CHEMISTRY

Determination of the Amount of Vitamin C in Various Fruits

Salma, Justine, Florence, Asmita, Abby

Objective

To measure the amount of Vitamin C in orange, kiwi, grapes and lemon by redox titration using iodate solution

To investigate the effect of temperature on the Vitamin C content in the fruits

Introduction

Vitamin C (a.k.a. ascorbic acid)An essential antioxidant needed by our

bodies

Principle

Iodate ions (IO3-) are added to an acidic solution with iodide ions (I-)

A redox reaction occurs 1) Iodate ions are reduced to form iodine IO3+6H+ 5e- ½ I2 + 3H20 2)Iodide ions are oxidised to form iodine 2I I2 + 2e-

PrincipleCombining the equations: 2IO3+ 10I- + 12H+ 6I2 + 6H20

The iodine formed by the reaction oxidize the ascorbic acid to dehydroascorbic acid while reduced to iodide ions

ascorbic acid+ I2 2I- + dehydroascorbic acid

Principle

The iodine formed is then immediately reduced to iodide when there is ascorbic acid

When all the ascorbic acid has been oxidized

The excess iodine reacts with starch indicator

Forming a blue-black starch-iodine complex (the end point of titration)

Principle

The chemical structure and antioxidant (reducing) action of ascorbic acid are illustrated in the redox half equation below:

Procedure

1) Measurement of the mass of potassium iodate (V)

a) A balance was set zero after a 60ml beaker was placed onto it. b) Around 0.6g of potassium iodate(V) was transferred carefully into the

bottle in the balance. c) The reading from the balance was recorded in Table 1. d) The potassium iodate(V) was then transferred into a 60ml beaker.

2) Preparation of potassium iodate (V) solution

a) Deionized water was added into the 60ml beaker b) The solution was stirred with a glass rod until the

solution the solid potassium iodate(V) was dissolved as much as possible.

c) The potassium iodate(V) solution was transferred into the 250ml volumetric flask.

d) Distilled water was added until the meniscus reached the 250ml line.

e) The flask was shaken upside down to mix the solution with the distilled water.

3) Preparation of sodium thisulphate solution

a) A balance was set zero after a 60ml beaker was placed onto it.

b) Around 3g of potassium iodate(V) was transferred carefully into the bottle in the balance.

c) The reading from the balance was recorded in Table 1.

d) The potassium iodate(V) was then transferred into a 60ml beaker.

4) Preparation of sodium thiosulphate solution a) Deionized water was added into the 60ml beaker. b) The solution was stirred with a glass rod until the solution the solid potassium

iodate(V) was dissolved as much as possible. c) The sodium thiosulphate solution was transferred into the 250ml volumetric flask. d) Distilled water was added until the meniscus reached the 250ml line. e) The flask was shaken upside down to mix the solution with the distilled water.

5) Preparation of fruit juice

a) The fruit was cut into pieces by a knife b) The juice was extracted by pressing the fruits into a beaker through a muslin

cloth. c) The juice was transferred to a clean 250ml volumetric flask. e) Distilled water was added until the meniscus reached the 250ml line. f) The flask was shaken upside down to mix the solution with distilled water. g) The fruit juice mixture was transferred into a 250ml beaker.

6) Titration between fruit juice and sodium thiosulphate solution

a) 25cm3 of the fruit juice mixture was transferred to a clean conical flask by a clean pipette.

b) 25cm3 of the potassium iodate(V) solution mixture which had been prepared was added into the flask and then 5cm3 of potassium iodide.

c) The solution was then titrated immediately with sodium thiosulphate solution. d) A few drops of starch solution when the reaction mixture turns to pale yellow. e) Then the mixture was titrated to the end point, which the solution would

become colourless. f) The volume of sodium thiosulphate solution used to titrate was recorded in

Table 3.

7) Investigation with other fruit juices

a) The above steps were repeated using different fruits (orange, lemon, grape, and kiwi)

8) Investigation of vitamin C with fruits at 50◦C

a) Steps 1-5 were repeated b) 25cm3 of the fruit juice mixture was transferred to a clean boiling tube by a clean

pipette. c) The boiling tube was placed in a hot water bath at a temperature of 50 ◦C. d) 25cm3 of the fruit juice mixture was transferred to a clean conical flask from the

boiling tube. b) 25cm3 of the potassium iodate(V) solution mixture which had been prepared was

added into the flask and then 5cm3 of potassium iodide. c) The solution was then titrated immediately with sodium thiosulphate solution. d) A few drops of starch solution when the reaction mixture turns to pale yellow. e) Then the mixture was titrated to the end point, which the solution would become

colourless. f) The volume of sodium thiosulphate solution used to titrate was recorded in Table

3. g)The above steps were repeated using different fruits (orange, lemon, grape, and

kiwi)

Results (titre for untreated fruits)

Orange Trail 1st 2nd AverageInitial 22.7 7 0 Final 6.3 41.2 34.4  Change 33.6 34.2 34.4 34.0666

7

Lemon Trail 1st 2nd AverageInitial 21 1.35 9.8 Final 4.7 35 43.2 Change 33.7 33.65 33.4 33.5833

3

Grape Trail 1st 2nd AverageInitial 0 33.9 0 Final 33.3 17 31.7 Change 33.3 33.1 31.7 32.7

Kiwi Trail 1st 2nd AverageInitial 17.5 0 30.2 Final 50.3 30.2 15.3 Change 32.8 30.2 35.1 32.7

Results (titre for fruits at 50°C)

Orange Trail 1st 2nd AverageInitial 0 0 0 Final 36.1 36.6 36.8 Change 36.1 36.6 36.8 36.5

Lemon Trail 1st 2nd AverageInitial 12.3 0 0.03 Final 46.8 32.3 35.38 Change 34.5 32.3 35.41 34.07

Grape Trail 1st 2nd AverageInitial 0 0 0 Final 35.9 35.7 35 Change 35.9 35.7 35 36.53

Kiwi Trail 1st 2nd AverageInitial 10 0 13.4  Final 46.52 37.43 49.1  Change 36.52 37.43 35.7 36.55

Amount of Vitamin C

Fruit used At room temperature At 50 °C

Orange 5.752 x 10-4 mols 5.365 x 10-4 mols

Lemon 5.797 x 10-4 mols 5.364 x 10-4 mols

Grape 5.672 x 10-4 mols 5.364 x 10-4 mols

Kiwi 5.672 x 10-4 mols 5.364x10-4 mols

Discussion

When the fruit samples were heated, they all decreased in vitamin C content and had a similar amount of vitamin C. This may be because the content of Vitamin C content is decomposed at high temperature.

Discussion

Starch solution acts as an indicator to indicate the end point of titration.

Added only when the reaction mixture becomes pale yellow.

If it is added too early, the high concentration of iodide react with iodine to form a water insoluble complex.

As a result, the amount of iodine available for reacting with sodium thiosulphate becomes smaller.

Discussion

After adding sulphuric acid, titration must be preformed immediately as iodine is volatile

It will vaporize and the amount of iodine available for reacting with sodium thiosulphate becomes smaller.

In addition, iodide ions will be oxidized by oxygen in air

4I-(aq) + O2(g) + 4H+(aq) 2I2(aq) + 2H2O(l)

Then, more iodine is available for reacting with sodium thiosulphate.

Discussion

Sources of error: Humidity Vitamin C is an acid which is very water

soluble and may dissolve in air vapour Freshness of fruit vitamin C would decompose in air Human error Amount of titre not exact, end-point detected

by naked eye only and residues of the fruits in juice after extraction

Discussion

Sources of error: Unstable solution Iodine is volatile and may evaporate Thiosulphate is unstable in acidic medium

under sunligh

Discussion

This method may be used to carry out other investigation such as

Vitamin C content of fruits/vegetables at different stages of ripeness.

Preserving the content of Vitamin C in fruits/vegetables when cooking.

Conclusion

From the results, the vitamin C content is most abundant in Kiwi, then orange, lemon and finally grape. Also, at high temperature the vitamin C content would decrease as the vitamin C content inside the fruit juice denature.