It can be prepared by the following methods
1.Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.
BaO2.8H2O (s) + H2SO4 (aq) → BaSO4 (s) + H2O2 (aq) + 8H2O (l)
2.Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.
This method is now used for the laboratory preparation of D2O2.
K2S2O8 (s) + 2D2O (l) → 2KDSO4 (aq) + D2O2 (l)
3.Industrially it is prepared by the autooxidation of 2-alklylanthraquinols.
In this case 1% H2O2 is formed. It is extracted with water and concentrated to ~30% (by mass) by distillation under reduced pressure.
It can be further concentrated to ~85% by careful distillation under low pressure. The remaining water can be frozen out to obtain pure H2O2.