Oleum

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Oleum refers to a solution of sulfur trioxide in sulfuric acid or sometimes more specifically to pyrosulfuric acid, disulfuric acid.

Oleums can be described by the formula xSO₃.H₂O where x is the total molar sulfur trioxide content. The value of x can be varied, to include different oleums. They can also be described by the forumula H₂SO₄.xSO₃ where x is now defined as the molar free sulfur trioxide content. Oleum is generally assayed according to the free SO₃ content by weight.

A value for x of 2 gives us the empirical formula H₂S₂O₇ of pyrosulfuric acid also called disulfuric acid. Pure pyrosulfuric acid itself is a solid at room temperature, melting at 36 °C and rarely used either in the laboratory or industrial processes.

Oleum is an important intermediate in the manufacture of sulfuric acid. If mainly due to its high enthalpy of hydration when SO₃ is added directly to water, rather than simply dissolving, it tends to form a fine mist of sulfuric acid which is difficult to manage. If SO₃ is added to concentrated sulfuric acid instead, it dissolves readily forming oleum which can then be diluted with water to produce additional concentrated sulfuric acid.

An advantage of oleum is that it can be shipped in bulk liquid railcars, because it is solid in room temperature and thus enhances the safety of the shipment. It can be converted into a liquid at the destination by steam heating. This requires some care to prevent overheating and thus causing evaporation of sulfuric acid gas and overpressurizing. This can cause the overpressure to release from the security valve, causing a general health hazard in the area.

Like concentrated sulfuric acid itself, the acid is such a strong dehydrating agent that if poured onto powdered glucose or virtually any other sugar, it will draw the elements of water out of the glucose in an exothermic reaction, leaving nearly pure carbon as a solid. This expands out of the container, hardening as a solid black substance with gas bubbles in it. Oleum is highly corrosive.

The lead chamber process is notable here because it cannot produce sulfur trioxide or even concentrated sulfuric acid directly due to corrosion of the lead, and absorption of NO₂ gas. So during the reign of this otherwise very successful process and up until the development of the contact process much later, oleum had to be obtained by separate means. Industrially the biggest source of this acid was by the distillation of iron sulfates at Nordhausen, from which the historical name Nordhausen sulfuric acid is derived.