Testing for vitamin C in fresh fruits

1. Testing for vitamin C in fresh fruits

2. Links to Curriculum for Excellence Level 3 Through experimentation, I can identify indicators of chemical reactions having occurred. SCN 3-19a Through research and discussion, I have contributed to evaluations of media items with regard to scientific content. SCN 3-20b • Level 4 • Having selected scientific themes of topical interest, I can critically analyse the issues, and use relevant information to develop an informed argument. • SCN 4-20b

3. What is Vitamin C? Ascorbic Acid A Common enzyme co-factor. Deficiency leads to Scurvy Scottish surgeon in the Royal Navy, James Lind, first proved scurvy could be treated with citrus fruit in experiments he described in his 1753 book, A Treatise of the Scurvy.

4. Aims of the Practical Workshop • To use reference samples to create a standard curve • To carry out a simple titration experiment • To show chemical reactions are taking place by the colour changes observed • To find out differences between the amounts of vitamin C in a range of fruits

5. How it works The test involves the reaction of vitamin C in the fruit with iodine. Mixing starch with iodine creates a complex with an intense blue/black colour that is much easier to see than iodine alone. The iodine reacts with vitamin C, reducing the iodine to colourless iodide ions. When all the iodine has reacted with the vitamin C present, there will be no blue-black coloured complex left, so the mixture is colourless. This is the end-point of the reaction.

6. The experiment The procedure is a very simple version of a titration. Instead of expensive and fragile glassware, we use cheap Pasteur pipettes. The ‘volume’ is measured by counting the drops. Should you wish to calculate a real volume, it is quite possible. Although there is very slight variation, a 1cm3 pasteur pipette will give 23 – 24 drops per cm3

7. Step 1 – The standard curve You will use standard solutions of ascorbic acid (vitamin C) to create a curve that you can use for analysis of the fruit juices. Drops of Vit C/juice Concentration of Vit C (mg/100ml)

9. Step 2 – Analysis of the fruit juices Now you will extract juice from some fruits – and use some pre-packed fruit juices. Using exactly the same technique as before, determine the number of drops of juice it takes to turn the iodine/starch complex clear. Use your reference graph to see what concentration of vitamin C that corresponds to.

10. Aims of the Discussion Activity • To analyse data to establish understanding of a scientific issue • To be critical of data presented as making a specific claim • To consider how the work of one scientist can influence our thinking

11. Discussion Activity • Use the sort cards ‘Are Vitamins Needed’ with the class divided into groups for the activity. • You are given 4 statements about vitamin supplements. • Discuss these in your group and rank them according to how much you agree with them.

12. Discussion Activity 2. Each group can then be given one type of the colour coded information sheets to discuss and form opinions on. The Vitamin sheets are colour coded as each colour contains a genuine article on vitamins, but they present somewhat different views on their worth as a supplement. It is important that students only see and discuss one of the opinions at this stage.