Uncertainty of volume of lemon juice used = uncertainty of final reading + uncertainty of initial reading
Below is an example of calculation made:
The volume of lemon juice sample used in trial 1 for fresh lemon juice = (45.10 – 3.20) cm3
= 41.90 cm3
Uncertainty = (0.05 + 0.05) cm3
= ± 0.10 cm3
The data calculated can be presented in following table:
Table 2 the volume of lemon juice sample used in each trial.
Next, we shall need to calculate the average volume of lemon juice sample used. Below is an example of calculation made:
Average volume of fresh lemon juice sample titrated = trial 1 + trial 2 2
= 32.80 cm³ + 32.40 cm³2
= 32.60 cm3
Uncertainty = ∑ǀ x-x* ǀ2 where x = volume of lemon juice sample titrated
x* = average volume of lemon juice sample titrated
= ǀ32.80 cm3- 32.60 cm3ǀ + ǀ(32.40 cm3- 32.60 cm3)ǀ2
= ± 0.20 cm3
Therefore, the average value of all lemon juice samples with their respective uncertainties can be presented in the following table:
Table 3 the average volume of lemon juice sample titrated to decolourise DCPIP together with their respective uncertainties.
According to international data standard (July 1980), the mass of ascorbic acid equivalent to 1 cm3 of DCPIP is 0.05 mg. This means that the volume of lemon juice sample titrated to decolourise DCPIP contains 0.05 mg of ascorbic acid.
Let Cs be the concentration of ascorbic acid in diluted lemon juice sample.
Therefore, Cs = 0.05 mg / volume of diluted lemon juice sample titrated.
Let Cj be the concentration of ascorbic acid in pure lemon juice sample.
Hence, a formula can be derived as follows:
CjCs = concentration of ascorbic acid in pure lemon juice sampleconcentration of ascorbic acid in diluted lemon juice sample
It is assumed that the concentration of ascorbic acid in pure lemon juice sample is 100%. Therefore, the concentration of ascorbic acid in diluted lemon juice sample is 4% (because 4 cm3 of pure lemon juice sample is diluted in 100 cm3 of solution). Hence:
CjCs = 100%4%
Cj = 25 × Cs
Where Cs = 0.05 mg / volume of diluted lemon juice sample titrated.
From the formula derived, the concentration of vitamin C in lemon juice sample (Cj) can now be calculated. Below is an example of calculation made to find the concentration of vitamin C in fresh lemon juice.
Cj (fresh lemon juice) = 25 × 0.05 mg32.60 cm³
= 3.834 × 10-2 mg cm-3
Uncertainties:
Percentage uncertainty of volume of diluted lemon juice titrated = 0.10 cm³32.60 cm³ ×100%
= 0.307%
Percentage uncertainty of DCPIP = 0.1 cm³1.0 cm³ ×100%
= 10.0 %
Total percentage uncertainty of the concentration of vitamin C in fresh lemon juice,
= 0.307% + 10.0 %
= 10.307%
Therefore, the concentration of vitamin C = 3.834 × 10-2 mg cm-3 ± 10.31%
in fresh lemon juice
Hence, the concentration of vitamin C in all samples of lemon juices can be presented in the following table:
Table 4 the concentration of vitamin C in different lemon juice samples.
The data in table 4 can be analysed much clearer by plotting a bar chart.
Graph 1 the concentration of vitamin C in lemon juice samples
DISCUSSION
2,6 – dichlorophenolindophenol (DCPIP) is a blue chemical compound widely used as redox dye. The blue colour indicates that it is in oxidised condition, while it will change to colourless when reduced. DCPIP is widely used as Vitamin C indicator. The presence of vitamin C (a good reducing agent) will cause DCPIP to change its colour from blue to pink due to acidic condition, and eventually turn colourless compound after being reduced by ascorbic acid. Below are the chemical equations involved in the reaction:
DCPIP (blue) + H+ ----------> DCPIPH (pink)
DCPIPH (pink) + VitC ----------> DCPIPH2 (colourless)
C6H8O6 + C12H7NCl2O2 ----------> C6H6O6 + C12H9NCl2O2
Vitamin C is one of the least stable of all vitamins in solution and is oxidised readily in light, air and when heated. It is also water soluble. This means that heating in water, (like cooking broccoli in boiling water) causes the vitamin to leach out of the food into the water and also to be oxidised, first to dehydroascorbic acid and then to diketogulonic acid. This last compound has no Vitamin C activity at all and is irreversible. So, the longer the food is cooked and the higher the temperature is used, the more oxidation of vitamin C occurs thus reducing the concentration of vitamin C. As a matter of fact, cooking can reduce the Vitamin C content of vegetables by around 60% possibly partly due to increased enzymatic destruction as it may be more significant at sub-boiling temperatures. In this experiment, a sample of lemon juice is boiled for 10 minutes while another sample for an hour. A longer cooking time will cause more vitamin C molecular structures to be destroyed, which result to a low vitamin C concentration.
From the bar chart plotted, it can be observed that 1 hour boiled lemon juice contains the highest concentration of vitamin C, followed by fresh lemon juice and 10 minutes boiled lemon juice. However, this contradicts with the theory, in which fresh lemon juice is supposed to have the highest concentration of vitamin C, while 10 minutes boiled lemon juice should be the second followed by an hour boiled lemon juice. This is because during the experiment, the 1 hour boiled lemon juice sample is used straight away right after being boiled instead of leaving it to cool down for a while. Since DCPIP is highly sensitive to heat, the warm diluted lemon juice sample titrated will cause the molecular structure of DCPIP to alter, thus causing it to decolourise faster (in fact, much faster than the fresh lemon juice). This explains why the trend observed is not the same with the theoretical trend.
CONCLUSION AND EVALUATION
- Conclusion
1 hour boiled contains the highest vitamin C concentration, followed by fresh lemon juice and 10 minutes boiled lemon juice. Hence, it can be concluded that the longer the cooking time of lemon juice, the lower the concentration of vitamin C. However, when the lemon juice is cooked for a long period of time, i.e. 1 hour, the concentration of vitamin C will be high. This situation contradicts with the theory explained in the discussion part, and may be due to some limitations of the experiment that will be discussed later on.
Therefore, the hypothesis is rejected.
- Evaluation
REFERENCES
- Nutrition in Perspective, Patricia A. Kreutler
- Nutrition Concepts and Controversies, Sizer and Whitney
- http://www.en.wikipedia.org
-
Campbell, Reece. Biology Eighth Edition. Pearson Benjamin Cummings, CA (2008). Page 877.
by
aisyahmaryamroji (student)
