Iodometric titration of vitamin C (ascorbic acid)

general remarks

Vitamin C - or ascorbic acid - is an important antioxidant. In the cells it is easily oxidized to dehydroascorbic acid, removing oxidizing agents before they can do damage to other substances present. This reaction is the basis of the iodometric titration of ascorbic acid - it is quantitatively oxidized by iodine.

reaction

Reaction taking place during titration is:

C₆H₈O₆ + I₂ → C₆H₆O₆ + 2I^- + 2H⁺

where C₆H₈O₆ is ascorbic acid (vitamin C) and C₆H₆O₆ is dehydroascorbic acid.

sample size

For 0.05 M titrant and assuming 50 mL burette, aliquot taken for titration should contain about 0.61-0.80 g of ascorbic acid (3.5-4.5 millimoles). Note, that such amounts of the ascorbic acid are present in vitamin C tablets, but they may require unusually large samples of juices and fruits. Thus in the case of analysis of natural products it may be reasonable to use more diluted titrant solution or smaller burette. If you have no idea what amount of vitamin C can be present in the sample, start with titration of small volume of the juice or small mass of the fruit pulp, and adjust amount used in the final titrations accordingly to the initial result.

end point detection

To detect titration end point we will use a standard indicator for iodine titrations - starch. As initially there is no iodine present, there is no risk of iodine strongly bonding with the starch, so we can add indicator at the very beginning of the titration. See iodometric titration end point detection for a more detailed explanation.

solutions used

To perform titration we will need titrant - 0.05 M iodine solution, and indicator - starch.

procedure

Depending on the sample type, preparation procedure will differ.

• In the case of tablets, simply weight them and dissolve in distilled water.
• In the case of juices, just transfer the required volume to the beaker. If there are solid parts, filter them before titration.
• In the case of fruits, blend them with distilled water (about 100g of fresh fruits and 50 mL of water). Filter the mixture. Wash the filtrate with a few milliliters of distilled water, filter again. Fill up to 100 mL in the volumetric flask.

Remember, that you may need to do a test titration before deciding on the sample size.

• Pipette aliquot of the ascorbic acid solution into Erlenmeyer flask (100 or 200 mL, depending on the sample volume).
• Add 5 mL of starch solution.
• Titrate with iodine solution until a faint blue color persists after 20 seconds of swirling the solution.

result calculation

According to the reaction equation given above 1 mole of ascorbic acid reacts with 1 mole of iodine and this ratio have to be used for titration result calculation.

sources of errors

Apart from general sources of titration errors, when titrating with iodine we should pay special attention to titrant. Iodine solutions are not stable and they should be standardized every 2-3 months.