CORE PRACTICAL 2: INVESTIGATE THE VITAMIN C CONTENT IN FOOD AND DRINK

Question 1

AIM

Answer 1

To investigate the vitamin C content in different fruit juices.

Question 2

Independent Variable

Answer 2

Type of fruit juice

Question 3

Dependent Variable

Answer 3

Volume of juice (cm³) required to decolourise 1cm³ of DCPIP

Question 4

Control Variables

Answer 4

1. Temperature – measure temperature with thermometer. Carry out in same place
2. Concentration of DCPIP – 1% solution used each time
3. Volume of DCPIP – 1cm³ of DCPIP solution used each time
4. Shake each tube same number of times (e.g. 3 times)
5. Same end point colour – until the blue colour of DCPIP just about disappears

Question 5

Describe the method for this investigation:

Answer 5

1. Pipette 1cm³ of 1% blue DCPIP into a conical flask .
2. Fill up a burette with the first type of fruit juice to be used and take a note of the start value.
3. Use the burette to slowly add the fruit juice to the DCPIP drop by drop. Swirl the contents of the conical flask with one hand whilst controlling the tap with the other.
4. Close the tap as soon as the DCPIP loses its blue colour and note the end value.
5. Work out how much volume of the fruit juice was needed to decolourise the DCPIP and note this down in an appropriate table.
6. Repeat this procedure for the other fruit juices available. Repeats can be carried out 2 times to obtain mean results.

Question 6

Conclusion

Answer 6

We would expect to see that different volumes of the different fruit juices are required to decolourise the DCPIP. This is due to the varying levels of vitamin C in the different fruit juices. You should see that as the vitamin C content increases, the volume required to decolourise the DCPIP decreases; a negative correlation.

Question 7

Results & Calculations

Answer 7

Results can be recorded in a table as well as a bar chart. Mean values should be calculated from the repeats to produce a more accurate result. We can also work out the mass of vitamin C required to decolourise 1cm³ of DCPIP along with the mass of vitamin C present in the fruit juice samples. We know that 1cm³ of 1% vitamin C solution should contain 10mg of vitamin C:

Mass of vitamin C to decolourise 1cm³ of DCPIP = 10mg × volume of vitamin C used

Mass of vitamin C in fruit juice sample = mass of vitamin C to decolourise 1cm³ of DCPIP × volume of sample required to decolourise 1cm³ of DCPIP

Question 8

WHAT ARE SOME RANDOM ERRORS THAT MAY OCCUR:

Answer 8

1. Difficulty in controlling temperature – use a thermometer beforehand to make sure it is constant and not affected if the fruit juice was refrigerated and is cold, for example
2. Misjudgement of end point – stop adding fruit juice once blue just about disappears. Carry out repeats to minimise the effect of misjudgement errors

Question 9

WHAT ARE SOME SYSTEMATIC ERRORS THAT MAY OCCUR:

Answer 9

1. Too much shaking adds oxygen which will slightly restore the DCPIP to blue – shake contents to mix reactants but keep it to a minimum
   Accuracy of measuring equipment – use equipment with more precise measurements