Science Project for ICCE General Level

Transcription

1 Science Project for ICCE General Level Investigation into the distribution in foodstuffs and health benefits of Vitamin C Vitamin C is an important vitamin long associated with good health. In this project you will be working through a number of work sheets based on the Royal Society of Chemistry s resource, Contemporary Chemistry for Schools and Colleges. Background Taking Vitamin C is claimed to have a role in preventing or limiting the effects of the Common Cold. This possible association was examined by the biochemist, Linus Pauling in his book, Vitamin C and the Common Cold, written in Another belief associated with Vitamin C is, when taken in large quantities, helps prevent the development of certain cancers. Vitamin C is the chemical ascorbic acid with the formula C 6 H 8 O 6. Ascorbic acid has two key roles: 1 helps the body absorb iron 2 production of the protein collagen For good health, the body requires around 60mg of Vitamin C per day and this is available from various foods particularly in fruit juices and some vegetables such as broccoli and potatoes. Vitamin C can be stored by the body but after about a month the store becomes depleted and needs to be replaced. There is also a known link between stress and the rate of depletion of Vitamin C reserves: the greater the stress, the greater the rate of depletion of the vitamin. One interesting effect of tobacco smoking is that smokers require more Vitamin C in their diet because smoking places a stress on the smoker s body thus increasing the vitamin s depletion rate. Ascorbic acid can be easily decomposed so boiled and treated vegetables contain much reduced levels than in the corresponding fresh product. Structural formula of ascorbic acid (Vitamin C)

2 Testing for Vitamin C Ascorbic acid is freely soluble in water so is quite easy for to be taken into the body. With its availability in fruit juices there is only a need to drink a glass of fruit juice a day to provide the body s daily Vitamin C requirement. In this project a comparison will be made of the levels of Vitamin C available in various fruit juices by chemical reaction with iodine. The reaction equation between Vitamin C and iodine is as follows: Iodine + ascorbic acid dehydroascorbic acid + iodide ions I 2 + C 6 H 8 O 6 dehydoascorbic acid + 2I - Blue-black colour no blue-black colour The iodine and ascorbic acid react together producing dehydroascorbic acid and iodide ions. Iodine has a brown colour in solution where the iodide ions are colourless. The colour of iodine can be seen more clearly with the addition of starch, where a dark blue-black complex is formed. As long as there is iodine present in the reaction then some of the blue-black complex will be visible. When all the iodine has reacted with the ascorbic acid then all the blue-black colouring will fade completely. This becomes the end-point of the reaction. Preparation of reagents Iodine / potassium iodide solution The recipe for producing a 0.05Molar iodine solution in potassium iodide is as follows: Dissolve 1.5gms of potassium iodide in as little distilled water as possible (approximately 20cm 3 ). Add 1.27gms of iodine crystals to this solution. Iodine rapidly dissolves in a concentrated potassium iodide solution. Make up the final volume of the solution to 100cm 3 with distilled water. Starch solution Mix 0.1gm of starch in a little cold distilled water to form a paste. Then dilute to 100cm 3 with boiling distilled water. Stir and allow to cool. Standard ascorbic acid (Vitamin C) solution Produce a 1mg/cm 3 solution of ascorbic acid by dissolving a 100mg Vitamin C tablet in 100cm 3 of distilled water. Equipment A clean, dry Pyrex glass test tube (eg 150mm x 16mm)for each test, test tube rack, a 250cm 3 Pyrex glass beaker, glass stirring rod, eye protectors, dropping pipettes (or Pasteur pipettes), 100cm 3 glass measuring cylinder, 10cm 3 graduated pipette or 10cm 3 glass measuring cylinder, distilled water, starch solution, iodine in potassium iodide solution, Vitamin C solution, various fruit juices. Reagents and equipment may be purchased from: Timstar Laboratory Supplies, Timstar House, Marshfield Bank, Crewe, Cheshire CW2 8UY Tel: or sales@timstar.co.uk

3 Method Remember to wear eye protectors when carrying out this experiment 1. Place 1cm3 of starch solution into a clean test tube 2. Add 5cm3 of distilled water and mix gently by shaking the tube 3. Add one drop of iodine solution (a blue-black colour will form) 4. Place a white card behind the tube to help see the colour change 5. Add the standard ascorbic acid solution drop by drop, keeping a note of how many drops are being added, shaking the tube after each drop. Keep adding until the blueblack colour disappears leaving a gray/white colour in the tube. Report the number of drops of ascorbic acid added to the tube. To ensure all the iodine has been reacted, add a further drop or so to see if there is a further colour change. If there is one then add this number of drops of ascorbic acid solution to the previous number. When the colour eventually stops changing then the end-point of the reaction has been reached. Write down the total number of drops added in the results table. Keep this tube as a check for the same colour in all the fruit juice tests. This becomes the reference test tube 6. Start again with a fresh, clean, dry test tube 7. Repeat steps one to three producing a blue-black colour 8. Select one of the fruit juices and add drop-by-drop, making note of how many drops are being added, until the blue-black colour disappears and is the same colour as that in the reference test tube. 9. Repeat steps one to three and test the next fruit juice 10. Create and complete the results table showing the number of drops of each fruit juice required to reach the end-point along with the result from the ascorbic acid standard test An example of a completed table is shown below. The results may differ from the ones you achieve due to, for example, the condition of the fruit selected or quality of the fruit juice. Do not be afraid to experiment with many more fruits. The bigger the list the more significant the determination of Vitamin C levels across a variety of different fruits becomes. NOTE Fruits are best bought fresh on the day of the experiment. Small amounts of juice can be extracted using a simple orange squeezer. Try not to get any fruit flesh in the extract, only juice!

4 Results table (example) Fruit Juice, Ascorbic acid solution Number of drops to reach end-point Ascorbic acid solution (1mg/cm3) 15 Lemon juice 30 Lime 35 Orange juice (carton) 1 day old 40 Kiwi 15 Melon 20 Mango 30 Knowing the number of drops of standard solution of ascorbic acid needed to produce an end-point, it is then possible to calculate the concentration of ascorbic acid present in each of the fruit juices tested. Concentration of Vitamin C (mg/cm3) = number of drops of standard mg/cm3 number of drops of test solution For example, from the result for lemon juice in the table above: Number of drops of 1mg/cm3 solution of ascorbic acid to reach the end-point = 15 Number of drops of lemon juice to reach the end-point = 30 Therefore the concentration of ascorbic acid in lemon juice (as tested) = 15/30 = 0.5mg/cm3 Table showing concentration of ascorbic acid (Vitamin C) present in each fruit juice Fruit juice or ascorbic acid solution Ascorbic acid mg/cm3 Ascorbic acid mg/100cm3 Ascorbic acid solution (1mg/cm3) Lemon juice Lime Orange juice (carton) 1 day old Kiwi Melon Mango The list of fruit juices for testing could include: apple, apricot, avocado, blackberry, blackcurrant, blueberry, grape, grapefruit, kiwi, lemon, lime, lychee, mandarin, mango, melon (water), melon (honeydew), melon (cantaloupe), orange, papaya, pawpaw, passion fruit, pomegranate, peach, pear, pineapple, plum, raspberry, redcurrant, strawberry, tangerine, and tomato.

5 Questions Why does the ascorbic acid or fruit juice solution have to be added drop-wise? Which juice has the most Vitamin C? Which had the least Vitamin C? Give an explanation of how you came to your conclusions. If you used a sample from a carton of juice, does the information on the juice package match your results? If not, why do you think this may be? Which juice(s) provide(s) the recommended daily allowance (RDA) in one 100cm3 glass?

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