Cpac 3: Vitamin C

Question 1

Method for vitamin C concentrations

Answer 1

1. Start with 100cm^3 of 0.1%
2. Titrate 50cm3 of this as the initial concentration
3. Place the other 50cm3 of this into a 100cm3 measuring cylinder and top up to the 100ml mark with distilled water, you now have 0.05% vitamin C.
4. Repeat steps b & c so that you obtain concentrations of 0.025, 0.0125 & 0.00625

Question 2

Method for main vitamin C experiment

Answer 2

1. Using a syringe (2ml) place 2ml of 0.15 DCPIP into a conical flask
2. Fill the burette with a known concentration of vitamin C (ascorbic acid)
3. Slowly (drop by drop) titrate the acid into the DCPIP
4. Note the volume taken to decolorise the DCPIP
5. Repeat until you obtain concordant results
6. Repeat for all other concentrations of DCPIP
7. Plot a graph of volume of ascorbic acid versus known concentration.
8. Now titrate the different fruit juices against the DCPIP, recording the volume required to decolourise the DCPIP

Question 3

Name of indicator used and colour change

Answer 3

DCPIP, turns from blue to colourless when it is reduced.

Question 4

Independent variable

Answer 4

Type of fruit juice used

Question 5

Dependent variable

Answer 5

The volume of juice (cm^3) required to decolourise 2ml of 0.15 DCPIP

Question 6

Control variables

Answer 6

• same concentration of DCPIP used
• same end point colour (until blue DCPIP has disappeared completely)

Question 7

Conclusion

Answer 7

From the results collected, we can see that different volumes of different fruit juices are required to decolourise the DCPIP. This is due to the varying levels of vitamin C in the fruit juices. Also, it is shown that as the vitamin C content increases, the volume of that juice, required to decolourise the DCPIP decreases. Therefore, the graph shows a negative correlation.