Iodometric titration of vitamin C (ascorbic acid)
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 taking place during titration is:
C6H8O6 + I2 → C6H6O6 + 2I- + 2H+
where C6H8O6 is ascorbic acid (vitamin C) and C6H6O6 is dehydroascorbic acid.
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.
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.
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.
To calculate vitamin C solution concentration use EBAS - stoichiometry calculator. Download determination of vitamin C concentration reaction file, open it with the free trial version of the stoichiometry calculator.
Clickbutton above I2 in the input frame, enter volume and concentration of the titrant used. Click button. Read number of moles and mass of ascorbic acid in the titrated sample in the output frame. Click button in the output frame below ascorbic acid, enter volume of the pipetted sample, read ascorbic acid concentration.
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.