Hydrogen peroxide (H2O2)

Hydrogen peroxide, H2O2, is a low-cost, clean, mild oxidising agent, commercially sold as a 30% aqueous solution. It can be prepared by four distinct routes: (i) treating hydrated barium peroxide with ice-cold dilute sulphuric acid at low temperature (273 K), which precipitates barium sulphate (filtered off) and leaves H2O2 solution behind, BaO2.8H2O(s) + H2SO4(aq) →(273 K) BaSO4(s) + H2O2(aq) + 8H2O(l); (ii) the Merck process — adding a small quantity of sodium peroxide to ice-cold dilute sulphuric acid with stirring, Na2O2(aq) + H2SO4(aq) →(273 K) H2O2(aq) + Na2SO4(aq); (iii) subjecting a 50% sulphuric acid solution to electrolytic oxidation, which forms peroxydisulfuric acid at the anode, 2HSO4⁻ →(electrolysis) H2S2O8 + 2e⁻, and then hydrolysing that acid to release hydrogen peroxide, HO-SO2-O-O-SO2-OH + 2H2O → 2H2SO4 + H2O2 (this same electrolytic method can also be extended to prepare heavy hydrogen peroxide, D2O2, in the laboratory); (iv) industrially, by air-oxidising 2-ethylanthraquinol to give H2O2 plus 2-ethylanthraquinone, with the anthraquinol regenerated afterwards by catalytically hydrogenating the anthraquinone (Pd catalyst) so the cycle can repeat.

Properties: pure H2O2 is a very pale blue liquid, boiling point 272.4 K; it is miscible with water and forms a hydrate, H2O2.H2O. Because concentrations vary, the strength of an aqueous H2O2 solution is conventionally expressed in 'volume' units — the commercially sold 30% (by mass) solution has a volume strength of 100 volume, meaning 1 mL of that solution liberates 100 mL of oxygen gas at STP (worked out for a different concentration in Problem 8.8). H2O2 can act as either a mild oxidising agent or a mild reducing agent depending on what it reacts with: in acidic medium it oxidises Fe²⁺ to Fe³⁺, 2Fe²⁺(aq) + 2H⁺(aq) + H2O2(aq) → 2Fe³⁺(aq) + 2H2O(l); and in acidic medium it reduces permanganate, 2MnO4⁻ + 6H⁺ + 5H2O2 → 2Mn²⁺ + 8H2O + 5O2. …