Sodium Thiosulphate (0.1M)
Molecular formula: Na2S2O3
Molecular weight: 158.11
Preparation: Dissolve 25 g of sodium thiosulphate and 0.2 g of sodium carbonate in carbon dioxide-free water and dilute to 1000 mL with the same solvent.
Standardization: Dissolve 0.2 g of potassium bromate, weighed accurately, in sufficient water to produce 250 mL. To 50.0 mL of this solution add 2 g of potassium iodide and 3 mL of 2 M hydrochloric acid and titrate with the sodium thiosulphate solution using the starch solution, added towards the end of the titration, as an indicator until the blue color is discharged [I2 forms a blue colored complex with starch]. Re-standardize the solution frequently.
KBrO3 + HI → HIO3 + KBr
IO−3 + 5I− + 6H+ → 3I2 + 3H2O
3[2Na2S2O3 + I2 → Na2S4O6 + 2NaI]
6Na2S2O3 ≅ 3I2 ≅ HIO3 ≅ KBrO3
6 moles of Na2S2O3 ≅ 1 mole of KBrO3
6000 mL of 1M Na2S2O3 ≅ 167 g of KBrO3
1 mL of 0.1M Na2S2O3 ≅ 0.002783 g of KBrO3
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