Volumetric Analysis of Ascorbic Acid Present in Citrus Fruits

8/10/2019 Volumetric Analysis of Ascorbic Acid Present in Citrus Fruits

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TABLE OF CONTENTS

Certificate

Acknowledgements

INTRODUCTION.......................................................................................................

.......................1

Structure.......................................................................................................................

.................2

Biological

Purpose.................................................................................................................3

Acidity.........................................................................................................................

..................4

Oxidation.....................................................................................................................

................4

Factors affecting rate of oxidation of Ascorbic acid..............................5

Explanation of

titration..................................................................................................5

EXPERIMENT............................................................................................................

..........................6



TABULATION...........................................................................................................

.........................8

CONCLUSION...........................................................................................................

........................12

TIME

RECORD.....................................................................................................................

..............13

SCOPE FOR FURTHER

INVESTIGATION.............................................................14

BIBLIOGRAPHY.......................................................................................................

.........................15

ACKNOWLEDGEMENTS

This project would not have been possible without the support and guidance of our

teacher Mr. Jaganatha Shenoy, our laboratory in-charge teacher Mrs. Sudha and the

laboratory assistants, who were always there to give us a helping hand and sort out the

nuances of the procedures, experimentation and Chemistry.

I would also like to thank the Principle of our school, Mrs. Sumitra Das, and the school

itself, for providing us an conducive environment for the study and pursuit of ideas.

Finally, I thank the Central Board of Secondary Education for incorporating the

experimentally rigorous parts of the Chemistry into our curriculum so that we may

graduate with an all-round knowledge of the subject, and be better prepared in pursuing

multidisciplinary endeavours in the future.



INTRODUCTION

Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It

is a white solid, but impure samples can appear yellowish. It dissolves well in water to

give mildly acidic solutions. Ascorbic acid is one form of vitamin C. It was originally

called L-hexuronic acid. The new name, ascorbic acid, is derived from a- (meaning "no")

and scorbutus (scurvy), the disease caused by a deficiency of vitamin C. Because it is

derived from glucose, many animals are able to produce it, but humans require it as

part of their nutrition.



Structure

Molecular Formula: C6H8O6 Molecular Mass: 176

IUPAC name: (5R)-[(1S)-1,2-dihydroxyethyl]-3,4-dihydroxyfuran-2(5H)-one



The Ascorbic acid molecule contains four hydroxyl groups present in positions 2,3,5,6;

the -OH group in position 3 is acidic (pKa = 4.3), the hydroxyl group in position 5 has

pKa = 11.2, while those in position 5 and 6 act as secondary and primary alcohol residue

respectively.

It exhibits tautomerism, where the C1=O and the C3-OH groups interchange with the

shift of double bond.

Biological Purpose

Ascorbic acid is required for the growth and repair of tissues in all parts of your body. Ithelps the body make collagen, an important protein used to make skin, cartilage, tendons,

ligaments, and blood vessels. Vitamin C is needed for healing wounds, and for repairing

and maintaining bones and teeth.

It is an antioxidant, and it blocks some of the damage caused by free radicals,

substances that damage DNA. The build-up of free radicals over time may contribute to



the aging process and the development of health conditions such as cancer, heart disease,

and arthritis.

The Reichstein-Grüssner synthesis of ascorbic acid provides a supply of pure

crystalline ascorbic acid for industrial uses; it also makes a person’s requirements of daily

ingest manageable. Because of the extent of its use and humans´ need for ascorbic acid,

the

chemistry of the oxidation of this compound merits thorough research. It has been

suggested that, when added to processed foods as a means of improving its beneficial

properties, ascorbic acid’s tendency to act as a reducing agent results in numerous

derivatives whose toxicity is (unlike that of ascorbic acid) such as in synthetic drinks is,

controversial yet partially supported by evidence..

Acidity

Ascorbic acid is classed as a reductone. The ascorbate anion is stabilized by electron

delocalization, through resonance. It has two canonical forms. For this reason, ascorbicacid is much more acidic than would be expected.



Oxidation

Ascorbic acid is sensitive to heating, to light, and to the action of oxidizing agents and

metal ions. It is readily oxidized in aqueous solutions by reacting with atmospheric

oxygen, and behaves as a electron pair donor.

The deprotonation of ascorbic acid is given by the following equilibrium:

This results in semihydroascorbic acid, which then releases another proton when it gives

off an electron pair to a present oxidant.



Ascorbic acid loses a proton because the electron pair is transferred to the more

electronegative oxygen. This leads to the first equilibrium between ascorbic acid and

semihydroascorbic acid. The enol group on ascorbic acid becomes an enolate groupafter the deprotonation. The second equilibrium is a result of electron movement

because of the delocalization that occurs in the dicarbonyl system (a carbonyl functional

group is a carbon atom sharing an electron pair with an oxygen atom). The fact that

delocalization occurs could be because the system is conjugated (there are alternating

double and single covalent bonds from one carbonyl group to the other).

The negative charge on the oxygen atom of semihydroascorbic (due of delocalization of

charge) becomes a π bond between carbon and oxygen, which then results in two new

carbonyl groups because oxygen cannot have a second σ bond, and so must therefore give

off the σ electrons, in this case to an oxidant X+ . The breaking of the σ bond implies that

the hydrogen atom is given off. The electron movement in the second equilibrium results

in a neutrally charged compound, dehydroascorbic acid.

It can be concluded that the oxidation of ascorbic acid involves the loss of

two electrons and two protons. One can then write the oxidation of ascorbic acid by

oxygen as follows:

Factors affecting rate of Oxidation of Ascorbic Acid

1. Concentration of oxygen [O2].

2. Temperature3. Catalysts:

a. The enzyme ascorbic acid oxidase catalyses the oxidation of ascorbic acid.

b. Transition metal ions increase the rate of oxidation.

4. Acidity: the rate of oxidation of ascorbic acid is greater in a neutral or basic

medium.



Explanation of Titration

The traditional way to analyze the ascorbic acid content is the process of titration, with

an oxidizing agent. Several procedures have been developed, mainly relying

on iodometry. Iodine is used in the presence of a starch indicator. Iodine is reduced by

ascorbic acid, and, when all the ascorbic acid has reacted, the iodine is then in excess,

f forming a blue-black complex with the starch indicator. This indicates the endpoint of

the titration. As an alternative, ascorbic acid can be treated with iodine in excess,

followed by back titration with sodium thiosulfate using starch as an indicator.

Experiment



Aim: To prepare N/10 solution of Iodine, and use this solution to find the concentration

of ascorbic acid present in citrus fruits.

Materials Used: Electronic balance, conical flask, standard flask, burette, pipette, iron

stand, white tile, test tubes, Iodine crystals, distilled water, starch indicator.

Theory:

Preparation of Standard solution

Iodine : I2

Molar Mass : 252 g mol-1

Equivalent mass : 126 g equivalent-1

To prepare 1l of N/10 solution of Iodine, 12.6g is required. To prepare 100 ml of N/10

Iodine solution,

12.6/10 = 1.26g

is required.

Law of equivalents: It states that the number of equivalents of substance to be titrated is

equal to the number of equivalents of titrant used.



Mathematically,

N1V1=N2V2

Chemical equation

C6H8O6→ C6H6O6 + 2H+ + 2e-

I2 + 2e-→2I

-

C6H8O6 + I2→ C6H6O6 + 2H+ + 2I-

Procedure

➢ 100 ml of N/10 Iodine solution was prepared by dissolving 1.26g of iodine crystals

in 100 ml of distilled water in a standard flask.

➢ The pipette and conical flask were rinsed with the citrus fruit juice.

➢ The burette was rinsed and filled with the prepared Iodine solution.

➢ 10 ml of the fruit juice was taken in the conical flask, and a few drops of starch

indicator were added.

➢ The Iodine solution was added dropwise to the titration flask till the colourless

solution turned blue black. The initial and final reading of the burette were noted. ➢ The experiment was repeated till two concordant readings were obtained.

➢

Observation

→ Burette solution : N/10 Iodine solution

→ Conical flask solution : Fruit juice

→

Indicator : Starch indicator

→ End point : Colourless → Blue Black

TABULATION



1.

Orange juice

BURETTE

READING

VOL OF JUICE

TAKEN(ml)

FINAL

VOLUME(ml)

INITIAL

VOLUME(ml)

VOLUME

USED(ml)

1.4 0 1.4 10

2.6 1.4 1.2 10

3.4 2.6 1.2 10

Concordant reading : 1.2 ml

Calculations

N1V1=N2V2

1 - Iodine solution

2 - Orange juice

N1 = N/10 V1 = 1.2ml V2= 10ml N2=x

N/10×1.2=x×10

N2=0.012 N

2.

Lime juice

BURETTE

READING

VOL OF JUICE

TAKEN(ml)



6.2 5.8 0.4 10

6.5 6.2 0.3 10

6.8 6.5 0.3 10


Calculations

N1V1=N2V2

1 - Iodine solution

2 - Synthetic drink

N1 = N/10 V1 = 0.3ml V2 = 10ml N2=x

N/10×0.3=x×10

N2=0.003 N

4.

Grape juice

BURETTE

READING

VOL OF JUICE

TAKEN(ml)

FINAL

VOLUME(ml)

INITIAL

VOLUME(ml)

VOLUME

USED(ml)



0.7 0 0.7 10

1.3 0.7 0.6 10

1.9 1.3 0.6 10


Calculations

N1V1=N2V2

1 - Iodine solution

2 - Grape juice

N1=N/10 V1=0.6ml V2=10ml N2=x

N/10×0.6=x×10

N2=0.006 N

CONCLUSION

The given samples of fruit juice were analysed, and the highest concentration of ascorbic

acid was found to be present in orange juice, while the lowest concentration was found to

be present in synthetic drink. The second highest concentration was found to be in lime

juice followed by grape juice.



TIME RECORD



SCOPE FOR FURTHER

INVESTIGATION

A paradox in metabolism is that, while the vast majority of complex life on

Earth requires oxygen for its existence, oxygen is a highly reactive molecule that

damages living organisms by producing reactive oxygen species. Consequently,

organisms contain a complex network of antioxidants and enzymes that work together to

prevent oxidative damage to cellular components. However, reactive oxygen species also

have useful cellular functions, such as redox signaling. Thus, the function of antioxidant

systems is not to remove oxidants entirely, but instead to keep them at an optimum level.

These oxidants can damage cells by oxidizing DNA or proteins. Damage to DNA can

cause mutations and possibly cancer, if not reversed by DNA repair mechanisms, while

damage to proteins causes enzyme inhibition and protein degradation.

Ascorbic acid is one important antioxidant required by both plants and animals. As one of

the enzymes needed to make ascorbic acid has been lost

by mutation during primate evolution, humans must obtain it from the diet; it is therefore

a vitamin. Therefore, the study of oxidation of ascorbic acid is vital to help control the

aforementioned diseases and cure them.



BIBLIOGRAPHY

1.

Wikipedia

2.

Khan Academy

3.

http://chemed.chem.purdue.edu/genchem/lab/techniques/titration/ 4.

http://www.dartmouth.edu/~chemlab/techniques/titration.html

5. http://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titratio

n

6.

Youtube

7.

Google Images

http://chemed.chem.purdue.edu/genchem/lab/techniques/titration/




http://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titration









Volumetric Analysis of Ascorbic Acid Present in Citrus Fruits

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