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Iodometric titration of vitamin C (ascorbic acid)

Potentiometric titrations

Permanganate titration

Iodometry

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Potentiometric titration » Iodometric titration of vitamin C (ascorbic acid)

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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.

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

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.

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.

Click n=CV button above I2 in the input frame, enter volume and concentration of the titrant used. Click Use button. Read number of moles and mass of ascorbic acid in the titrated sample in the output frame. Click n=CV 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.


Page was last modified on September 09 2009, 12:33:59.

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