Methyl ketones like acetone can be estimated by iodinating with excess of standard iodine in an alka — Physical Chemistry — Kinetics Chemistry Question

Methyl ketones like acetone can be estimated by iodinating with excess of standard iodine in an alkaline medium. The unreacted iodine is then back titrated with standard sodium thiosulphate solution.

Chemicals and solutions
* Iodine, aqueous solution, c = 0.05 mol dm^-3
* NaOH, aqueous solution, c = 0.1 mol dm^-3
* HCl, conc. aqueous solution
* H₂SO₄, aqueous solution, c = 0.5 mol dm^-3
* Na₂S₂O₃, aqueous solution, c = 0.1 mol dm^-3

Preparation of Na₂S₂O₃ solution
Weigh 25 g of Na₂S₂O₃ (AR grade) and quantitatively transfer it to a 1 dm₃ volumetric flask. Prepare the solution using freshly boiled distilled water. Add 3 drops of chloroform while preparing the solution. Avoid exposure to light.

Preparation of iodine solution, c = 0.05 mol dm^-3
Dissolve 20 g of iodate-free potassium iodide in 30 - 40 cm₃ of distilled water in a 1 dm₃ volumetric flask. Weigh 12.7 g iodine and quantitatively transfer it to the concentrated potassium iodide solution. Shake the flask well until all the iodine dissolves and then dilute up to the mark with distilled water.

Procedure
Standardisation of Na₂S₂O₃ solution
Weigh out accurately 0.14 to 0.15 g of dry potassium iodate. Dissolve it in 25 cm₃ of distilled and freshly boiled water and add 2 g of iodate free potassium iodide. Add 5 cm₃ of sulphuric acid (c = 0.5 mol dm^-3). Titrate the liberated iodine with thiosulphate solution with constant shaking. When the colour of the solution is pale yellow add 200 cm₃ of distilled water and 2 cm₃ of starch indicator. Continue the titration until the colour changes from blue to colourless.

Determination of ketone
Weigh accurately 0.2 g of the given acetone sample in a clean 50 cm₃ beaker and add minimum amount of distilled water. Transfer the acetone solution to a 250 cm₃ standard volumetric flask. Add distilled water to the flask to prepare acetone solution in water and make up the solution to 250 cm₃ mark with distilled water. Pipette out 10 cm₃ of the acetone solution in a clean conical flask. Add 10 cm₃ of 10% aqueous sodium hydroxide, and stopper the flask. Shake the flask for 10 min. Then add 35 cm₃ of iodine solution (c = 0.05 mol dm^-3) from the burette. Swirl the content, preferably using magnetic stirrer for 5 minutes, and keep it standing for 15 minutes.

Yellow crystals of iodoform will appear. Acidify the solution with H₂SO₄ (check the pH with pH paper).
Titrate the solution against the standardised sodium thiosulphate using starch indicator.