iodometric titration of peracids – determination of active … titration.pdf · iodometric...
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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.