Iodometric Titration of Peracids – Determination of Active Oxidant

Procedure:

1. Dissolve ~1.5 g NaI in 50 mL distilled water.

2. Add 5 mL chloroform and 5 mL glacial acetic acid

3. Accurately weigh 0.1 g peracid and add to the above solution. Make sure the

mixture is stirring vigorously.

4. Titrate the mixture with 0.10 M Na2S2O3 solution (NB you need to know the

concentration of this solution accurately – just diluting a random bottle of 1 M

solution, etc. is not ok...)

5. Brown iodine colour fades to yellow then disappears (may be easier to see with

starch). As a rough guide, 0.1 g mCPBA should need 9-11 mL 0.100 M Na2S2O3.

Chemistry:

RCO3H + 2I- + 2CH3CO2H → RCO2H + I2 + 2CH3CO2- + H2O (I2 liberation by peracid)

I2 + 2 S2O32- → 2 I- + S4O6

2- (Titration of I2 by thiosulphate)

So, mol RCO3H = mol I2 = ½ mol S2O32-

Example calculation for mCPBA (MW 172.57):

• 0.1007 g mCPBA = 0.584 mmol (if concentration is 100%)

• Titrated with 9.00 mL of 0.100 M Na2S2O3 solution

• mol Na2S2O3 = conc. x vol. = (0.100 mol dm-3) x (9.00 x 10-3 dm3) = 0.900 mmol

• mol mCPBA = mol I2 = ½ mol S2O32- = (0.900mmol / 2) = 0.450 mmol

• % mCPBA = 0.450 mmol / 0.583 mmol = 77%

Starch indicator solution (0.3% w/v)

• Add 150 mg starch (Chemstore 19:4) to 50 mL distilled water and heat until almost

transparent.

• Allow to cool to room temperature before use. Store in the dark / prepare

immediately before use.

• Add ~2 mL to the titration mixture just before the endpoint.