Ceric Ammonium Sulphate (0.1 M)
Molecular formula: (NH4)2Ce (SO4)2⋅2H2O
Molecular weight: 632.56
Preparation: Dissolve 65 g of ceric ammonium sulphate, with gentle heat, in a mixture of 30 mL of sulphuric acid and 500 mL of water. Cool, filter the solution, if turbid, and dilute to 1000 mL with water.
Standardization: Weigh about 0.2 g of arsenic trioxide accurately, previously dried at 105°C for 1 hour, and transferred to a 500 mL conical flask. Wash down the inner walls of the flask with 25 mL of an 8.0% w/v solution of sodium hydroxide, swirl to dissolve, add 100 mL of water and mix. Add 30 mL of dilute sulphuric acid, 0.15 mL of osmic acid solution, 0.1 mL of ferrroin sulphate solution and slowly titrate with the ceric ammonium sulphate solution until the pink colour is changed to a very pale blue, adding the titrant slowly towards the end-point.
Oxidation: As2O3 + 2H2O ⇌ As2O5 + 4H+ + 4e−
Reduction: 4[Ce+4 + e− ⇌ Ce+3]
4 Ce+4 ≅ As2O3
4 moles of (NH4)2Ce(SO4)2⋅2H2O ≅ 1 mole of arsenic trioxide
4000 mL of 1M (NH4)2Ce(SO4)2⋅2H2O ≅ 197.81 g of As2O3
1 mL of 0.1 M (NH4)2Ce(SO4)2⋅2H2O ≅ 0.004946 g of As2O3
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