Volumetric Analysis of Ascorbic Acid present in Citrus Fruits
Saurav Sachidanand
Class 12 A (2014-2015) National Public School Rajajinagar
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. It helps 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, ascorbic acid 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 group after 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 [O 2]. 2. Temperature 3. 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 -1 Molar Mass : 252 g mol 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+ + 2eI2 + 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 Conical flask solution Indicator End point
: N/10 Iodine solution : Fruit juice : Starch indicator : Colourless → Blue Black
TABULATION
1.
Orange juice
BURETTE READING
VOL OF JUICE TAKEN(ml)
FINAL VOLUME(ml) 1.4
INITIAL VOLUME(ml) 0
VOLUME USED(ml) 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. BURETTE READING
Lime juice VOL OF JUICE TAKEN(ml)
FINAL VOLUME(ml) 4.8
INITIAL VOLUME(ml) 3.8
VOLUME USED(ml) 1
10
5.8
4.8
1
10
Concordant reading :
1 ml
Calculations
N1V1=N2V2 1 - Iodine solution 2 - Lime juice N1 = N/10
V1 = 1ml V2 = 10ml N2=x N/10×1= x×10 N2=0.01 N
3.
Synthetic Drink
BURETTE READING FINAL VOLUME(ml)
VOL OF JUICE TAKEN(ml) INITIAL VOLUME(ml)
VOLUME USED(ml)
6.2
5.8
0.4
10
6.5
6.2
0.3
10
6.8
6.5
0.3
10
Concordant reading :
0.3 ml
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 FINAL VOLUME(ml)
VOL OF JUICE TAKEN(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
Concordant reading :
0.6 ml
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. 2. 3. 4. 5. 6. 7.
Wikipedia Khan Academy http://chemed.chem.purdue.edu/genchem/lab/techniques/titration/ http://www.dartmouth.edu/~chemlab/techniques/titration.html http://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titratio n Youtube Google Images
