Commonly contains added MeOH. Addition of KOH soln (1 mole KOH: 100 moles (CAS NO. )) to 40% formaldehyde soln, or evaporation to dryness, gives paraformaldehyde polymer which, after washing with water, is dried in a vacuum desiccator over P₂O₅ or H₂SO₄. Formaldehyde is regenerated by heating the paraformaldehyde to 120 °C under vacuum, or by decomposing it with barium peroxide. The monomer, a gas, is passed through a glass-wool filter cooled to -48 °C in CaCl₂/ice mixture to remove particles of polymer, then dried by passage over P₂O₅ and either condensed in a bulb immersed in liquid nitrogen or absorbed in ice-cold conductivity water.