The control variables are the temperature,the volume of water used in grinding the fruits and the volume of DCPIP solution used in each test tube.The experiment have to be carried out under constant room temperature because heat destroys the reducing property of vitamin C and the result might vary if the temperature is not constant.The volume of water added and the volume of DCPIP solution used can be measured and control by using pipette.
Assumptions: The fewer drops of juice used to decolourise the DCPIP solution, the higher the concentration of Vitamin C in the fruit juice.
Apparatus: Towel,razor blade,white tile,mortar & pestle,filter funnel,filter cloths,test tubes,test tube rack,100ml beaker for collecting distilled water,250ml beaker for discarding waste,10ml pipette,1ml pipette,pipette filler,droppers.
Materials:30ml of 1% DCPIP solution,kiwifruit,orange,green grapes,lemon
Procedure:
Cut the kiwifruit,orange,green grapes and lemon into small pieces.
Grind the small pieces of each fruit with 2ml of distilled water
Squeeze the extract of each fruit through a filter cloth and put the extracts into respective test tubes.
Pipette 1ml of 1%DCPIP solution to each test tube.
Suck each sample of fruit juice by using a dropper and add in each sample of juice drop by drop to the DCPIP solution in each test tube until the blue colour fades to colourless.
Record the drops of each fruit juice used to decolourise the DCPIP solution
Repeat steps 4 to 6 two times to record the average number of drops of juice used.
Precautions:
The pipette must be cleaned with distilled water and rinsed by the DCPIP solution before used to transfer the solution.
The size of each drop of juice added should be constant.
When adding the drops of juice into the DCPIP solution,make sure the dropper is held vertically so that the juice would not flow to the side of the test tube.
Results:
Table:The drops of fruit juice needed to decolorize 1ml of DCPIP solution
Key: The sign of /’states that the drops of green grape juice needed to turn DCPIP solution from blue to colourless is undetermined. That is due to the drops required to decolorize the DCPIP solution is too many.
Discussion:
The results shows the number of drops of different fruit juice needed to add to decolorize the DCPIP solution, which indicates the different concentration of vitamin C in different fruits. The fewer drops of juice used to decolourise the DCPIP solution, the higher the concentration of Vitamin C in the fruit juice.It shows that orange has the highest vitamin C concentration , then it is lemon and kiwifruit.Green grape has the least vitamin C concentration.
The following are the source of error of the experiment:
There might be personal error when adding drops of juice as the size of drops might not be the same.
A lot of drops of green grape juice are needed to turn the DCPIP solution to colourless which is hard to record and determine.
There might be heat transfer to the juice during the experiment which would affect the vitamin C content.
To increase the accuracy of the experiment,we can do the followings:
1.Repeat the experiment a few times and take the average reading.
2.Dilute the DCPIP solution to a certain concentration before adding fruit juice into it.
Conclusion:
Orange has the highest vitamin C content among the fruits we had examined.