Good sources of vitamin C from vegetables are broccoli, Brussels sprouts, cauliflower, cabbage, green leafy vegetables, green and red peppers, chillies, watercress, parsley, sweet and white potatoes. Tomatoes, blackcurrants, strawberries, blueberries, cranberries, pineapples, kiwi fruit, papaya, mangoes, guavas and citrus fruits are also excellent sources of vitamin C.
Image 3: Examples of good sources of vitamin C
DCPIP solution may be used to investigate the amount of vitamin C in a solution. 2,6 dichlorophenolindophenol (DCPIP) is a blue chemical compound that is used as a redox dye and indicator for vitamin C. DCPIP is blue when oxidized and colourless when reduced. Vitamin C acts as an oxidizing agent to reduce DCPIP causing the blue oxidized DCPIP, which turns pink in acidic solutions to turn colourless when reduced.
DCPIP (blue) + H+ ——→ DCPIPH (pink)
DCPIPH (pink) + Vitamin C ——→ DCPIPH2 (colourless)
C6H8O6 + C12H7NCl2O2 ——→ C6H6O6 + C12H9NCl2O2
During titration, when all the ascorbic acid has been used up, the solution will remain pink due to the DCPIPH present. Thus, the end point of titration is not a colourless solution but a pink solution in which the colour remains for 10 seconds or more.
Problem Statement:
Which type of fruit juice has the highest Vitamin C content?
Hypothesis:
Orange juice has the highest vitamin C content when compared with lime juice and star fruit juice
Variables:
Manipulated variable: Type of fruit juice used
Responding variable: Volume of fruit juice required to decolourise DCPIP solution
Constant variable: 0.5ml of 1% DCPIP solution
Materials:
1% dichlorophenolindophenol (DCPIP) solution, 1000mg vitamin C tablet, fresh orange, fresh lime, fresh star fruit, commercial orange juice, commercial lime juice, commercial star fruit juice, distilled water
Apparatus:
150ml measuring cylinder, 3ml syringe, 100ml beakers, test tubes, knife, white tile, glass rod, test tube rack
Technique:
Record the volume of fruit juice and ascorbic solution needed to decolourise 0.5ml of 1% DCPIP solution.
Procedure:
- PREPARATION OF STANDARD SOLUTION & STANDARD CURVE
- 100ml of distilled water is measured by using a measuring cylinder and poured into a 100ml beaker.
- 1/8 of the vitamin C tablet is cut by using a knife on a dry white tile.
- The 1/8 of the vitamin C tablet is dissolved into the distilled water to produce a vitamin C solution of concentration 0.125g/100ml.
- The solution is stirred thoroughly to ensure that all the vitamin C has been dissolved.
- The beaker containing the 0.125g/100ml vitamin C solution is then labelled.
- Steps 1 to 5 are repeated by using ¼, ½, ¾ and 1 tablet of vitamin C to produce vitamin C solution of concentration 0.250g/100ml, 0.500/100ml, 0.750g/100ml and 1.000g/100ml respectively.
- 0.5ml of 1% DCPIP is measured by using a 3ml syringe and placed in a test tube.
- 3ml of the prepared vitamin C solution of concentration 0.125g/100ml is measured using a clean 3ml syringe.
- The 0.125g/100ml vitamin C solution is added drop by drop into the test tube containing the DCPIP solution until the DCPIP solution has been decolourised.
- The volume of 0.125g/100ml vitamin C solution required to decolourise the DCPIP solution is observed and recorded in a table (Table 1).
- Steps 7 to 10 are repeated two times for each concentration of vitamin C solution and the average of the result is obtained.
- Steps 7 to 11 are repeated by using vitamin C solution of concentration 0.250g/100ml, 0.500g/100ml, 0.750g/100ml and 1.000g/100ml.
- The results are tabulated and a graph of vitamin C required to decolourise DCPIP solution against concentration of vitamin C solution is plotted (Graph 1).
- DETERMINATION OF VITAMIN C CONTENT OF FRUIT JUICE
- Fresh orange is squeezed to prepare fresh orange juice.
- This procedure is repeated with the fresh lime and fresh star fruit to prepare fresh lime and fresh star fruit juice.
- 0.5ml of 1% DCPIP is measured by using a 3ml syringe and placed in a test tube.
- 3ml of the prepared fresh orange juice is measured using a clean 3ml syringe.
- The fresh orange juice is added drop by drop into the test tube containing the DCPIP solution until the DCPIP solution has been decolourised.
- The volume of fresh orange juice required to decolourise the DCPIP solution is observed and recorded in a table. ( Table 2)
- Steps 3 to 6 are repeated two times for each type of fruit juice and the average of the result is obtained.
- Steps 3 to 7 are repeated by using fresh lime juice, fresh star fruit juice, commercial orange juice, commercial lime juice and commercial star fruit juice.
Results:
Table 1: Volume of vitamin C solution required to decolourise DCPIP solution
Table 2: Volume of fruit juice required to decolourise DCPIP solution
Graph 1: Standard curve
From graph,
When concentration of vitamin C solution is 0.750g/100ml, the volume of vitamin C solution required to decolourise DCPIP solution is 0.40ml.
Discussion:
The higher the volume of vitamin C or fruit juice required to decolourise DCPIP solution, the lower the content of vitamin C in the solution. Based on the result obtained, we can observe that orange juice has the highest vitamin C content with 0.300g/100ml among all the fruit juice used. This is due to the fact that least volume of orange juice is needed to decolourise and reduce the blue DCPIP solution. It is followed by lime juice with a vitamin C content of 0.136g/100ml. Meanwhile, star fruit juice has the least amount of Vitamin C content that is, 0.025g/100ml compared to orange juice and lime juice.
Graph shows the volume of vitamin C solution required to decolourise DCPIP solution against the concentration of vitamin C solution used. From the graph, we can deduce that the volume needed to decolourise 0.5ml of 1% DCPIP solution is inversely proportional to the concentration of vitamin C. As the concentration of vitamin C increases, the volume required decreases.
From Table 2, we can observe that most commercial fruit juice have lower Vitamin C content than fresh fruit juice. In the process of making commercial fruit juices, fruits are transported through long distances from its origin country has cause the freshness of the fruit to deteriorate and hence the significant loss of vitamin C. Furthermore, the fruit juice is added with preservatives, colourings and other additives which affected the vitamin C content of the fruit juices. The fruit juice is also consumed after a long period after being harvested, thus vitamin C is loss through oxidation during this long period of time. However, as with the commercial orange juice, some commercial fruit juices may have more vitamin C than fresh fruit juice because they may have been fortified, that is to manufacture a product with a higher vitamin and mineral content than before it is refined.
Evaluation:
The experiment has to be carried out in a careful manner. Some steps must be taken to further improve the validity and accuracy of this experiment. The vitamin C tablet should have been pounded first with a pestle and mortar and the powdered vitamin C should be weighed out accordingly using an electronic balance to before being dissolved in 100ml distilled water to produce the vitamin C solutions of different concentrations. This is to improve the accuracy of the concentration of the vitamin C solutions prepared. By using a knife to cut, the amount of vitamin C dissolved in the distilled water is inaccurate and frequently less than the theoretical amount as parts of the vitamin C tablet is crushed into powder when force is applied on the tablet. Another method would be to dissolve a whole tablet in 100ml distilled water in a beaker before diluting the solution accordingly by using a measuring cylinder to measure out the exact amount of water required.
Besides, the fresh fruit juices used in the experiment cannot be prepared a day earlier before the experiment or exposed to air for a day. This is to prevent the vitamin C in the fruit juices from being destroyed as it is easily oxidized by the air. Thus, the fruit juices should only be prepared right before beginning the experiment to ensure the freshness of the fruit juices and that the vitamin C content of the fruit juices are highest during the experiment.
DCPIP solution oxidizes easily when exposed to air. The colour of the DCPIP solution becomes darker when oxidised and may result in difficulty to obtain the observation at the end of the experiment. This is because the DCPIP may have been significantly oxidised causing the colour change to be not very visible. During the experiment, when vitamin C or fruit juice is added drop by drop into the test tube containing DCPIP solution, the test tube should not be shaken too vigorously as this may introduce oxygen into the solution causing the oxygen to oxidise the DCPIP and partially restore the dark colour of the DCPIP solution resulting in an inaccurate observation of the volume of vitamin C or fruit juice required to decolourise the DCPIP solution.
Each experiment carried out is also repeated twice or thrice to ensure the reliability of the result. The average value for each type of fruit juice is determined by dividing the sum of all the reading for that particular fruit juice by the number of readings taken. By repeating the same procedure, random errors such as parallax error can be minimized. The content of vitamin C of each type of fruit juice can be determined more accurately and may be closer to the theoretical value.
A syringe with a needle or a smaller scale such as a 1ml syringe may also be used to improve the accuracy of the volume of vitamin C solution or fruit juice used. This is because an instrument with a smaller scale is more precise than an instrument with a bigger scale.
Safety Precautions:
The experiment must be carried out in a careful manner. A lab coat should be worn to prevent clothes from being stained dark blue by the DCPIP solution. Closed shoes should be worn in the lab to provide adequate protection in case of any mishaps. When cutting the tablet into smaller parts, the knife should be handled with care.
Conclusion:
The lower the volume fruit juice used to decolourise the DCPIP solution, the higher the concentration of vitamin C in the fruit juices. Orange juice contains the highest amount of vitamin C when compared with lime juice and star fruit juice. The hypothesis is accepted.
References:
Vitamin C
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DCPIP
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Wikipedia. Dichlorophenolindophenol. Available at: [Accessed on 9 August 2010]
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