Question

Brown iodine stains can be removed (I2 to I-) from clothes using a concentrated solution of sodium thiosulfate, Na2S2O3. In this reaction, the thiosulfate ion is converted to the tetrathionate ion, S4O62-. Both of these ions are colourless.

a) Explain how the thiosulfate ion is able to remove the iodine stain. Accompany your answer with a balanced equation.

b) Which species in this reaction is acting as the reducing agent? Why?

Answer 1

a) I₂ + 2e^- $\to$ 2I^-

2S₂O$_3^{2-}$ $\to$ S₄O$_6^{2-}$ + 2e^-

2Na₂S₂O₃ + I₂ $\to$ 2NaI + Na₂S₄O₆

b) SO$_3^{2-}$ act as a reducing agent

Because the electrons lose by thiosulphate can be accepted by iodine .