulfate salts can generally be prepared by dissolution of metals in aqueous H2SO4, neutralization of aqueous H2SO4 with metal oxides or hydroxides, oxidation of metal sulfides (a sulfide contains S2−) or sulfites (SO32−), or decomposition of salts of volatile acids, such as carbonates, with aqueous H2SO4. Some important soluble sulfate salts are Glauber’s salt, Na2SO4 · 10H2O; Epsom salt, MgSO4 · 7H2O; blue vitriol, CuSO4 · 5H2O; green vitriol, FeSO4 · 7H2O; and white vitriol, ZnSO4 · 7H2O.
Reactions and uses
Pure H2SO4 undergoes extensive self-ionization (sometimes called autoprotolysis).
2H2SO4 → H3SO4+ + HSO4−
This autoprotolysis reaction is, however, only one of the equilibrium reactions that occur in pure H2SO4 to give it an extremely high electrical conductivity. There are three additional equilibrium reactions that take place because of the ionic self-dehydration of sulfuric acid.
2HSO4 ⇌ H3O+ + HS2O7−
H2O + H2SO4 ⇌ H3O+ + HSO4−
H2S2O7 + H2SO4 ⇌ H3SO4+ + HS2O7
Thus, there are at least seven well-defined species that exist in “pure” H2SO4. The value of the dielectric constant of the acid is also quite high (ε = 100).
Concentrated sulfuric acid is not a very strong oxidizing agent unless it is hot. When it acts as an oxidizing agent, however, it can be reduced to several different sulfur species, including SO2, HSO3−, SO32−, elemental sulfur (S8), hydrogen sulfide (H2S), and the sulfide anion (S2−). Concentrated sulfuric acid is a good dehydrating agent, as it reacts with many organic materials to remove the elements of water.
The amount of sulfuric acid used in industry exceeds that of any other manufactured compound. In the United States approximately 67 percent of the acid is utilized to convert phosphate rock to phosphoric acid. The phosphoric acid is then converted to phosphate fertilizers. Other major uses include the refining of petroleum, the removal of impurities from gasoline and kerosene, the pickling of steel (the cleaning of its surface), and the manufacture of other chemicals, such as nitric and hydrochloric acids. It also is utilized in lead storage batteries and in the production of paints, plastics, explosives, and textiles.
Sulfurous acid and sulfite salts
When sulfur dioxide is dissolved in water, an acidic solution results. This has long been loosely called a sulfurous acid, H2SO3, solution. However, pure anhydrous sulfurous acid has never been isolated or detected, and an aqueous solution of SO2 contains little, if any, H2SO3. Studies of these solutions indicate that the predominant species are hydrated SO2 molecules, SO2 · nH2O. The ions present in these solutions are dependent on concentration, temperature, and pH and include H3O+, HSO3−, S2O52−, and perhaps SO32−. However, “sulfurous acid” has two acid dissociation constants. It acts as a moderately strong acid with an apparent ionization of about 25 percent in the first stage and much less in the second stage. These ionizations produce two series of salts—sulfites, containing SO32−, and hydrogen sulfites, containing HSO3−. Only with large cations, such as Rb+ (rubidium) or Cs+ (cesium), have solid HSO3− salts been isolated. Attempts to isolate these salts with smaller cations tend to yield disulfites as a product of dehydration. 2HSO3− ⇌ S2O52− + H2O
With the exception of the alkali metal sulfites, these salts are relatively insoluble. The HSO3− ion has an interesting structure in that the hydrogen atom is bonded to the sulfur atom and not to the oxygen atom, as might be expected. There is some suggestion that in solution both the sulfur-hydrogen and oxygen-hydrogen structures may exist in equilibrium with one another, but there is no concrete evidence for this phenomenon. Heating solid hydrogen sulfite salts (shown by the equation above) or passing gaseous sulfur dioxide into their aqueous solutions produces disulfites. HSO3−(aq) + SO2 → HS2O5−(aq) Disulfite ions possess a sulfur-sulfur bond and are therefore unsymmetrical. Addition of acid to the solution of HS2O5− above does not produce “disulfurous acid” (H2S2O5) but instead regenerates HSO3− and SO2. “Sulfurous acid” solutions can be oxidized by strong oxidizing agents, and oxygen in the air slowly oxidizes the solution to the more stable sulfuric acid. 2H2SO3 + O2 + 4H2O → 4H3O+ + 2SO42− Likewise, solutions of sulfites are susceptible to air oxidation to produce solutions of sulfates. Sulfites and hydrogen sulfites are moderately strong reducing agents. For example, the reaction with iodine (I2) is quantitative (i.e., proceeds nearly to completion) and can be used in volumetric analysis. HSO3− + I2 + H2O → HSO4− + 2H+ + 2I− Sodium sulfite is used in the paper-pulp industry and as a reducing agent in photographic film development.