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There is no doubt that sulfuric acid, H₂SO₄, is one of the crucial technical compounds in modern civilisations. The breakthrough for the industrial production of the acid was achieved in the late 18th Century when the so-called ‘lead chamber technique’ was invented. Nowadays, the world production of sulfuric acid exceeds 200 million tonnes, and most of the compound is produced by the so-called ‘contact process’. The acid is needed for the production of fertilisers, as an electrolyte for accumulators, for the processing of mineral oars, and for various other purposes. With respect to the importance of sulfuric acid, one could argue that there should be a large knowledge of its properties. However, many aspects of the chemistry of sulfuric acid and its salts are still not known and further studies are necessary. Some examples should illustrate this statement.

Reactivity

Sulfuric acid is a highly acidic and oxidising medium. Thus, the acid is usually handled in chemical inert equipment made from glass or noble metals. However, as we could show, even these materials can be attacked by the acid. For example, at high temperature even elemental platinum, a typical metal for crucibles, reacts with sulfuric acid under formation of the platinum sulfate Pt₂(SO₄)₂(HSO₄)₂. The same is true for palladium and rhodium, which are oxidised under comparable conditions to sulfates like Pd(S₂O₇) and Rh₂(SO₄)₃. Glass, the typical lab equipment for handling sulfuric acid, may also react with it, especially if the SO₃-rich variety of the acid, oleum, is used. These reactions lead to the formation of the unique tris-(disulfato)-silicate anion [Si(S₂O₇)₃]^2-.

Polysulfates

Sulfuric acid is able to dissolve quite large amounts of its own anhydride, namely sulphur trioxide, SO₃. It is believed that the reaction of sulfuric acid and SO₃ leads to the formation of larger sulfuric acids of the general composition H₂S_nO_3n+1. They are called polysulfuric acids, but in fact, besides H₂SO₄ (n=1), only disulfuric acid H₂S₂O₇ (n=2) is known up to now. However, the long chain acids and most of their salts are still elusive. We could show that at least polysulfates could be prepared if high SO₃ concentrations are applied in the reactions. We have thus far reported polysulfates up to a chain length of six, and theoretical investigations revealed that this chain length might be the limit of stability for these types of compounds. Thus, it will be interesting to figure out what the chemical nature of even more SO₃-rich sulfuric acid is.

Taming sulfuric acid

The oxidation power of sulfuric acid might cause problems for applications. For purposes which first require a strong acidity and a low oxidation power, it is fashionable to use a derivate of sulfuric acid, the so-called ‘methane sulfonic acid’ CH₃SO₃H (MSA). The compound is especially used for electroplating processes because it provides high proton conductivity but does not oxidise the deposited metals. However, the full elucidation of its reactivity and a study of its compounds are still scarce for this acid. We could show that MSA might still be a remarkable oxidant when used at high temperature. For example, elemental tin, a typical metal for electroplating, is readily oxidised by the acid, leading to the methanesulfonate Sn(CH₃SO₃)₂. Further derivatisation of sulfuric acid in order to modify its properties is achieved if the CH₃ group of MSA is substituted by other organic scaffolds. The larger scaffolds also bear the option to introduce further sulfonic acid groups. In this way, polysulfonic acids Y(SO₃H)n (Y=organic scaffold) are gained, a new class of compounds for building co-ordination polymers.

Conclusion

Sulfuric acid and its derivatives are of utmost importance for technical processes. Therefore it is mandatory to explore the chemistry of these acids and their salts in great detail. These investigations are an ongoing process for our group, and our recent report gave a hint of what might be discovered in the next years.

Professor Dr Mathias S Wickleder
University of Giessen
Institute of Inorganic and Analytical Chemistry

www.uni-giessen.de/cms/fbz/fb08/inst/iaac/wickleder