Determination of vitamin c by redox titration with iodate

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5 – Determination of Vitamin C Concentration using a redox titration method

Introduction

Vitamin C, (ascorbic acid), is an essential antioxidant needed by the human body. You will

determine the vitamin C concentration in a solution by a redox titration using iodine. As the iodine

is added during the titration, the ascorbic acid is oxidised to dehydroascorbic acid, while the

iodine is reduced to iodide ions. Reaction 1 shows the overall reaction whereas reaction 2 and 3

represent the two half-reactions.

Ascorbic acid + I2 → 2 I −

+ Dehydroascorbic acid (1)

I2 + 2e- → 2 I

-

The iodine formed is immediately reduced to iodide as long as there is any ascorbic acid present.

Once all the ascorbic acid has been oxidised, the excess iodine is free to react with the starch

indicator, forming the blue-black starch-iodine complex. This is the endpoint of the titration.

You will use this method to determine the vitamin C content in various sources of vitamin C, such

as vitamin C tablets, fresh fruit juice, packaged fruit juice and solid fruits and vegetables.

Materials (per student pair)

Equipment Chemicals

Volumetric flask (100 or 200 mL)

20 mL pipette

50-mL burette, stand and clamp;

Cheese cloth

Mortar and pestle

Food processor

Conical flask (250 mL)

Iodine Solution

Starch Indicator solution

Vitamin C tablets (1 per pair)

Orange juice

Apple juice

Fresh fruit/vegetables

DI water

Health and Safety:

Iodine stains badly – gloves should be worn in addition to lab coat and safety

glasses.

(3)

(2)

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Sample Preparation

For vitamin C tablets:

Dissolve a single tablet in 200 mL of distilled water (in a volumetric flask if possible).

For fresh fruit juice:

Strain the juice through cheesecloth to remove seeds and pulp which may block pipettes. Make

the extracted solution up to 100 mL with distilled water (if needed).

For packaged fruit juice:

This may also need to be strained through cheesecloth if it contains a lot of pulp or seeds. Make

the extracted solution up to 100 mL with distilled water (if needed).

For fruits and vegetables:

Cut a 100 g sample into small pieces and grind in a mortar and pestle. Add 10 mL portions of

distilled water several times while grinding the sample, each time decanting off the liquid extract

into a 100 mL volumetric flask. Finally, strain the ground fruit/vegetable pulp through cheesecloth,

rinsing the pulp with a few 10 mL portions of water and collecting all filtrate and washings in the

volumetric flask. Make the extracted solution up to 100 mL with distilled water.

Alternatively the 100 g sample of fruit or vegetable may be blended in a food processor together

with about 50 mL of distilled water. After blending, strain the pulp through cheesecloth, washing

it with a few 10mL portions of distilled water, and make the extracted solution up to 100 mL in

a volumetric flask.

Solutions

Iodine solution: (0.005 M). Weigh 2 g of potassium iodide into a 100 mL beaker. Weigh 1.3 g of

iodine and add it into the same beaker. Add a few mL of distilled water and swirl for a few

minutes until iodine is dissolved. Transfer iodine solution to a 1 L volumetric flask, making sure to

rinse all traces of solution into the volumetric flask using distilled water. Make the solution up to

the 1 L mark with distilled water.

Starch indicator solution: (0.5 % (w/v)). Weigh 0.25 g of soluble starch and add it to 50 mL of

near boiling water in a 100 mL conical flask. Stir to dissolve and cool before using.

Method: Titration

1. Pipette 20 mL of the sample solution into a 250 mL conical flask and add about 150 mL of

distilled water and 1 mL of starch indicator solution.

2. Rinse and fill the burette with the 0.005 M iodine solution. Record the initial volume in the

table below.

3. Titrate the sample with the iodine solution. The endpoint of the titration is identified as the

first permanent trace of a dark blue-black colour due to the starch-iodine complex. Record the

final volume in the burette in the table below

4. Calculate the volume of 0.005M iodine solution added during the titration.

5. Repeat the titration with further aliquots of sample solution until you obtain 3 titres that agree

within 0.1-0.2 mL of each other.

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Table 1: vitamin C tablet titration

Initial Burette Reading Final Burette Reading Volume of 0.005 M iodine

solution added (mL)

Average volume used

Table 2: Orange juice titration

Initial Burette Reading Final Burette Reading Volume of 0.005 M iodine

solution added (mL)

Average volume used

Table 3 Apple juice titration

Initial Burette Reading Final Burette Reading Volume of 0.005 M iodine

solution added (mL)

Average volume used

Table 4: Solid fruit/vegetables titration

Initial Burette Reading Final Burette Reading Volume of 0.005 M iodine

solution added (mL)

Average volume used

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Calculations

For each of the different juices, perform the following calculations:

Vitamin C tablets

1. Calculate the average volume of iodine solution used from your titres.

Average volume = mL

2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine

solution in the burette.

3. Using the equation of the titration (below) determine the number of moles of ascorbic acid

reacting. Show your calculations below:

Ascorbic acid + I2 → 2 I −

+ Dehydroascorbic acid

Moles of ascorbic acid reacting =

4. What is the number of moles of ascorbic acid in the original vitamin C tablet?

5. Calculate the amount of ascorbic acid, in mg, in the vitamin C tablet.

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Apple Juice

1. Calculate the average volume of iodine solution used from your titres.

Average volume = mL

2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine

solution in the burette.

3. Using the equation of the titration (below) determine the number of moles of ascorbic acid

reacting. Show your calculations below:

Ascorbic acid + I2 → 2 I −

+ Dehydroascorbic acid

Moles of ascorbic acid reacting =

4. Calculate the molar concentration of ascorbic acid in the original apple juice solution.

5. Calculate the concentration, in mg/100mL of ascorbic acid, in the original apple juice

solution.

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Orange Juice

1. Calculate the average volume of iodine solution used from your titres.

Average volume = mL

2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine

solution in the burette.

3. Using the equation of the titration (below) determine the number of moles of ascorbic acid

reacting. Show your calculations in the box below

Ascorbic acid + I2 → 2 I −

+ Dehydroascorbic acid

Moles of ascorbic acid reacting =

4. Calculate the molar concentration of ascorbic acid in the original orange juice solution.

5. Calculate the concentration, in mg/100mL of ascorbic acid, in the original orange juice

sample.

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Solid fruit/vegetables:

1. Calculate the average volume of iodine solution used from your titres.

Average volume = mL

2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine

solution in the burette.

3. Using the equation of the titration (below) determine the number of moles of ascorbic acid

reacting. Show your calculations below:

Ascorbic acid + I2 → 2 I −

+ Dehydroascorbic acid

Moles of ascorbic acid reacting =

4. Calculate the molar concentration of ascorbic acid in the original fruit/vegetables solution.

5. Calculate the concentration, in mg/100mL of ascorbic acid, in the original fruit/vegetables

solution.