Determination of vitamin C in food by iodometry

Vitamin C or ascorbic acid (C ₆ H ₈ O ₆ ) is one of the essential nutrients in our diet. In addition to being essential for the prevention of scurvy, a serious disease that can cause death, this substance represents a powerful antioxidant capable of protecting us against cancer, it gives a boost to our immune system by protecting us from infectious diseases and is even capable of prevent cataracts.

All of these benefits, plus others we didn’t mention, make determining the amount of vitamin C in different foods extremely important. Despite the fact that there are different very precise and fast instrumental analytical techniques, the determination by means of redox titration continues to be one of the most widely used standard methods thanks to its low cost and easy implementation.

Here, we present a protocol for the determination of ascorbic acid by means of the iodometry technique (also called iodimetry, iodometry, or iodometric titration), a form of direct redox titration that uses molecular iodine as a titrating agent for the determination of a reducing agent. (in this case ascorbic acid), using starch as an indicator of the end point.

Fundamentals of the method

What is iodometry?

Iodometry is a particular type of redox titration. In turn, redox titration is a technique that is based on the rapid and quantitative reaction between an oxidizing agent and a reducing agent, any of which can be the analyte and either the titrant.

The titling agent

In the case of iodometry, the titrating agent is a solution of molecular iodine (I ₂ ) of known concentration, which has been solubilized in the form of the triiodide ion (I ₃ ^– ). This is achieved through the reaction of iodine with iodide ions, which increases its solubility in water:

Although this reaction facilitates the dissolution of elemental iodine in water, triiodide is generally generated in situ through the oxidation of iodide ions using potassium iodate as the oxidizing agent, according to the following reaction:

This reaction can be used to quantitatively generate triiodide to thus obtain a solution of known concentration of this oxidizing species to be used as a titrant for ascorbic acid.

The analyte – ascorbic acid or vitamin C

Ascorbic acid is an organic compound with the molecular formula C ₆ H ₈ O ₆ . Its structure is shown in the following figure:

This compound is easily oxidizable thanks to the structure that has two neighboring enols that are easily oxidized to the respective ketones, even with mild oxidizing agents.

The titration reaction

Like all volumetry, iodometry is based on a rapid chemical reaction that occurs quantitatively. This means that the reaction progresses to completion, completely consuming all of the limiting reactant, allowing exact stoichiometric calculations to be made.

In the case of the present experiment, the oxidizing agent is iodine, which is found in the solution in the form of triiodide, as mentioned above. The reduction half-reaction of iodine is:

For its part, the analyte is ascorbic acid, which is oxidized to dehydroascorbic acid through the oxidation of two of its hydroxyl groups and releasing two electrons and two protons, as shown below:

Then, the overall reaction of the degree would be:

Indicator operation

As in all degrees, there must be some way to detect the end point of the degree. For this, indicators are usually used, which are chemical substances that react with the excess titrant or that undergo some other observable change when reaching the equivalence point. In the case of iodometry, the indicator consists of a starch solution. This compound forms a complex with the triiodide ions that has a very intense dark blue color, almost black.

During the titration, the triiodide added from the burette reacts with the analyte, transforming it into iodide, thus preventing the formation of the colored complex with the starch. However, upon reaching the equivalence point and consuming all the ascorbic acid present, the next drop of titrant will have an excess of triiodide that will not be reduced to iodide, so the black complex will immediately form giving a dramatic color change to the dissolution.

Materials and REACTIVES

For the determination of vitamin C in food using iodine titration, the following reagents are required:

• Potassium iodate (KIO ₃ )
• Potassium iodide (KI)
• sulfuric acid 1M
• Starch
• Food samples. These can be citrus fruit juices, vegetables, fruit pulp, etc.

In addition to reagents, the following laboratory supplies and instruments are also needed:

• 250 mL Erlenmeyer vials or flasks.
• Volumetric flasks of 100 mL, 200 mL and 500 mL.
• Volumetric pipets of 10mL and 20mL.
• 50 mL graduated pipette.
• 500 mL and 100 mL beakers.
• 25 mL burette.
• Porcelain mortar.
• Glass funnel.
• Filter paper.
• Weigh substances.
• Analytical balance.
• Heating plate.

Ascorbic acid titration protocol by iodometry

Preparation of the titrating agent (I ₂ 0.01 M)

500 mL of 0.005 M potassium triiodide solution will be prepared as follows:

1. Weigh 7 g of potassium iodide and transfer it to a 500 mL beaker or beaker containing 200 mL of deionized water. Shake until completely dissolved.
2. Using a graduated pipet, transfer to the above beaker 100 mL of 1 M sulfuric acid to the above solution and shake.
3. Accurately weigh 1.0700 g of potassium iodate in a weigher and transfer it to the same beaker , making sure to wash away all the salt with deionized water using a shaker. Dissolve completely.
4. Transfer the solution quantitatively to a 500 mL volumetric flask, making sure to thoroughly rinse the sides of the beaker with deionized water repeatedly.
5. Fill the balloon up to the mark using first a beaker and then a bottle. Shake, cover with a glass stopper and reserve.

NOTE: For more accurate results, it is recommended to standardize this solution before or after titrating the samples. This can be done, for example, by titrating a vitamin C solution of known concentration.

Indicator Preparation

The indicator is a 0.25% m/V starch solution. This should be prepared shortly before titration as follows:

1. Weigh out 0.25 g of starch and transfer to a 100 mL beaker.
2. Add 50 mL of deionized water with a graduated pipet.
3. Heat to boiling on a hot plate under constant stirring until completely dissolved.
4. Allow to cool and reserve for use during the titration. If desired, it can be transferred to a closed container to avoid contamination.

Preparation of the samples to be analyzed

This protocol can be used to determine the concentration of vitamin C both in samples of vitamin C tablets, juices or liquid extracts of fruits and other vegetables, or in solid samples such as fruit pulp, roots, etc.

The sample preparation process varies from case to case. The following is the procedure for three typical cases:

Sample of vitamin C tablets

1. Weigh a vitamin C tablet, crush it, and transfer it to a 200 mL beaker.
2. Add 150 mL of water and shake until completely dissolved. It is essential to ensure that no residue remains in the bottom of the beaker .
3. Fill the flask up to the mark, cover, stir and reserve for analysis in a dark place, since vitamin C is sensitive to sunlight.
4. Quantitatively transfer this solution to a 200 mL volumetric flask, rinsing with deionized water through a bottle. Be sure not to add too much water during rinsing to avoid going over 200mL

fruit juice sampler

Fruit juices and extracts can be titrated directly without dilution. However, regardless of whether they are natural or industrial juices, it is advisable to filter the solution before titrating, to prevent any remaining pulp from clogging the pipettes used during the experimental procedure.

Samples of fresh fruit or fruit pulp

1. Cut and weigh 100 g of fruit pulp and add it to a mortar, crushing vigorously.
2. Add 10-mL portions of deionized water while grinding, decanting the supernatant solution into a 100-mL beaker by first passing it through a filter.
3. Repeat the above procedure at least 5-6 times, making sure to grind well between additions of deionized water.
4. Transfer the filtered extract to a 100 mL volumetric flask and fill up to the mark with deionized water using the bottle.

Degree procedure

1. Transfer a 10 mL aliquot of the respective sample (prepared according to the previous steps) to a 250 mL vial using a volumetric pipet.
2. Add 100 mL of deionized water and 1 mL of the starch indicator.
3. Titrate the aliquot with the triiodide solution using a buret until the solution in the vial changes color to a dark blue, almost black color.
4. Take note of the volume of titrant spent and repeat the procedure 2 more times to obtain an average volume of titrant.

Calculations of the ascorbic acid content in the samples

Since the stoichiometric ratio between the titrant and the analyte is 1:1, at the equivalence point we can say that the moles of both are equal, that is:

In the previous formula, the volume of the titrant must be placed in mL. This concentration can be transformed to a percentage or to a content per unit depending on the type of sample in question, as shown below.

Sample of vitamin C tablets

The concentration of the aliquot obtained thanks to the titration can be converted into the amount of vitamin C in the tablet, taking into account that it was dissolved in a total volume of 200 mL and that the molar mass of ascorbic acid is 176.12. g/mole:

fruit juice sampler

In this case, the molar concentration of vitamin C in the juice is directly that obtained for the aliquot, so the mass per 100 mL of juice can be obtained as follows:

This formula gives grams of vitamin C per 100 mL of juice or extract.

Samples of fresh fruit or fruit pulp

As in the case of the tablet, in this case it is taken into account that the vitamin C of the 100 g of pulp is dissolved in 100 mL of solution, from which aliquots were taken for analysis. Then, the concentration of vitamin C per 100 g of pulp is determined by:

If you want to convert this amount to grams of vitamin C for each unit of a fruit, you must multiply the previous result by the total mass of pulp of a unit of fruit and divide by 100 g.

References

Ikewuchi, C.J. (2011). Iodometric determination of the ascorbic acid (Vitamin c) content of some fruits consumed in a university community in Nigeria | Global Journal of Pure and Applied Sciences . Global Journal of Pure and Applied Sciences. https://www.ajol.info/index.php/gjpas/article/view/78733

Royal Society of Chemistry. (2018, August). Measuring the amount of vitamin C in fruit drinks . CSR.org. https://edu.rsc.org/download?ac=11742

Se, C. (nd). Iodimetry – PDF Free Download . Qdoc.Tips. https://qdoc.tips/yodimetry-pdf-free.html

Silva, CR, Simoni, JA, Collins, CH, & Volpe, PLO (1999). Ascorbic Acid as a Standard for Iodometric Titrations . An Analytical Experiment for General Chemistry . Journal of Chemical Education, 76 (10), 1421. https://pubs.acs.org/doi/10.1021/ed076p1421

Skoog, D. (2021). Analytical Chemistry (7th ^ed .). MCGRAW HILL EDUCATION.

University of Canterbury. (nd). Determination of Vitamin C Concentration by Titration . www.canterbury.ac.nz. https://www.canterbury.ac.nz/media/documents/science-outreach/vitaminc_iodine.pdf