Vitamin C Lab
Stephanie Chan 12HT
Biology HL – Mr. Etheridge
Vitamin C Investigation
Ascorbic acid, commonly known as Vitamin C, is a water soluble, essential vitamin for the body (meaning that it has to be acquired from our diet). It has the molecular formula of C6H8O6 and the structural formula can be seen on the right. Vitamin C is vital for the formation of collagen, a main component of connective tissues and the basis for the general shape and structure of the body. Vitamin C not only serves as an integral part of maintaining the function of blood vessels, bones, teeth and many other various connective tissues, it is also an antioxidant. This function allows the Vitamin C to protect the body from other water soluble molecules that could create free radicals (atoms which may cause damage and mutation to cells) when oxidised. Furthermore, it has also been shown in research that Vitamin C stimulates the immune system, and coupled the antioxidant function, it may help prevent and treat infections and diseases (generally used to combat the common cold).
Because Vitamin C is a strong reducing agent – hence its function as a good antioxidant – it can also be easily changed by oxidising agents. This is particularly evident when Vitamin C is exposed to atmospheric oxygen, the concentration will be reduced due to the ascorbic acid oxidising with the surrounding air, and in addition, ascorbic acid is also sensitive to light. This experiment will study temperature, an abiotic factor, on how it affects Vitamin C by calculating its concentration in lemon juice by titrating lemon juice against dichlorophenolindolephonol (DCPIP) and see how much DCPIP is needed to decolourise a controlled amount of lemon juice (2cm3).
Design a quantifiable investigation to look at the effect of at least one abiotic factor – in this case temperature – on Vitamin C.
I think that as the temperature increases, the Vitamin C concentration within the lemon juice will decrease. This is so because when Vitamin C is exposed to the air, it oxidises, modifying the vitamin into dehydroascorbic acid, hence reducing the concentration of ascorbic acid in a solution. Therefore I think that when the temperature of the lemon juice increases, the Vitamin C’s reaction with atmospheric oxygen will increase, lessening the concentration of Vitamin C as it oxidises into dehydroascorbic acid – therefore, the higher the temperature, the smaller the concentration of Vitamin C in the lemon.
Independent – temperature (10°C, 25°C, 40°C, 55°C)
Dependent – amount of DCPIP needed to decolourise 2cm3 lemon juice (cm3)
This is a preview of the whole essay
Controlled – amount of lemon juice used (2cm3)
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
- Gather and set up all equipment as shown
- Label one 500cm3 beaker, “waste”
- Label the other 500cm3 beaker, “lemon juice”
- Pour some DCPIP from the big jug into 100cm3 beaker
- Pour DCPIP from the 100cm3 beaker into a funnel into the burette, with the DCPIP just over the 0.00cm3 reading
- 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
- Halve the lemon with a knife and squeeze out the juice from the halves into 500cm3 beaker labelled “lemon juice”
- Pipette 2cm3 of freshly squeezed lemon juice into a clean test tube
- Record the temperature of the test tube with a thermometer
- Pour the test tube of lemon juice into a conical flask
- Record the initial reading of the DCPIP from the burette
- 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
- Stop releasing DCPIP immediately after the colour of the lemon juice decolourises into a faint pink
- Record the new burette reading
- Throw out the mixture of lemon juice and DCDPIP in the conical flask into “waste”
- Acquire a new conical flask
(if there are no clean conical flasks readily available, rinse out the conical flask previously used with deionised water)
- Repeat steps 8-16 two other times
- Repeat step 8 and place the test tube into a 50°C water bath
- Wait a while and retrieve the test tube
- Read the temperature of the test tube and wait until the temperature drops to 40°C
- 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)
- 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
- 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
Raw Data Table
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
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%
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.
Results Data Table
The Vitamin C concentration uses the mean titre of each temperature.
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.
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.
Wikipedia. (2007). Ascorbic acid. Retrieved October 25, 2007, from http://en.wikipedia.org/wiki/Ascorbic_acid