Oleum

Oleum (Latin oleum = "oil"), or fuming sulfuric acid, is a solution of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Oleum is idenfied by the CAS number 8014-95-7.

Oleums can be described by the formula ySO₃.H₂O where y is the total molar sulfur trioxide content. The value of y can be varied, to include different oleums. They can also be described by the formula H₂SO₄.xSO₃ where x is now defined as the molar free sulfur trioxide content. Oleum is generally assessed according to the free SO₃ content by mass. It can also be expressed as a percentage of sulfuric acid strength; for oleum concentrations, that would be over 100%. For example, 10% oleum can also be expressed as H₂SO₄.0.13611SO₃, 1.0225SO₃.H₂O or 102.25% sulfuric acid. The conversion between % acid and % oleum is: % acid = 100 + 18/80 × % oleum

A value for x of 1 gives the empirical formula H₂S₂O₇ for disulfuric (pyrosulfuric) acid. Pure disulfuric acid itself is a solid at room temperature, melting at 36 °C and rarely used either in the laboratory or industrial processes.

Oleum is produced in the contact process, where sulfur is oxidized to sulfur trioxide which is subsequently dissolved in concentrated sulfuric acid. Sulfuric acid itself is regenerated by dilution of part of the oleum.

The lead chamber process for sulfuric acid production was abandoned partly because it could not produce sulfur trioxide or concentrated sulfuric acid directly due to corrosion of the lead, and absorption of NO₂ gas. Until this process was made obsolete by the contact process, oleum had to be obtained through indirect methods. Historically, the biggest production of oleum came from the distillation of iron sulfates at Nordhausen, from which the historical name Nordhausen sulfuric acid is derived.

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