The general purification methods listed under are applicable. The o- and p-isomers can be removed by their selective oxidation when a rn-xylene sample containing them is boiled with dilute HNO₃ (one part conc acid to three parts water). After washing with water and alkali, the product can be steam distd, then distd and purified by sulfonation. m-Xylene (CAS NO. ) is selectively sulfonated when a mixture of xylenes is refluxed with the theoretical amount of 50-70% H₂SO₄ at 85-95 °C under reduced pressure. By using a still resembling a Dean and Stark apparatus, water in the condensate can be progressively withdrawn while the xylene is returned to the reaction vessel. Subsequently, after cooling, then adding water, unreacted xylenes are distd off under reduced pressure. The m-xylene sulfonic acid is subsequently hydrolysed by steam distn up to 140 °C, the free m-xylene being washed, dried with silica gel and again distd. m-Xylene is stored over molecular sieves Linde type 4A.