Iodine (0.05 M)
Molecular formula: I2
Molecular weight: 126.90
Preparation: Dissolve about 14 g of iodine in a solution of 36 g of potassium iodide in 100 mL of water, add three drops of hydrochloric acid and dilute with water to 1000 mL.
Standardization: Weigh accurately about 0.15 g of arsenic trioxide, previously dried at 105 oC for 1 hour, and dissolve in 20 mL of 1 M sodium hydroxide by warming, if necessary. Dilute with 40 mL of water, add 0.1 mL of methyl orange solution and add drop-wise dilute hydrochloric acid until the yellow colour is changed to pink.
Add 2 g of sodium carbonate dilute with 50 mL of water and add 3 mL of starch solution. Titrate with the iodine solution until a permanent blue colour is produced. Store the solution in amber-coloured, glass stoppered bottles.
As2O3 + 2I2 + 5H2O → 2AsO3−4 + 4I− + 10H+
As2O3 + 6NaOH → 2Na3AsO3 + 3H2O
Na3AsO3 + I2 + H2O → Na3AsO4 + 2I− + 2H+
Starch + I2 → Starch-Iodine (blue colour)
2 moles of I2 ≅ 1 mole of arsenic trioxide
2000 mL of 1M I2 ≅ 197.84 g of AS2O3
1 mL of 0.05M iodine ≅ 0.004946 g of As2O3
Make sure you also check our other amazing Article on: How do you make 0.1 M sodium thiosulphate solution?