Two methods for the determination of hydrogen peroxide.

 Hydrogen peroxide can behave either as an oxidising agent or as a reducing agent; this is because the oxygen is in oxidation state (-1), which can go up or down. The ordinary decomposition of hydrogen peroxide into oxygen and water is a disproportionation reaction.

The reducing properties of H₂O₂ are used in its determination with potassium manganate(VII), its oxidising properties in the determination via an iodine titration.

Prepare a solution of hydrogen peroxide HP for use in both titrations by pipetting 10.0 cm³ of 20-volume hydrogen peroxide into a 250 cm³ graduated flask. Make to the mark with pure water and mix well.

 Potassium manganate(VII) method.

The half-reactions are:

MnO₄ ^- + 8H⁺ + 5e ^- à Mn²⁺ + 4H₂O

H₂O₂ à O₂ + 2H⁺ + 2e ^–

        Method:

        Pipette 25.0 cm³ of HP into a 250 cm³ conical flask, add about 25 cm³ of bench sulphuric acid, and titrate with standard 0.02 mol dm^-3 potassium manganate(VII) solution until the mixture just becomes permanently pink.

        Repeat to obtain three consistent titres.

        Results:

Mean titre/cm³:

Hence find the concentration of the original hydrogen peroxide solution in mol dm^-3.

 Iodometric method.

The half-reactions are:

2I^- à I₂ + 2e^-

H₂O₂ + 2H⁺ + 2e^- à 2H₂O

The reaction of iodine with thiosulphate ions is

I₂ + 2S₂O₃^2- à S₄O₆ ^2- + 2I ^–

         Method:

        Pipette 25.0 cm³ of HP into a 250 cm³ conical flask, and add about 40 cm³ bench sulphuric acid, about 1 g of solid potassium iodide, and 3 drops of a 3% solution of ammonium molybdate. The latter reagent catalyses the otherwise rather sluggish reaction. Swirl to dissolve the potassium iodide and liberate the iodine, and titrate the mixture with 0.1 mol dm ^-3 sodium thiosulphate solution until the mixture is a pale straw colour. Add a little starch solution, and continue the titration until the deep blue colour changes to colourless. (If the starch is added too soon in iodine titrations, an insoluble complex is formed between the starch and the iodine and the titre is too low.)

        Repeat to obtain three consistent titres.

        Results:

Mean titre/cm³:

Hence find the concentration of the original hydrogen peroxide solution in mol dm^-3. Explain what is meant by the term ‘volume strength’ for hydrogen peroxide. This idea is not used for any other material.

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