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Food Chemistry Practical Notes

Ascorbic acid analysis by redox titration

Introduction

Ascorbic acid is a reducing agent and on oxidation forms dehydroascorbic acid. Both these

chemicals have Vitamin C activity although often chemists erroneously assay for ascorbic acid

only. In this class you will assay for ascorbic acid in 2 different foods using the oxidizing agent

dichlorophenolindophenol (DCPIP). DCPIP is also an acid-base indicator having a pink colour in

acid solution and a blue colour in alkaline solution. The reduced form of DCPIP is colourless.

+

C C

C

O

H C O

O O

+ 2H3O+ + 2e –

H H

2 H2O

CHCH2 OH

OH

ascorbic acid dehydroascorbic acid

C C

C

O

H C O

O O

CHCH2 OH

OH

Materials and methods

Foods to be tested

The ascorbic acid content of orange juice is to be determined. By law orange juice must have a

minimum of 400 mg/L Vitamin C – this is the total of ascorbic acid and dehydroascorbic acid. As

well a green vegetable will be tested.

Part I Preparation of ascorbic acid solution.

This section is to be done as a group.

1. Weigh approx 0.1 g ascorbic acid accurately.
2. Transfer the ascorbic acid into a 100 mL volumetric flask and dissolve in about 20 mL 1 %

metaphosphoric acid. Make to the mark with 1 % metaphosphoric acid.

3. Transfer a 25.0 mL aliquot of the ascorbic acid solution into a 250 mL volumetric flask.

Make to the mark with 1 % metaphosphoric acid.

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Part II Standardization of DCPIP.

1. Pipette a 10.00 mL aliquot of the approx 0.1 mg/ mL ascorbic acid solution (prepared above)

into an Erlenmeyer flask.

2. Titrate with DCPIP until a permanent pink end point. This is not always easy to determine.
3. Repeat until triplicate results are obtained.
4. Calculate the mass of ascorbic acid that reacts with 1.00 mL DICPIP.

Part III Assay of Ascorbic Acid in Cabbage

1. Cut 4 or 5 thin slices of cabbage and chop roughly. You will need about 50 g. Weigh the

cabbage (use the top loading balance set to 2 decimal places).

2. Place the cabbage into a blender and add a known mass of 1 % metaphosphoric acid (weigh

about 200 mL – but make sure it is enough to cover the blades of the blender).

3. Blend for a minute or two.
4. Add a known mass of the extract (about 20 g) into a plastic centrifuge tube.
5. Repeat with a second centrifuge tube so that both tubes weigh the same. You must know the

mass of extract in each tube.

6. Centrifuge for 5 to 10 minutes.
7. Pour the supernatant (the liquid) from each centrifuge tube into the same 200 mL volumetric

flask.

8. Add a few mL of metaphosphoric acid to each centrifuge tube to resuspend the pellet.
9. Add about 20 mL metaphosphoric acid to each tube so that they are balanced.
10. Recentrifuge.
11. Carefully pour the supernatant into the same 200 mL volumetric flask (step 7 above).
12. Make the flask up to the mark.
13. Pipette a 10.0 mL sample into a flask and titrate with DCPIP.
14. Consult your demonstrator to determine an appropriate sized aliquot to test next time. Repeat

the titration.

5. Calculate the ascorbic acid in the solution and hence in the cabbage itself.

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Part IV Assay of Ascorbic Acid in Orange Juice

1. Filter about 30 mL orange juice through glass wool.
2. The orange juice is too concentrated to use directly so dilute it 1 in 5 (i.e. 20.0 mL into a 100

mL volumetric flask). In this case for dilution to the mark use the metaphosphoric acid

solution provided.

3. Pipette a 10.0 mL aliquot of the diluted orange juice into a flask.
4. Titrate with DCPIP until a permanent pink colour is obtained. If the colour is difficult to see

you may add 10 mL water.

5. Decide whether the titre is reasonable (is it just right, too high or too low?). Check with your

demonstrator.

6. Repeat the titration using a suitable sized aliquot.
7. Calculate the amount of ascorbic acid in the diluted orange juice and hence in the orange

juice itself.

Questions for report

1. Compare the results with values from Food Composition tables as well as any available label

data.

2. Are your answers sensible?
3. Comment on the precision of your results.