Edexcel A Biology A level core practical Write ups. All include an introduction, hypothesis, variables, method, results, evaluations...
CP2 - Investigate the vitamin C content of food and drink
Core practical 2 - Investigating the Vitamin C content of food and drink
Aim: to investigate the vitamin C content of different fruit juices using 0.01% DCPIP
solutions
Introduction
Vitamin C (ascorbic acid) is a water soluble vitamin that is naturally present in some foods
such as citrus fruits, berries, potatoes, tomatoes, peppers, cabbage, Brussels sprouts,
broccoli and spinach. Humans, unlike most animals, are unable to synthesise Vitamin C
naturally so it is an essential dietary component. (National Institutes of Health, 2021). It
Ascorbic acid is an electron donor which makes it a strong antioxidant which can counteract
unstable molecules called free radicals (Padayatty et al., 2003). Free radicals are electron
deficient that have an unpaired electron making it unstable and can cause damage to human
cells, DNA which plays a role in heart disease, cancer and other diseases. Antioxidants
neutralise free radicals by giving up some of their own electrons which helps break the chain
reaction (Harvard Health Publishing, 2019).
Vitamin C has a major role in the treatment of cold and flu, protecting against cancer, heart
disease and stress and helping in maintaining a healthy immune system by supporting
various cellular functions of both the innate and adaptive immune system. Ascorbic acid is
well known for its role in collagen synthesis. Collagen deficiency, also known as scurvy, is
characterised by loose teeth, superficial bleeding, poor healing and compromised immunity.
Maintenance of daily dietary intake of vitamin C leads to the prevention of scurvy (Ugbe,
Ikudayisi and Amusan, 2017)
Dichlorophenolindophenol (DCPIP) is a redox dye commonly used as an indicator for vitamin
C as it is an electron acceptor that changes from blue to colourless when reduced. Ascorbic
acid is a good reducing agent as it is an electron donor so will reduce DCPIP to colourless if
present. In a titration, when all the ascorbic acid in the solution has been used up, there will
not be any electrons available to reduce the DCPIP and the solution will remain pink due to
the DCPIP. The end point is a pink colour that persists for 10 seconds or more.
(www.liquisearch.com, n.d.)
Hypothesis : The orange juice will have more vitamin C than the apple juice as it is a citrus
fruit and the juices will have a lower vitamin C content than the pure vitamin C solution.
Variables
Independent variable : type of fruit juice (orange, apple and vitamin C solution)
Dependent variable : volume of DCPIP(cm3) reduced by 25cm3 of vitamin C solution
Control variables :
- Temperature by recording the temperature in the room and preventing from touching
the equipment as hands are slightly warm—> vitamin C is water soluble and
temperature sensitive. High temperatures have been found to cause losses of
vitamin C. In addition, temperature increases the rate of oxidation.
, Core Practical 2 - 23/02/2022
- Concentration of DCPIP (0.01%) —> a higher concentration would be require more
vitamin C to reduce it so needs to be kept the same for each solution
- Volume of vitamin C → larger volume requires more DCPIP to colourise it
- Same end point colour (eg 3 times) —> to have standard for each titration which can
be used for all the titrations and keeps the conditions the same
- Shake each tube the same number of times - more shaking would cause the colour
to fade more so shake the same number to make sure variations in results are not
due to using different method
Risk assessment
- When pouring the DCPIP into the burette, make sure the burette is at eye level to
avoid spillage.
- DCPIP can cause skin and eye irritations so contact with skin should be avoided.
Wear eye protection and gloves. Do not ingest.
- Broken glass can cut so take care when handling glassware and keep away from the
edge of the desk.
Equipment
● 0.01% DCPIP solution - Indicator for vitamin C which can be used to calculate
amount of vitamin present
● 0.005% vitamin C (ascorbic acid) solution - solution which we will be testing
● Apple juice (1:400) - solution which we will be testing
● Orange juice (1:400) - solution which we will be testing
● Conical flask - to hold the vitamin C during the titration
● Burette (50ml) - to control the amount of DCPIP which can be released drop by drop
for accurate titration
● Measuring cylinder (25ml) - to measure 25cm3 of vitamin C
Method
1. Fill the Burette with 0.01% DCPIP solution and take a note of the starting value
2. Using a measuring cylinder, add 25cm3 of 0.005% vitamin C solution in a conical
flask.
3. Use the burette to add the DCPIP solution drop by drop. Swirl the contents of the
conical flask with one hand whilst controlling the tap with the other
4. Continue to add drops of DCPIP solution until the blue colour of the DCPIP has just
appeared and note down the value on the burette
5. Record the end volume in the burette. Minus the final reading from the original
reading. This is the volume of DCPIP reduced by 25cm3 vitamin C solution. Repeat
the procedures and average the result
6. Repeat this procedure and replace the vitamin C with the fruit juices provided.
Repeat each fruit juice 3 times to get a mean result.
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