Standardisation of 0.1M sodium thiosulphate using potassium iodate.

Prepare an approximately 0.1M solution of sodium thiosulphate, Na₂S₂O₃.5H₂O, by dissolving approximately 6.2g of the salt in 250ml of distilled water. Store the solution in a dark place.

Weigh about 0.89g the potassium iodate accurately into a 250ml standard flask and dissolve it with distilled water before making it up to the mark of the flask. Pipette 25ml of the potassium iodate solution, add about 1g of potassium iodide and 5ml of 1M sulphuric acid titrate with sodium thiosulphate until it is only slightly dark in colour. Add a drop or two of starch indicator. Iodine-starch complex is dark-blue in colour. Continue the titration until the solution turns clear.

From the equations it is obvious that the active compound is I₂ but it is very difficult to prepare an iodine solution of an accurately known concentration, as iodine solid is very volatile. Potassium iodate is no problem as it can be purchased in a highly pure form and is stable at ambient condition.

• HYDROGEN PEROXIDE

  H₂O₂ + 2I‾ + 2H⁺   ®   I₂ + 2 H₂O

• ANTIMONY (III) in the presence of sodium bicarbonate.
  Sb₂O₃ + 2I₂ + 2 H₂O   ®   Sb₂O₅ + 4 HI

  This reaction is reversible and the bicarbonate is used to remove the HI to prevent the reaction proceeding to the left.

  HI + NaHCO₃   ®   NaI + H₂CO₃

  The carbonic acid is easily removed by warming the solution to give carbon dioxide gas and water.

• TIN(II)
  

  Sn⁺² + I₂   ®   Sn⁺⁴ + 2 I‾

• IRON (III)
  

  2 Fe⁺³ + 2 I‾   ®   2 Fe⁺² + I₂

• SULPHITE
  

  SO₃‾² + I₂ + H₂O   ®   2 H⁺ + 2 I‾ + SO₄‾²