Answer to Question #66049 in General Chemistry for Ambre

Question

For a personnel project I did a Acid/Base titration of NaOH and orange juice, as well as a redox titration of iodine and the same juice.

My purpose was to find the amount of ascorbic acid. However I am extremely confuse on which confounding variable might affect my result. I knew citric acid (for example) might interfere in the Acid/Base titration. However, I though it would not be a problem for the Iodine titration as Ascorbic acid was a greater reducing agent. Apparently not. Apparently citric acid is too.

How can I verify this? Is there a way to make sure Citric acid do not interfere, and does it interfere at all?

Thank you very much for your help.

Answer 1

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.
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.
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.