Turnitin Final Determination of Vitamin C by Redox Titration PDF

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Determination of Vitamin C by redox titration Abstract Vitamin C (ascorbic acid), is an important nutrient essential for normal growth and development. A powerful antioxidant which slows down damage to cells in a human’s body and plays an important role in healing infections and a creator of collagen Braun P (2001). In this experiment, a solution containing ascorbic acid was titrated with an iodine solution to determine the concentration of ascorbic acid in various samples of (i) vitamin C tablet, (ii) (ii) orange juice; (iii) apple juice; and (iv) fresh fruit juice (carrot-sieved). The concentration of ascorbic acid calculated was then compared to the reported values. The values obtained were lower than the established values, which, may have been due to errors during the experiment. Introduction Vitamin C, also known as ascorbic acid is an essential micronutrient essential for normal metabolic functioning of the body. (Jaffe 1984). Vitamin C is generally obtained from our food, without this, humans develop scurvy, a disease characterised by bleeding gums and the loss of teeth, slow wound healing and subcutaneous haemorrhages. Scurvy was a recurrent and incurable problem on lengthy sea voyages until late in the eighteenth century, until Scottish physician, James Lind, discovered that if the sailors ate citrus fruits and fresh vegetables the disease could be prevented. As a standard provision limes were made readily available by the British Admiralty for all voyages “and British sailors have been called “limeys” ever since. When the active ingredient in limes was isolated, it was named ascorbic acid (“without scurvy”) acid”. (Sadava 2016). For adult non-smoking men and women, the current recommended dietary allowance for vitamin C to prevent the deficiency disease scurvy is shown in Table 1 below (Institute of Medicine. Food and Nutrition Board. 2000). Vitamin C a water-soluble vitamin, is defecated by the body through urine and sweat and consequently needs to be replaced each day Braun P (2001). Table 1: Recommended Dietary Allowances (RDAs) for Vitamin C Age Male Female Pregnancy Lactation * Adequate Intake (AI) 0–6 months 40 mg* 40 mg* 7–12 months 50 mg* 50 mg* 1–3 years 15 mg 15 mg 4–8 years 25 mg 25 mg  Vitamin C - C ₆H ₈O 9–13 years 45 mg 45 mg ₆ 14–18 years 75 mg 65 mg 80 mg 115 mg  molar mass: 19+ years 90 mg 75 mg 85 mg 120 mg 176.12g/mol Smokers Individuals who smoke require 35 mg/day  melting more vitamin C than non-smokers point: 190 °C  Boiling point: 553 °C  Density: 1.65 g/cm³ 1



Soluble in: Water, Glycerol, Ethanol, Propylene glycol. (Figure 1).

Figure 1 Structure of vitamin C

Given the health risks associated with not meeting the daily recommended allowance of vitamin C, accurate measuring of vitamin C in fruit and vitamin C tablets is required to ensure the general population is consuming enough vitamin C daily. It can be measured by using a redox titration using iodine which determines the amount of vitamin C in food/vitamin C tablet. In this experiment four known sources of vitamin C were tested by using redox titration to determine the concentration of vitamin C, which were then compared to the concentration stated in the guidelines. A reaction of a substance which transfers one or more electrons to another substance is called an oxidation reduction (redox reaction). Oxidation and reduction always exist together. Oxidation is the loss of one or more electrons (oxidising agent). Reduction is the gain of one or more electrons (reducing agent). (Figure 2). (Sadava 2016). Vitamin C transfers electrons to reactive free radicals, this process is called “oxidative stress”. The loss of one electron will result in the oxidation of vitamin C to ascorbate the free radical which is not as reactive as the other free radicals. The ascorbate free radical is reduced to vitamin C by gaining one electron. The loss of a further electron will be oxidised to dehydroascorbic acid. Vitamin C removed from the diet of a healthy human after 30 days will therefore be oxidised to dehydroascorbic acid, which is irreversible. (Figure 3) (Linus Pauling Institute, 2019).

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Figure 2 oxidation and reduction in a redox reaction (Sadava 2016)

Ascorbic acid is oxidized to dehydroascorbic acid. Rose RC (1988) (figure 3).

Figure 3 oxidation of ascorbic acid

In the presence of ascorbic acid, Iodine is reduced to Iodide and ascorbic acid is oxidised to dehydroascorbic acid, any excess iodine turns blue /black in starch indicator. (1)

C6H8O6 + I2

(2)

C6H8O6 oxidised  C6H6O6 + 2e

(3)

I2 + 2e reduced  2I¯

 2I¯ C6H6O6

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If vitamin C is present in the solution, triiodide will be converted to iodide immediately. When all the vitamin C is oxidised, triiodide and iodine will be present, reacting with starch to form a blue-black compound which is the endpoint of the titration. This titration can be achieved on its own using an iodine solution, although using an iodate solution is more constant and gives a more accurate result (Helmenstine 2018). Materials and Methods

Health and safety Lab coats, safety glasses, nitrile gloves, were always worn in the laboratory and avoid any contact with iodine solutions, as they will stain your skin. Potassium iodate solution (KIO3) must be disposed of in the “Waste oxidants” container and the Na2S2O3 solution into the “waste reducing agent” container.

Figure 4: titration apparatus

A series of titrations were performed using vitamin C tablet, fresh fruit juice (carrot-sieved) and packaged orange juice/apple juice. The burette was rinsed with distilled water and filled to the 0.00 mL mark with iodine solution. 100mL of fruit juice was added to a conical flask and titrated with iodine solution until the endpoint had been reached. 20mL of an aliquot sample solution was transferred to a 250 mL conical flask and 1mL of starch indicator solution was added (figure 4). The sample with 0.005 mol L with -1 iodine solution was titrated until a deep blue colour was obtained that lasted 60 seconds. Once a permanent trace of dark blue-black colour had been identified (which is due to the starch-iodine complex) the endpoint of the titration had been reached. This titration should be repeated with further aliquots of sample solution until concordance agreeing within 0.1-0.2 mL is obtained. The final number of mL iodine supplemented was recorded. Results Results are given in tables 1-4. Summary of calculations: table 5. Please refer to Appendix 1 for calculations.

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Table 1: vitamin C tablet titration

Initial Burette Reading

Final Burette Reading

0.0

32.7

Volume of 0.005 M iodine solution added (mL) 32.7

32.7

22.4

22.4

22.4

25.7

25.7

Average volume used

26.8

Table 2: Orange juice titration Initial Burette Reading

Final Burette Reading

Volume of 0.005 M iodine solution added (mL)

0.0

38.0

12.0

12.0

39.0

11.0

39.0

38.5

10.5

Average volume used

11.2

Table 3: Apple juice titration Initial Burette Reading

Final Burette Reading

0.0

3.5

Volume of 0.005 M iodine solution added (mL) 3.5

3.5

5.5

2.0

5.5

8.5

3.0

Average volume used

2.83

Table 4: Solid fruit/vegetables titration 5

Initial Burette Reading

Final Burette Reading 11.1

Volume of 0.005 M iodine solution added (mL) 2.6

15.3

4.2

17.1

1.8

8.5 11.1 15.3 Average volume used

2.8

Table 5 summary of the calculations

Aver vol Iodine mL No moles of iodine mol No of moles of ascorbic acid mol Molarity of ascorbic acid in sample M Concentration of calculated ascorbic acid per tablet or mg/100ml sample Stated concentration of sample Per tablet or mg/100ml Difference mg

Tablet

Orange Juice 11.17 5.585x10-5

Apple juice 2.83 1.415x10-5

26.8 1.34x10-4

carrot 2.86 1.43x10-5

1.34x10-4

5.585x10-5

1.415x10-5

1.43x10-5

1x10-3

1.117x10-6

2.83x10-7

2.86x10-7

943.3

49.1

12.4

12.5

1000

25

0.9

8.5

24.148

>11.552

>4.084

Discussion From this experiment, the concentration of the calculated ascorbic acid in the vitamin C tablet was determined 943.36mg which is lower compared to the amount stated in the reported manufacturers content in which the ascorbic acid content is 1000mg. However, you need to consider sources of error associated with the practical and a large source of error is a consistent colour change, trying to get the end point colour with the starch and iodine is very difficult to achieve. There is also parallax error and uncertainty in glass ware. These errors may have been why the readings were lower than it was. The orange juice recorded was 49.148mg/100mL, compared to 25mg/100mL (difference of 24.148), the apple 6

juice 12.452mg/100mL compared to the stated concentration of 0.9mg/100ml, obviously there is a discrepancy with these calculations which could be due to experimental error, the sample may not have been made up correctly, or perhaps the starch and iodine getting old and not as effective. The final titration was conducted using the fresh fruit juice (carrot) 12.584mg/100mL compared to 8.5mg/100mL (difference of 4.084mg/100mL). Three international papers in the international medical community were used in conjunction with this research, each determined that the daily vitamin C intake should be 90mg for a male and women 75mg per day. The vitamin C tablet which was tested had 1000mg, an excess of (56.4mg) of which will be defecated in sweat and urine. (Jacob 2002, Carr et al 2013, Mangels et al 1993). To avoid any recurring errors in this experiment, an electronic PH probe could be used which would give a consistent endpoint ( Encyclopaedia Britannica Online 2016). By experimenting on different batches of vitamin C tablets (one at a time) and fresh carrot juice, packaged orange/apple juice from around the country at different times of the year as the vitamin C content could be affected by season or the region it originated from. We all presume that obtaining vitamins from fresh fruit and vegetables, remains the best source of vitamin C. A study which was published in 2013 Nutrients Journal (Carr et al 2013), identified some shocking results. 36 men were given 50g of Vitamin C either in the form of fresh fruit or a vitamin C supplement. The results showed no significant difference in the quantities of vitamin C measured in tissues and body fluids, regardless of the form of vitamin C they took. Two other studies proved that natural food sources of vitamin C were not better than synthetic sources (Mangles et al 1993), (Gregory 1993). This is not to suggest that humans should forego fresh fruits and vegetables. A study which was undertaken by the Journal of American Nutrition (2004) identified that many fresh fruits and vegetables are less nutrient dense than those from a generation ago, a bite of a broccoli spear still contains not only vitamin C but also vitamin K, iron, vitamin A, calcium, and several other nutrients (Mangels et al 1993). Each fruit and vegetable are, in their own way classed as a multivitamin. Dietary supplements would seem to be the obvious way to plug gaps in a human diet, but humans cannot function properly without a healthy balance of fresh fruit and vegetables. Conclusion Analysis of vitamin C content of the Vitamin C tablet, orange juice, apple juice and fresh carrot juice was carried out by iodometric titration. The results from the experiment showed different values from the manufacturer’s specifications. The manufacturer claims that each vitamin C tablet contains 1000 m/g of ascorbic acid while this experiment shows that it had 943.36m/g which is below the manufacturer’s specifications. However, this could be a result of different factors or the experiment contained errors.

References

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Braun P (2001). The antioxidant saga: why we need vitamins C and E http://blog.insidetracker.com/the-antioxidant-saga-why-we-need-vitamins-c-and-e (accessed 04/04/2019) Cisternas, P., Alvarez, S., Martinez F., (2014), Journey of Neurochemistry the oxidized form of vitamin C, dehydroascorbic acid, regulates neuronal energy metabolism: International Society for Neurochemistry: Vol 129, 4th Ed. Gregory, J.F. Ascorbic acid bioavailability in foods and supplements. Nutrition Reviews. 1993; volume 51: pages 301-309. (PubMed) Harvard Health Publishing, Harvard Health Medical School (2015): https://www.health.harvard.edu/staying-healthy/should-you-get-your-nutrients-from-foodor-from-supplements (accessed 21/03/2019) Helmenstine, A.M., Ph.D. "Vitamin C Determination by Iodine Titration." ThoughtCo, Oct. 22, 2018, https://www.thoughtco.com/vitamin-c-determination-by-iodine-titration-606322 (accessed on 04/04/2019) Institute of Medicine. Food and Nutrition Board. (2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press Jacob RA, Sotoudeh G. (2002) Vitamin C function and status in chronic disease. Nutr Clin Care: 66-74. [PubMed abstract] Jaffé GM (1984) Vitamin C. In: Machlin LJ (ed) Handbook of vitamins. Nutritional, biochemical, and clinical aspects. M Dekker, New York, Google Scholar Journal of the American College of Nutrition.2004. Davis Dr, Epp MD, Riodan HD (2004) (6):669-82. Changes in USDA food composition data for 43 garden crops, 1950 to 1999. Linus Pauling Institute, (2019). Vitamin C: Oregon State University, USA https://lpi.oregonstate.edu/mic/vitamins/vitamin-C Machlin L. (1984) Handbook of vitamins. New York: Marcel Dekker Inc, 199–244. Madhusha (2017) Difference Between Standardization and Titration: Pediaa.Com. http://pediaa.com/difference-between-standardization-and-titration/ (accessed 28/03/2019) Mangels R, Block G, Frey C.M, Patterson B H, Taylor P R, Norkus E P, Levander O, (1993). The Bioavailability to Humans of Ascorbic Acid from Oranges, Orange Juice and Cooked Broccoli Is Similar to That of Synthetic Ascorbic Acid, The Journal of Nutrition, Volume 123, Issue 6, June, Pages 1054–1061, https://doi.org/10.1093/jn/123.6.1054 (accessed 02/03/2019)

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Encyclopaedia Britannica Online. "pH meter". 2016 (accessed on 03/04/2019). https://www.britannica.com/technology/pH-meter Rose RC. (1988) Transport of ascorbic acid and other water-soluble vitamins: Biochim Biophys Acta 947:335-366 Sadava D., 2016, Life The Science of Biology, 2nd edn. p 1073: Sinauer Associates Inc., USA Scientific Advisory Committee on Nutrition (2011) Dietary Reference Values for Energy. The Stationery Office. London. https://www.gov.uk/government/publications/sacndietaryreference-values-for-energy (accessed 02/04/2018)

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Appendix 1 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 26.8mL 2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine solution in the burette. Moles = M x Litres 0.005M x 0.0268 =1.34x10¯4 mols 3. What is the number of moles of ascorbic acid in the original vitamin C tablet? 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 = 1.34x10-5 mols 4. Calculate the amount of ascorbic acid, in mg, in the vitamin C tablet.

Moles = M x Litres 5mL 0f 200ml, 200/5=40mL/Litre 1.34x10¯⁴ x40=5.36x10⁻³mols 5. Calculate the amount of ascorbic acid, in mg, in the vitamin C tablet. 176g/mol (G₆H₈O₆) x 5.36x10⁻³mols = 0.94336g x1000(convert to mg) =943.36mg Apple Juice 1. Calculate the average volume of iodine solution used from your titres. Average volume = mL 2.83mL

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2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine solution in the burette. 2.83/1000=0.00283L x0.005M=1.415x10⁻⁵mols 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 = 1.415x10-5 4. Calculate the molar 1.415x10⁻⁵concentration of ascorbic acid in the original apple juice solution. 20mL of apple juice = 0.020L x 1.415x10⁻⁵mols=2.83x10⁻⁷M 5. Calculate the concentration, in mg/100mL of ascorbic acid, in the original apple juice solution. 1.415x10⁻⁵ per 20ml therefore 20ml x 5 = 100ml, 1.415x10⁻⁵ x5=7.075x10⁻⁵x 176g/mol(G₆H₈O₆) =0.012452g/ml x 1000 = 12.452mg/100ml Orange Juice 1. Calculate the average volume of iodine solution used from your titres. Average volume = mL 11.17 2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine solution in the burette. 11.17ml/1000 =0.01117L x 0.005M=5.585x10⁻⁵mols 3. Using the equation of the titration (below) determine the number of moles of ascorbic acid reacting. Show your calculations in the box below Moles of ascorbic acid reacting = 5.585x10-5 4. Calculate the molar concentration of ascorbic acid in the original orange juice solution. 20ml /1000=0.02L x 5.585x10⁻⁵mols=1.117x10⁻⁶M

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5. Calculate the concentration, in mg/100mL of ascorbic acid, in the original orange juice sample. 20ml has 5.585x10⁻⁵mols therefore 100ml will have 5.585x10⁻⁵mols x5=2.7925x10⁻⁴ x 176g/mol=0.049148g/ml x1000=49.148mg/100ml Solid fruit/vegetables: 1. Calculate the average volume of iodine solution used from your titres. Average volume = mL 2.86 2. Calculate the number of moles of iodine in this average volume using the molarity of the iodine solution in the burette. 2.86/1000=0.00286 x 0.005M = 1.43x10⁻⁵mol 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 = 1.43x10-5 mol 4. Calculate the molar concentration of ascorbic acid in the original fruit/vegetable solution. 20ml =0.02l x1.43x10⁻⁵mol = 2.86x10⁻⁷M 5. Calculate the concentration, in mg/100mL of ascorbic acid, in the original fruit/vegetable solution. 1.43x10⁻⁵mol x5 = 7.15x10⁻⁵x176g/mol = 0.012584gx 1000=12.584mg/100mL

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