Lab Write Up 5- Redox Reactions
Student ID- 1794251
Lab partner- George Cann
Date of Lab Experiment- 24/11/17
Lab Experiment: Determination of
vitamin c content found in orange juice and vitamin c tablet
reaction is where one component in the reaction is reduced and the other
component is oxidised. A redox titration is a reaction that occurs between the
analyte, a substance which is being measured and identified, and a titrant
which is a solution used in the reaction to determine the concentration of an
unknown solution. The amount of titrant needed to titrate the analyte can be
measured and dispensed using a burette which can then be compared. In a redox titration,
the analyte is titrated when a significant colour change in observed, this is
known as the end point.
experiment, it involved the measurement of the amount of vitamin c found in
multiple solutions. In order to do this iodine (the titrant) was used as an
oxidising agent to titrate the vitamin c solutions (analyte). The scientific
name for vitamin C is L-Ascorbic acid, and is known as an antioxidant which is
required by the body in order to protect the body from infection and disease. The
use of iodine as a titrant in this reaction, causes the ascorbic acid to become
oxidised and form dehydroascorbic acid.
Oxidisation is the addition of oxygen to a compound and a loss of
electrons. After the ascorbic acid has been completely oxidised, the free
iodine molecules react and combine with the starch indicator molecules in the
analyte solution, and the solution forms a blue-black colour, this is known as
the end-point of the reaction.
is the balanced half reaction equation for the reaction:
6 H+ + 5 e- ? 0.5 I2 + 3 H2O
The simple reaction
equation that takes place is: Ascorbic Acid + I2- ? Dehydroascorbic acid
that as the ascorbic acid is titrated using iodide, iodine that is formed is immediately
reduced to iodide. This will occur until all the ascorbic acid has been
oxidised to form dehydroascorbic acid. Once this occurs the free excess iodine
then reacts with the starch indicator and a colour change is observed and this
is when we know that the reaction has reached its end point as a starch-iodine
complex has been formed.
· To use redox titration method to
determine the vitamin C content of a vitamin C tablet.
· To use redox titration to determine
the vitamin c content of packaged fruit juice.
· To determine and understand which of
the species is oxidised, which is reduced and to identify which component in
the reaction is the oxidising agent.
that the orange juice will have considerably lower levels of vitamin c compared
to the vitamin c supplement, as the orange juice will likely have already been
diluted before the experiment.
Determination of vitamin c content
in fruit juice
1. Firstly, the mass of the vitamin c
tablet is recorded using a weighing scale.
2. The standard solution is then
prepared in a 100ml volumetric flask by dissolving the vitamin c tablet into deionised
3. 20 ml of the standard solution is
then added to a 20ml conical flask, and 150cm3 of deionised water is
4. Then 5mL of 3 solutions is added of
potassium iodide solution, hydrochloric acid and starch indicator.
5. A burette is then used, which is
filled with 0.002 mol dm-3 potassium iodate solutions is used to
titrate the analyte solution.
6. Steps 3-7 are then repeated until
the results become consistent within 0.2ml range of each other.
Determination of vitamin c content
in fruit juice
1. Firstly, the fruit juice is filtered
to remove any pulp or seeds.
2. 50mL of juice is measured using a
measuring cylinder and deionised water is added to make 100mL of solution.
3. Steps 3-7 in part are then followed to titrate
20cm3 aliquots of the orange juice using the potassium iodide
solution in the burette.
4. Measurements of the titrant needed
to see a visible significant colour change are then recorded once they are
of the vitamin c content of a food supplement tablet.
· Average titrant needed= (65.6+64.1+64.3+64)/4=64.5mL
· 0.002Mol x 0.0645L=1.29×10-4mol of ascorbic acid in 20mL
1.29×10-4×5=6.45×10-4mol of ascorbic acid in 100mL
· 6.45×10-4×176.14=0.11g of ascorbic acid
· 0.11g x1000=110mg of ascorbic acid
in 1.26g of vitamin c supplement
· 110mgx3.7=407mg of ascorbic acid in a
whole 4.7g vitamin c supplement
Determination of vitamin C content in fruit juice
· 20ml= 0.002×6.8×10-3=1.36×10-5mol of ascorbic acid
· 100ml= 1.36×10-5×5=6.8×10-5mol of ascorbic acid
· grams of ascorbic acid in solution=6.8×10-5×176.14=0.01g
· 0.01×1000=10mg/100ml of ascorbic
carrying out the experiment, a colour change was observed when the titrant had
fully reacted with the vitamin c solution. This demonstrated the ascorbic acid
being oxidised to become dehydroascorbic acid. As the iodate and iodide reacted,
iodine is produced and then it is immediately reduced to iodide by the ascorbic
acid that is found in both the vitamin c tablet and the fruit juice. The colour
change seen occurred because once all the ascorbic had been oxidised, free
iodine molecules combine with the starch indicator and thus forms the blue-black
colour change that was observed. This gave the optimum amount of titrant that was
needed to form the dehydroascorbic acid.
From the results,
it can see that in the portion of the vitamin c tablet that Is used in the
standard solution had an ascorbic acid content of 110mg. Using this, it is
estimated that a whole vitamin c tablet would have 407mg of ascorbic acid content
within it, which is significantly lower than expected, as the actual vitamin c
content of the vitamin c tablets tested is around 1000mg according to the
container of the vitamin c tablets.
looking at the vitamin c content found in the fruit juice it was much lower
than the vitamin c tablet at 10mg/100mL, compared to the vitamin tablet at
407mg. This is likely to be because the orange juice is already fairly diluted with
water and so the concentration of vitamin c will be much less, compared to the
concentrated vitamin c tablet.
was a success and from the amount of titrant needed to see the colour change,
the concentration of the vitamin c content of both the orange juice and the supplement
could be calculated successfully. One of the main potential limitations to the
accuracy of the experiment, was making sure that the end point colour was consistent
for each of the titrations. However, the amount of titrant that was used to
titrate the vitamin c tablet was fairly consistent with each other, and was within
0.3mL of each other. In order to ensure that the readings were consistent with each
other, test titrations were carried out which ensured our data was accurate. Another
potential limitation was that ascorbic acid was susceptible to the oxygen in
the atmosphere which could have caused oxidation and therefore affect the amount
titrant that was needed to react with the ascorbic acid and therefore affect
the reliability of the results. When using the burette to dispense the titrant,
it was made sure that there were no bubbles present, that could affect the
reading of the amount of titrant that was required to fully titrate the
conclude, the vitamin c tablet had a high vitamin c content at 110mg for the
1.26g sample that was analysed. When estimating the vitamin c content of the whole
tablet this was 407mg, which is significantly higher. The orange juice had a
significantly lower vitamin c content at 10mg/100mL, which could have been due
to the dilution of the juice prior to the experiment when the juice was produced.
My hypothesis, that the orange juice would be significantly lower due to being
diluted before the experiment was correct, and was very low at 10mg, compared
to the vitamin c supplement at 407mg for the whole tablet.