Measuring the content of Vitamin C in fruit juice
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What is the independent variable in the experiment?
Fruit juice/ type of food
What is the dependent variable in the experiment?
volume of juice required to decolourise 1cm3 of DCPIP
Describe an experiment needed to measure the amount of vitamin C is present? ( 9 marks)
- Firstly you need to create a calibration curve. This is done by firstly creating 6 different solution of known vitamin C concentration
- Then you measure out at set volume of DCPIP into the test tube.
- Then using a pipette, chose a vitamin c solution, extract it and drop by drop add it to the DCPIP, shaking it for a set period of time each time its added.
- When the solution turns from blue to colourless, record the volume of Vitamin C that has been added.
- Repeat the experiment twice more, with the same solution, and take an average of all 3 readings.
- Repeat the above procedure with each solution.
- Use the results to make a line graph, showing volume of vitamin C solution against its concentration.
- Then with your unknown solution, using a pipette, add drop by drop the unknown solution to a test tube with DCPIP until it becomes colourless.
- Then using your calibration curve you can find the concentration of Vitamin C in your solution.
What variables need to be controlled in the experiment? ( 4 marks)
- Concentration of DCPIP solution (1%)
- Shake each tube same number of times
- Same end point colour. i.e. until the blue colour of DCPIP just disappears
What sources of errors could arise from the DCPIP experiment (5 marks)
- Difficulty in controlling temperature
- Amount of shaking (too much adds oxygen which will slightly restore the DCPIP to blue)
- End point difficult to judge as needs to be just when blue colour disappears especially in highly coloured juices
- Some loss of solution when transferring from one beaker to another
- Accuracy of measuring equipment
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