Vitamin C Lab

        Controlled – amount of lemon juice used (2cm3)

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Method

Equipment – one 100cm3 and three 500cm3 beakers, labels, a fresh lemon, a knife, 10cm3 pipette, burette, retort stand, safety mat, conical flasks, test tubes, test tube rack, water baths at 50°C and 80°C, thermometer, DCPIP (1% aqueous solution), deionised water, ice, funnel, pipette bulb

1. Gather and set up all equipment as shown

2. Label one 500cm3 beaker, “waste”
3. Label the other 500cm3 beaker, “lemon juice”
4. Pour some DCPIP from the big jug into 100cm3 beaker
5. Pour DCPIP from the 100cm3 beaker into a funnel into the burette, with the DCPIP just over the 0.00cm3 reading
6. Let some DCPIP pour through the mouth into the 500cm3 beaker labelled “waste”, leaving the DCPIP in the burette to the nearest whole cm3 and ensure that there are no air bubbles throughout the tube
7. Halve the lemon with a knife and squeeze out the juice from the halves into 500cm3 beaker labelled “lemon juice”
8. Pipette 2cm3 of freshly squeezed lemon juice into a clean test tube
9. Record the temperature of the test tube with a thermometer
10. Pour the test tube of lemon juice into a conical flask
11. Record the initial reading of the DCPIP from the burette
12. From the burette, slowly release the DCPIP drop by drop into the conical flask with lemon juice in it, simultaneously gently swivel the conical flask around to ensure that the lemon juice and DCPIP is mixed
13. Stop releasing DCPIP immediately after the colour of the lemon juice decolourises into a faint pink
14. Record the new burette reading
15. Throw out the mixture of lemon juice and DCDPIP in the conical flask into “waste”
16. Acquire a new conical flask
    (if there are no clean conical flasks readily available, rinse out the conical flask previously used with deionised water)
17. Repeat steps 8-16 two other times
18. Repeat step 8 and place the test tube into a 50°C water bath
19. Wait a while and retrieve the test tube
20. Read the temperature of the test tube and wait until the temperature drops to 40°C
21. Immediately repeat the steps of seeing how much DCPIP is needed to decolourise the heated lemon juice (steps 10-16), repeat twice along with steps 18-20 (heating the lemon juice)
22. Repeat step 21 three times but place the test tube into an 80°C water bath and wait for the temperature to drop to 55°C
23. Repeat step 21 three times but place the test tube into a 500cm3 beaker filled with ice and wait for temperature to drop to 10°C.

*If at any point the burette runs out of DCPIP, repeat steps 5-6

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Raw Data Table

Results/Calculations

Because in our experiment, we reversed the titration and found the amount of DCPIP needed to decolourise a fixed amount of lemon juice, the calculations would be as follows:

Sample calculation using mean titre for lemon juice at 40°C

The standard shows that 2.00cm3 of 1% DCPIP decolourises 4.00mg of 0.1% Vitamin C

And from one titration, 2.10cm3 DCPIP decolourises 2.00cm3 of Vitamin C,

To calculate concentration of Vitamin C, the amount of DCPIP must equal to 2cm3 for the data to be quantifiable and equivalent to the standard, hence

In order for the 2.10cm3 DCPIP to equal to 2.00cm3, it must be multiplied by

(2/2.10) cm3 DCPIP = 0.95

And to keep the ratio of 2.00cm3 of 1% DCPIP decolourises 4.00mg of 0.1% Vitamin C the same,

Lemon Juice = 0.95 × 2.00cm3

         = 1.90 cm3

Hence, 1.90cm3 of lemon juice contains 4mg is required to decolourise 2cm3 of DCPIP,

4.00mg Vitamin C/1.90cm3 lemon juice = 2.10mg Vitamin C/1 cm3 Lemon Juice

Because 1mg of Vitamin C is in 1cm3 of solution, or 0.1%,

Therefore the concentration of 1.05mg/cm3 Vitamin C in 1cm3 lemon juice is

= (2.10 × 0.1%) × 100

= 0.210%

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Uncertainties

Pipette uncertainties:

The pipette used in this experiment measures 2.00cm3 of lemon juice ± 0.04cm3. The uncertainty due to the pipette is thus

(0.04/2) × 100 = 2%

Burette uncertainties:

Sample calculation with overall uncertainties

If Vitamin C concentration in lemon juice at 10°C is 0.16%, the uncertainty is 2.93% (calculated above) or 0.16 × 2.93% = 0.004688 or 4.69 × 10-3.

The final answer would then be 0.16% ± 4.69 × 10-3.

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Results Data Table

The Vitamin C concentration uses the mean titre of each temperature.

Graph

Attached.

Discussion and Conclusion

At first look of the graphs, there can be no specific trend seen. However, the data still partially corresponds with my hypothesis – as the temperature increases, the concentration of Vitamin C decreases. Conversely, it is interesting that the concentration is lowest when the lemon juice is at 10°C, contradicting with my hypothesis. This may suggest that the Vitamin C oxidises and decomposes faster than in a hotter temperature, but the most likely reason is firstly, the data acquired for 10°C lemon juice is quite unreliable and inconclusive as we only managed to do one actual titration. Furthermore, another reason why the concentration is particularly low at 10°C is perhaps because we had left the freshly squeezed lemon juice totally exposed to air and light, and by the time we tested the concentration at 10°C, it was nearly an hour after the juice was squeezed out. In addition to leaving the beaker of lemon juice exposed to atmospheric oxygen and light, it may not only affect the lemon juice at 10°C, but with every other titration done as well, rendering all our data slightly unreliable.

However, just from looking at the data points of lemon juice at 25°C – 55°C in the graph, it can be seen that as the temperature increases, the amount of DCPIP needed to decolourise the fixed amount of lemon juice decreases, showing that less DCPIP is needed because there is a lesser amount of ascorbic acid for the DCPIP to react with. Furthermore, the concentration also shows the same trend – the higher than temperature, the lower the concentration of Vitamin C in lemon juice.

As seen from the data, the error bars are close to minimal, showing that Kenneth and I were quite consistent with our method, hence the precision of our data. However, there is an exception with the titrations of room temperature lemon juice. The reason for this could be environmental – the classroom’s temperature could have fluctuated, hence altering the lemon juice and its Vitamin C concentration. Another reason why the mean titre of lemon juice at 25°C (room temperature) may have an error bar compared to the other temperatures could be Kenneth and I were unfamiliar with which point the lemon juice decolourised, and we did our first titrations at room temperature. This leads to one important aspect regarding the reliability of our data - the determination of the end of point the titration. It is hard to determine fully whether or not the lemon juice within the conical flask is decolourised, as the lemon juice itself has its own faint yellow tint.

Another reason why our data might be unreliable is because there were instances where the glassware, particularly the conical flasks used could be contaminated with traces of lemon juice and DCPIP, as we had to reuse the conical flasks, and we could have been careless when rinsing it out.

However, in conclusion, the data still moderately corresponds with my hypothesis that temperature has a relatively significant effect on ascorbic – the higher it is, the faster the oxidation process, the smaller concentration. This is explained when the Vitamin C gets heated up, the molecules get more ‘excited’, increasing collisions with each other and surrounding atoms, hence increasing the number of reactions with surrounding atmospheric oxygen.

Evaluation

One important aspect Kenneth and I could have improved upon was making sure that the lemon juice we squeezed out into a beaker was covered up (perhaps with aluminium foil, at the same time also protecting the lemon juice from exposure to light), because we had left the beaker of lemon juice completely exposed throughout the entire experiment. This mistake could lead to a great unreliability in our data (particularly the titrations done in the latter parts of the entire experiment) due to the addition of oxidised Vitamin C from being exposed to the air.

Another aspect we could have improved upon was the amount of time each test tube of lemon juice spent inside the water bath, as the decomposition of the concentration of Vitamin C can vary depending on how long it has been heated for. Another problem with our method regarding the heating of the lemon juice was that the temperature of our lemon juice was not constant, particularly when we placed the lemon juice in the 80°C water bath and waited for the temperature to drop to 55°C. This method an unreliable way to see how 55°C would affect Vitamin C because the oxidation rate would have been different at 80°C than it would have been if the temperature had stayed at 55°C. In addition, a possible further experiment could be done to see the relationship between Vitamin C concentrations in a solution to the different amounts of time of heating it at a constant temperature (not at room temperature).

Another way to make this experiment easier to calculate the results is to filter the lemon juice through filter paper so we could titrate the lemon juice in a burette like we normally have in previous determining Vitamin C concentration experiments.

Bibliography

Wikipedia. (2007). Ascorbic acid. Retrieved October 25, 2007, from http://en.wikipedia.org/wiki/Ascorbic_acid